The Times about time: Geochronology themed article in the NY Times

No real analysis here, but I'd like to draw your attention to this recent article by Kenneth Chang in the New York Times about recent work that might shed light on the very early earth. I am highlighting the article both because it deals with geochronology (U-Pb zircon geochronology to be exact), and because it is a rare article dealing explicitly with geology; not modern climate change or some geoscience themed hazard, but straight up geology. The article also has some good quotes from some of the grand high mucky mucks of geology including Mark Harrison, John Valley, and Norm Sleep. I've seen a talk version of some of this data before, and when I get into my office tomorrow I'll check out the Nature paper as well and try to comment, although in all honesty I seriously doubt I'd come up with much more insightful than Professors Harrison, Valley, Sleep, and Mojzsis.

In consideration of self promotion, if you'd like some background on geochronology to help with the NY Times article, check out this earlier post of mine, or any of the background geology posts I have listed on my sidebar.

FT2008 - Alaska chapter 4 and a big thanks

Before I write any more about FT2008, the International Conference on Thermochronometry, I need to take care of one blog related item. Last week I was named a "blog of note" on blogger.com, and have since seen a drastic increase in my readership. Above is a bar graph of my daily page loads from 10/10/2008 until today. Take the statistics challenge, see if you can tell what day I was named a blog of note. I removed the actual numbers, well, mainly because I have been shamed into realizing I was letting my blog suffer tremendously and therefore are unwilling to admit my average readership. But, thanks to the recognition, my page loads really spiked, that first day they were 2 orders of magnitude higher than average, and although they have settled down, are still 20 times what I am used to. Now, based on the comments, many people want to attribute this to my pretty pictures from Alaska, but that is probably only because they are embarrassed to admit how addictive thermochronology can be. I understand gentle readers, but don't be ashamed, it is OK to admit that you are fascinated by thermochronology, that you now want to quit your career and pursue this new passion, that you now try to work in the phrase "thermally activated volume diffusion" into everyday conversations, and you are constantly frustrated when reviewing papers that compare apparently phase-independent "⁴⁰Ar/³⁹ ages" to U-Pb zircon ages like they are the same thing.

Seriously though, thank you to whoever named me a blog of note, and to all of the people who've had so many nice things to say about the blog and my pictures. I appreciate the kind words.

So back to FT2008, the International Conference on Thermochronmetry. In a previous post, I discussed some of the methodoligical advancements I was most interested in. Today I just wanted to highlight a few of the case studies I found most intriguing. Again, if you are interested in these topics, make sure to check out the free and downloadable extended abstracts from the meeting, available from the Union College FT2008 website. Of course, these will be interspersed with random pictures from the field trips, in no particular order.

• There was one talk and a few posters that dealt with apatite fission-track and (U-Th)/He ages from tunnels in the alps. The talk was by Reinecker, and I apologize for not remembering his first name, and the posters were by Glotzbach and Spiegel. All of these papers were in the Alpine Orogen session on the Thursday of the talk. So why tunnels? Well, these tunnels go straight through significant topographic peaks. Isotherms, or surfaces of equal temperature in the earth, tend to mimic topography, especially at relatively shallow levels. In some ways this is a problem in thermochronology. We often would like to know how fast things came to the surface, but that depends on the depth of the closure temperature isotherm, which in turn depends on toppgraphy (and many other things), which we don't necessarily know. Isotherms are deflected up under large topographic peaks, meaning that if you drill sideways through a mountain, you will experience hotter and hotter temperatures towards the core of the mountain. So I mentioned that the deflection of isotherms is a problem for us brave thermochronologists, but used correctly, it could also be a relatively powerful tool. If topography can affect isotherms, then topography should also be recorded in thermochronometers. The tunnel studies should see evidence for the topography being recorded in the low-temperature thermochronometers. Turns out it isn't so obvious, but I'll leave the abstracts for you to read.

A Blue Grouse (I think, correct me if I am wrong)

• In the last few years there have been a number of studies investigating the link between climate and tectonics. Specifically, which drives which? My own personal belief is that it just isn't an either or, but the idea that climate (namely erosion) could drive crustal processes is kind of hard to swallow for many geologists. Some of the evidence for this involves correlations between erosion rates, rainfall, and uplift rates in active mountain belts. This isn't supposed to work everywhere, there are plenty of places that get tons of rain but where nothing is being uplifted (like the Amazon basin), but many people think of it as a major driver in mountainous regions. Frank Lisker presented a paper on some of his results from Sri Lanka, and what struck me is that the southern part of the island has a rather large mountain (2000+ meters) and gets buckets of rain, but has i n c r e d i b a l l y s l o w uplift rates, slow enough they are reported in meters per million years (typically we report uplift rates in kilometers per million years).
  

More massive piles of Late Miocene - Pliocene conglomerates

So I think that is all I'm going to write on this. It gets difficult to decide what talks to highlight and what talks not to highlight. If you have found any of the things I've discussed intriguing, download and enjoy the abstract volume.

More pillow basalts from the Kenai Peninsula. Seriously, they actually look like pillows!

And my last Alaskan fall picture

FT2008 - Alaska chapter 2

As I mentioned in my last post, the scientific program at FT2008 (The International Conference on Thermochronometry in Anchorage) was overall pretty impressive. I thought I'd highlight a few of the presentations that I found most interesting. As a side note, the extended abstracts for this meeting can be downloaded for free from the official meeting website here. The abstracts vary in length, but most are true extended abstracts with color figures. My discussion is by no means exhaustive, and who knows, I might augment it later. Here are some of my highlights, interspersed with random pictures of mine from the field trips. I am primarily sticking to methodological highlights today, I'll save the others for later posts.

• There were many discussions and presentations by one of the meeting sponsors, Autoscan. Autoscan is an Australian company that has been working to develop an automated fission-track counting system. I am still in the early stages of learning to count tracks, but I've observed the process and know most of the basics. Counting fission tracks is exactly what it sounds like, fission-tracks are etched in acid, and then using a microscope you count the number of tracks in your grain (gross oversimplification, I know, but to make a point). So counting tracks can be tedious, you need to count hundreds of them from dozens of grains to beef up your statistics. Anyways, as nice as it would be to have an automated counting system, the mechanics and potential complications of the process make me wary of trusting an algorithm. That being said, the Autoscan demonstrations are pretty convincing. You can download the demonstration and demo images from the Autoscan website here. Andy Gleadow gave the presentations on Autoscan and led the discussions. He went into detail about how the software deals with some of the more specific problems, comparing reflected and transmitted light images, evaluating overlapping tracks, distinguishing tracks from scratches and dust, etc. By the end I was sold. Again, I am not a certified fission track counter [yet], and therefore am undoubtably missing some important caveats, but Autoscan impresses me.
• Speaking of fission-tracks, there were another set of talks and posters by the group from Union College/SUNY Albany (John Garver and his student Matt Montario) about their recent work using a scanning electron microscope to date high track density zircon samples. The problem is this: Fission-track dating works because with time, tracks form in U-bearing minerals due to the spontaneous fission of ²³⁸U. Old and/or U-rich samples can accumulate so many tracks that they become impossible to count; they overlap and obscure each other too much. The Union/Albany group has developed techniques that allow them to count very high density samples. They do this by using a modified etchant (super secret recipe, well, until they get it published that is) and a scanning electron microscope. Typically, fission-tracks are etched with acid so they become large enough to see with an optical microscope. But, if you have a lot of tracks, this is a problem. So by using a less aggressive etchant, and more powerful microscope, they are able to effectively count samples that would otherwise be useless. I am assuming this will all be published soon, so I'll keep you updated.
• Barry Kohn presented some work he has been doing attempting to reduce single-grain apatite (U-Th)/He age spread in quickly cooled samples. Apatite (U-Th)/He thermochronology has been in widespread use for a little over a decade now, and as more and more data sets are collected, we are starting to identify and grapple with recurring problems. Perhaps the most significant issue are irreproducible single-grain ages. These are samples that appear well-suited for analysis, and have easily measurable quantities of U, Th, Sm, and He. Despite this, it is not uncommon for grains from the same hand sample to show significant scatter, well beyond what you'd expect from simple analytical uncertainty. There are many reasons why you'd actually expect significant single-grain scatter in slowly cooled samples. I won't go into it, but instead refer you to Fitzgerald et al., (2006) for a review. For quickly cooled samples, however, there shouldn't be as many complicating factors. Kohn presented results from his experiments where grains are abraded prior to analysis. Air-abrasion removes the outer rind of the crystals, leaving just a rounded core. Air-abrasion has the potential to deal with the "bad neighbor" problem in apatite (U-Th)/He thermochronology. "Bad neighbors" are U, Th, and/or Sm bearing phases that are close to or in contact with the apatite crystal in the rock. Because the He atoms move about 20 microns or so when they are expelled from their parent atom, He produced in neighboring phases can be implanted into the apatite. You end up with "parentless" He, which gives you artificially old ages. So, the idea is that if you abrade off the rind, you remove the region that could have had "parentless" He implanted into it. Kohn isn't trying to say that this is the only answer or that it always works, but in some of the samples he analyzed it certainly had the desired effect. Namely, abraded grains showed less scatter and were more consistent with fission-track ages and/or other constraints. Obviously still a lot of work to be done, but again, very intriguing.
  

So those are some of the presentations I have thought about the most since I got back from Alaska. I'll have more highlights in later posts. I'll also have more pictures, including a special Alaskan wildlife post, and a brief discussion of our stop at the Wasilla town hall. Yes, we stopped in Wasilla. But before I leave, here is a picture I took of the Exit Glacier, I tried to get the glacial striae in the foreground with the big looming wall o' ice in the background. Unfortunately I couldn't Photoshop out the guard rope and warning sign.

Fitzgerald, P. G., S. L. Baldwin, L. E. Webb, and P. B. O'Sullivan (2006), Interpretation of (U-Th)/He single grain ages from slowly cooled crustal terranes: A case study from the Transantarctic Mountains of southern Victoria Land, Chemical Geology, 225, 91-120.

SoCal Shakeout 2008

When I TA'd introductory geology for my adviser, he'd always have an assignment early in the course that focused on earthquake safety. My graduate school was in Northern California (read "the chosen land"), in an area that straddles one of the most famous faults in the world, and has experienced many destructive historical earthquakes. The assignment was simple; he told the students to go back to their dorm room and do one thing that would make them safer during the next earthquake. They didn't have to turn anything in, but after a week he'd ask for a show of hands to see how many people followed through. Participation was minimal, to say the least. Those people that did raise their hands put in less-than-impressive effort. No one made an earthquake kit (bottled water, batteries, non-perishable food, diet coke, etc..), and just a few removed heavy things from above their bed or double checked their emergency exits. I was even surprised how few graduate students, fellow geologists had proper earthquake kits. I had one, never had to use it, but it took a total of 20 minutes and about $10 to put together.

This is all a preamble to advertise what appears to be a fantastic earthquake preparedness event that I was told about this afternoon. It is called the Great Southern California ShakeOut. The ShakeOut is a series of events that focus on preparing Southern Californians (read "heartless water-stealers") for the inevitable; the next big quake. The ShakeOut has both a blog and official website, and culminates in the largest planned earthquake drill ever in the history of the known universe, on November 13th 2008 at 10 am. Now, I proudly hail from the cultural, intellectual, and political capital of California. I spent graduate school in Northern California, where folks in L.A.L.A.Land get most of their water. Needless to say I have many beefs with the ne'er-do-well neighbors to the south, but I can give the ShakeOut event nothing but support. Contrary to some reports, Southern California is not all bad, and the fact that they are putting so much effort and energy into what is essentially geoscience education and public outreach can make me even [temporarily] forget the 2002 NBA playoffs (also here and here). They also have a great motto on the blog, "because shift happens."

So check out the sites, and if you live in Southern California, get ready for the Big One!

Brian Greene in the NY Times

The better half and I just returned from vacation this week (contrary to the comment I recently received, the blog break has not been NSF-induced, but rather from an incredible week + vacationing.) I am adjusting to the time and lifestyle change fine, and spent most of the weekend getting back on track (including a few visits to a lab undergoing a bakeout). This morning I ran across this article in the NY Times by Brian Greene (physicist and author of many popular science books.) It starts with a description of a letter he received from a soldier stationed in Iraq, the kind of letter anyone who fancies themselves a writer must be thrilled to receive. He then spends some time discussing the status of science education in the US. As a scientist who is interested in science education, I thought it was worth linking to.

My only beef is that when discussing the issue of science education Green writes:

It’s much the same story in classes for biology, chemistry and mathematics.

You might notice that once again geology and/or earth science is not given a place on the list of important sciences. I am sure Greene has no problem with geology, that is not what I mean, I just think it is sad how often the "important" sciences are listed off with nary a mention of the science that studies the planet we live on. Other than that, the article is worth a read. He makes the point that science education is "unassailably vertical [paraphrased]" and too often focuses on teaching skills and forgets to involve the grand ideas, the things that might inspire students to want to learn the skills. Or as he writes:

Like a music curriculum that requires its students to practice scales while rarely if ever inspiring them by playing the great masterpieces, this way of teaching science squanders the chance to make students sit up in their chairs and say, “Wow, that’s science?”

This is the second time I've blogged about a prominent scientist somehow appearing in the NY Times and forgetting to include geology as an important science. The first time is here, in an article that quotes nobel laureate Leon Lederman.

Thermochronologic Sampling

I've posted about thermochronology and mineral separations (twice), but I realized that I haven't yet gone over the field work aspect of thermochronology.

Now, I'll admit up front that the field work I do is not very hard core. There are plenty of thermochronologists who spend months in the classic style, mapping and sampling in some remote part of the planet, I am really not one of them. The most rugged field work I've done was when I was in college, and while the helicopter rides and grizzly bears gave me some geo-street cred, my field work since then has been rather tame. The truth is, as a thermochronologist, you rely on good maps. Your data is meaningless outside of geologic or structural context. Now, it is pretty commong for people doing thermochronology to provide the necessary structural and geologic underpinnings. For much of my work, I've been fortunate enough to work in places that have been mapped very well, and I'm often in field parties with the people who've done the mapping.

Anyways, what I am going to be blogging about assumes you either have or are making the maps necessary for actually understanding your data.

OK, first things first, you can take a thermochronologic sample anywhere there are rocks, but when you plan your sampling, there are some important things to consider.

First, although you can get important information from single samples, the most powerful sampling method involves collecting multiple samples from different structural positions. If cooling and exhumation have been vertical (as is the case with my current project, see this for a more in depth discussion), then this means collecting samples at different elevations. For reasons I'll explain in a later post, it helps to cover as much structural relief as possible without moving too much laterally. The ideal sampling surface in a region with vertical exhumation would be a sheer cliff. In regions like the basin and range, where faults and structures have rotated lithologic units, structural relief might be almost horizontal (like Yerington). Again, this is why you need to already understand your field area.

So lets just say that exhumation has been vertical. Your sampling transect will try to cover the greatest amount of relief possible.

In almost every case, the best lithologies to sample for thermochronology are granites or granodiorites. So to summarize, you are looking for a big granite mountain.

Unless you are attempting some rather specialized analyses, you also want fresh and underformed rocks to sample. Obviously then, avoid altered rocks, weathering varnishes, and rocks that have been faulted or otherwise chewed on. You will also want to avoid the outer few cm's of outcrop because of potential problems caused by forest fires (see Mitchell and Reiners, 2003, Geology v.31, n.12, pp.1025-1028; summary if you don't have access, fires can reset apatites, so don't sample the outer few cm's. They do very interesting things with the data, but I won't go into that right now).

OK, so find your enormous granite mountain made entirely of fresh outcrop (this sounds pipe dreamy, but actually describes some of my current field areas).

So you want to take a whole slew of samples from different elevations. It is pretty standard to sample every 100 meters or so of relief (structural relief or elevation). So, first step, find a place at the right elevation that has fresh outcrop. As below

If the rocks aren't that well exposed or fresh, it helps to spend some quality time with your chisel. I love the ones with the hand guards. If you are sampling granitoids, bring a sledge hammer, not a pick hammer. Sledge hammers (3 or 4 lb head, metal or wood handle) make the job much easier.check out Dr. New Helvetia chiseling away at an oriented sample


If you are interested in mica, hornblende, or K-Feldspar, you usually don't need much sample at all. You can often see the minerals you want to analyze with your hand lens, and can sometimes "field dress" your sample, and take out what you want with your knife (works well especially for books of mica). For apatite, zircon, and monazite, you have a lot more work ahead of you. since the first step in that mineral separation might mean feeding your sample through a jaw crusher, I find it useful to break your sample into bite sized chunks on the outcrop. If you wait to bash up a large sample into smaller pieces back in the lab, you have to spend all kinds of time cleaning up. You don't have to clean up or worry about contamination if you bash things up on the outcrop. Bashing things up also lets you remove the weathered rinds you want to avoid.

Many rocks seem to be either feast or famine when it comes to apatite and zircon abundance. Some rocks I've worked on yield piles and piles of apatite, where I could have gotten away with much less sample. Others yield nothing, and it wouldn't have mattered if I would have brought home a barrel's worth. I usually go for about 3-4 kgs of sample.
So you've now got your mandarin orange sized chunks of amazingly fresh granite, time to bag them. I prefer heavy cloth bags, like those available here at Miners Inc. I don't reuse them, so I label the bajeesus out of the bags with a Sharpie.

I write the sample name, elevation, and some basic info about the geologic context of the sample. I also like to take a picture of the sample bag (with the sample name readable) on the outcrop, kind of a way to help me remember the sample and where I took it. I also take a GPS reading, and plot myself manually on a topographic map. I'll actually check my map location against the GPS whenever possible.

This is also a good way to trick otherwise camera shy members of a field party to have their picture taken. Again, Dr. New Helvetia.
Taking pictures like this also helps me keep my photos organized. For example, I'll have pictures like this below, that I don't write a ton about in my field book (because I am an idiot who is still acquiring skills), but I have it sandwiched in between pictures I have GPS coordinates for, so I'm all good. I was able to recreate the outcrop where I saw these enormous K-feldspars, even though it was towards the end of the day and my field notes weren't as complete as they should have been.

The K-feldspars remind me of salmon swinning upstream.

Now, I usually have a back-and-forth sampling route. Meaning, I'll hike up to the highest point in my field area, take a sample, and then head down, sampling as I go. This saves you from having to carry rocks up hill, which is usually a good thing. I also find it easy to plan time this way. I plan on eating lunch at the highest point, and I know that I can cover ~1300 m of elevation in a day (13 samples) with a good field party (at least 1 other person). You can also note good outcrops on your way up, making the way down more efficient.

Spring [Field Trip] Fever

Here at ESRU we've been enjoying some unseasonably warm weather, even hitting the 80's last week. Although the cold nights have returned, this amuse-bouche of spring has sent my brain into desert field trip mode.

As a undergraduate, my first extended field trip was to the Colorado Plateau. The trip was at the end of my freshman year, we left TMLAC in early May, and spent the next two weeks all over the Plateau. As you might imagine this was a watershed moment in my professional life, although my geoscience experience at the time was limited to one introductory course, but by halfway through the trip (camping in the snow on the north rim of the Grand Canyon), I knew I was going to be a geologist. I've returned to the Colorado Plateau a few more times, including a family trip (my graduate present from my parents), and my honeymoon (belated, but incredible).

In graduate school I either TA'd or hung out on a Death Valley/Owens Valley/White Mountains spring field trip many times. Depending on the course and professor this trip would change specifics, but at its base always involved a week in the desert. The timing of this trip is really perfect. It was usually smack dab between two academic quarters, a time when you were definitely ready for a break. More importantly, the weather this time of year is amazing. If you are lucky, your trip will coincide with the brief but colorful bursts of wildflowers. The high elevations are still cool (cold even, you can find snow fields once you get over a few thousand feet), and the low elevations are not yet absurdly hot. Most of the campsites cool off enough at night to allow for comfortable sleeping, but once the sun rises you can get away with shorts and Tevas. Plus, you beat most of the crowds. Win-win-win.

The desert is one of the things I've missed most since moving out here. In truth the region around ESRU is beautiful and scenic, especially this time of year, don't get me wrong. But it is not the same. Thoughts of these trips have inspired me to post some of my random pictures from the deserts, a sort of virtual field trip.

To fully put myself into the desert, I suppose I could wake up early, make coffee, and fry a bagel in butter (seriously, try this, fry the cut side of the bagel, then add cheese and guacamole, and make into a sandwich.....most amazing breakfast ever.)

So the pics

The end of Monarch Canyon in the Funeral Mountains, looking west into Death Valley.

Racetrack playa, Death Valley National Park

Mono Craters, Owens Valley California

Wildflower preserve east of Arvin, CA, California Golden Poppies in the foreground

Picture taken by moonlight (long exposure) in Arches National Park. Thanks to my brother for suggesting the technique.

Standard issue picture of Delicate Arch, Arches National Park

View looking into Death Valley

Ripples in sand dunes in Death Valley

The Boundary Canyon Detachment Fault, Funeral Mountains, Death Valley National Park. View is looking north, fault is at a low angle (it has actually been overturned) right at the break in color.

Great Gift Idea for that Geochronologist in Your Life

I just wanted to mirror a post on the Arizona Geology Blog in case you know any geochronologists with upcoming birthdays, or wanted to get a head start on the Holiday shopping season. This could be the very first gift item aimed directly at us "something"chronologists. And, seeing that this is the world's greatest thermochronology blog, I figured it was appropriate for me to point you here, where you can learn all about the PaleoClock!

Farallon Plate Part 2

Next in my hall o' fame of geoscience images is the Farallon Plate - North American Plate animtion put together by Tanya Atwater (UCSB). This is one of a series of excellent earth science animations available at this site. This cartoon continues last week's theme, the fate of the Farallon plate, and follows (in map view) the evolution of the western U.S. plate boundary from 38 Million years ago until present. I have long considered this to be one of the best and most useful geoscience illustrations ever produced.

I've seen this animation in a wide variety of talks and lectures. As someone who went to graduate school in California, every discussion of the San Andreas fault would begin with this movie. But I've also seen it used when discussing general plate tectonics, basin and range extension, or this history of magmatism in California.

The movie was made primarily using plate reconstructions. Basically you "unspread" the oceans, using the time and velocity constraints provided by sea-floor spreading anomalies to step things back in time. This is augmented and checked using terrestrial records of deformation. Although it is slightly simplified, it is a fantastic view of the development and evolution of the western U.S., especially California, and the development of the San Andreas Fault.

In the movie you will see a number of things. First off, the thick red lines are the plate boundaries. Separating the Pacific Plate from the Farallon Plate is a divergent plate boundary, where sea floor spreading is making new oceanic crust. The toothed red line separating the Farallon Plate from the North American Plate indicates a subduction plate boundary, where the Farallon Plate is being subducted underneath North America. This plate configuration begins at roughly the same time as the Humphreys figure I blogged about here. The big white arrows are plate velocity vectors relative to a stable north america. The thin black lines on North America are rough outlines of the state boundaries, prior to Basin and Range extension.

So at the beginning of the movie, the western plate boundary of North America is a subduction zone. Beginning in southern California (at about the latitude of Santa Barbara) at ~30 Ma, the divergent plate boundary (or spreading ridge) separating the Pacific and Farallon Plates is subducted. This results in in the formation of a strike slip boundary, the margin we now refer to as the San Andreas Fault.

As more of the spreading ridge is subducted, the transform San Andreas fault gets longer and longer. You also see the initiation of significant extension throughout western North America. These regions are colored a salmon color in the movie. This extension roughly doubles the present width of the western U.S. This extension leads to the formation of the Basin and Range province.

As the San Andreas continues to form, you can watch parts of southern California rotate (the transverse ranges) and/or be translated northwards. Finally, the movie finishes with the modern plate set up.

As simple as the cartoon now looks, it represents a great deal of effort to put together. And, if you are looking to discuss any geologic event in the western U.S. in the past 40 Ma or so, this cartoon provides a fantastic framework.

I've used this movie every time I've TA'd or taught a course, or given lectures to audiences unfamiliar with Cordilleran geology. The figure has the rare ability to talk to all levels of earth scientists. People with no background can really see the development of a very complex plate margin.

Now a quick disclaimer. The website that hosts these movies says they can be used for free for educational purposes. My blog is non-profit and I consider it to be primarily an educational endeavor, which is why I am posting the video. If anyone connected to the video does not appreciate that, please just email me.

Enjoy! And check out the site I reference for a whole boatload of geoscience movies.

Future Geologists

This story appeared in our local paper this morning, and I thought it was worth highlighting. It tells the story of a 5th grader who noticed that a display at the Smithsonian identified the Pre-Cambrian as an "era." He knew this was wrong; the term "era" has a specific use in geologic time, and according to standard time scales the Pre-Cambrian does not qualify. The Cenozoic, Mesozoic, and Paleozoic are eras, the Pre-Cambrian is not. Anyways, many geoscience education posts, especially those dealing with the state of geoscience education in America today, are usually negative or at the very best worrisome. This is good!

And, the fact that there is a 5th grade teacher somewhere that gives proper lessons on geologic time made my day. I doubt I knew anything about geologic time back then. In 5th grade I was an expert on the Chicago Cubs and the 1985 Bears. If the Smithsonian had incorrectly labeled the Cubs 2nd baseman as Ryan Sandberg (instead of) Ryne Sandberg, well hell, I would also have filled out a comment card and made the AP wire. I probably also would have complained about the snack bar.....and so ends the window into 10 year old thermochronic.

Lab Ninjas - ANIMAL edition

I like to think of myself as a lab person. I think on average, I have a decent knowledge of the technical aspects of noble gas analysis. I have built a He extraction line from scatch, I've been involved with upgrades, repairs, routine maintenance, and exploratory surgery on both He and Ar extraction lines. I feel confident in my ability to put together noble gas labs quickly without paying a technician or lab mercenary (that is a special note for all you search committee members out there). I can dumpster dive, resuscitate broken pumps, and leak test with the best of them. Well, that is what I used to think.

Thanks to Dr. Chèvre I recently realized just how much more there is to learn. The good Dr. recently told me about the ANIMAL facility at Auburn University. ANIMAL is an acronym (one that many thermochronologists must be jealous of) for Auburn Noble Isotope Mass Analysis Laboratory. It is the noble gas (mainly Ar/Ar) facility at Auburn, run by Willis Hames and Mehmet Billor (can't find his web page). What makes it amazing though is not just the acronym, it is the fact that the entire apparatus, magnetic sector mass spectrometer included, was built by hand at Auburn, specifically for their lab. Honestly I'd never considered building a mass spectrometer, I figured it was something I'd buy (if I was ever in that position). I've now spent a lot of time on their website, looking at the pictures they have from the development and construction of the machine, looking at their calibration data, it is just amazing. It appears to be a great collaborative effort, involving geologists, material scientists, chemists, physicists, and engineers. Combining experience, resources, and skills, they put this things together from scratch.....and it works! There are even all of these great pictures of students helping to build the thing, polishing the flight tube, wrapping the coil of the magnet, etc.

There are a few things I love about this. First, the design of the machine is actually pretty innovative. Both the extraction line and the mass spectrometer have been optimized in such a way as to significantly reduce the internal volume of the extraction line. The primary modification of the extraction line that impresses me is the lasing system. Most lasing systems have a laser port and sample holder that are attached to the extraction line via stainless steel flex tubes. The laser port is mounted on a motorized stage, so it can be moved under a fixed laser, and samples can be maneuvered into place. this works well, but flex hoses are kind of a pain to deal with. By length, they have much more internal volume than straight tubes, and they are also blessed with much more internal surface area. Both of these facts are negatives from the point of view of signal size and blanks. Some flex hoses are necessary, but it is nice to avoid them when possible. At ANIMAL, the laser port is fixed, using short straight tubing, while the laser beam is mobile.

They call this set up the "flying optic." Basically, the optics of the laser are set up so that both the laser itself and the samples remain stationary, but the mirrors that aim the laser are movable. You can see in the picture above that one of the samples in the sample chamber is glowing hot.

The second great innovation deals with the size of the flight tube. The flight tube of a mass spectrometer can be one of the larger contributions to the overall volume of the extraction line. Large volumes are harder to pump out, but more importantly, the size of your signal is a function of the partial pressure of the gas in your system. Large volumes mean lower pressures, and smaller signals. Many commercial mass spectrometers have flight tube volumes of ~2 liters. ANIMAL is 80% smaller, with an internal volume of ~0.4 liters (that data comes from their website).

So this is all fantastic. One other thing that I think is cool is that they built a viewport into the ion source of the mass spectrometer. That means you can actually see the filament and electrical doohickeys working. I am not sure if this helps in the operation of things, but it is really cool, and doesn't seem to compromise their blanks at all.



So the ANIMAL lab is a good example of Lab Ninjas at work. If you have some time and are interested at all in mass spectrometers it is worth spending some time on their site.

Anyone have a spare Andy Goldsworthy?

I've been a definite blogging rut lately, more than a rut really, more like a chasm, thalweg, canyon, trench, graben........the list goes on. The trouble is that I have some ideas, but I am still unsure of the direction of Apparent Dip. As I've mentioned before, the department I currently work in does an amazing job of bringing in outside speakers. This means that every week I see a new talk, some excellent, most OK, and a few that were truly terrible. I mean embarrassingly bad. Same went for AGU, I saw dozens of talks and posters, had a lot of interaction and feedback, and plenty of blog ideas, but no posts.

My problem is that although I like the idea of blogging about peer reviewed research, I am not sure if my blog is currently a great place for that. I am pseudo-anonymous, meaning I am amazed how many people tell me they like my blog, even though I don't associate my name with it anywhere. I tend to dislike anonymous reviews, so it seems like if I wanted to comment on other people's work, I'd have to remove anonymity completely before posting. But, I am not sure I necessarily like that yet. I will be on the job market again in the not-too-distant future, and I am not sure if random critiques of invited talks is the best thing for the resume.

It is like deciding whether or not to give anonymous reviews for papers. I am always proud of the reviews I've written, but end up chikening out and checking the "anonymous" box. Might it hurt me, might it help me, who knows.

So I was thinking about this dilemma and realized how analogous my blog chasm was to my collection of copper ConFlat gaskets. Let me explain. In order to connect equipment in ultra-high vacuum lines, you can either weld things (huge pain), or use special fittings that use metal gaskets to create amazingly tight connections. One of the most common systems is called ConFlat (when you have a leak in a ConFlat fitting it is called ConFlatulence.) Anyways, these fittings use special copper gaskets, and these gaskets are single use only. For some reason when I started working in a noble gas lab I started collecting the used gaskets. At first I had no idea why, but as time when on I decided that I'd collect all of the gaskets used for my PhD and make some sort of installation art out of the whole thing. This was also inspired by my affinity for the art of Andy Goldsworthy. Andy Goldsworthy is one of the most amazing artists I have ever come across. As an earth scientist who love the outdoors and the beauty of the natural world, Goldsworthy strikes a particularly strong chord. I was actually introduced to his work by a professor in my graduate department. He uses materials that he finds locally, and spends hours and days and weeks constructing the work. Many of his pieces are left to erode naturally, that is actually one of the tools he uses, showing the response of the art to time and the elements.

Anyways, if there was a local material in a noble gas lab that one would try to make some sort of art with, I'd have to guess it would be copper gaskets. Every lab I've worked in has a pile of these things, waiting for someone to figure out a good way to recycle them, or for the price of copper to skyrocket. I want to make something out of them, but I'd like suggestions.

So like my blog, I have all these ideas, but for various reasons have yet to pull the trigger. Below are some images of the gaskets, if you have any inspiration by all means pass it along. And if you have suggestions on how to deal with the problems of blogging about peer research pre-tenure, by all means I'd love to hear it.




For scale the viewport (steel ring with the window) is ~3 inches in diameter.

Closure Temperature

I've alluded in some of my previous posts (aquí und hier) to one of the fundamental concepts of thermochronology, the idea of a closure temperature. Today I ended up re-reading some of the first thermochronology papers I ever read, back in the day. Long story short I was thinking about what the term closure temperature means today, and decided that after a year of blogging it was high time the the world's foremost thermochronology blog did something with this fundamental idea.

Most people trace the idea of a closure temperature, or at least the term, to a classic 1973 paper written by Martin Dodson (Dodson, 1973). The issue, as Dodson describes, is that some radioactive daughter products (he was concerned with Sr and Ar, but this also applies to other daughter products) are mobile long after a rock has crystallized. By mobile, I mean that they can readily move out of a crystal. If the daughter products are lost, there is no way to calculate a radiometric age. Well, I take that back, you would calculate an age of zero. Once a mineral cools enough, then the daughter product can accumulate, and the radiometric "clock" has started. Before I go on, I will make another self reference to a post where I point out that OG, Original Geochronologist R.J. Strutt mentioned this problem a century ago. OK, that's done.

It would be nice if this transitional temperature were finite. That is, that there was a set and known temperature above which the daughter product diffused readily, and below which the daughter product is entirely retained. This theoretical temperature is called the closure temperature (usually written as T_c); the temperature below which the mineral is closed to diffusion (in the past few years I've noticed this called a Dodsonian closure temperature, in part recognizing Dodson's contribution but also recognizing the necessary simplifying assumptions his 1973 paper made, more on that in the next paragraph.) In the real world, however, there is actually a broad range of temperatures over which daughter products can, and do, diffuse. This range of temperatures is often referred to as a partial retention zone.

When you start trying to pin down exactly what that means, physically, it gets tricky. The truth is, things like Ar and He never really stop diffusing out of crystals. Even at room temperature, for example, Ar and He diffuse, just so slowly that it really doesn't make much of a difference. Dodson of course realized this, and provides a very zen definition of what he thinks the term closure temperature means:

Closure temperature of a geochronological system may be defined as its temperature at the time corresponding to its apparent age.

So you see, grasshopper, it is what it was. This of course gives no direct way to assign a temperature to an age. Dodson showed the idea graphically this way:

The top graph shows the thermal history of a sample, in the form of temperature decreasing with time. The bottom graph shows the daughter to parent ratio of the sample over the same time interval, with the assumption that at high temperatures the daughter escapes easily, so the ratio is zero. At some temperature, daughters start to be retained, and as cooling continues, the daughter/parent ratio increases with time. If you use the present day ratio to calculate an age, and then see what temperature the rock was at at that time, well, that is the closure temperature of your system. Unfortunately, as I'll talk about in a bit, rocks don't come with the top graph, or even the bottom one for that matter, all we have is the present day daughter to parent ratio.

Mathematically, Dodson worked out a way to calculate the closure temperature, given a few assumptions and with a few other knowns thrown in. Here is the Dodsonian closure temperature equation

The best description I've ever heard of a closure temperature was when I was in grad school, a visiting über-thermochronologist said that a closure temperature is really just thermodynamic shorthand for a whole suite of physical and chemical properties. In the equation above, R is the gas constant, E_a is the activation energy of the daughter product's diffusion in the mineral of interest, A is a geometric factor considering the shape of the host mineral the daughter is diffusing out of, D_o is the frequency factor (another diffusive constant describing the rate the daughter product), a is the size of the diffusion domain in the mineral of interest, and dT/dt is the cooling rate. D_o and E_a are things that are determined through laboratory diffusion experiments, A and a are specified depending on the situation. I'd like to point out a few things about the equation. First, of all the inputs, E_a is the strongest control on the closure temperature; the other variables are locked up in a natural log term. Second, you have to input a cooling rate. But how do you know the cooling rate without thermochronologic ages and closure temperatures? Laboratory measurements of of the diffusive parameters are actually pretty good, but assigning a closure temperature to a system (say He in apatite, or Ar in biotite) still requires knowing a priori the cooling rate. In practice, people often quote a closure temperature assuming an average cooling rate, which is almost as meaningless as saying average geothermal gradient, and which should read geologically reasonable cooling rate (just like it should be a geologically reasonable geothermal gradient,) but I digress. In truth, for a wide range of geologically reasonable cooling rates, the range of closure temperatures isn't that great, certainly within an acceptable range of uncertainties for most applications. Where you run into problems are on the extreme slow cooling end of the spectrum. Minerals that cool very slowly can have drastically reduced closure temperatures (some slowly cooled Chinese samples I worked on have apatite He closure temperatures of about 57°C, about 30% lower that the quoted closure temperature of He in apatite.) The situation gets even more tricky, because this definition of a closure temperature, at least the derivation of the equation, assumes cooling that is linear in 1/T (or that in a time versus inverse temperature plot the thermal history is linear.) Dodson explains why he did this in the paper, and why it is a reasonable assumption for the purpose of the paper, but you can probably guess that rocks that have experienced very complex thermal histories that include perhaps prolonged time at moderate temperatures and/or cycles of heating and cooling will not fit this model.

The upshot of all of this, in my opinion, is that by themselves, thermochronologic ages are meaningless. They are non-unique, and therefore just numbers. Assigning meaning to a thermochronologic age requires external constraints, namely a good understanding of the geology of your study area (good maps, cross sections, and tectonic reconstructions) and multiple thermochronologic controls. There are some excellent software packages that allow you to use many different thermochronometers at once to iteratively constrain each other, usually considering many potential variables are deciding what thermal histories are consistent with all of the available data. My personal favorite is Rich Ketcham's HeFTy. But it all really starts with the geology.

Like many complications in science, the problems inherent in assigning a temperature to a thermochronologic age are increasingly being turned into tools savvy thermochronologists can actually use to their advantage. As I've said before, calculating a geochronologic or thermochronologic age is simple and straightforward. Assigning geologic meaning to that age takes time. It turns out that this problem is especially important to the understanding of the "low temperature" thermochronometers. Since these have seen a huge boom in use and importance in the past decade, it is no wonder that the problem of what closure means is
receiving more and more attention.

I hesitated at including this as the final figure, but decided to include it with a few disclaimers. First, these are all approximate range of closure temperatures for common thermochronologic systems assuming cooling rates of ~10-100 °C/My, and, I do not mean to imply that U-Pb zircon has a classic closure temperature, only that Pb is retained in zircon at exceptionally high temepratures, and typically is more reflective of the time of crystallization than of cooling through a specific temperature. Also, deciding what thermal importance to assign to your particular thermochronologic age requires a great deal of thinking, that is, these temperatures are relevant for the kinds of systems and tectonic settings I typically study, not necessarily those you happen to study. For example, if you work in hydrothermally altered areas, all bets are off. That being said.

References, etc..

Dodson, M.H., 1973, Closure Temperature in Cooling Geochronological and Petrological Systems, Contributions to Mineralogy and Petrology, v. 40, pp. 259-274.

And, although it is focused on the ⁴⁰Ar/³⁹Ar world, Ian McDougall and T. Mark Harrison's excellent book Geochronology and Thermochronology by the ⁴⁰Ar/³⁹Ar Method (Oxford University Press, 1999) gives an excellent discussion of closure temperatures, their meaning, and their derivation.

Deskcrop #4

Embarrassingly I did not post to The Accretionary Wedge centered on Deskcrops. I have some lame-o excuse, but that's all it is. So, belatedly, here is the next installment of my deskcrops.


As an undergraduate I was the beneficiary of an amazing alignment of geologic planets. My first year as a major (sophomore year) was also the first year a new professor was teaching at TMLAC. He happened to be a co-PI on a large NSF funded project, and had included funding for undergraduate research in his portion of the proposal. I turned out to be a good student, and for two summers (after sophomore and junior year) was able to do field work with him, and an excellent cast of high-falootin' geologists in northwest British Columbia. It was really an amazing experience for me. The first summer I went from school ending, to a 2-week geology field trip in the northeast, to 6 weeks of field camp, and then straight to British Columbia. Incidentally, that summer I spent all but 2 weeks total living out of tents and sleeping bags. It was awesome.

My main project, what later became my senior thesis, focused on a large tonalitic pluton near the town of Prince Rupert. I was mapping the contact, and looking at variations in the magmatic fabric of the pluton as it related to a major bounding structure. The thesis is embarrassing to read now, but it did end up as a talk at GSA my senior year (Salt Lake City, 1997.) Anyways, even though I was mainly looking and sampling the tonalites, we did venture into the country rock every now and then. The sample above is one of my favorites. This is one of many chunks I brought back, although I've never seen it in place. This particular chunk was whacked from a big piece of float while waiting for the helicopter to come and pick us up. Yes, perhaps the coolest thing about the field work was that we got to use helicopters, not typically for day trips, but we'd get flown in and left for a few weeks of field work, then picked up and given a few days to shower and get away from the black flies (and hit Cowpuccino's). I digress, back to the deskcrop.

This is a piece of the country rock, a garnet-sillimanite gneiss. How much sillimanite you ask? Well, roughly 70%. I basically see this rock as a metamorphosed beer can, the amount of aluminum is really astounding. I was on a field trip to the Funeral Mountains in Death Valley where one of the big stops was where we could see sillimanite (you can, in Monarch Canyon, walk from a brittle detachment fault into sillimanite grade schists and gneisses.) When we got to the outcrop, I was a tad unimpressed. The sillimanite we spent the afternoon hiking to occurred as fuzzy growths on biotite, something you needed a hand lens to see. Previous to this my entire field experience with sillimanite was where it was the dominant mineral in the rock. I have since learned to appreciate all forms of sillimanite, but this is still my favorite. I have yet to find many reference for this rock, although it is kind of famous among the people who work up there. There was one outcrop that was mapped and sampled just prior to being blasted away to make a road, but other than that it has only been found as float. As such, I cannot provide much insight into it's geologic significance, but I have about 15 pounds of it in my office.

O.G. (Original Geochronologist) R.J. Strutt

Blogger's Note - Today we are digging into the archives of Apparent Dip. I am working on my AGU poster and not up for a brand new post, so I thought I'd re-post one of my earliest entries. My audience has grown since I first put this up (1/16/2007), and it is one of my favorites. So enjoy.

As much as I like the idea of being a field geologist, anyone who knows me also knows that the bulk of my graduate (and most likely post-graduate) geology career took place in a lab. Not just any lab, mind you, but a noble gas thermochronology lab. I primarily worked on (U-Th)/He thermochronology. In the past decade, (U-Th)/He thermochronology has exploded in popularity and has become a relatively common and useful thermochronologic tool. Of course, the more we learn the more potential problems and pitfalls we see, which is good, because that means there are plenty of papers left to write. To show you how the techqnique has really taken off, below is a chart showing the number of georef hits for (U-Th)/He by year. (I compiled this data myself rather quickly, so I am sure I am missing some relevant papers.)



In many ways, (U-Th)/He thermochronology is a cutting edge technique. But, it is also the first radiometric geochronometer. As far back as 1905, super-scientists like Ernest Rutherford and R.J. Strutt were estimating the age of rocks and minerals based on their measurements of U, Th, and He. I'd like to focus on one of these papers today, the one that I am most amazed with. It was written by R.J. Strutt in 1910:

Strutt, R.J., 1910, Measurements of the Rate at Which Helium is Produced in Thorianite and Pitchblende, with a Minimum Estimate of their Antiquity: Proceedings of the Royal Society of London, Series A, Containing Papers of a Mathematical and Physical Character, Vol. 84, n. 571, pp. 379-388

I found this paper on JStor, which most academic libraries have access to. Reading this paper and those it references I am first blown away that they could measure U, Th, or He in the first place, especially He. I spent months and months with very fancy equipment trying to accurately measure the amount of He trapped in apatite crystals. Of course, I was trying to measure much smaller quantities with much higher precision, but I am still astounded by the ingenuity with which these labs were built. For example, to meaure the rate at which He was produced, Strutt first dissolved the material in various liquids (usually combinations of acids), and placed the solutions in this contraption


The solution was allowed to sit for some period of time for the helium to accumulate. Then, the helium was gently boiled off and collected in a test tube inverted into a pool of mercury. (I'd love to try to get this experiment approved by the Health and Safety folks at the University nowadays). The collected helium was transferred into this set up



Here, the helium, in the test tube on the left. The gas would be let into the apparatus (evacuated with a mercury pump), and then the tubing would be filled up with more mercury, pushing the helium along until it was confined to area c, which is a cooled charcoal trap used to clean up the gas (an idea still used today in He thermochronology thanks to nifty devices like this



from Janis Cryogenics). After a while the helium is "drawn" into part "d" (not sure how that is done), and part "d" is filled with even more mercury, pushing the helium into the capillary "g" where the volume of helium can be measured using the length of tube the gas occupies and the pressure of mercury that is pushing it up there. As someone who regularly complains about high-tech devices that dare to come without GPIB ports or LabView drivers, this is slightly humbling. So, amazing fact #1 is that they could actually accurately measure helium in the first place.

Amazing fact #2 is that they could measure helium production rates from both U and Th with decent reproducibility.

Amazing fact #3 is that they all didn't die from Mercury poisoning (curiously, however, the lab assistants are never named)

But, the most amazing fact, that would be #4, is that the ages Strutt calculated, and most importantly the conditions he applied to interpreting that age, are really pretty good.

Below are his results from that paper

Strutt refers to these as "minimum ages," according to him "...because helium leaks out from the mineral, to what extent it is impossible to say"

In earlier papers, specifically one called Leakage of Helium from Radio-Active Minerals (Same journal as above, v. 82, n. 553, pp. 166-169), Strutt discusses some of the reasons helium "leaks" out of geologic materials, spending significant time talking about temperature. Thermally activated helium diffusion is of course now the basis for He thermochronology, something he alluded to in 1909.

So, he realizes that these are minimum ages, and his reasons make perfect sense. But his minimum ages are really not that bad. Realistically they are all good minimum ages for the time period they represent (8.4 Ma for a minimum age for the Oligocene, 31.0 Ma for the Eocene, 150 Ma for the Carboniferous, and 710 Ma for the Archean). This both blows me away and makes me wonder why it took me so long to get a lab running! It also makes me thankful that there are now good alternatives to mercury filled McLeod gauges and mercury pumps.

So, tonight I raise a toast to the O.G.'s of this world, the Original Geochronologists. I'll put another plug in for JStor, they have so many of these early papers there for the downloading.

Predicting Science News

Andrew over at About.com (the geology page) posted his fantastic idea for the upcoming Fall AGU meeting. Prior to the meeting he tries to predict what ten research projects presented at AGU would get the most print. That is, which talks or posters would show up in newspapers, magazines, and on-line. He batted an astonishing .900 last meeting (see this for his write-up on the process.) Both the prediction and the post-meeting media analysis can serve as good starting off points for science and society discussion. If I were in a position where I had students, I could also see this as a good way to make the connection between current research and the science page of their local paper.

Andrew is soliciting entries to either be posted on his blog, or linked to from his About.com page. Links are above or in my sidebar.

As a note, I know that most of you when compiling your lists will search through the abstracts for exciting terms like "thermochronology," "k-feldspar MDD modelling," or "apatite (U-Th)/He." For some unknown reason, print media has been slow to pick up on the Thermo-craze, so while I know that those are the stories you all think SHOULD be reported on the most, I'd suggest trying other themes. Someday print media will understand what the rest of the earth science community already knows (low-temperature thermochronology is hot), but for now, things like earthquakes, hazards, and climate change get all the ink.

Deskcrop (?) #2 - Mantle Xenoliths

As pointed out by Ron and Kim, the next samples from my rock collection for posting are my mantle xenoliths. Xenoliths are pieces of a pre-existing rock that get incorporated into a magma but for any variety of reasons, do not melt. When the magma crystallizes the xenoliths appear as distinct bodies and are usually pretty recognizable. You find xenoliths in all sorts of igneous rocks: basalts, granites, andesites....really almost anything. Somtimes the xenoliths are pretty local in origin. This summer while sampling granites, for example, we found xenoliths a few kilometers from the pluton-country rock contact that were easily identifiable as pieces of one of the wall rock units.

Magmas that rise through the crust relatively quickly can incorporate pieces of all of the rocks they pass through. Magmas with deep origins, therefore, can bring up pieces of the lower crust, or in some cases, even the mantle.

The mantle xenoliths I have in my office are from Kilbourne Hole, a maar in New Mexico that is part of the Pleistocene Potrillo Volcanic field. Maars are explosive volcanoes that form when magma flash heats groundwater. In the case of Kilbourne Hole, a basaltic magma carrying pieces of the lower crust and mantle erupted sometime between 80 and 17 thousand years ago. The xenoliths are medium to coarse grained peridotites, with P-T-ometery suggesting origin depths up to 67 km (Thompson et al., 2005). One of the things I always think about with chunks of the mantle is how odd our perspective is as geologists. I like to show these rocks off, even to non-geologists, because they are odd-looking and distinct. But, if you assume that some flavor of peridotite (or related olivine- and pyroxene-rich ultramafic rocks) makes up the entire mantle, then this is volumetrically the most abundant lithology on earth. Most of us make our living studying the dynamics of the outermost scum of the planet. The lithologies we regard as common, ones that I wouldn't even bother displaying on my window ledge, are really some of the rarest. It is just our limited surficial perspective that makes mantle rocks seem rare, and granites or shales seem abundant. There are of course good reasons for this, but it always sticks in my head.

First, a field map (taken from Thompson et al., 2005.) I collected these rocks on a field trip in 1996 while in college. We spent two weeks going up and down the Rio Grande Rift. Kilbourne Hole is on the west side of the rift, where the obvious rift features start to give way to the Basin and Range province. The xenoliths occur as volcanic bombs, often with thin basalt crusts. Collecting the xenoliths is pretty simple, you basically walk around on the rim of the crater, picking up bomb shaped items, and cracking them open. Well worth the drive if you are in the area.


And a google maps view, you can see Kilbourne Hole as the bluish splotch in the middle of the field of view. The Potrillo Volcanic field includes the big pockmarked region to the west of Kilbourne Hole. Las Cruces is the city in the northeast part of the view, at the intersection of the interstates. Las Cruces is the home of the Whole Enchilada Fiesta. Man, I miss mexican food.

Now the mantle peridotite xenoliths!

Thompson, R.N., Ottley, C.J., Smith, P.M, Pearson, D.G., Dickin, A.P, Morrison, M.A., Leat, P.T., and Gibson, S.A., 2005, Source of the Quaternary Alkalic Basalts, Picrites and Basanites of the Potrillo Volcanic Field, New Mexico, USA: Lithosphere or Convecting Mantle? Journal of Petrology, v. 46, n. 8, pp. 1603-1643; doi:10.1093/petrology/egi028. Available here.