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 3 - Alaskan Wildlife

So no geology this post, but instead I thought I'd post some of the wildlife pictures I took on the Denali and Kenai Peninsula field trips. First from the Kenai peninsula fjords cruise, some Stellar Sea Lions, hanging out on pillow basalts. I guess I lied, I said no geology, and here I am tossing around the term pillow basalts. This species is endangered, and if you look at the guy in the center, you can see he has a numbered brand. The decline in sea lion population is a little confusing, allegedly, and the tatoos help track them throughout their range. The pillow basalts, while not endangered, are still fantastic. This cruise actually included some of the best pillow basalt exposures I've ever seen firsthand.

Next up, a black bear! To attempt to head off any scolding comments or emails, I did not approach this bear. I was walking down a path when I came on a student who had stopped. She had been there for a few seconds. She had walked around a corner, and a youngish black bear had seen her and ran into a tree. I had my camera out, snapped this picture, and quietly walked away, trying not to attract anyone else down the walkway.

What would Alaska be without a moose? What this picture doesn't show is the other 50 people on the side of the road snapping pictures of this moose.

I took this last picture for my Mom, just to say that we did not see any of the famous Alaskan "chikens" on the trip.


Enjoy!

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.

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.

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.

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.

Deskcrop #3 - Ventifacts

Today's deskcrop is a ventifact. A ventifact is a rock that has been abraded by the windblown particles: sandblasted if you will. They typically have a heavily grooved or polished surface. Large ventifacts can have the grooves aligned with the prevailing wind direction. Small ventifacts, like mine, appear to not remain stationary, and therefore can develop a "brainy" texture. Ventifacts are not all that common, reflecting the relatively small importance of wind erosion. Water (both solid and liquid) is far and away the most powerful erosive agent around; wind is much less significant. In order to see well developed ventifacts, it helps to look in places where the wind is absurdly strong, there are abundant loose and abrasive particles (i.e. sand), erosion from water is relatively minor, and you don't have to worry about plants or soil covering things up. My samples are from Ventifact Ridge, in Death Valley National Park. (legal note, these were collected by someone who had the proper permit to take a few chunks). The rock itself is a basalt, Pliocene in age I believe (but cannot confirm right now).


Besides general interest, ventifacts and other eolian-erosion related features are the best earth analogs we have for many of the images sent back from the different martian landers. Below are images taken by the Viking and Pathfinder landers thought to represent Martian ventifacts (from Greeley et al., 2002.) Greeley et al., is an interesting paper that compares images sent back from the Martian landers to eolian features in the Mojave Desert and in Iceland.


In addition to dry environments, ventifacts need time to form. I haven't found any references for the average amounts of time needed to create well-developed ventifacts, but I imagine it is strongly dependent upon wind speed, lithology, and the type of particulate matter being thrown through the air. References would of course be appreciated.

Greeley, R., Bridges, N.T., Kuzmin, R.O, and Laity, J.E., 2002, Terrestrial analogs to wind-related features at the Viking and Pathfinder landing sites on Mars. Journal of Geophysical Research, v. 107, n. E1, 10.1029/2000JE001481.

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.

The outcrop I've been carting around Part 1

For the past year I've been living in the academic equivalent of the penthouse apartment. I was the sole inhabitant of a two-room office; the outer room has two enormous windows looking out over the campus. It is a first floor office, but elevated enough that I have an excellent view. This past week, the postdoc population of my group, and consequently my office, doubled. This is an excellent development on a lot of different levels, one of which is that I actually tend to work better with someone else in the office (well, I am sure there are exceptions to that, but in general it is true.)

So when I moved into the office, I had way more space than I needed. Consequently my crap kind of exploded all over the place. My rock collection was no exception. I ended up having them all piled on the heating grate under the window. This served double duty, not only was it a place to pile the rocks but it also helped regulate the air, which in the winter is set at "Thermochronic total fusion", and in the summer "Cryogenic cold trap."

So I had to rearrange all of the crap in my office, including the rocks. I've decided to photograph and post about some of my favorites.

First, I want to draw a distinction between my rock collection and my sample collection. I've been collecting rocks since my first Introductory Geology field trip (heh, some of those first Wisconsin Ordovician dolomites and glacial erratics are still in a bucket in my parent's garage.) Rocks in my collection were all picked up on field trips, during field work, while hiking, plus a few gifts. I rarely took very good notes on the samples when I picked them up, or if I did the notes are long gone. This was especially a problem early in my career as a geologist, I've been trying to work on it, but the end result is I have a lot of rocks with vague pedigrees.

Samples, on the other hand, I could take back to their place of origin and more than likely put them exactly where they came from. Samples all have GPS coordinates, they are marked on maps, and I have pictures of the outcrops before and after sampling. Why I can't replicate that for random rocks I pick up I have no idea.

So I moved all of my collection over to my side of the window, it is a little more compact, but I think it still works. Notice how I am covering up about a third of the air vent.



Today's sample is one of my personal favorites. It is something I picked up during my first year in graduate school, when I was a field assistant for my office-mate. Incidentally, Clastic Detritus provided the original inspiration for putting this rock on a post. (And, as a general inspiration, Loose Baggy Monsters recent post about her desk also inspired me to both clean and post a general shot of my workspace. Unlike her post, however, you can actually see my desk in these pictures.) As far as I remember, it is an Early Cambrian shale that I collected along the Transcanadian Highway in eastern British Columbia, just east of Golden B.C., and about as close to the Burgess Shale as you can get on the highway. I remember my office mate telling me we were actually in a late Pre-Cambrian unit, but perusing as many geologic maps as I can find makes me think we were firmly in the Cambrian. I'll hunt some more, but we'll see. Anyways, these are burrow trails in a really old shale. A trace fossil! Our ancestors!



This has always been one of my favorites to show off. Whenever I've participated in people's practice qualifying exams I use this as one of the test rocks. It is amazing how many people want to call it a metamorphic rock, staurolite schist or something.

So more to come, some with better explanations. But I bet that most professional geologists reading this blog have similar piles in their office. Show them off! Let me know and I'll link in this post.

Also, there are outcrops and roadcrops.... so what are these called? Deskcrops? Workcrops? Any suggestions? I am trying to play with the word diamictite, but it is just not working right now.

Red Rock Canyon State Park

Red Rock Canyon State Park, in southern California, is usually the first stop on the Death Valley field trip my graduate department used to run over spring break. I had the chance to TA this trip once, and went on the trip a few other times. These pictures are from the apring of 2006, the last time I went on the trip, and the last time I've been in the area.

The Red Rock area has some spectacular exposures of Miocene fluvial sandstones, eroded in many areas into badlands style topography. During the Miocene, basin and range style extension was active all throughout western North America, eventually creating the corrugated morphology we see today. As I mentioned in this post, as the crust was stretched apart, it was broken into upflited ranges and downdropped basins.

The ranges are beautifully exposed, from California to Utah. Many geologists have made their careers understanding the ranges. The basins though, are buried, and therefore much more difficult to study. There are a few places where the basins have been dissected or are otherwise well exposed, and Red Rock Canyon is one of them.

The pictures are various views of the area, showing the Ricardo Formation dipping to the west. The beds have variable dips, reflecting their deposition during basin formation. In the first picture, I am standing on one of the interbedded basalt flows. In the background of the image you can see a more gently dipping surface.




And a close-up of the actual rocks

The Ricardo Formation is famous for it's fossils. This page is dedicated to the paleontology of Red Rocks, and includes boatloads of images of some of the finds. And to finish, some scenery from near the Ricardo campground.

A Joshua Tree

And a student on his way back to camp.

The Barton Garnet Mine and Fall in the Adirondacks

This Saturday I was able to tag along on a mineralogy field trip to Gore Mountain, in the Adirondack Highlands, home of the Barton Garnet Mine. I was especially excited about this, even as far back as my mineralogy course (12 years ago?) I remember seeing samples of these garnets. First a little background.

The Barton Garnet Mine is an open pit garnet mine that operated for more than 100 years (1878-1983). The mining level is now below the water table, so to avoid the costs of pumping water out of the pit the operational mine has moved about 4 miles away. What is left at the original mine is now open to the public. The biggest part of the tourist attraction is the first pit (shown below), where you can search for gem-quality chunks of garnet. The garnets themselves can be enormous (10cm or so), but small chips of them, especially some of the darker red chips, are gem quality, can be cut, and are therefore potentially valuable. That is the main draw it seems, you can keep whatever you find (well, at $1 a pound, but that is almost nothing for chips of garnet).

The picture above is from the main pit, where you can collect the chips. You can see it was a superb fall afternoon. The mine is at about 2600 feet, and with the wind made it pretty chilly, but that just kept other folks away.

The garnets are entirely hosted by a hornblende-rich garnet amphibolite unit that is itself in fault contact with a meta-syenite to the south, and grades into an olivine meta-gabbro to the north. The garnets can be enormous, up to 35cm in diamter, even though the modal percent of garnet is not all that unusual for the Adirondacks (5-20%). I think the most impressive thing about these garnets is how the are often mantled with thick haloes of hornblende (shown below). The larger the garnet, the larger the hornblende halo. The garnets are largely pyrope (Mg-garnet 37-43%) and almandine (Fe-garnet 40-49%)

Although it is pretty and would make an exceptional counter top and/or stone fireplace, the primary reason this garnet is mined is as an industrial abrasive. Garnet is hard (8 or so on Mohs scale, although according to our guide this particular garnet can top out over 9), and because garnet does not have any cleavage (which means it is a self-sharpening abrasive, every time it breaks it is a conchoidal fracture, which creates a new sharp edge), garnet is a very good abrasive. Most sandpaper is made with garnet, the polishes used on the lens of the Hubble telescope were garnet based, most sandblasting is done with garnet, the list is endless. And (again according to our guide), 95% of the world's industrial garnet comes from this mine. The founder of the mine (Henry Hudson Barton) was actually a jeweler who married into a wood-working family, and used his knowledge of minerals to pioneer the use of garnet as the abrasive in sand paper.

Aside from the garnets it was a great day outside. Fall has arrived in the Adirondacks, and the colors were exceptional. Just some pictures from right near the visitor's center at the mine.
To finish with the pictures of fall, I'm just going to end with this. Fall colors always remind me of this poem, and in particular of cruising down M street on the back of my Dad's bike on my way to school.

October's Party by George Cooper

October gave a party;
The leaves by hundreds came-
The Chestnuts, Oaks, and Maples,
And leaves of every name.
The Sunshine spread a carpet,
And everything was grand,
Miss Weather led the dancing,
Professor Wind the band.

The Chestnuts came in yellow,
The Oaks in crimson dressed;
The lovely Misses Maple
In scarlet looked their best;
All balanced to their partners,
And gaily fluttered by;
The sight was like a rainbow
New fallen from the sky.

Then, in the rustic hollow,
At hide-and-seek they played,
The party closed at sundown,
And everybody stayed.
Professor Wind played louder;
They flew along the ground;
And then the party ended
In jolly "hands around."