Tag: pet rock

Categories
feminism pet rock

Diamonds are interesting, but I have no desire to take them to the movies.

Interesting post here about “The facts about diamonds.” The author of the post mostly focuses on the cultural/social aspects of diamonds, and for the most part I agree with him. I’ve always found jewelry commercials in general irritating, and even more so the ones that dig up the rotting corpse of “diamonds are a girl’s best friend” and display it on national television. I don’t like the message that women are shallow beings that can be bought off with a shiny bauble; it’s demeaning for women (we’re coin-operated sex bots) and men as well (since apparently men have nothing going for them except their ability to give us shiny things.) It’s not any better if you approach it from the angle of “jewelry as a means for men to show off their wealth” since that places women squarely in to the category of an ornament for men, the vehicle by which they do their social posturing.

Bah. Bah, I say.

I actually do own two pieces of jewelry that involve diamonds. One of them is a small pair of earrings that a good friend of the family gave me for my birthday several years ago. I bring them out for special occasions. The other is actually my engagement ring. It wasn’t something I asked for; I always told Mike that if he wanted to get married, I’d be just as happy with a plastic ring out of a vending machine, or no ring at all. But Mike is an earnest, wonderful guy, who likes to feel as if he’s doing things properly when he’s moved to do them. In this case, that meant finding a really cool looking ring (no standard gold band with a rock on it for him) and giving it to me at the most bizarre moment imaginable. I think that’s what makes me feel okay about the outward appearance of tradition, there; I didn’t demand anything, I didn’t expect1 anything, and Mike did what he did because he had the financial means and wanted to. As anti-diamond and anti-jewelry as I tend to be, I also respect that in the great game of give and take that is a relationship, I’ve got to do my share of giving.

I like the shiny diamond ring and wear it every day because I love Mike to bits and know how important it is to him. Not the other way around.

I’m always left wondering, between the slime of advertising campaigns and these little events that make up my own life, where I sit relative to other women. Are there actually women whose affection can be bought by jewelry? I hope not, and I’ve never personally known any, but I also don’t think I’d be friends with someone like that to begin with. I’ve already learned far more about the seedy underbelly of human relationships than I ever wanted to know, just while trying to plan a wedding.

Social stuff aside, diamonds themselves are, I think, pretty interesting rocks. If nothing else, they intersect nicely with my favorite non-sedimentary rock, kimberlite. As far as anyone has ever seen, you don’t get diamonds unless there’s an Archean-age craton for the kimberlitic eruption to punch through; what we get from those kimberlites are the little bits and bobs that the magma carried up with it. This is why you get diamonds in Canada (and even in Wyoming), but not in Colorado. We’re just a bit too far south of the remaining, long-buried Archean age rocks.

So, there was something about geological conditions back in the Archean (about 2.5-3.7 Ga) that allowed diamonds to form then and not since. So any “natural” diamond is quite old. There was much higher heat flow and there was full mantle melting back then, as opposed to the partial melting we get today. This different melting/depletion of the mantle probably is what allowed diamonds to grow.

Cratons are actually part of the lithosphere, the basement that the crust sits on top of. They’re also remarkably stable; it’s actually a matter of great interest how the Archean cratons have managed to hang in there so long. So the majority of diamonds – which haven’t been dragged to the surface by a kimberlitic freight train – “live” more than 100 km below the surface.

Which is why Steven Shirey says:

“Diamonds aren’t just for spectacular jewelry,” commented Shirey. “They are scientific gems too.”

Jewelry? Meh. Science? WOOHOO!

1- Literally. He caught me completely by surprise, the brat.

Categories
pet rock

Using Geochemistry to find Kimberlites

I found this article pretty interesting: The GOPE 25 Kimberlite Discovery, Botswana, Predicated on Four Mg-Ilmenite Grains from Reconnaissance Soil Samples: A Case History

Basically, some kimberlites were identified using indicator minerals that came from the soil above the pipes. One of the important indicator minerals was ilmenite (which I had in my own pet rock from Green Mountain). Even more interesting, the scientists used the chemical signatures of these ilmenites to infer if the kimberlites in question were likely to contain diamonds. That’s some pretty cool stuff.

Categories
backyard geology igneous stuff pet rock

A girl and her pet rock (4)

So I just realized that I owe a post for today. I spent my entire day working on my geology term paper, so I have nothing all that clever or interesting to say. So I guess instead of falling down on the job completely, I shall share the rough draft of my paper. Yay?

Unfortunately I can’t really include the pictures, so you will miss the treat of my extremely awful, juvenile-looking hand-drawn cross section. It’s one step up from MS Paint, but a small step at that.

I’ve also now submitted my two grad school applications. Keep your fingers crossed for me!

Also as a note, the pictures of what I’m referring to in the petrochemistry section can be found right here.

Genesis of the Green Mountain Kimberlite

Introduction
The Green Mountain Kimberlite is located in a mountain park/open space near the city of Boulder, Colorado, at approximately latitude and longitude 39º59.431’N, 105º18.09’W.

The Green Mountain Kimberlite intrudes in to the Boulder Creek Batholith, which is primarily composed of Precambrian granodiorite. There are no rocks other than the granodiorite and kimberlite exposed in the immediate area, and no evidence of other intrusions at the surface.

The exposed kimberlite contains no identifiable rock fragments that are younger than Precambrian in age. Larson and Amini (1981) attempted to track the age of the kimberlite using fission track ages on apatite and sphene within the rock. The apatite fission tracks yielded the highly suspect age of 77.1 ±5 million years, while the sphene fission tracks yielded a more reliable age of 367 ±15 million year. This number agreed with other kimberlite emplacements near the Colorado-Wyoming border and was considered reasonable at the time, under the assumption that all kimberlites in the region were emplaced at approximately the same time during the Devonian. However, a later study used 40Ar/39Ar of phlogopite from the kimberlite to determine a maximum emplacement age of ~865 million years, though that age was considered suspect within the study due to problems with Ar degassing and anomalously low initial 40Ar/39Ar ratios. Using 147Sm/144Nd ratios taken from megacryst samples from the kimberlite, the same study found an age of 572 ±49 million years for emplacement (Lester et al, 2001). This dating of the Green Mountain Kimberlite agrees with that of the Chicken Park Dike in the same study; the two kimberlite intrusions are compositionally similar to each other, while being significantly dissimilar to other kimberlites in the area, making the difference in age seem both reasonable and logical. At this time, the evidence points to the Green Mountain Kimberlite being emplaced in the Paleozoic, at 572 ±49 million years ago.

Lester et al (2001) have suggested that their dating of the Green Mountain Kimberlite as Neopaleozoic in age puts the emplacement in line with the break up on the Rodinia supercontinent and suggests a tentative link between the two events. If this is the case, the Kimberlite resulted from an extensional tectonic setting, in which the kimberlitic magma flowed up through deep fissures and zones of crustal weakness related to the extension. The formation of the kimberlite came from the melting of mantle peridotite mixed with volatiles, most importantly CO2, though the source of these volatiles is not immediately apparent in the scenario of the Rodinia breakup. Another possible scenario for the generation of the kimberlitic magma is hot spot activity, though the evidence for such activity in North America is so thin as to be nonexistent (McCandless 1999).

Petrochemistry
The Green Mountain Kimberlite is a porphyritic, with a fine-grained ground mass surrounding large phenocrysts. The phenocrysts in the thin section examined were serpentanized olivine sometimes with apparent remnant olivine, phlogopite, biotite, and large calcite crystals. There was also a 1-2mm in diameter opaque of unknown type in the sample, and infrequent but identifiable orthopyroxene. The ground mass is fine grained and rich in calcite, as well as opaques. Boctor and Meyer (1979) identify the major mineral components of the kimberlite as diopside, ilmenite, Cr-rich and Cr-poor almandine, olivine (serpentanized and not), orthopyroxene, biotite, phlogopite, and calcite. No large garnets were identified in the thin section, but it is very possible that some small garnets exist in the ground mass, which remains mostly dark at all angles under crossed polars. Ilmenite is an opaque mineral and as such cannot be identified with true certainty in the thin section, but considering its abundance within the kimberlite, it is likely that a significant percentage of the opaques in the ground mass are ilmenite. The ground mass is also rich in calcite.

Boctor and Meyer also note the presence of Perovskite within the Green Mountain Kimberlite, though it is a mineral not easily identified within the thin section. However, the presence of the perovskite does suggest that the mantle peridotite source of the kimberlite interacted with CO2-rich fluid, which allowed the chemical interactions to create the abundance of Nb and REE in that mineral.

Conclusions
The formation mechanism for kimberlite magmas in particular is still a topic of great discussion among geologists (Heaman et al, 2004), and unfortunately the genesis of the Green Mountain Kimberlite remains murky. In general, the kimberlitic magma that produced the Green Mountain Kimberlite must have formed due to the interaction of mantle peridotite with volatiles, particularly CO2 and water. This volatile interaction is further supported by the abundance of calcite phenocrysts and in the ground mass of the kimberlite, as well as the Nb and REE-rich Perovskite found within the kimberlite by Boctor and Meyer (1979). Probably prior to the partial melting, the peridotite had undergone at least one episode of metasomatism. The source of the volatiles for this metasomatism and melting is unclear; there is little evidence for a mantle plume in the area, and the existence of a nearby subduction zone is likewise unclear (Heaman et al, 2003). After the formation, the magma was forced upward under high pressure, most likely following deep crustal fissures or zones of weakness related to the break up of the Rodinia supercontinent. This rapid, pressurized intrusion (and ultimately extrusion) of the kimberlitic magma explains the existence of granodioritic xenoliths within the kimberlite, taken from the surrounding Boulder Creek Batholith during the kimberlite’s intrusion. With even the age of the Green Mountain kimberlite still a matter for debate, little more can be said about the rock’s formation with any degree of certainty.

References
Boctor, N. Z., Meyer H. O. A. Oxide and sulfide minerals in kimberlite from Green Mountain, Colorado. In: The mantle sample – inclusions in kimberlites and other volcanics (F. R. Boyd and H. O. A. Meyer, editors), Proceedings of the Second International Kimberlite Conference, AGU, Washington DC, v. 1 (1979), pages 217-229.

Heaman, L. M., Bruce A. Kjarsgaard, Robert A. Creaser, The temporal evolution of North American kimberlites, Lithos, Volume 76, Issues 1-4, Selected Papers from the Eighth International Kimberlite Conference. Volume 1: The C. Roger Clement Volume, September 2004, Pages 377-397, ISSN 0024-4937, DOI: 10.1016/j.lithos.2004.03.047.

Heaman, L. M., B. A. Kjarsgaard, R. A. Creaser, The timing of kimberlite magmatism in North America: implications for global kimberlite genesis and diamond exploration. Lithos, Volume 71, Issues 2-4, A Tale of Two Cratons: The Slave-Kaapvaal Workshop, December 2003, Pages 153-184, ISSN 0024-4937, DOI: 10.1016/j.lithos.2003.07.005.

Larson, E. E., M. H. Amini. Fission-track dating of the Green Mountain Kimberlite diatreme, near Boulder, Colorado. The Mountain Geologist, v. 18 (1981), pages 19-22.

Lester, A. P., E. E. Larson, G. L. Farmer, C. R. Stern, and J. A. Funk. Neoproterozoic kimberlite emplacement in the Front Range, Colorado. Rocky Mountain Geology, v. 36, no. 1 (2001), pages 1-12.

McCandless, T.E. Kimberlites: mantle expressions of deep-seated subduction. In: J.J. Gurney, J.L. Gurney, M.D. Pacsoe and S.H. Richardson, Editors, Proceedings of the Seventh International Kimberlite Conference vol. 2 (1999), pp. 545–549.

Categories
pet rock

A girl and her pet rock (3)

Well, I did my photomicrographs of my thin section today, and I think they turned out really well! There’s some beautiful examples of what calcite looks like in thin section if nothing else. I’m still sad that all of my olivines have been altered by weathering, though; normal olivines are very bright and pretty. Altered, they’re just interesting shades of gray.

Pictures of my thin section!

Also, I’ve got some nice phlogopite mica in my sample. Phlogopite is a hydrated mica; in the pictures that I have, it’s very hard to tell it apart from biotite, actually. The tip off is that it’s in kimberlite (very hydrated) and that under plane light it looks different as you rotate it. It goes from honey-colored to almost transparent, which is unfortunately not something you can really show with photos.

Categories
pet rock

A Girl and Her Pet Rock (2)

I went back to the rock lab on Monday to finish up my thin section. Which I should have picked up from the rock lab this morning as a matter of fact. Except that I forgot. You see, I had a brain once. Then I forgot it somewhere and I’ve been screwed ever since. Hopefully I’ll remember to pick it up tomorrow, so I can get on with the important business of doing the photo micrographs and examining it in detail under the microscope.

Over the weekend, Paul (the guy in charge of the rock lab) glued the wonderful flat face that I’d made on my pet rock onto a glass slide with epoxy. Epoxy is the go-to glue for rocks, and has the benefit of having a very index of refraction, since everything is in face viewed through a cloud of glue once you get your slide put together. This is important since one of the identifiers used to discern minerals is what kind of relief they have – which is to say how much they stand out from the epoxy. Quartz and feldspar barely stand out at all, while garnet and olivine have very sharp outlines against the epoxy. So this basically means that quartz and feldspar have indexes of refraction very close to epoxy, while garnet and olivine don’t.

My task was to get rid of most of the rock glued on to the slide, then. All but about 30 microns of it, to be more exact. I started out by cutting most of the rock off using another diamond saw. Then over the course of about 15 minutes, I used a grinding wheel to take layers off of the remaining rock until what I was left with was basically transparent. The point of a thin section, of course, is to have a piece of rock so thin that light can shine through it; hard to image a translucent piece of rock, huh? I was doing pretty well with it. The rock I chose, Kimberlite, is actually very soft as igneous rocks go, partially due to its high calcite and hydrate mineral content.

The hardest, scariest part was actually manually grinding down the last little bit, using the glass plate and very fine carbide grinding powder. What makes that the scary part is that you have to constantly stop and check to make certain that you haven’t taken off too much, and that you’re polishing it up evenly. I actually lost a strip off of one of the edges of my thin section because I didn’t stop to check soon enough. Kimberlite is soft, soft stuff. I had to be much, much more careful going forward, since if I’d lost much more off the slide I would have had to start over and make a new thin section. In the end, I managed to get it fairly even, thankfully!

Before I left the lab, Paul let me check the section under his microscope, using cross-polarized light. It was pretty indeed – lots of calcite and phlogopite mica, as well as a gigantic opaque. All in all I’m very pleased with how it turned out, except that almost all of the olivine in my specimen has been serpentinized – which is to say that it’s been exposed to enough weathering that the original olivine has altered in to serpentine. There’s a spot of remnant olivine here and there, but most of it’s serpentine, which isn’t nearly as pretty. I was hoping that I’d get more olivine, but I would have had to really dig in to the outcrop to get some, and I didn’t want to do that. At least the rest of my minerals still look really pretty!

Categories
pet rock

A Girl and Her Pet Rock (1)

One of the cool things we’re doing for this field class is cutting a thin section of a rock that we picked up on one of the field trips. Chuck, our teacher, has been calling these rocks our “pet rocks.”

I picked my pet rock up at the Green Mountain Kimberlite. And named it Bobby. Yesterday it was time to cut Bobby up to start the creation of the thin section. This involved going to the rock lab in the basement of the geology building, which is an interesting place filled with all sort of intimidating power tools. Some of the saws would probably be more at home in horror movies.

Actually, I started out with three chunks of kimberlite, then showed the benevolent dictator of the rock lab, Paul, all three. Since I’ve never cut a thin section before, I had no idea which would be best. He immediately picked the smallest of the samples, which was also the “chunkiest” since it would be easiest to cut.

I put on a plastic apron (flecked with rock dust rather than the horror movie alternative, thankfully) and some extremely silly looking protective glasses that fit over my own. Paul turned on the saw, which was very, very scary looking. It was a water-cooled affair, so there was a constant drip of water on to the blade. He explained that it was a diamond-bladed saw, though different from the ones most people are used to. The strangest part is that it’s actually very difficult to cut yourself with this particular saw. Paul even touched the blade a couple times while it was running, just to show this. At least as far as flesh goes, it’s yielding enough that your skin and wobbly bits will just flex out of the way of the blade. So if you want to cut yourself, you really have to jam your finger on to it. Or apparently come at it fingernail first, because the saw will just rip through anything solid like tissue paper.

I was very glad I’d recently trimmed my fingernails.

I was pretty intimidated by the saw at first, but it helped that one of my classmates went before me so I could see how he was doing things. I’m not a big fan of power tools, and I’m not a hands-on kind of person.*** My version of being handy is, when forced by circumstance, fishing out the little tool kit my dad gave me and picking up a screw driver. This only happens when my fingernail, my scissors, and my fiance’s pocket knife have all failed to defeat a screw. The only power tool I’ve ever used is a screwdriver. I’ve seen UHF way too many times to be comfortable around saws.

But anyway, once I worked myself up to actually using the saw, it went really well. I sliced Bobby in half length-wise, then trimmed the half the stayed intact down to the right size to fit on a slide. The other half (the thinner half, I think) broke apart as I was running the rock through the saw. I even kept the cut pretty even.

Though of course, a circular saw is not nearly a delicate enough tool to make the sort of even cut necessary for a thin section. When all is said and done, the thin section is going to be so thin that light shows through it. The sort of thin that you measure with microns. (Like your average runway model.) So once the gross cutting was done with the saw, I powered it down and then basically sanded the surface completely flat. You use two different grades of grit on a glass plate, coarse then fine, and basically just sand the thing down until it’s absolutely flat and smooth.

It’s funny, but Paul spent a lot of time telling me and my classmate to not “pet” the smooth surface when rinsing the grit off. And as laughable as that is, it’s hard to do. When something’s that smooth and polished, your fingers just itch to touch it. It’s bad to do so, though, since oils from your hands interfere with the epoxy that gets used later.

So that was the first step. On Monday, step two!

*** Unless you count the time in Fire Academy, but I’d still say there’s a big difference between cutting a car apart with hydraulic sheers and getting your fingers anywhere remotely close to a spinning saw blade.

Categories
backyard geology igneous stuff pet rock

Backyard Geology: The Green Mountain Kimberlite

Unfortunately, I can’t provide very good directions to this one, and there’s a good reason for it. We drove up to Green Mountain (near Boulder, Colorado), got on one of the trails, and then at a random time just sort of bombed off into the underbrush. It involved going down and back up an extremely steep stream valley where there wasn’t even the hint of a track. Steep, like I’m clinging to trees to keep myself from tumbling down the slope steep. It was a very, very, very rough hike for someone with bad knees and an often embarrassing lack of balance. It’s only about a mile and a half, but it feels much, much longer.

About the best I can do right now is give you the lat/lon of the outcrop: 39º59.431’N, 105º18.09’W. These coordinates should have about a 20 foot accuracy if you believe the claims of the GPS unit’s manufacturers.

That said, the hike is very, very worth it. Big important note, though: the kimberlite is in park land. I honestly have no idea what trouble if any there could have been for us going off trail the way we did, but I know for certain that you’re not supposed to bring in a rock hammer and whack samples off the outcrop.

The kimberlite itself is very interesting. It intrudes through the Boulder Creek granodiorite, which is a holocrystalline intrusive rock with large crystals of quartz, feldspar, and mafic minerals. If you run across Boulder Creek outcrops, they have a distinctive “salt and pepper” appearance. In comparison, the kimberlite is a porphyritic extrusive rock where the ground mass is extremely dark. The samples we found contained large garnets, ilminites, and olivines. The weathered surface of the kimberlite is gray rather than black, with the chemically altered phenocrysts much more obvious by color difference.

The outcrop is mid to upper slope and stands out fairly well from the landscape. There are no trees growing in it. The outcrop itself is about 100 feet in diameter, though on the down slope it elongates into a teardrop-like shape due to the erosion of the slope.

So, a tough hike, but very cool rocks.

Kimberlite is actually one of my favorite igneous rocks, mostly because it’s very cool to look at in thin section. Much of the fine-grained ground mass in kimberlite isn’t actually silicate minerals – it’s calcite. This makes it incredibly colorful when looked at with crossed polars.

The story behind kimberlites is also very cool. They are effectively volcanic dikes, but rather unusual ones. Kimberlitic magma is produced when there’s a critical mass of volatiles in an area of the mantle, normally carbon dioxide and water. (The large amount of carbon dioxide present is the reason kimberlites contain so much calcite.) The volatiles lower the melting point of the surrounding mantle material, and with the sudden pressure on a body of volatile-filled magma, the results are explosive. The magma exits the mantle upward and comes exploding out of the crust, in some cases at the speed of sound. This incredibly explosive, violent eruption of magma under high pressure is what gives kimberlites their characteristic carrot or funnel-like shape.

Also, since the eruption of a kimberlite is so violent, they often carry significant chunks of everything they went through to get to the surface. This includes pieces of mantle peridotite – most of what we know about the mantle composition came from samples brought up in kimberlites. In certain areas, this also means that the kimberlite brings up pieces of old continental crust – most importantly, pieces of the remaining cratons from the Archean. And these craton bits are where diamonds come from. Kimberlites can be small (like the one on Green Mountain) or enormous, like the ones that are mined for diamonds in Africa.

And the best part? Technically speaking, we could get another one erupting out of the ground at any time. There’s no way of knowing. There’s just something cool about that thought, though I wouldn’t want to be standing on top of one when it made its appearance.