A girl and her pet rock (4) 1

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.