THE EARTH
| Larry Taylor's Group has been collaborating with Scientists at the Russian Academy of Science, Siberian Branch in Novosibirsk (Academician Nick Sobolev, Dr. Nick Pokhilenko) and the ALOSA Diamond Company, Mirny (Dr. Zdislav Spetsius) for the last 20 years. The precious samples used in the research endeavors of Taylor and his group come mostly from the Yakutian diamond fields in the Siberian Craton. The extensive international collaborations and the ensuing research continue to be funded by National Science Foundation. |
Inasmuch as the Earth is a planet, a significant portion of our studies address its early evolution, particularly the mantle and interaction with its derivatives, the crust. All of our studies of the Earth are guided by a single goal, to understand the origin and evolution the Earth's mantle and derivative melts. In all of these endeavors, we bring a host of analytical techniques to bear on the problem to be solved: most notably, the electron microprobe for major-element mineral chemistry, instrumental neutron activation analyses (INAA) and inductively-coupled plasma mass-spectrometer for trace-element analyses of bulk samples, the ion probe for trace-element analyses of individual minerals, and thermal ionization mass-spectrometers for radiogenic isotopic (Rb-Sr, Sm-Nd, Lu-Hf, Re-Os) analyses of whole-rocks and mineral separates. We have collaborated with a plethora of distinguished scientists to generate other important sets of data: including oxygen isotopic analyses of silicate minerals (Prof. Robert N. Clayton, Univ. of Chicago; Prof. John W. Valley, Univ. of Wisconsin), trace elements of various eclogites and inclusions (Prof. Ghislaine Crozaz, Washington University), carbon isotopic analyses of diamonds (Prof. Peter Deines, Penn. St. Univ.), FTIR analyses of water in silicate minerals (Prof. George Rossman, Cal Tech), and Secondary Ion Microprobe
- Yakutian Eclogites (Udachnaya, Mir, Obnazhennaya pipes)
Over 15 years ago, we began a study of eclogite xenoliths from the Bells Bank kimberlite, South Africa and were among the first groups to suggest that such xenoliths could represent fragments of ancient subducted oceanic crust. Since 1991, we have extended these studies to eclogites from several kimberlite pipes -- Mir, Obnazhennaya, Zagodachnaya, and Udachnaya pipes -- within the Archean Siberian craton. These xenoliths have allowed us to study eclogite formation from widely spaced locales and have given us an unprecedented look at the lateral variation of subcontinental lithosphere beneath an Archean craton.
Figure 1. Our oxygen isotopic analyses of Udachnaya eclogites have added to our understanding of mantle eclogites as possible oceanic crust. Here eclogites are compared to a possible modern analog, the Samail ophiolite. - Diamond Research
Here at UTK we are studying diamonds, inclusions in diamonds, and rocks that contain diamonds; all in an effort to understand how diamonds form.Diamondiferous Rocks
With our colleagues (Bill Carlson and Richard Ketcham) at the University of Texas, Austin, we are using a high-resolution X-ray technique to see inside of diamond-bearing rocks. This technique is called high-resolution X-ray computed tomography, and is similar to the medical CAT-scan technique. Because diamonds have a slightly lower density and mean atomic number compared to silicate minerals, they do not attenuate X-rays as much as the silicate minerals do. The sample is scanned in a series of two-dimensional "slices" (Figure 1), which can be assembled into a three-dimensional digital model of the sample. We are using these 3-D models to precisely locate the diamonds within the sample (Figure 2) so that we can best plan how to cut the sample to extract the diamonds without destroying the minerals around the diamonds. We can also use volume visualization techniques to determine spatial relationships between the diamonds and the other minerals in the rock.
Figure 1. This is a 2-dimensional X-ray tomography "slice" through an eclogite xenolith from the Udachnaya kimberlite in Siberia. The dark gray shapes near the center and left edge are diamonds. The light gray shapes are garnets, and the small white spots are sulfides. The medium gray matrix is clinopyroxene. A series of these slices, evenly spaced through the sample, provide a 3-D model of the xenolith. These data helped us locate more than 30 diamonds within this rock. The sample is about 4 x 6 cm where this slice was taken. 
Figure 2. This is a 0.2 carat diamond still embedded in the same Siberian eclogite. Comparing the chemical compositions of the minerals in the xenolith to the compositions of the inclusions tells us how the composition of the xenolith changed since the diamonds grew. Diamonds and Their Inclusions
Diamonds often contain inclusions of other minerals. These inclusions were trapped within the diamonds as they grew, and as long as the diamond remained intact (no cracking), the inclusions are isolated from the rest of the rock. The diamond is then similar to a time capsule. This makes the inclusions scientifically valuable because they contain crucial clues to the chemical and physical characteristics of the environment under which the diamond formed. Once we have removed the diamonds from the rock (Figure 3) we can begin to study these inclusions. There are basically two ways to gain access to inclusions inside diamonds: a) destroy the diamond by crushing or burning it, or b) grind away a corner of the diamond until an inclusion is exposed. We have chosen the latter technique because it preserves the diamond, although it is more time consuming.
Figure 3. This is a 0.6 carat diamond extracted from the eclogite xenolith from Siberia. The black spots in the upper left portion of the diamond are mineral inclusions. These inclusions, along with the diamond's irregular shape, make it worthless as a gemstone. - Mantle Metasomatism of Yakutian Lithosphere
Using our extensive, and incomparable, set of eclogite xenoliths from the well-studied Udachnaya kimberlite pipe, we are attempting to decipher secondary processes that could have acted in the subcontinental lithosphere. Through whole-rock trace-element and major-element reconstructions, we hope to identify (and eventually "see through") the character and nature of kimberlitic metasomatism. Once we can characterize the kimberlitic metasomatism, we will be able to "screen out" these effects in order to evaluate earlier metasomatic events in the Earth's mantle and possibly even earlier effects produced by metamorphism in the original subducting and descending oceanic crustal slab
Figure 4. Comparison of Ion Probe trace-elements in silicate inclusions in diamonds and to those of host eclogite silicates from Udachnaya, Siberia. - Early Proterozoic Mafic Magmatism in Western Russia.
Rift-related magmatism within preserved Archean crustal segments during the earliest Proterozoic allows a unique view into the mantle, the degree of melting, an estimate of its composition, and its division into various geochemical domains. Furthermore, comparison of the character of earliest Proterozoic magmatism with that in the Archean should allow us to evaluate various hypotheses of evolution of these mantle domains. Recently, workers have considered that the isotopic compositions of mafic intrusions in the eastern Baltic Shield are due to either 1) extensive crustal contamination of mantle-derived magmas, or 2) direct derivation from enriched mantle. We are currently in the process of evaluating these two hypotheses by looking at the trace-element chemistry of early Proterozoic layered mafic intrusions (the Burakovsky, Monchegorsk, Imandra, and Pana-Tundra intrusions) and associated volcanics in western Russia - Mantle Xenoliths from Kimberlites of NWT, Canada :
Mineralogy and Petrology.
Our Canadian colleagues have recently delivered to us several important peridotite xenoliths from the Northwest Territories. In light of recent important diamond discoveries, study of the mineralogy, petrology, and mineral chemistry of these mantle-derived rocks should help in the understanding the nature of the mantle beneath this region.
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