GRADUATE STUDENTS' RESEARCH
Chondrules are small, generally-round "balls" that are the dominant constituent of chondrite meteorites. While the formation of chondrules is still largely unknown, we do know that heating of the material in the nebula was involved. Something else we know is that chondrules were not quite the first materials to have formed in our solar system. We know this because chondrules can be found to contain "relict grains" - little bits of crystals whose composition and/or texture don't fit with the other crystals in the chondrule that are "left-overs" from the chondrule-forming process. So, if we know that there were materials that existed before chondrules, what were they? That's what I'm trying to answer with my research. It turns out that the materials that became chondrules were heated to varying degrees - some completely melted, some experienced quite a bit of partial melting, and others experienced only slight partial melting. Since the ones that were only slightly melted look most like the chondrule precursors, I'm studying them to develop an inventory of materials that were the precursors to chondrules. That's the first part of my research.
The second part of my research has to do with sorting of nebular particles in the nebula. It's been shown that the distribution of sizes of chondrules in one meteorite differs from the distribution of chondrule sizes in another meteorite, and this is one of the observations that have led to the idea that there was some sort of sorting mechanism in the nebula. Previous work has shown that the sorting is probably occurring aerodynamically rather than by mass, so my goal is to see how much the shape of a nebular particle (primarily metal grains and chondrules) affected its sorting. To do this, I'm employing X-ray tomography (more commonly known as "cat-scans?), though at much higher resolution that the scans that hospitals use. This way we can get highly-detailed, 3D measurements of the meteorites without having to destroy them. And again, this should give us some exciting new details about the way our solar system formed.
"Lithium, Beryllium, and Boron in Basalts from Phase 2 of the Hawaii Scientific Drilling Project"
My dissertation research is composed of projects on Hawaiian and Martian basalts and ordinary chondrite meteorites. I measured Li, Be, and B abundances in Hawaiian basalts from the Hawaii Scientific Drilling Project drill core and in select Martian basaltic meteorites. My dissertation goal is to determine how these elements behave in mantle plume environments on Earth and Mars and whether they indicate the presence of a crustal component in the source regions (as do other geochemical discriminants). My project on ordinary chondrites addressed the problem of determining peak temperatures reached by their parent asteroids during thermal metamorphism. After assessing the limitations of other geothermometers as applied to these meteorites, I determined that orthopyroxene-clinopyroxene pairs and the QUILF95 geothermometer record the best peak temperature estimates for ordinary chondrites. My results were recently accepted for publication in Meteoritics & Planetary Science.
"Geochemistry of eclogites at Alp de Trescolmen and other localities
in the adula nappe, Central Alps, Switzerland".
In the central Swiss Alps, the former continental basement rocks from the European margin underwent subduction
during the early Tertiary and reached eclogite facies conditions. Relict high P-T assemblages are mainly found
in the structurally upper part of the Adula nappe, particularly in mafic lenses. The aim of this research is to
use geochemical data from eclogites and metapelites in the Adula nappe and try to shed light on the physico-chemical
nature of the protolith, trace element mobility, P-T condition, and possible fluid-rock interaction that might
have taken place during subduction.
Techniques such as EMP analyses of major elements, oxygen isotopes of mineral separates from ecligites and metapelites, SIMS, and Laser ICP-MS analyses will be utilized to obtain the geochemical data.
"Petrology and Geochemistry of Upper Mantle and Lower Crustal Xenoliths
from Thailand"
Rare spinel-lherzolite xenoliths occur in late Cenozoic alkali basalts in the SE-Asia region of Thailand.
They offer a unique opportunity to understand the physico-chemical nature of the mantle within two contrasting
lithospheric fragments, the Shan-Thai (western side) and Indochina (eastern side), which were joined by a Mesozoic
Nan-River suture. Major-element chemistry in whole rocks varies considerably (e.g., ~33-49 wt% MgO and 1-6 wt%
Al2O3), with the Indochina xenoliths being more fertile than those of the Shan-Thai. Xenoliths from both sides
of the suture, however, yield comparable estimated temperatures (~890-1170oC). Trace-element compositions suggest
variable extents of basaltic-melt extraction and metasomatism. Clinopyroxene trace-element modeling indicates that
the majority of lherzolite xenoliths experienced small degrees (<6%) of partial melting, although up to 16%
melting is necessary for a depleted sample (5% modal Cpx). In a unique xenolith, clinopyroxenes are significantly
LREE-depleted (La/Lu = 0.01), possibly reflecting their derivation from garnet breakdown. In addition, the modeling
of LREE-enriched samples, with pronounced HFSE (Ti, Zr, Hf) depletion, suggests metasomatic modification by carbonate-rich
fluids/melts, unrelated to the host basalts. This metasomatic enrichment was more intense in xenoliths from the
eastern microcontinent (up to 30%), compared to that occurred in samples of the western counterpart. In the Chantaburi-Trat
basaltic field of SE Thailand, granulite-facies xenoliths represent the only known occurrence of lower crustal
material in this region. Important geochemical features of these granulite xenoliths include: (1) mafic compositions
(43-49% SiO2), (2) high normative olivine, plagioclase, and diopside, and (3) low and generally flat REE abundances
with positive Eu anomalies. These characteristics indicate cumulate origins, possibly from underplating basaltic
magmas. Mineralogical and geochemical chemical evidence suggests that the original cumulate mineralogy (Ol, Pl,
and Cpx) was transformed to granulite-facies assemblages (Ol, Pl, Cpx, and Gt) at ~1100 to 1200o C and 15 to18
kbar. Corundum inclusions were also produced at the upper limit of this P-T range. The petrological and geochemical
characteristics of spinel-lherzolite and granulite-facies xenoliths in the region of Thailand may reflect the combined
effects of Mesozoic subduction, and asthenospheric upwelling and magmatism in the Cenozoic.