Tomohiro Usui

MY RESEARCH

My research focuses on the petrology and geochemistry of terrestrial and extraterrestrial rocks.  During my PhD at ISEI, I mainly studied eclogite xenoliths from the Colorado Plateau as a proxy of the subducted oceanic crust in order to understand material recycling through subduction zones [3 & 4].  Now at UTK, I am studying the petrology and geochemistry of Martian meteorites and rocks analyzed by the Mars Exploration Rover Spirit [1].  I am also involving in the NASA’s ongoing asteroid exploration mission (Dawn mission) [2].

1. Studies of petrogenesis of igneous rocks on Mars

1.1. Petrogenesis of olivine-phyric shergottite Yamato 980459 (Usui el al. 2008)

We analyzed major and trace element compositions of minerals and glasses in the most magnesian olivine-phyric shergottite Yamato 980459.  These geochemical datasets were interpreted based on results from multiple saturation experiments on the Yamato 980459 whole-rock composition and elemental partitioning studies.  We concluded that Yamato 980459 whole-rock composition closely approximates a Martian primary melt composition.  

1.2. Petrogenesis of high-phosphorous alkaline rocks in Gusev crater, Mars (Usui et al., in press)

We developed a petrogenetic model for high-phosphorous tephrites (Wishstone class) found at the Sprit landing site, Gusev crater.  We proposed that the Wishstone-class represents a silica-saturated, plagioclase-rich rock that formed in association with carbonatitic melt/fluid.  Our model supports the hypothesis that primordial carbon resided in the Mars mantle.

1.3. Petrogenesis of alkaline basalts in Gusev crater, Mars (Usui and McSween 2007, AGU abstract) 

Dr. McSween and I investigated the geochemistry of relatively unaltered alkaline basalts in Gusev crater in order to reconstruct magmatic processes occurred in Gusev crater.  We suggested that most Gusev basalts could have formed by different degrees of melting of a primitive Marian mantle compositionally similar to the Dreibus-Wänke mantle.

2. Prospective study for interpretation of gamma ray and neutron spectra for the Dawn mission

2.1. Mixing model (Usui and McSween, 2007)

The Gamma Ray and Neutron Detector (GRaND) on the Dawn spacecraft mission will provide elemental compositions of the surface of asteroid 4 Vesta with spatial resolution significantly coarser than spectrally observed surface heterogeneity.  Dr. McSween and I developed a linear mixing model based on the compositions of howardite, eucrite and diogenite meteorites (HED, thought to be Vesta samples) appropriate for interpretation of GRaND data.  Reliability of the mixing model was statistically investigated based on published whole-rock data for HED meteorites.  We demonstrated that the mixing model can accurately estimate the abundances of all the GRaND-analyzed major elements, as well as estimate minor elements (Na, Cr, and Mn) not analyzed by this instrument. 

2.2. K-Th systematics of HED and other achondrites (Usui et al. 2008, LPSC abstract)

A highly differentiated asteroid 4 Vesta is believed to be the parent body of the HED meteorite suite, but the HED suite is not the only achondrite suite representing asteroidal basaltic crusts.  My colleagues and I critically examined the variability of GRaND-analyzed elements, K and Th, in HED meteorites, and propose a method based on the K-Th systematics to distinguish between HED and other differentiated achondrites.

3. Studies of geochemical evolutions of subducted oceanic crust

3.1. Origin of eclogite xenoliths from the Colorado Plateau (Usui et al. 2003)

My colleagues and I determined U-Pb ages of zircons in the Colorado Plateau eclogite xenoliths.  Moreover, we presented the first finding of the high-pressure mineral coesite in the xenoliths.  These observations conclusively demonstrate that certain eclogite xenoliths from the Colorado Plateau originated as fragments of the subducted Farallon plate, which had been residing in the upper mantle since the Late Cretaceous. 

3.2. Petrology and geochemistry of eclogite xenoliths from the Colorado Plateau (Usui et al. 2006)

My colleagues and I presented geochemically enriched trace element and Sr-Nd-Pb isotopic compositions of minerals and whole rocks of the Colorado Plateau eclogite xenoliths.  Combining these geochemical datasets with mineralogical and petrographic observations suggested that the MORB-like protolith of the xenoliths was metasomatized by a fluid equilibrated with sediment in the fore-arc region of a subduction zone and that this metasomatic fluid produced continental crust-like isotopic compositions of the xenoliths.

3.3. Trace element microdistributions in a lawsonite eclogite xenolith from the Colorado Plateau (Usui et al. 2007)

My colleagues and I presented trace element microdistributions of minerals in a lawsonite eclogite xenolith from the Colorado Plateau.  Our observations suggested that Sr and rare earth elements could be retained in subducting oceanic crust even in the coesite stability field, if the slab is sufficiently cold enough to pass though the lawsonite eclogite facies.

4. Improvements of analytical methods

4.1. Improvement of U-Pb dating method (Usui et al. 2002)

My colleagues and I developed a U-Pb zircon dating method employing High-Resolution Secondary Ion Mass Spectrometry (HR-SIMS, Cameca ims-1270) by applying (1) preparation of zircon standards, (2) simultaneous analyses of Pb isotopes by multi-collection system, and (3) use of a highly focused primary ion beam as small as 5 µm in diameter.  Our method can provides an accurate age of zircon with a spatial resolution as high as 5 μm. 

4.2. Improvement of sequential U-Th-Pb, Rb-Sr, and Sm-Nd isotopic analytical method (Kuritani et al. 2006; Analytical methods section in Usui et al. 2006)

My colleagues and I improved a chemical separation method that can successively separate Rb, Sr, Sm, Nd, U, Th and Pb from whole-rock and mineral samples, using multi-ion exchange column chemistry.