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Lu-Hf and Sm-Nd Ages and Source Compositions for Depleted Shergottite Tissint

Thera E. Grosshans1, Thomas J. Lapen1, Rasmus Andreasen1, and Anthony J. Irving2
1Dept. of Earth and Atmospheric Sciences, University of Houston, TX, USA
2Dept. of Earth & Space Sciences, University of Washington, Seattle, WA

Tissint is the fifth witnessed Martian meteorite fall and was collected in Morocco in 2011. The quick recovery of this meteorite limits the possibility of weathering or contamination by terrestrial components. Tissint is classified as a depleted olivine-phyric shergottite similar in bulk composition to Dar al Gani 476 and EETA79001A. It has a porphyritic texture with primary mineral phases of olivine, plagioclase (maskelynite), and pyroxene (augite and pigeonite). Other phases include oxides (chromite, ilmenite, magnetite) and phosphate (Irving et al. 2012).

We performed in situ and isotope analyses at the University of Houston. Preliminary in situ trace element analyses of plagioclase and pyroxene show similar REE patterns to bulk Tissint and other depleted shergottites. The plagioclase, pyroxene, and whole rock fractions yielded a Lu-Hf age of 570 ± 84 Ma. The initial 176Hf/177Hf isotope ratio is 0.284060 ± 0.000046, and the initial ε176Hf of Tissint is +58.0. The Sm-Nd age of Tissint determined from the plagioclase, pyroxene, and whole rock fractions is 616 ± 67 Ma. The initial 143Nd/144Nd isotope ratio is 0.51397 ± 0.00016, and the initial ε143Nd is +41.6. The internal isochron Lu-Hf and Sm-Nd ages of this study are within error of each other as well as a previously published 143Nd/144Nd age of 596 ± 23 Ma [Brennecka et al. 2012). Initial ε176Hf and ε143Nd values indicate Tissint was derived from a highly depleted source like that of other depleted shergottites, and the ages indicate Tissint is one of the oldest known shergottites.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013