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Pyroxenes in Martian meteorites as petrogenetic indicatorsPyroxenes in Martian meteorites are important recorders of petrogenetic processes. Understanding the details of pyroxene major and minor element compositional variations can provide important insights into those processes. A combination of careful petrographic analysis of natural samples and experimental crystallization studies can lead to better understanding of the processes that gave rise to these samples on Mars. In addition, experimentally determined major, minor and trace element partition coefficients are important for using natural pyroxenes to estimate the compositions of the melts from which they crystallized and the oxidation conditions that prevailed during crystallization. We will report on minor element (Al, Ti, Cr) zoning in nakhlite pyroxenes and in synthetic pyroxenes that we have grown for the purposes of determining pyroxene/melt partition coefficients for Sr and REE. The natural pyroxenes have patchy Al zoning that, by analogy with our experimental pyroxenes, we interpret as sector zoning. The irregular patchy nature of the zoning is probably the result of the vagaries of growth kinetics and local environment during crystal growth. More slowly cooled nakhlites have the most distinct bimodal zoning, with one mode having Al2O3 around 0.5-0.6 wt%, and the other around 0.9 %. Average Al content increases with increasing cooling rate. This feature is puzzling, since the cumulus pyroxenes were almost certainly present at the time of eruption. Al and Ti are strongly correlated, but Cr is completely decoupled from those elements. The synthetic pyroxenes are distinctly sector zoned in Al and Ti, and the sector-to-sector variation in Al within a single crystal has important effects on trace element partition coefficients. Trivalent REE are strongly correlated with Al, while divalent elements (Sr, Eu+2) show a significantly weaker correlation. For example, as the Al2O3 content varies from 0.3 to 0.6 wt % from one sector to another, D(Gd) increases by approx.80% while D(Sr) and D(Eu) only increase by 20-30% at oxygen fugacities near IW. The different Al dependency of D for trivalent REE vs. divalent Eu and Sr considerably complicates the calibration of the Eu oxybarometer.
Document ID
20060013471
Acquisition Source
Johnson Space Center
Document Type
Conference Paper
Authors
McKay, Gordon
(NASA Johnson Space Center Houston, TX, United States)
Le, L.
(ESC Group Houston, TX, United States)
Mikouchi, T.
(Tokyo Univ. Japan)
Makishima, J.
(Tokyo Univ. Japan)
Schwandt, C.
(ESC Group Houston, TX, United States)
Date Acquired
August 23, 2013
Publication Date
January 1, 2006
Subject Category
Geophysics
Meeting Information
Meeting: 19th General Meeting of the International Minearlogical Association
Location: Kobe
Country: Japan
Start Date: July 23, 2006
End Date: July 28, 2006
Distribution Limits
Public
Copyright
Other

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