|Paper No. 32-5|
|Presentation Time: 2:00 PM-2:15 PM|
|EVALUATING EVIDENCE FOR MAGMATIC WATER IN MARTIAN BASALTS: SIMS ANALYSES OF LI AND B IN EXPERIMENTAL AND NATURAL PHASES|
HERD, Christopher1, TREIMAN, Allan1, MCKAY, Gordon2, and SHEARER, Charles3, (1) Lunar and Planetary Institute, 3600 Bay Area Blvd, Houston, TX 77058, email@example.com, (2) Astromaterials Research Office, SR, NASA Johnson Space Ctr, Houston, TX 77058, (3) Institute of Meteoritics, Department of Earth and Planetary Sciences, Univ of New Mexico, Albuquerque, NM 87131|
Lentz et al. (2001) searched for evidence of pre-eruptive water in the Shergotty and Zagami martian basalts through SIMS analyses of light lithophile elements (Li, Be, B) in their pyroxenes. They found that Li and B decrease from pyroxene cores to rims, contrary to the behavior expected during igneous evolution. Lentz et al. ascribed this decrease to a loss of magmatic water, based on the behavior of these elements in subduction zones and hydrothermal fluids.
However, recent work shows that DB(water-basaltic melt) < 1 (Hervig et al. 2002), so that water loss should have little effect on the B content of a magma. The uptake of Li and B by phases that co-crystallize with pyroxene, i.e. plagioclase and phosphates, has not been adequately examined, especially for martian basalt compositions. We performed crystallization experiments using a martian basalt composition in order to obtain relevant D values and we revisited Li, Be and B in pyroxene in the Zagami martian basalt using the Cameca 4f SIMS at the University of New Mexico (UNM).
One-atmosphere, Re-loop gas-mixing (CO/CO2) experiments were performed in Deltech furnaces at the Johnson Space Center (JSC). A starting composition near that of the QUE 94201 martian basalt was doped with Li2B4O7, yielding 500-1500 ppm Li and 1500-4700 ppm B. Runs were cooled at 1°C/hour to target temperatures yielding pyroxene, olivine, plagioclase, merrillite, and glass. We used the UNM SIMS to analyze for Li and B, and the Cameca SX-100 microprobe (JSC) for all other elements. Results corroborate previous work: Li and B are incompatible in these phases (Table).
Our SIMS results for Li, Be and B in Zagami pyroxene reproduce and extend the results of Lentz et al. (2001), and demonstrate that zoning of Li and B in Zagami pyroxene is complex. We found no well-defined decreases in Li or B from core to rim. As an anhydrous control, we analyzed zoned pyroxene in the (asteroidal) Pasamonte eucrite, which shows the expected trend of increasing Li and B with crystallization (Fe# or wt% TiO2), consistent with their incompatible behavior (Table). The role of phosphate minerals in the uptake of Li and B during pyroxene crystallization has yet to be elucidated.
2002 Denver Annual Meeting (October 27-30, 2002)
|Session No. 32|
Colorado Convention Center: C101/103
1:00 PM-3:45 PM, Sunday, October 27, 2002
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