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, herd@lpi.usra.edu, (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 1C/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).

Phase

Composition

DLi

DB

Olivine

Fo59-51

0.28(1)

0.007(1)

High-Ca pyroxene

~Fs30En35Wo35

0.20(3)

0.021(7)

Plagioclase

Ab68An32 / Ab15An85

0.38(5) / 0.48(4)

0.024(3)

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
Planetary Geology
Colorado Convention Center: C101/103
1:00 PM-3:45 PM, Sunday, October 27, 2002
 

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