EVALUATING EVIDENCE FOR MAGMATIC WATER IN MARTIAN BASALTS: SIMS ANALYSES OF LI AND B IN EXPERIMENTAL AND NATURAL PHASES

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 D_B(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).

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.