Determining the Origin of Phosphates in Lherzolitic Shergottites Through Phosphate Saturation Experiments

Phosphorous has been correlated with sulfur and chlorine in martian soils with some soils containing several wt % P2O5 and high-P2O5 layer covers many rocks. However, using the RAT to remove the top few mm of some rocks revealed zones of low-P2O5 in the outer mm relative to the interior composition. This suggests that some igneous rocks of Mars are high in P2O5 and that over time phosphorous has been mobilized into the martian soils. In martian meteorites, phosphorous has been detected in high concentrations relative to terrestrial basalts and cumulates. High intensity x-ray maps of lherzolitic shergottite ALH 77005 reveal phosphates in olivine-hosted melt inclusions and P2O5 zoning in the host-olivine. Analyses of rehomogenized olivine-hosted melt inclusions reveal high-P2O5 glasses (>5 wt %). As phosphates are the major reservoir of REE in martian meteorites, determining if the phosphates are primary igneous or secondary sedimentary minerals may have significant consequences for use of REE as oxybarometers and radiogenic-isotopic dating systems in these meteorites. Therefore, an experimental investigation was initiated to determine whether the phosphates are primary igneous minerals or secondary weathering products. The phosphate saturation curve in SNC magmatic compositions has been studied experimentally by synthesizing the parental melt composition of lherzolitic shergottite ALH 77005 and performing anhydrous crystallization experiments in TZM pressure vessels. The parental melt composition was then doped with 5 wt % P2O5 in the form of CaHPO4 and the crystallization experiments were repeated. ALH 77005's parental melt saturated phosphate near 7 wt % at 1165°C and 5 wt % at 1150°C. These saturation values illustrate how high-P2O5 would have to build up before phosphates would nucleate and are consistent with high-2O5 content found in rehomogenized olivine-hosted melt inclusions. These saturation values are higher than the reported saturation for lunar and typical terrestrial basalts, which may reflect differences in the composition, volatile content, and/or the oxidation state of shergottite magmas. The presence of phosphate-saturated melts in rehomogenized, olivine-hosted melt inclusions suggests that olivine contains previously unaccounted for high levels of phosphate. If the phosphates are magmatic in origin, the evidence suggests that phosphates saturated earlier in the crystallization of lherzolitic shergottite ALH 77005 than previous authors have indicated and therefore phosphates contained in olivines have a larger impact on REE evolution of the magma than if olivine had been crystallizing alone. If the included phosphates are secondary in origin, the presence of enough P2O5 to saturate a melt inclusion with phosphate during rehomogenization suggests that mineral separates of olivine would still carry enough REE to have a significant effect on studies that attempt to use REE for oxybarometers and radiogenic-isotopic dating.