HIGH CALCIUM (~80mol%) LATE STAGE CARBONATE IN ALH84001

Introduction: Secondary minerals within Martian meteorites have been the subject of intense scrutiny in order to expand our knowledge of surface and near surface processes on Mars. The coarse grained orthopyroxene cumulate meteorite ALH84001 is of particular importance for these studies, due to its ancient age (around 4.5Ga) and the complex mineralogical and chemical zoning of its approximately 1 vol% secondary carbonate mineral assemblage. To understand how the Martian atmosphere and environment evolved, it is necessary to determine the formation conditions and relationships between the secondary minerals in ALH84001. This information can be used to constrain the nature of the source fluids, the extent of elemental and isotopic exchange between phases and the extent of subsequent weathering. Ultimately, one can then make inferences regarding the evolution of martian volatiles in the near surface environment and test models of an early wet/warm Mars compared to an episodically wet Mars. In particular the chemically and mineralogically zoned carbonates, previously described by numerous authors [1-8], within ALH84001 have revealed a complex history of low temperature precipitation and possible high temperature alteration of the carbonate mineral assemblages. Description: This work expands on a recent paper by Holland et al, [9] detailing negative δO values in Allan Hills 84001 carbonates and aims to study in detail the petrographic relationships and chemical composition of a previously undescribed generation of carbonate relative to the zoned carbonates. The carbonate appeared to be a new, later stage, high calcium carbonate cement that surrounds Mg rich carbonates and rosette fragments. It also seems to be intimately intergrown with silicate minerals and is referred to as high Ca later stage stage cement carbonate. Method. This work will use data that has already been collected during the above mentioned study [9] which includes quantitative analysis by scanning electron microscope (JEOL JSM 6400 Analytical SEM) in secondary electron mode at the University of Manchester and chemical analyses by Cameca SX100 electron microprobe. However, in addition we will also obtain elemental abundance data from Time of Flight Secondary Ionisation Mass Spectrometry [10]. In addition to 2 of the samples used in [9] a new sample has already been prepared in a similar manner and another is currently being prepared. The following is a brief description based upon the data already collected. Results: Composition and textural description. The later stage carbonate has a much higher calcium content (up to ~80 molar%), than any previously described Ca rich carbonates (with one exception [8]) either within the rosette cores or within the massive ankeritic domains described previously [7]. Texturally the high Ca carbonate appears similar to ankeritic domains [7] in that these areas are typically larger than the rosettes (~200μm), intergrown and closely associated with silicate minerals and do not show any of the magnesite-siderite banding common to the rosettes. Two of the carbonate samples also contain grains of feldspathic glass and in the sample shown in figure 1 the high Ca later stage cement carbonate area to the left can be seen enclosing rosette fragments. Both types of carbonate appear to show a sharp contact with the pyroxene grain.