CONSTRAINTS ON THE RARE EARTH ELEMENT CHARACTERISTICS OF METASOMATIZING FLUIDS IN THE MARTIAN METEORITE ALH84001.  M. Wadwa1,2 and G. Crozaz1, 1McDonnell Center for the Space Sciences and Department of Earth and Planetary Sciences, Washington University, St. Louis MO 63130 USA, 2Scripps Institution of Oceanography, University of California, La Jolla CA 92093, USA.

Published in Lunar and Planetary Science XXVI, pp. 1451-1452, LPI, Houston.

Abstract:  ALH 84001, a newly identified SNC meteorite, shows clear petrographic and geochemical evidence of multiple phases of infiltration metasomatism which resulted in the formation of late stage minerals, such as maskelynite and apatite, and carbonates. We have analyzed the REE compositions of these minerals to constrain the geochemical characteristics of the infiltrating fluids from which they crystallized. We propose that their petrographic and geochemical features suggest that this meteorite underwent at least two major phases of infiltration metasomatism. During the first, a LREE-enriched silicate melt fluxed through it and formed the non-cumulus assemblage comprising minerals such as maskelynite and apatite; during the second, a CO2-rich aqueous fluid, which may have been LREE-depleted, infiltrated it and precipitated the carbonate.

ALH 84001 was recently identified as belonging to the SNC group of meteorites [1], believed to have originated on the planet Mars. Petrographically, one of the most interesting features of this meteorite is its abundance of carbonates. The presence of this mineral in two main textural occurrences, i.e., (1) early “preshock” carbonates (~100 µm) filling interstitial cavities, and (2) late “post-shock” carbonates (~10 µm) located in brecciated zones [1] provides evidence for multiple fluid fluxes through this meteorite. In fact, the significant role of infiltration metasomatism in its petrogenesis is also indicated by the REE compositions of early (i.e., cumulus orthopyroxene) and late (i.e., maskelynite and apatite) forming minerals. In our previous work [2], we estimated the parent melt of ALH 84001 to be slightly LREE-depleted, whereas the melts in equilibrium with maskelynite and apatite were considerably LREE-enriched. We also showed that no reasonable amount of orthopyroxene fractionation could have produced this LREE enrichment in this “intercumulus melt” component, thus indicating that these late minerals formed from a LREE-enriched infiltrating fluid. In this study, we have focused on the REE compositions of the late forming minerals (maskelynite and apatite) and of carbonates to constrain the REE characteristics of the different “generations” of fluids that fluxed through this meteorite and precipitated these minerals. With this approach, we hope to better understand the post-cumulus, metasomatic history of this rock.

We measured the REE concentrations in maskelynite, apatite and carbonates of ALH 84001 with the Washington University ion microprobe. As described by us previously [2], the REE patterns of both maskelynite and apatite are LREE-enriched (on average, La ~2.3 × CI and Dy ~ 0.11 × CI in maskelynite and ~400 × CI and Yb ~27 × CI in apatite). Additionally, we have now found two types of REE patterns in the apatite, one that has slightly higher REE abundances (La ~418-434 × CI) and no Eu anomaly (pattern A in Fig. 1) while the other has slightly lower REE abundances (La ~389-398 × CI) and a small Eu anomaly (pattern B in Fig. 1). The difference in these two apatite patterns can be interpreted in teens of a sequence of crystallization, where the apatites with pattern A crystallized from an intercumulus melt before the apatites with pattern B (which formed after the melt had been somewhat depleted in all the REEs, but especially in Eu, by substantial amounts of phosphate and maskelynite crystallization). Using suitable partition coefficients [3], the melts calculated to be in equilibrium with both the maskelynite and apatite were found to be LREE-enriched (on average, La/Yb ~5 for melts in equilibrium with apatite) and cannot be produced by fractionation of orthopyroxene from the parent melt (determined from the orthopyroxene compositions of [2] to have a La/Yb ratio of ~0.6). Therefore, the melt from which the non-cumulus assemblage in ALH 84001 (comprising chromite + maskelynite + apatite + pyrite) crystallized was a metasomatizing LREE-enriched silicate melt. As discussed by [4], an upward percolating melt through a LREE-depleted matrix such as a cumulus pile of LREE-depleted orthopyroxene can result in extremely fractionated, LREE-enriched melt compositions, and a similar process of upward melt migration through a compacting cumulus pile could have generated the above mentioned LREE-enriched silicate melt that infiltrated ALH 84001. All carbonate grains analyzed by us were about 50-100 µm in diameter and belonged to the “pre-shock” occurrence described above (“post-shock” carbonates in our section of ALH 84001 were too fine grained to be measured with the ion microprobe). Again, two types of REE patterns were found in these carbonates (Fig. 1), one (pattern A) which is LREE-depleted (La/Yb ~0.25) and the other (pattern B) which is almost flat, although still slightly LREE-depleted (La/Yb ~0.9). No significant Eu anomalies were found to exist in any of the carbonates analyzed. It has been recognized that REEs form complexes with carbonate ions in aqueous fluids and that HREEs have a greater tendency to do so than LREEs [5,6]. It is likely that both carbonate patterns A and B reflect the LREE depletion in the CO2-rich aqueous fluids (perhaps with XCO2 exceeding ~0.85 as indicated by [7]) from which they precipitated. The difference in the degree of LREE depletion in these two patterns may be understood in teens of precipitation sequence, where the carbonate with pattern A formed before the one with pattern B (which subsequently precipitated from a fluid that became less LREE-depleted due to preferential incorporation of HREEs in earlier formed carbonates).

Therefore, to explain the REE compositions of late stage minerals (i.e., maskelynite and apatite) and of “pre-shock” carbonates in ALH 84001, at least two phases of infiltration metasomatism are required, one involving a LREE-enriched silicate melt and the other involving a LREE-depleted CO2-rich aqueous fluid. Of course, subsequent formation of “post-shock” carbonates would require interaction with yet another generation of CO2-rich fluids.

References:  [1] Mittlefehldt (1994) Meteoritics, 29, 214. [2] Wadhwa and Crozaz (1994) Meteoritics, 29, 545. [3] Lundberg et al. (1988) GCA, 54, 2537. [4] Navon and Stolper (1987) J Geol., 95, 285. [5] Cantrell and Byrne (1987) GCA, 51, 597. [6] Lee and Byrne (1992) GCA, 57, 295. [7] Harvey and McSween (1995) LPS XXVI, this volume.