Published in Meteoritics, 29, p. 545.
ALH 84001, originally classified as a diogenite, was recently recognized by Mittlefehldt  as a new member of the clan of martian meteorites. It is a coarse-grained orthopyroxenite with the same O isotopic composition as the nakhlites . Most of this meteorite consists of orthopyroxene grains; it also contains maskelynite, chromite, and accessory minerals including apatite, augite, pyrite, and Mg-Ca-Mn-Fe carbonates . With the ion microprobe, we measured the concentrations of REEs and other selected minor and trace elements in individual grains of orthopyroxene, maskelynite, and apatite. In pyroxene, the REEs do not show the striking compositional zoning that is characteristic of all other SNCs [3-6]. This is consistent with the observation that orthopyroxene grains commonly join in 120° triple junctures and the suggestion that ALH 84001 cooled more slowly that the shergottites, nakhlites, or Chassigny . Only Ti and Zr, two of the incompatible elements, show correlated variations in ALH 84001. Titanium concentrations vary by a factor of 1.9 while those of Zr vary by a factor of 4.3. Mittlefehldt  noted that the orthopyroxene compositions in ALH 84001 are similar to those of the megacrysts from the lithology A of EETA 79001 and of Chassigny, but this similarity does not extend to their Ti and Zr concentrations. The C1 chondrite-normalized abundances in orthopyroxene vary from ~0.03 for La to ~2.0 for Lu, and Eu is strongly depleted. The maskelynite REE pattern, on the other hand, shows a steep decrease from La (~2.3× C1) to Dy (~0.11× C1) and a pronounced positive Eu anomaly (~13× C1). The apatite REE pattern is strongly LREE enriched as well (from La ~400 to Yb ~27) and has a small negative Eu anomaly.
Although in all SNCs phosphate is the mineral with the highest REE concentrations, it is not the major REE carrier in ALH 84001. It accounts for ~90% of the La, but only 30% of Eu, and less than 3% of the Yb in the whole rock. A mixture of 98.85% orthopyroxene, 1% maskelynite, and 0.15% phosphate, consistent with what is known about the modal composition of this meteorite, gives a perfect match with the REE whole-rock composition determined by Mittlefehldt .
Using the most appropriate partition coefficients for these minerals in SNCs , we estimated the compositions of the melts that may have been in equilibrium with the average orthopyroxene, the apatite, and the maskelynite. The orthopyroxene equilibrium melt is slightly LREE depleted (its REE pattern is similar to that of the whole rock), whereas the apatite and maskelynite equilibrium melts have higher REE concentrations and are strikingly LREE enriched. We tried unsuccessfully to derive the apatite and maskelynite melts from the orthopyroxene melt by fractional crystallization. This is the first instance for a SNC meteorite where it is not possible to derive by this process the melts that gave rise to the late-forming minerals (such as feldspar and apatite) from those that were in equilibrium with the earliest-formed REE-bearing mineral (i.e., pyroxene). We therefore suggest that an infiltrating fluid enriched in LREE is responsible for the formation of the apatite and maskelynite that occur as interstitial grains in ALH 84001. The presence of interstitial carbonates and pyrite also indicate that hydrothermal alteration played a significant role in the formation of this meteorite . The similarity of REE patterns for the parent melts of ALH 84001, Shergotty, and Zagami seems to indicate that the new SNC meteorite is more closely related to these two shergottites than to any of the other meteorites thought to have come from Mars.
References:  Mittlefehldt D. W. (1994) Meteoritics, 29, 214-221.  Clayton R. N. (1993) Antarct. Meteorite Newsletter, 16, 4.  Lundberg L. L. et al. (1990) GCA, 54, 2537-2547.  Harvey R. P. et al. (1993) GCA, 57, 4769-4783.  Wadhwa M. et al. (1994) GCA, in press.  Wadhwa M. and Crozaz G. (1994) GCA, submitted.