Published in Meteoritics, 29, p. 472.
One of the reported characteristics of the orthopyroxenite ALH 84001 (A84) is the absence of olivine  distinguishing it from the other martian meteorites, all of which are olivine-bearing. However, olivine is present in A84, as widely dispersed 5-10-µm grains enclosed within orthopyroxene. This olivine is compositionally homogenous with a distinctly Fe-rich composition (34.5% Fa). Another newly discovered mineral in martian meteorites is cumulate orthopyroxene in Chassigny . Both observations have profound implications for the magmatic and alteration histories of these meteorites.
A84 is dominated by nearly homogeneous cumulus orthopyroxene, and minor chromite found within cumulus orthopyroxene grains and interstitial areas . Olivine and orthopyroxene show disequilibrium compositions (Fe/Mg of 0.35 in olivine, 0.29 in orthopyroxene). Similar assemblages of olivine, chromite and orthopyroxene can be found in other martian meteorites, such as the nearly identical shergottites LEW 88516 and ALH 77005 (L&A) and the lithology A megacryst assemblage of EETA 79001 (EET-A)[1, 3-6]. The relatively consanguine mineralogy and texture of these lithologies suggests that they could have been derived from similar basaltic parental magmas. Calculated parents for L&A and EET-A are remarkably similar (Ex from ; i from ). Very early crystallization of such a liquid, from 1250° to 1200°C, produces an assemblage of olivine + chromite + orthopyroxene . Further equilibrium crystallization of such a parent results in the resorption of olivine to produce orthopyroxene, and later pigeonite. The accumulation of early crystallization products could produce the heter-adcumulates L&A, the orthocumulate A84, and the mesocumulate EET-A lithologies under various conditions of exclusion of evolving intercumulus liquid. Chassignys cumulus orthopyroxene requires that mineral to crystallize before plagioclase, suggesting a more shergottite-like composition, much different than previously determined parents for that meteorite [7,8]. Although complex scenarios of mixing or assimilation are required to account for the varying observed trace-element and isotopic compositions for these rocks, the possible parental similarities suggest that a generic brand of Al-poor basaltic magma was (or is) globally prevalent on Mars [7,9].
Although olivine in A84 may have been resorbed during magmatic evolution, another way to reduce its abundance is metamorphic reaction at much lower temperatures. The abundant carbonates in A84 are Mg- and Fe-rich and Ca-poor; they are estimated to have formed below 740°C during fluid influx through the shattered rock . The absence of anthophyllite and/or talc from A84 implies that H2O was a minor constituent of these fluids and suggests a possible method of forming the magnesite present. At temperatures around 550°C and very high values of XCO2 (>0.85), CO2 and olivine react to form magnesite and orthopyroxene. Magnesite can also be formed through reaction of CO2 with orthopyroxene, but would produce free SiO2 as well, which has not been found in A84 . Substantial olivine may have been present before its removal by magmatic resorption and metamorphic alteration.
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