Forging the fourth link between planetary worlds: Vesta and the HEDs

"Dar a1 Gani 4 7 6 L u c k y for some, unlucky for others" is the title of an abstract that was submitted to the LPSC conference in 1999 (Wright et al., 1999). Dar a1 Gani (DaG) 476, recovered in 1998 in the Sahara, is the thrteenth member of a group of meteorites which are widely believed to have come from Mars. In many respects the title of this abstract has been proven right. Unlucky were those scientists who wanted to find evidence for life on Mars in this meteorite. They found numerous hints for traces of life, but terrestrial desert weathering caused most of those. Lucky, however, are petrologists and isotope geochemists who study the rocky parts of meteorites. They are provided with a sample that gives insights into complex igneous processes on Mars, nicely illustrated in a paper by Mikouchi et al. (200 1) in the present issue of Meteoritics & Planetary Science. Of the 16 meteorites believed to have come from Mars, 1 1 belong to the shergottite group. Shergottites comprise two major rock types: basalts and lherzolites, which are chemically and texturally distinct. DaG 476 is a new member of the basaltic shergottites, yet it has a chemical composition, with high Mg and low A1 and Ca, that has strong affinities to lherzolitic shergottites such as Yamato (Y)-793605. In addition, DaG 476 contains megacrysts of olivine, orthopyroxene and chromite, resembling the unusual lithology A of Elephant Moraine (EET)A7900 1, another basaltic shergottite. Megacrysts in lithology A form clusters which strongly indicate that they are xenocrysts, possibly derived from lherzolitic shergottites. The relation of megacrysts in DaG 476 to their host rock is less clear, because they do not cluster. More recently found martian meteorites, DaG 48916701735 and Sayh a1 Uhaymir 005/0081 051, show similar textures. In order to understand the relationship of these rocks to other shergottites, it is essential to understand the origin of their megacrysts. Are they xenocrysts, similar to those in lithology A, or did they crystallize from a magma identical or related to that of their host rocks? The answers to these questions will help to constrain the process by which these rocks formed. Mikouchi et al. (2001) study takes a step in the direction of revealing these secrets. Based on microprobe analyses of megacrysts in DaG 476, Mikouchi et al. (2001) conclude that the olivine megacrysts are unrelated to their host rock but resemble lherzolitic compositions, similar to those of Y-793605. Therefore, these authors favor their origin as xenocrysts and propose that DaG 476 has an impact melt origin as was proposed for lithology A in EETA79001 (Mittlefehldt e f al., 1999). Furthermore, the preserved chemical zonation of olivines in DaG 476 allows Mikouchi et al. (2001) to constrain cooling rates during groundmass crystallization. Their estimates indicate rapid cooling of this rock near the surface (<3 m), conditions similar to those obtained for lithology A of EETA79001, and thus support the impact melt hypothesis. Whether this proposed model gives the final answer to the origin of DaG 476 remains to be seen. Further studies of trace elements in megacrysts and their melt inclusions, and the use of isotopes as geochemical tracers, may reveal this meteorite's secrets to some "lucky" scientists.