It is based on mineral compositions, textural characteristics and more recently, magnetic susceptibility. W.Ĭlassifying chondrites is relatively easy and the criteria are well documented. These regions could have been related by processes such as mixture of variable amounts of volatile-rich and volatile-poor components in planetary or nebular settings, or alternatively by variable varying degrees of volatile loss from volatile-rich materials.Ĭhennaoui-Aoudjehane, H. Similarly, the source regions of the parent magmas of the nakhlite and chassignite meteorites differed from those on the shergottite parent body by being still richer in this volatile-rich component. It is proposed that the peridotites which on partial melting generated the parent magmas of the shergottite meteorites differed from those which gave rise to eucritic magmas by being enriched in a component rich in alkalis and other volatiles. Petrological evidence is used to support the hypothesis that although the magma source regions and parent bodies of basaltic achondrite, shergottite, nakhlite, and chassignite meteorites are clearly distinct, they may be simply related. These anomalies might be explained using current models but the alternative hypothesis, that Fe/Mn ratio is controlled not by nebular volatility constraints, but by planetary differentiation should be explored.Ī petrogenetic model of the relationships among achondritic meteorites For example, Na is massively depleted in basaltic achondrites when compared to Mn. When the abundance in achondrites of elements of similar volatility is examined, anomalies appear. However, such volatile enriched components have not been identified in chondrites. Variation of Fe/Mn ratios based on the relative volatility of these elements in the early nebula provides a constraint for models by which the basaltic achondrites (with Fe/Mn ratios approximately = 25-50) are derived from mixtures of nebular components that were enriched in volatile components such as Mn. In contrast, however, Mn is more volatile than Fe in a nebular setting. The Fe/Mn difference between achondrites and chondrites is particularly significant since Fe and Mn are geochemically similar elements with similar partitioning behavior in familiar magmatic systems and are generally coupled during crystal-liquid fractionation. Most models of achondrite genesis involve magmatic differentiation of chondritic precursors. Fe/Mn ratios are suggested to be distinctive for samples from each achondrite parent body and for the Earth and Moon, but the correspondence between the Fe/Mn systematics of achondrites and chondritic precursors is unclear. Models of the origin of achondritic assemblages must, therefore, account for these ratios. Most achondritic meteorites have Fe/Mn ratios that are lower than those of carbonaceous chondrites and of course are lower than the solar system abundance ratio of these elements. The Fe/Mn constraint on precursors of basaltic achondritesĭelaney, Jeremy S.