Metamorphic Angrite Northwest Africa 3164/5167 Compared to Magmatic Angrites

21/07/2015 23:59 · by karmaka · in achondrite, angrite

Bashar Baghdadi, Albert Jambon, Jean-Alix Barrat

Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 21 July 2015

Northwest Africa (NWA) 3164 and 5167 are two angrites with a granulitic texture unlike that of other angrites, with a variable, up to millimeter grain size. Besides mineralogical and chemical similarities to other angrites, NWA 3164 and 5167 exhibit unique characteristics. Ca-rich olivine dominates (in NWA 3164: ∼49 vol%, Fo57; NWA 5167: 40 vol%, Fo59). Fassaitic clinopyroxene is the second major phase (in NWA 3164: 29 vol%; in NWA 5167: 36 vol%) with a significant Tschermak component. In addition, two Al-rich phases are present: plagioclase An99 and hercynitic spinel (∼6 and ∼7 vol% respectively for NWA 3164; 17 and 4 vol% respectively for NWA 5167). Heavily weathered iron sulfide and kamacite, (9 wt% in NWA 3164; 4 wt% in NWA 5167) are the remaining minor phases, a unique feature among angrites. All mineral phases are homogeneous. Like other angrites, NWA 3164 and 5167 exhibit a superchondritic Ca/Al ratio, with negligible amounts of alkalis and very low silica content. The presence of metal results from the incorporation of exogenous iron following impact. Subsequent annealing resulted in the observed granulitic texture. Major element composition indicates that both NWA 3164 and 5167 are derived from a picritic angrite precursor after incorporation of metal and annealing.

After correction for iron and Fo90 olivine incorporation, bulk rock REE abundances of both NWA 3164 and NWA 5167 appear lower than those of quenched angrites, showing the lowest absolute abundances among angrites. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analyses of clinopyroxene, olivine and anorthite indicate that difference of Rare Earth Element (REE) abundances between NWA 3164 and NWA 5167 can be explained by adding traces of phosphate to NWA 3164. NWA 3164/5167 trace elements abundances are similar, showing depletion in volatile elements and enrichment in refractory lithophile elements such as Ca, Ti, and Al. The most incompatible elements are depleted as well, unlike other angrites. This indicates that the source of these younger angrites was more depleted in incompatible elements when compared to the older magmatic angrites. The low Hf/W is understood as the result of exogenous iron incorporation and therefore the Hf/W and W isotopic heterogeneity of the Angrite Parent Body (APB) mantle is secondary. Comparison with other angrites suggests that iron incorporation may be necessary to explain their low Hf/W and W isotopic compositions.