Volatile degassing of basaltic achondrite parent bodies: Evidence from alkali elements and phosphorus

Abstract

The volatile alkali element (Na, K, Rb, Cs) and P abundances in eucrites, diogenites and basaltic clasts from polymict eucrites, howardites and mesosiderites were compared with an average of highly incompatible refractory (AHIR) elements (Ba, La, Ce, Nd, Sm, Hf, Th, U) normalized to cosmic abundances. Ratios of e.g. Rb AHIR remain constant during normal magmatic differentiation because the very low and approximately equal mineral melt partition coefficients for these elements are inefficient in causing fractionation in the melt. The Na AHIR ratio in eucrites is essentially constant at 0.050× cosmic. Basaltic eucrites and basaltic clasts show a positive correlation between K, Rb and Cs and alkali element AHIR ratios. The averages for basaltic eucrites are: K AHIR = 0.043 , Rb AHIR = 0.0088 , Cs AHIR = 0.0040 × cosmic . The positive correlations between alkali elements and alkali AHIR are due to progressive volatile loss of the alkali elements from the basalts and not to source region heterogeneities. This process probably had a minor effect on the parent body inventory of volatile elements. In diogenites, the alkali AHIR ratios are systematically greater than in basaltic eucrites and are more variable. The lower values are 1.4–2× the means for basaltic eucrites, while the higher values are similar to those of CV and CO chondrites for K, Rb and Cs. The highest alkali AHIR ratios were obtained on the most incompatible-poor of the diogenites and indicate minimum ratios for the parent body of Na AHIR = 0.24 , K AHIR = 0.49 , Rb AHIR = 0.55 and Cs AHIR = 0.14 × cosmic . There are well defined negative correlations between K, Rb, Cs and alkali AHIR ratio in the diogenites. The data are consistent with progressive alkali loss through volatile outgassing during crystallization of one or more parent magmas. This process resulted in >90% loss of the volatile element inventory from the parent body. Phosphorus also shows evidence for volatile loss from the basaltic eucrites and elevated P AHIR ratios in diogenites. The low P AHIR ratio in eucrites is largely controlled by volatile loss, rather than through equilibration with a metallic liquid as previously suggested. The parent body of the basaltic achondrites appears to have initially contained alkali element (and presumably other volatile element) abundances comparable to those in CO and CV chondrites, and perhaps higher.