Experimental and crystal chemical study of the basalt–eclogite transition in Mars and implications for martian magmatism

Abstract

This paper represents the start of a campaign to identify possible source regions for pyroxene-phyric “enriched” and “depleted” martian basalts using high pressure studies on martian meteorite compositions that represent liquids or near liquids. Our first experiments focus upon mineralogical and crystal chemical aspects of the basalt–eclogite transformation on Mars. It is anticipated that like Earth, eclogites are not the dominant upper mantle assemblage. However, like Earth they may be important hosts for P, Cl, F, OH, Ti, REE, Sr, Y, high-field-strength elements, Hf, and Zr in the upper mantle. This initial experimental study evaluates the major and trace element crystal chemistry of potential martian eclogite assemblages using a martian melt composition (QUE 94201). This composition is appropriate for this study because it is evolved, so it is pyroxene-phyric, contains abundant phosphate, which is important for storage of REE, and is very well studied. In the high pressure experiments, garnet and omphacitic pyroxene are the dominant phases. The garnet has a compositional range from Al56.9Gr+An24.6Py18.5 to Al41.5Gr+An28.1Py30.4 to Al40.3Gr+An22.6Py37.1 (where Al = almandine, Gr+An = grossular+andradite, and Py = Pyrope) and the pyroxene has jadeite and acmite components of up to 26.0 and 9.9%, respectively. The garnet is enriched in REE over omphacite; this is especially true for the HREE which likely remain sequestered in residual garnet during melting. This observation suggests that garnet may play a key role in producing the relatively flat REE patterns of enriched martian basalts. The prime substitution couple that incorporates REE into garnet is: REE3+X Site+Mg2+Y Site↔R2+X Site+Al3+Y Site, where R2+=Ca, Mn2+, Fe2+, and Mg. This is a very effective couple that accounts for both charge balance and ionic size restrictions in individual sites. The prime substitution couple that incorporates REE into omphacite is: REE3+M2 Site+Na1+M2 Site for 2 Ca2+M2 Site. This is a less effective couple than that for garnet because Na is pulled in two directions in omphacite; it must balance the Al in the M1 site in the jadeite component and it also has to provide a charge deficiency for each REE3+ cation in the M2 site. The potential for the presence of eclogite in the martian mantle, has implications for bulk trace element partitioning differences between eclogite and peridotite assemblages that are closely linked to garnet and pyroxene compositions. Further, the occurrence of eclogites may have implications for the capability of the martian mantle to produce magmas, and the major element composition of those magmas.