Melting of amphibole-apatite-rich metasomes in the continental mantle and comparison of melt compositions with natural igneous rocks

Abstract

The source regions of alkaline igneous melts are thought to be mixtures of peridotite, pyroxenite and hydrous ultramafic rocks, but relatively few melting experiments are available for the last of these rock groups. Here, we expand the scope of experimental results on hydrous ultramafic rocks to assemblages rich in Ca-amphibole and apatite, which are found as xenoliths in eastern Australia. Experiments were conducted at 15 kbar on five hydrous ultramafic mineral assemblages; four consisted of equal amounts of Ca-amphibole and apatite, some with minor clinopyroxene and/or phlogopite. A fifth mix consisted of 50 % each of mica and clinopyroxene, but with lower Mg# (Mg/(Mg + Fe)) than in previous experiments. Results show that Ca-amphibole melts rapidly and completely within 50–100 °C of the solidus, producing melts with melilitite-nephelinite compositions similar to the amphibole. Apatite contribution to the melt is minor (P₂O₅ = 2.1–4.7 wt%) and clinopyroxene crystallises as a peritectic phase. These melts are distinct from those produced from mixtures containing alkali amphibole+phlogopite in having much lower SiO₂ (35–40 wt%) and low K₂O/Na₂O, but higher CaO and Al₂O₃. Melts of mica clinopyroxenites have intermediate SiO₂ (41–47 wt%) and much higher K/Al.

Trace element patterns for incompatible elements show that abundances for most elements are much lower than in natural nephelinites and melilitites as long as residual apatite retains elements with high D^ap/melt, but incompatible trace element abundances approach those of natural rocks if apatite melts out. Hydrous minerals exert strong control on many first-row transition elements (especially Ni and Cr) and may sequester these in the proximal parts of vein systems, removing them efficiently from migrating melts. The melting points of all hydrous ulltramafic rocks, whether rich in phlogopite, Ca-amphibole or K-richterite, are below that of water-undersaturated peridotite. High degrees of melting occur quickly if amphibole is abundant, and so significant amounts of melt may be mobile at temperatures below or close to the solidus of peridotite.