Generation of rhyolitic melt in an artificial magma: Implications for fractional crystallization processes in natural magmas

Abstract

Abstract A large (1.3 × 107 g) artificial mafic melt with a bulk composition similar to an unusually calcic basalt (17 wt.% CaO) was produced by in-situ heating of soil, and subsequently cooled and crystallized. The final crystalline assemblage consisted dominantly of acicular diopsidic to hedenbergitic pyroxene and anorthitic feldspar, with a subordinate amount of potassic feldspar. Electron microprobe analyses reveal the presence of a small amount (∼10 vol.%) of rhyolitic glass (71% SiO2) within the crystalline network. This glass is the residual material left after crystallization of pyroxene and feldspar, analogous to rhyolitic melt which may be generated from fractional crystallization of a basaltic magma. Ion microprobe imaging indicates that the rhyolitic glass is generally present in isolated triangular- to rectangular-shaped interstices left by crystallization of acicular and tabular phases, and that the glass is enriched in trace elements (such as Zr and Cs) which are incompatible within the crystalline phases. No evidence of coalescence or migration of the rhyolitic melt is suggested by glass morphology. Although these observations were made in an artificial magmatic system, analogies to a natural system may be drawn. The isolated nature and low abundance of the rhyolitic glass highlights the difficulty of extracting and segregating evolved melts produced by high degrees of crystallization of a primitive parent, particularly one characterized by elongated, rather than equant, crystals. The distribution of the rhyolitic glass supports the suggestion that extraction of significant amounts of evolved melts from rocks may require repeated partial melting of crystallized material in order to allow the evolved liquid to physically separate from the parent, and also suggests that the geometry of crystals may be an important factor in melt segregation.