GENERATION OF RHYOLITIC MELT IN AN ARTIFICIAL MAGMA - IMPLICATIONS FOR FRACTIONAL CRYSTALLIZATION PROCESSES IN NATURAL MAGMAS

A large (1.3x 10(7) g) artificial mafic melt with a bulk composition s imilar to an unusually calcic basalt (17 wt.% CaO) was produced by in- situ heating of soil, and subsequently cooled and crystallized. The fi nal 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 p resence of a small amount (approximately 10 vol.%) of rhyolitic glass (71% SiO2) within the crystalline network. This glass is the residual material left after crystallization of pyroxene and feldspar, analogou s to rhyolitic melt which may be generated from fractional crystalliza tion of a basaltic magma. Ion microprobe imaging indicates that the rh yolitic glass is generally present in isolated triangular- to rectangu lar-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 evide nce of coalescence or migration of the rhyolitic melt is suggested by glass morphology. Although these observations were made in an artifici al magmatic system, analogies to a natural system may be drawn. The is olated nature and low abundance of the rhyolitic glass highlights the difficulty of extracting and segregating evolved melts produced by hig h degrees of crystallization of a primitive parent, particularly one c haracterized by elongated, rather than equant, crystals. The distribut ion of the rhyotitic glass supports the suggestion that extraction of significant amounts of evolved melts from rocks may require repeated p artial melting of crystallized material in order to allow the evolved liquid to phasically separate from the parent, and also suggests that the geometry of crystals may be an important factor in melt segregatio n.