Magmatic processes revealed by textural and compositional trends in Merapidome lavas

Syn-eruptive degassing of volcanoes may lead to syn-eruptive crystallizatio n of groundmass phases, We have investigated this process using textural an d compositional analysis of dome material from Merapi volcano, Central Java , Indonesia. Samples included dome lavas from the 1986-88, 1992-93, 1994 an d 1995 effusive periods as well as pyroclastic material deposited by the No vember 1994 dome collapse. With total crystallinities commonly in excess of 70% (phenocrysts + microlites), the liquids present in Merapi andesites ar e highly evolved (rhyolitic) at the time of eruption. Feldspar microlites i n dome rocks consist of plagioclase cores (Ab(63)An(29)Or(8)) surrounded by alkali feldspar rims (Ab(53)An(5)Or(42)). compositional pairs which are no t in equilibrium. A change in the phase relations of the ternary feldspar s ystem caused by degassing best explains the observed transition in feldspar composition. A small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical do me material and contain rimless plagioclase microlites, suggesting that the 1994 collapse event incorporated less-degassed, partially liquid magma in addition to fully solidified dome rock. As decompression drives volatile exsolution, rates of degassing and resulta nt microlite crystallization may be governed by magma ascent rate. Microlit e crystallinity is nearly identical among the 1995 dome samples, an indicat ion that similar microlite growth conditions (PH2O and temperature) were ac hieved throughout this extrusive period, However, microlite number density varied by more than a factor of four in these samples, and generally increa sed with distance from the vent. Low vent-ward microlite number densities a nd greater microlite concentrations down-flow probably reflect progressivel y decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome. Comparison between dome extrusion episodes ind icates a correlation between lava effusion rate and microlite number densit y, suggesting that extrusion slowed during 1995, Crystal textures and compo sitions in the 1992-93 and 1994 domes share the range exhibited by the 1995 dome, suggesting that transitions in crystallization conditions (i.e., rat es of undercooling determined by effusion rate) are cyclic. (C) 2000 Elsevi er Science B.V. All rights reserved.