Experimental Phase Relations of Basaltic Andesite from Hole 839B Under Hydrous and Anhydrous Conditions

Abstract

Experimental phase relations were used to assess the role of volatiles and crustal level fractional crystallization in the petrogenesis of lavas from Hole 839B in the central Lau Basin. Melting experiments were performed on Sample 135-839B-15R-2, 63-67 cm, at 1 atm, anhydrous, and 2 kbar, H 2 O-saturated (~6 wt% H 2 O in the melt) to determine the influence of variable pressure and H 2 O content on phase appearances, mineral chemistry, and liquid line of descent followed during crystallization. The effects of H 2 O are to depress the liquidus by ~100°C, and to suppress crystallization of Plagioclase and orthopyroxene relative to olivine and high-Ca clinopyroxene. At 1 atm, anhydrous, olivine and Plagioclase coexist near the liquidus, whereas ortho-pyroxene and then clinopyroxene appear with decreasing temperature. Crystallization of 50 wt% produces a residual liquid that is rich in FeO* (10.8 wt%) and poor in A1 2 O 3 (13.6 wt%). At 2 kbar, H 7 O-saturated, the liquidus phases are olivine and chromian spinel, with high-Ca clinopyroxene appearing after-10% crystallization. Plagioclase saturation is suppressed until-20% crystal-lization has occurred. The residual liquid from 35 wt% crystallization is rich in AI2O3 (17.4 wt%), and poor in MgO (4.82 wt%); it contains moderate FeO* (8.2 wt%), and resembles the low-MgO andesites recovered from Hole 839B. On the basis of these experiments we conclude that the primitive lavas recovered from Hole 839B have experienced crystallization along the 01 + Cpx saturation boundary, under hydrous conditions (an ankaramitic liquid line of descent), and variable amounts of olivine and chromian spinel accumulation. The low-MgO andesites from Hole 839B are the products of hydrous fractional crystallization, at crustal pressures, of a parent magma similar to basaltic andesite Sample 135-839B-15R-2, 63-67 cm.