Determining the composition of high-pressure mantle melts using diamond aggregates

Abstract

We present a new experimental technique for circumventing the quenching problems that have plagued high-pressure peridotite melting studies. A thin layer of ~50 μm diamonds is placed above a layer of peridotite powder. Partial melt extracted from the peridotite layer collects in the pore spaces between the diamonds and equilibrates diffusively with the residual peridotite mineralogy. Isolated from the crystalline residue, the melt quenches to a glass that records the composition of the liquid coexisting with the residual crystalline phases under the conditions of the experiment. We have used this technique to investigate partial melting of a fertile mantle composition at 10 kbar and a temperature range of 1270–1390°C. Oxide concentrations in the liquids from the longest duration runs (up to 151 hours) vary systematically with increasing temperature: TiO_2, Al_2O_3, and Na_2O decrease monotonically, while Cr_2O_3, FeO^∗, and MgO increase steadily. CaO shows more complicated behavior, first increasing and then decreasing, with the crest in the temperature-CaO trend approximately coincident with the disappearance of clinopyroxene from the residue between 1330 and 1350°C. Overall variation in silica content with temperature is small, and there appears to be a minimum at about 12% melting. The compositions of liquids produced in time series, temperature reversal, and two-stage experiments (conducted to test the technique) all indicate that our experimentally determined liquid compositions represent close approaches to equilibrium. Calculated melt fractions (F) also vary systematically with temperature. The slope of the T(°C)-F curve is not constant over the spinel lherzolite melting interval, but decreases as temperature increases from 1270 to 1330°C. Extrapolating the curve back to zero melt suggests that the anhydrous solidus temperature for our peridotite starting composition is ~ 1240°C. At temperatures below the cpxout curve, melt generation occurs via the reaction, 0.38 opx + 0.71 cpx + 0.13 sp → 0.22 oliv + 1.0 liq, and the proportions of minerals that enter the melt appear to be independent of temperature. At temperatures above cpx-out, the less well constrained melting reaction is: 1.06 opx + 0.04 sp = 0. 1 oliv + 1 liq. The fact that all of the 10 kbar melts have FeO^∗ contents that are substantially lower than those reported in any primitive MORB glasses further strengthens the conclusions that these glasses are not 10 kbar primary melts, that they involve a component of higher pressure partial melting, and that they have evolved by significant olivine fractionation from more primitive liquids. Our experimental data also provide an independent check of the results of recent peridotite partial melting calculations. Efforts to parameterize the experimental database on peridotite melting, and to calculate melt compositions as a function of P, T, and F are partially successful in reproducing the compositional trends determined in this study.

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