Open AccessHarzburgite melting with and without H 2 O: Experimental data and predictive modeling
Author(s) -
Parman Stephen W.,
Grove Timothy L.
Publication year - 2004
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2003jb002566
Subject(s) - anhydrous , andesite , partition coefficient , olivine , analytical chemistry (journal) , geology , mineralogy , materials science , volcano , chemistry , volcanic rock , geochemistry , organic chemistry , chromatography
The effect of H 2 O on harzburgite‐saturated melts has been quantified with a series of hydrous and anhydrous melting experiments using a piston‐cylinder device. Experimental conditions were 1.2–2.2 GPa and 1175–1500°C. Melt H 2 O contents range from 0 to 10 wt %. The effects of temperature, pressure, and bulk composition (including H 2 O) on the SiO 2 content of the experimental melts have been evaluated using SiO 2 activity coefficients. The results suggest a two‐lattice‐type model for the melt phase in which H 2 O mixes nearly ideally with other network modifiers (MgO, FeO, etc.) but does not mix on the network‐forming lattice site and so has little effect on SiO 2 activity coefficients. The effect of H 2 O on SiO 2 activity is too small to produce the high SiO 2 contents observed in mafic andesite magmas. It is proposed that the SiO 2 ‐rich character of hydrous, subduction‐related magmas is the result of the low temperatures at which hydrous melting occurs relative to anhydrous melting. Partition coefficients for MgO and FeO increase at lower temperatures, while the partition coefficient for SiO 2 is nearly constant and is buffered by olivine‐orthopyroxene equilibria. Therefore the SiO 2 /(MgO + FeO) ratios of harzburgite saturated melts increase as temperature falls in both hydrous and anhydrous systems. The results suggest that H 2 O contents of andesitic magmas may be far higher (>7 wt %) than is generally accepted. Experimentally measured mineral/melt partition coefficients (this study and literature data) have been parameterized in terms of pressure, temperature, and melt H 2 O content. These expressions have been used to construct a Gibbs‐Duhem‐based numerical model that predicts the compositions of hydrous and anhydrous olivine‐orthopyroxene‐saturated melts. Comparisons with experimental data not included in the model indicate that it is the most accurate model available for predicting the compositions of high‐degree mantle melts, with or without H 2 O.