The effect of water on partial melting in the upper mantle.

Abstract

This thesis presents experimental constraints on incipient melting of mantle peridotite under hydrated conditions. High P-T experiments were performed at pressures of 3 to 13 GPa, and at temperatures of 1200-1450°C. These experiments measure mineral/melt H 2 O partitioning and storage capacity of peridotite components, as well as determine melting phase relations and the compositions of partial melts and residues of hydrated peridotite. Incipient melt H 2 O concentrations are estimated by peridotite/melt H 2 O partitioning ([Special characters omitted.] ). To parameterize [Special characters omitted.] , mineral/melt H 2 O partition coefficients were determined for all crystalline phases of the peridotite solidus assemblage (Chapter 2). Combining these [Special characters omitted.] values with corresponding modal abundances along the solidus yields a [Special characters omitted.] of 0.005-0.010 from 1 to 5 GPa, which is dependent on pressure due to varying garnet and pyroxene modal abundances, and to variable pyroxene Al content. This [Special characters omitted.] range predicts that incipient melts of MORB source (50-200 ppm H 2 O bulk ) and OIB source (300-1000 ppm H 2 O bulk ) upper mantle contain 0.5-3.8 wt.% and 3-20 wt.% dissolved H 2 O, respectively. The amount of dissolved H 2 O in incipient melt dictates hydrous solidus depression, Δ T , which ultimately controls the stability of hydrous melts at P and T . This Δ T -H 2 O melt relationship was investigated at 3.5 GPa by partially melting hydrated peridotite from 1200-1450°C (Chapter 3). Mass balance of phases allows for determination of melt fractions ( F ) from experiments, as well as estimation of H 2 O melt . Δ T values are quantified as the difference in melting temperature between dry and wet peridotite at a particular F . Parameterization of Δ T as a function of H 2 O melt predicts that solidus melts with 1.5, 5, 10, and 15 wt.% dissolved H 2 O generate Δ T values of 50, 150, 250, and 300°C, respectively. Combination of this paramterization with [Special characters omitted.] (Chapter 2) insinuates that 500 ppm H 2 O bulk is necessary to stabilize melt across the observed seismic low velocity zone (LVZ) beneath oceanic lithosphere, which is significantly greater than the MORB source upper mantle H 2 O bulk of 50-200 ppm. This observation argues against suggestions that hydrous melting is solely responsible for the LVZ. At higher pressures the aforementioned parameterizations are difficult to constrain experimentally, but the onset of hydrous melting can be determined by the peridotite H 2 O storage capacity, defined as the maximum H 2 O concentration that peridotite can store without stabilizing a hydrous fluid or melt. A new method of determining a minerals H 2 O storage capacity is employed, in which a hydrated monomineralic layer is equilibrated with a layer of hydrated peridotite and a small amount of melt (Chapter 4). Experiments were carried out at conditions near the 410 km transition zone (TZ) depth to investigate hydrous melting due to the H 2 O storage capacity contrast between the TZ and upper mantle. Measured olivine and orthopyroxene H 2 O storage capacities, combined with estimates of garnet H 2 O storage capacity, and P -dependent lherzolite modes, yields a peridotite H 2 O storage capacity of 700-1100 ppm directly above 410 km. This is not consistent with pervasive melting above 410 km, as this range is several times greater than MORB source upper mantle H 2 O bulk . However, regional melting in areas such as H 2 O-rich OIB source, or areas of recent subduction may likely occur, leaving residues with ∼1000 ppm H 2 O bulk .