Robert W. Luth

EXPERIMENTAL STUDY OF THE SYSTEM PHLOGOPITE-DIOPSIDE FROM 3.5 TO 17 GPA

Abstract On the basis of both natural samples and experimental studies, clinopyroxene is a potential reservoir for potassium in the Earth’s mantle. The amount of K partitioning into clinopyroxene depends on the phase assemblage present, the bulk composition, pressure, and temperature. To investigate some of these dependencies, subsolidus and melting phase relations in the system phlogopite-diopside have been studied to 17 GPa. In this system, phlogopite becomes unstable with increasing pressure, breaking down to potassium richterite, which in turn breaks down to another K-bearing hydrous phase (phase X), such that a K-rich phase coexists with clinopyroxene to 17 GPa. Clinopyroxenes contain ≤ 1.3 wt% K2O in assemblages of phlogopite + clinopyroxene ± olivine ± liquid at 3-5 GPa, phlogopite + clinopyroxene + garnet ± olivine ± liquid at 7-9 GPa, clinopyroxene + garnet + olivine ± potassium richterite ± liquid at 11 GPa, and clinopyroxene + olivine + garnet + phase X at 14 and 17 GPa. In these assemblages, K is partitioned into hydrous phases or liquid, rather than into the clinopyroxene. By inference, phlogopite (or its higher-pressure breakdown products) is the primary host for K in the mantle (if H2O is present), and any coexisting clinopyroxene has very low concentrations of K. Conversely, the natural occurrence of clinopyroxene with >> 1 wt% K2O requires that phlogopite, potassium richterite, or phase X is not stable in the local source environment of such samples.

Diamonds, Eclogites, and the Oxidation State of the Earth's Mantle

The reaction dolomite + 2 coesite →← diopside + 2 diamond + 2O2 defines the coexistence of diamond and carbonate in mantle eclogites. The oxygen fugacity of this reaction is ∼1 log unit higher at a given temperature and pressure than the oxygen fugacities of the analogous reactions that govern the stability of diamond in peridotite. This difference allows diamond-bearing eclogite to coexist with peridotite containing carbonate or carbonate + diamond. This potential coexistence of diamond-bearing eclogite and carbonate-bearing peridotite can explain the presence of carbon-free peridotite interlayered with garnet pyroxenites that contain graphitized diamond in the Moroccan Beni Bousera massif at the Earths surface and the preferential preservation of diamond-bearing eclogitic relative to peridotitic xenoliths in the Roberts Victor kimberlite.

Experiments on CaCO3-MgCO3 solid solutions at high pressure and temperature

Abstract Multi-anvil experiments have been performed in the system CaCO3-MgCO3 at pressures of 5.0-7.0 GPa and temperatures of 800-1600 °C. The reaction dolomite = aragonite + magnesite has been reversed and located near 750 °C at 5.0 GPa, 900 °C at 6.0 GPa, and 1000 °C at 7.0 GPa. Between 5 and 6 GPa, the reaction boundary is strongly curved and its dP/dT slope increases from 2 to 12 MPa/°C, expanding the stability field of dolomite. This increase is attributed to increasing Ca-Mg disorder in the dolomite. In addition to the reaction boundary, an isobaric section of CaCO3-MgCO3 has been determined at 6.0 GPa. There is a two-phase field of aragonite + dolomite on the Ca-rich side, which closes before minimum melting temperatures of 1350 °C are reached. The two-phase field on the Mg-rich side, where dolomite + magnesite coexist, intersects with the solidus. Inferred minimum melts are close to dolomite composition suggesting congruent melting of dolomite at 6 GPa. The melt compositions and temperatures in the pure carbonate system are surprisingly similar to solidus phase relations in the CMAS-CO2 system, implying that minimum melting conditions in carbonated peridotite at high pressures are dominated and controlled by the carbonate component.

The graphite-COH fluid equilibrium in P, T, \(f_{O_2 } \) space

AbstractThe oxygen fugacity (

Ferric iron in mantle-derived pyroxenes and a new oxybarometer for the mantle

f_{O_2 }

Constraints on the oxidation state of the mantle: An electrochemical and 57Fe Mössbauer study of mantle-derived ilmenites

) of a C-O-H fluid in equilibrium with graphite has been determined in the range 10–30 kbar by equilibrating solid

Spectroscopy of the cation distribution in the schorlomite species of garnet

f_{O_2 }