Near mantle solidus trace element partitioning at pressures up to 3.4 GPa

Abstract

[1] We present new experimental partitioning data for a range of petrogenetically important elements at pressures up to 3.4 GPa. The experiments are designed to mimic low degrees of anhydrous melting beneath mid-ocean ridges. The available data indicate that the partition coefficients are pressure, temperature, and composition dependent. Therefore partitioning behavior over the appropriate range of pressure, temperature, and composition must be quantified, in order to model continuous extraction of melt during the adiabatic rise of mantle material. For this purpose, we have parameterized the partitioning behavior of the REE, Hf, Zr, U, and Th based on a simple thermodynamic model. Although these parameterizations cannot be used for retrieving thermodynamic constants yet, they do yield accurate descriptions of the partitioning behavior that are useful for modeling decompression melting. Our parameterizations show that the partitioning of trace elements is strongly dependent on the Ca and Alcontent of the clinopyroxene (cpx) and REE are always incompatible in cpx on the peridotite solidus at pressures up to 3.4 GPa. For garnet the data indicate that the heavy REE partition coefficients decrease with increasing pressure. Our data also indicates that Pb is more incompatible than Ce in clinopyroxene; Ce and Pb have similar partition coefficients in garnet. Therefore the presence of a residual phase with high Pb partition coefficients is required to produce the near-constant Ce/Pb ratios in MORB and OIB. Sulfides are the most likely phase to buffer the Pb content in the melt. Except at small porosities (<0.3%), clinopyroxene on the peridotite solidus is unable to fractionate U from Th significantly (15% 230 Th-excess), whereas garnet can fractionate U from Th effectively at porosities up to 1%. Therefore if the 230 Th-excesses in midocean ridge basalts are melting phenomena, then melting with garnet residual is required in order to be compatible with physical observations on porosities and upwelling rate at mid-ocean ridges. New model calculations that include the compositional dependent partitioning of the trace elements show that the predicted physical characteristics (depth and extent of melting, upwelling rate, porosity) of the MORB melting regime are similar for the Lu/Hf, Sm/Nd, and U-Th systems.

Mantle Pb paradoxes: the sulfide solution

“…Four experimental studies of Pb and REE partitioning between cpx, garnet and silicate melt show D Ce /D Pb values ranging from 3-10 for cpx, and from 3-500 for garnet Lundstrom et al 1998;Salters et al 2002;Schmidt et al 1999). In contrast, data from Klemme et al (2002) and Keshav et al (2005) Hofmann et al (1986) proposal that Ce and Pb have similar partition coefficients during mantle melting, at least if Ce and Pb partitioning are controlled simply by silicate phases.…”

Quantifying garnet-melt trace element partitioning using lattice-strain theory: assessment of statistically significant controls and a new predictive model

“…Note that both of these experiments lack data for Sc, and hence were not included in the statistical database. A slight mismatch persists between the model predictions and the data from the Salters et al (2002) experiment, although we note that the mismatch is less than it was for the WvW01 model. This panel illustrates that the new model performs as well or better than did the original WvW01 formulations for generally basaltic systems typical of melting in Earth's mantle.…”

“…Possible correlations between these variables were first assessed to find combinations in which all variables were independent of one another. Then, each individual lattice-strain fit parameter (D 0 r 0 , and E) was (2000) 1-3 GPa, 900-1,150°C, hydrous, andesitic to tonalitic melts Klemme et al (2002) 3 GPa, 1,400°C, anhydrous Nicholls and Harris (1980) 2-3 GPa, 940-1,420°C, dacitic melts, hydrous Salters and Longhi (1999) 2.8 GPa, 1,500-1,550°C, anhydrous Salters et al (2002) 2.8-3.4 GPa, 1,465-1,660°C, anhydrous Shimizu and Kushiro (1975) 3 GPa, 1,275°C, anhydrous Walter et al (2004)~23 GPa,~2,300°C, anhydrous, majoritic garnet multiply regressed against combinations of these independent variables using both standard and ''stepwise'' approaches. Multiple linear regression results for these permutations show which are robust predictors and which are not.…”

Volatile and trace elements in basaltic glasses from Samoa: Implications for water distribution in the mantle

“…For comparison, N-MORB La/Sm and Sm/Yb ratios are each 1.0 (Hofmann, 1988). The steepness of the rare-earth element slopes indicates melting within the stability field of garnet lherzolite or melting to small degrees within the stability field of spinel lherzolite (Hauri et al, 1994;Salters et al, 2002), as to be expected to occur beneath an old tectonic plate where the bottom of the lithosphere (e.g. Li et al, 2004) is near the garnet to spinel transition (Kogiso et al, 1998;Robinson and Wood, 1998;Klemme and O'Neill, 2000).…”