Pyroxenes as tracers of mantle water variations

Abstract

The concentration and distribution of volatiles in the Earth's mantle influence properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, concentrations of H 2 O, P, and F were measured in olivine, orthopyroxene, and clinopyroxene in mantle peridotites by secondary ion mass spectrometry. Analyzed peridotites are xenoliths (Pali Aike, Spitsbergen, Samoa), orogenic peridotites (Josephine Peridotite), and abyssal peridotites (Gakkel Ridge, Southwest Indian Ridge, Tonga Trench). The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H 2 O concentrations, in agreement with mineral diffusion data. Olivines have diffusively lost water during emplacement, as demonstrated by disequilibrium between olivine and coexisting pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high-temperature water contents, and their partitioning agrees with published experiments and other xenoliths. Hence, olivine water concentrations can be determined from pyroxene concentrations using mineral-mineral partition coefficients. Clinopyroxenes have 60-670 ppm H 2 O, while orthopyroxenes have 10-300 ppm, which gives calculated olivine concentrations of 8-34 ppm. The highest olivine water concentration translates to an effective viscosity of 6 × 1019 Pa s at 1250°C and~15 km depth, compared to a dry effective viscosity of 2.5 × 10 21 Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20-220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late-stage melt addition, during which only hydrogen diffuses fast enough for reequilibration.

Fluorine and chlorine in mantle minerals and the halogen budget of the Earth’s mantle

“…f. Ol-Opx inter-mineral partition coefficients from this 651 study only. Black solid lines in c, d, e, and f denote inter-mineral partition coefficients using 652 data from Warren and Hauri (2014 Tables 2, 3 and 4. 685 686 687 688 689 …”

On water in nominally anhydrous minerals from mantle peridotites and magmatic rocks

“…In addition, water in NAMs from most mantle xenoliths could be analyzed using FTIR. Therefore, a great number of mantle peridotite xenoliths have been systematically studied for water in NAMs (e.g., Li et al, 2008;Yang et al, 2008;Peslier et al, 2010Peslier et al, , 2012Xia et al, 2010Xia et al, , 2013aDoucet et al, 2014;Warren and Hauri, 2014;Hui et al, 2015;.…”

“…Other hypotheses have also been proposed to make olivine more easily to lose H via diffusion during the magma ascent, such as rearrangement of defects in olivine, partial association with planar defects of microscopic hydrous phases, and distribution of hydrous nanophases along deformation structures (Peslier, 2010, and references therein). In addition, the fast hydrogen diffusion in olivine may be due to charge compensation by a flux of electron (Kohlstedt and Mackwell, 1998), while the hydrogen diffusion in pyroxene is rate limited by the coupled exchanges of Al 3+ and H + for Si 4+ (or 2Mg 2+ ), or exchange reaction governed by Fe content (Hercule and Ingrin, 1999;Woods et al, 2000;Warren and Hauri, 2014). Nevertheless, it is generally believed that pyroxene might not have exchanged water with its host magma, in contrast to the significant H loss of olivine during magma ascent (e.g., Peslier et al, 2002;Gose et al, 2009;Warren and Hauri, 2014).…”

“…Therefore, pyroxene water contents typically represent the mantle values. (e.g., Peslier et al, 2002;Gose et al, 2009;Xia et al, 2013a;Warren and Hauri, 2014;Hui et al, 2015;. In addition, covariation between water contents of pyroxene and those of cations with low diffusivity, such as pyroxene Al concentrations, could also be used as evidence that pyroxene may have preserved mantle water content (e.g., .…”

“…This may partly explain that the H 2 O/Ce ratios in mantle peridotite vary significantly (Figure 3), while the range of hydrogen isotopes in mantle NAMs is rather limited (Bell and Ihinger, 2000). Furthermore, the observation that the water contents in some mantle peridotites are too high relative to their rareearth-element concentrations, has been interpreted as water reequilibration due to fast diffusion during late-stage melt addition (Warren and Hauri, 2014).…”