Low water contents in pyroxenes from spinel-peridotites of the oxidized, sub-arc mantle wedge

water content of the metasomatic medium is similar to that of carbonatitic-kimberlitic melts/fluids. Eclogites contain more water than peridotites recorded in the literature (341 ± 161 vs 122 ± 54 ppm) and represent an important water reservoir in the lithospheric mantle wherever they occur. This is an important parameter to be considered in the interpretation of mantle processes and geophysical data such as seismic wave speeds and electrical conductivity, and in geodynamic modeling.

Section: Implications Of the Datamentioning

confidence: 99%

Water contents of Roberts Victor xenolithic eclogites: primary and metasomatic controls

Huang

Griffin

et al. 2014

“…Clinopyroxene has the highest hydrogen solubility of the major mantle minerals (Ingrin and Skogby 2000) and therefore likely contributes significantly to the upper mantle water budget (Bell and Rossman 1992), which may in turn control the occurrence of melting and plate tectonics (Green et al 2006;Hirschmann 2006;Kushiro et al 1968). Hydrogen concentrations measured in mantle xenolith clinopyroxene are commonly interpreted to represent the hydrogen content of the mantle from which the xenolith originated (Demouchy et al 2006;Denis et al 2013;Ingrin and Skogby 2000;Li et al 2008;Peslier 2010;Peslier et al 2002Peslier et al , 2012. However, if hydrogen diffusion in clinopyroxene is fast relative to the rate of magma ascent bringing the xenolith to the surface, then these measured hydrogen concentrations may reflect at least partial equilibrium with the external magma rather than an original mantle signature.…”

Section: Introductionmentioning

confidence: 99%

Site-specific hydrogen diffusion rates during clinopyroxene dehydration

Ferriss

Plank

Walker

2016

peaks at 3620, 3550, 3460, and 3355 cm −1 in the thickness direction at 800 °C. Bulk hydrogen diffusivity in the Jaipur diopside is consistent with previous work, and hydrogen diffusivity in augite PMR-53 is slightly lower than the fast direction diffusivities measured || [100] and [001]* in Jaipur diopside. Both diopsides show 1-2 orders of magnitude differences in the peaks-specific diffusivities, with the fastest diffusivities at 3450 cm −1 and the slowest at 3645 cm −1 . However, the hydrogen diffusivities in Jaipur diopside are 2-4 orders of magnitude higher than those in Kunlun diopside for bulk hydrogen and all peaks. Thus, peak-specific differences cannot by themselves adequately explain the 5 orders of magnitude range in hydrogen diffusivities observed experimentally in different diopsides. The results are broadly consistent with a previously proposed increase in hydrogen diffusivity in diopside with Fe up to 0.6-0.8 a.p.f.u., although there may be an opposing relationship with Al(IV). The results of this study and others predict high water diffusivities in Fe-bearing mantle xenolith clinopyroxene, on the order of ~10 −9 to 10 −11 m 2 /s at 1000 °C. The common observation of hydrogen zonation in mantle xenolith olivine, but not in clinopyroxene implies that hydrogen diffusion is much faster in olivine than in pyroxene, which then requires the operation of the fastest diffusion mechanism quantified in olivine and diffusivities in clinopyroxene at the lower end of this 2 orders of magnitude range. Such high diffusivities strongly suggest that water in mantle xenoliths has at least partially equilibrated with the host magma, and that the diffusion profiles observed in mantle xenolith olivine reflect only the final stage of ascent after water begins to exsolve. AbstractThe rate of hydrogen diffusion in clinopyroxene is relevant to interpreting hydrogen ("water") concentrations in xenoliths, phenocrysts, and clinopyroxene-hosted melt inclusions to provide insight into the deep-earth water cycle and volcanic explosivity. Here, we determine bulk and site-specific hydrogen diffusivities in two diopsides and an augite by heating initially homogeneous water-bearing samples in a 1-atm CO/CO 2 gas-mixing furnace at 800-1000 °C and oxygen fugacity at the quartz-fayalite-magnetite buffer and observing H-loss profiles. The O-H stretching range between wavenumbers 3000 and 4000 cm −1 in FTIR spectra is resolved into 4-6 peaks, each of which is assumed to represent a distinct defect site for the hydrogen, to determine peakspecific diffusivities using our previously published whole-block method. For the diopside from the Kunlun Mts. in China, Arrhenius relations are reported for peaks at 3645, 3617, 3540, 3443, and 3355 cm −1 based on measurements at 816, 904, and 1000 °C. Bulk and sitespecific diffusivities are determined for the same set of peaks at 904 °C for the second diopside (Jaipur). The augite (PMR-53) was a triangular thin slab, and hydrogen diffusivities were determined for bulk hydrogen and Communicated by Gor...

“…subduction settings) may be more limited than previously thought. Low water contents in pyroxenes from the sub-arc mantle wedge have previously been interpreted to be controlled by the redox state of the mantle rock (Peslier et al 2002), since a negative correlation between oxygen fugacity and water content of the pyroxenes could be established. On the basis of our experimental results the salinity of the fluid phase could be an alternative explanation.…”

Section: Implications For the Earth's Mantlementioning

confidence: 99%

“…More than half of this hydrogen inventory is hosted in orthopyroxene (Grant et al 2007), which exhibits a rather narrow range around 200 ppm (wt) H 2 O in mantle xenoliths from continental alkali basalts and kimberlites (Grant et al 2007). In contrast, natural mantle orthopyroxenes from sub-arc settings show a much higher variability (40-260 ppm H 2 O) and generally lower water contents (Peslier et al 2002). The incorporation of water in orthopyroxene has also been investigated in many experimental studies (Rauch and Keppler 2002;Stalder 2004;Stalder et al 2005;Mierdel et al 2007), where up to several thousands ppm H 2 O have been reported.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen incorporation in enstatite in the system MgO–SiO2–H2O–NaCl

Stalder

Kronz²,

Simon³

2008

The incorporation of hydrogen in enstatite in a hydrous system containing various amounts of NaCl was investigated at 25 kbar. The hydrogen content in enstatite shows a clear negative correlation to the NaCl-concentration in the system. The most favourable explanation is the reduction of water fugacity due to dilution. Other reasons for the limited hydrogen incorporation at high NaCl levels, such as a significant influence of Na + on the defect chemistry or an exchange between OH -and Cl -in enstatite, appear much less important. A partition coefficient D Na En/Fluid = 0.0013 could be determined, demonstrating that Na is less incompatible in enstatite than H. The new results support the idea that dissolved components have to be considered when the total hydrogen storage capacity in nominally anhydrous minerals is estimated, especially in geological settings with high levels of halogens, such as subduction zones.