Dehydration experiments on natural omphacites: qualitative and quantitative characterization by various spectroscopic methods

Koch‐Müller, Monika; Abs-Wurmbach, I.; Rhede, Dieter; Kahlenberg, Volker; Matsyuk, S. S.

doi:10.1007/s00269-007-0181-7

Cited by 29 publications

(21 citation statements)

References 23 publications

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“…The alternative is to directly analyze water in nominally anhydrous mantle minerals of dismembered peridotite bodies (e.g., Miller et al, 1987). Experiments have shown that the substitution type by which hydroxyl is bound in a mineral's structure has a major impact on the diffusivity of hydrogen and, thus, the tendency toward water loss or gain (Blanchard & Ingrin, 2004;Ferriss et al, 2016;Hercule & Ingrin, 1999;Koch-M€ uller et al, 2007;. The greatest problem-water loss due to decompression-applies to any approach because water loss may affect both mantle minerals and melts (plus melt inclusions).…”

Section: General Backgroundmentioning

confidence: 99%

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Water in Abyssal Peridotite: Why Are Melt‐Depleted Rocks so Water Rich?

Schmädicke

Gose

Stalder

2018

Geochem Geophys Geosyst

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Evidence for the heterogeneity of the mantle's water content is generally acknowledged but it remains unclear if the variability of data is influenced by decompressional water loss. Here based on high water contents in peridotite, we explore if water gain may play a role. The study investigates water in abyssal peridotite by analyzing oriented orthopyroxene with polarized infrared radiation. Orthopyroxene from ODP‐Leg 153 (Mid‐Atlantic Ridge) contains 100 ppm more H2O (range: 220–323 ppm) than Leg 209 samples (range: 121–231 ppm). Possible explanations for high water concentrations are (1) water supply by gabbroic dykes, (2) reequilibration with ancient, H2O‐rich melts or melt‐derived fluid, (3) diffusional exchange between depleted peridotite and fertile rock volumes, or (4) high initial (premelting) water contents. Water in orthopyroxene is unrelated to the proximity of gabbroic dykes, and the inferred low H2O content of the latter renders possibility (1) unlikely. Possibilities (2) and (3) require that the rocks remained at mantle depth for an extended time span. This idea of a prolonged “mantle residence time” is corroborated by (i) literature data on isotopes and (ii) mantle equilibrium temperatures. Peridotite from Leg 153 reequilibrated at lower temperature (950–1,000°C) than Leg 209 peridotite (1,100–1,200°C) implying that more time elapsed between melt removal and exhumation—extending the time span for refertilization and explaining the its higher water content. The alternative that the measured concentrations are residual (4) requires high H2O amounts in original orthopyroxene (1,500 ppm) and peridotite (700 ppm)—values that are typical of an OIB‐source.

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Section: General Backgroundmentioning

confidence: 99%

“…Hydrogen is highly diffusive in mantle minerals (e.g., Demouchy & Mackwell, 2006;Koch-M€ uller et al, 2007;Stalder & Skogby, 2003). Hydrogen is highly diffusive in mantle minerals (e.g., Demouchy & Mackwell, 2006;Koch-M€ uller et al, 2007;Stalder & Skogby, 2003).…”

Section: Water Diffusion From Hydrous Mantle Volumesmentioning

confidence: 99%

Water in Abyssal Peridotite: Why Are Melt‐Depleted Rocks so Water Rich?

Schmädicke

Gose

Stalder

2018

Geochem Geophys Geosyst

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“…Hydrogen in clinopyroxene is typically observed as one or more of four separate bands in infrared spectra at wavenumbers 3640 ± 10, 3540 ± 10, 3450 ± 10, and 3355 ± 10 cm −1 (Ingrin et al 1989;Skogby 2006;Skogby et al 1990). Despite decades of effort, including systematically doping hydrous diopside (Karimova and Stalder 2013;Purwin et al 2009;Stalder and Ludwig 2007), correlating hydrogen concentration with composition in natural samples (Koch-Muller et al 2004;Skogby et al 1990;Smyth et al 1991), and experimentation Koch-Muller et al 2007;Yang et al 2015), many details of these four hydrogen incorporation mechanisms remain uncertain. Broadly, high-wavenumber bands around 3600 cm −1 bands are most likely related to silica undersaturation and H + charge-balanced by the resulting vacancies and/or trivalent cation substitutions on the tetrahedral site (Karimova and Stalder 2013;Stalder and Ludwig 2007;Su et al 2008).…”

Section: Introductionmentioning

confidence: 98%

Site-specific hydrogen diffusion rates during clinopyroxene dehydration

Ferriss

Plank

Walker

2016

Contrib Mineral Petrol

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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...

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“…Partition coefficients for OH between coexisting garnet and omphacite, using the primary structural water contents of garnet, are 0.07–0.23 (Table ), similar to other studies (discussed by Schmädicke & Gose, ). However, because dehydrogenation of garnet (Wang, Zhang, & Essene, ) and omphacite (Su, Zhang, Redfern, & Bromiley, ) may occur during decompression, the mechanism of hydrogen loss from the crystal structure is different from one to the other (Blanchard & Ingrin, ; Koch‐Müller, Abs‐Wurmbach, Rhede, Kahlenberg, & Matsyuk, ), and the equilibrium partition coefficient is dependent on composition (Bell, Rossman, & Moore, ), the empirical partition coefficients determined in this study may not record equilibrium values related to the peak of UHP metamorphism.…”

Section: Resultsmentioning

confidence: 99%

On the survival of intergranular coesite inUHPeclogite

Wang

Brown

et al. 2017

Journal Metamorphic Geology

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Coesite is typically found as inclusions in rock‐forming or accessory minerals in ultrahigh‐pressure (UHP) metamorphic rocks. Thus, the survival of intergranular coesite in UHP eclogite at Yangkou Bay (Sulu belt, eastern China) is surprising and implies locally “dry” conditions throughout exhumation. The dominant structures in the eclogites at Yangkou are a strong D2 foliation associated with tight‐to‐isoclinal F2 folds that are overprinted by close‐to‐tight F3 folds. The coesite‐bearing eclogites occur as rootless intrafolial isoclinal F1 fold noses wrapped by a composite S1–S2 foliation in interlayered phengite‐bearing quartz‐rich schists. To evaluate controls on the survival of intergranular coesite, we determined the number density of intergranular coesite grains per cm2 in thin section in two samples of coesite eclogite (phengite absent) and three samples of phengite‐bearing coesite eclogite (2–3 vol.% phengite), and measured the amount of water in garnet and omphacite in these samples, and also in two samples of phengite‐bearing quartz eclogite (6–7 vol.% phengite, coesite absent). As coesite decreases in the mode, the amount of primary structural water stored in the whole rock, based on the nominally anhydrous minerals (NAMs), increases from 107/197 ppm H2O in the coesite eclogite to 157–253 ppm H2O in the phengite‐bearing coesite eclogite to 391/444 ppm H2O in the quartz eclogite. In addition, there is molecular water in the NAMs and modal water in phengite. If the primary concentrations reflect differences in water sequestered during the late prograde evolution, the amount of fluid stored in the NAMs at the metamorphic peak was higher outside of the F1 fold noses. During exhumation from UHP conditions, where NAMs became H2O saturated, dehydroxylation would have generated a free fluid phase. Interstitial fluid in a garnet–clinopyroxene matrix at UHP conditions has dihedral angles >60°, so at equilibrium fluid will be trapped in isolated pores. However, outside the F1 fold noses strong D2 deformation likely promoted interconnection of fluid and migration along the developing S2 foliation, enabling conversion of some or all of the intergranular coesite into quartz. By contrast, the eclogite forming the F1 fold noses behaved as independent rigid bodies within the composite S1–S2 foliation of the surrounding phengite‐bearing quartz‐rich schists. Primary structural water concentrations in the coesite eclogite are so low that H2O saturation of the NAMs is unlikely to have occurred. This inherited drier environment in the F1 fold noses was maintained during exhumation by deformation partitioning and strain localization in the schists, and the fold noses remained immune to grain‐scale fluid infiltration from outside allowing coesite to survive. The amount of inherited primary structural water and the effects of strain partitioning are important variables in the survival of coesite during exhumation of deeply subducted continental crust. Evidence of UHP metamorphism may be preserved in similar isolated st...

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Dehydration experiments on natural omphacites: qualitative and quantitative characterization by various spectroscopic methods

Cited by 29 publications

References 23 publications

Water in Abyssal Peridotite: Why Are Melt‐Depleted Rocks so Water Rich?

Water in Abyssal Peridotite: Why Are Melt‐Depleted Rocks so Water Rich?

Site-specific hydrogen diffusion rates during clinopyroxene dehydration

On the survival of intergranular coesite inUHPeclogite

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