Abstract
The supracrustal metamorphic rocks of the Dora-Maira Massif, western Alps, have been intensively studied. Certain ultra-high-pressure lithologies contain coesite and nearly end-member composition pyrope, Mg3Al2Si3O12, making this locality petrologically and mineralogically unique. Structural OH-, loosely termed “water”, in pyrope crystals of different composition has been investigated numerous times, using different experimental techniques, by various researchers. However, it is not clear where the minor OH- is located in them. IR single-crystal spectra of two pyropes of composition {Mg2.79,Fe2+0.15,Ca0.04}Σ2.98[Al]2.02(Si)2.99O12 and {Mg2.90,Fe2+0.04,Ca0.02}Σ2.96[Al]2.03(Si)3O12 were recorded at room temperature (RT) and 80 K. The spectra show five distinct OH- bands located above 3600 cm-1 at RT and seven narrow bands at 80 K and additional fine structure. The spectra were curve fit and the OH- stretching modes analyzed and assigned. It is argued that OH- is located in microscopic- and nano-size Ca3Al2H12O12-like clusters. The basic substitution mechanism is the hydrogarnet one, where (H4O4)4- ⇔ (SiO4)4-, and various local configurations containing different numbers of (H4O4)4- groups define the cluster type. The amounts of H2O range between 5 and 100 ppm by weight, depending on the IR calibration adopted, and are variable among crystals. Hydrogrossular-like clusters are also present in a synthetic pyrope with a minor Ca content grown hydrothermally at 900 °C and 20 kbar, as based on its IR spectra at RT and 80 K. Experiment and nature are in agreement, and OH- groups are partitioned into various barely nano-size hydrogrossular-like clusters. This proposal is new and significant mineralogical, petrological, and geochemical implications result. Ca and proton ordering occur. Hypothetical “defect” and/or coupled-substitution mechanisms to account for structural OH- are not needed to interpret experimental results. OH- incorporation in pyrope of different generations at Dora-Maira is discussed and OH- could potentially be used as an indicator of changing P_{{{text{H}}_{{text{2}}} {text{O}}}} {text{(}}a_{{{text{H}}_{{text{2}}} {text{O}}}} {text{) - }}T conditions in a metamorphic cycle. Published experimental hydration, dehydroxylation, and hydrogen diffusion results on Dora-Maira pyropes can now be interpreted atomistically.
Highlights
Estimates of the amount of H 2O held in Earth’s mantle range between 0.5 and 5.5 oceans worth (Hirschmann et al 2005)
The key reservoir for H 2O in the upper mantle lies in the small amounts of OH- that are structurally bound in the nominally anhydrous minerals (NAMs) olivine, orthopyroxene, clinopyroxene and garnet, as shown by Bell and Rossman (1992a)
The anhydrous crystal-chemical formulae of the Dora-Maira garnets were calculated, using the formulation of Locock (2008), giving {Mg2.79,Fe2+0.15,Ca0.04}2.98[Al]2.02 (Si)2.99O12 for Mas-2b and {Mg2.90,Fe2+0.04,Ca0.02}2.96[Al]2.03 (Si)3.00O12 for Ch-2. The latter has, upon renormalizing, about 98% pyrope component making it, as best we know, one of the most pyrope-rich garnets found to date on Earth
Summary
Estimates of the amount of H 2O held in Earth’s mantle range between 0.5 and 5.5 oceans worth (Hirschmann et al 2005). The key reservoir for H 2O in the upper mantle lies in the small amounts of OH- that are structurally bound in the nominally anhydrous minerals (NAMs) olivine, orthopyroxene, clinopyroxene and garnet, as shown by Bell and Rossman (1992a). These four silicates can hold small concentrations of O H- that, when integrated over their volumes contained in the upper mantle, give rise to such large-scale, global amounts of water. On the other hand, understanding is still lacking in some most fundamental questions
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