OH defects in coesite and stishovite during ultrahigh-pressure metamorphism of continental crust

Abstract

The high-pressure silica polymorphs coesite and stishovite were synthesized under water-saturated conditions from a natural granitic composition doped with Li and B. Experiments were performed in a Multi-Anvil apparatus between 4 and 9.1 GPa and 900 and 950 °C, based on the conditions of a subducting continental crust as realistic for the ultrahigh-pressure metamorphic units Dora Maira and Kochetav massifs. Run products consisted of coesite/stishovite + kyanite ± phengite ± omphacite, and quench material. The synthesized silica polymorphs were successively analyzed by infrared spectroscopy, electron microprobe, and Secondary-Ion Mass Spectrometry (SIMS). No hydrous defects were observed in coesite synthesized at 4 GPa and 900 °C, whereas coesite grown at higher pressures revealed a triplet of infrared absorptions bands at 3575, 3523, and 3459 cm − 1 , two minor bands at 3535 and 3502 cm − 1 , and a small band at 3300 cm − 1 that was only visible at 7.7 GPa. The total amount of Al was charge-balanced by H and the other monovalent cations. However, the band triplet could not be associated with AlOH defects, while the band doublet was inferred to BOH defects and the small band probably corresponded to interstitial H. Stishovite displayed one dominant band at 3116 cm − 1 with a shoulder at 3170 cm − 1 , and a minor band at 2665 cm − 1 , probably all associated with AlOH defects. BOH defects were not observed in stishovite, and LiOH defects were neither observed in coesite nor stishovite, probably because of preferentially partition of Li in other phases such as omphacite. The total amount of defect protons increased with pressure and with metal impurity concentrations. The general increase in OH defects in silica polymorphs with increasing pressure (this study) contrasted the negative pressure trend of OH in quartz observed previously from the same starting material, and revealed an incorporation minimum of OH in silica polymorphs around the quartz/coesite phase transition.

OH point defects in quartz – a review

“…This review summarises early findings on "water" in quartz and its influence on physical properties, followed by a review of theoretical and experimental considerations on OH point defects, and finally reports recent data on OH point defects in common quartz as a rock-forming mineral from igneous rocks, metamorphic rocks, and sedimentary archives. As will be outlined in this study, molecular water follows different incorporation laws than OH point defects, and, consequently, the wealth of studies on fluid inclusions in quartz, as well as on OH species (molecular water and OH point defects) in microcrystalline and/or amorphous silica such as opal and chalcedony (Langer and Flörke, 1974;Frondel, 1982;Adams et al, 1991;Chauviré et al, 2017) and moganite (Flörke et al, 1984;Heaney and Post, 1992;Hardgrove and Rogers, 2013), are not considered in this review.…”

Experimental constraints on trace element partitioning between coesite and hydrous silicate melt at 5 GPa and 1500–1750°C

“…Unlike the high-P SiO 2 polymorph stishovite which can host some extra components such as Al 2 O 3 (Liu et al, 2006;Liu et al, 2007), Coe is well known as a nearly pure phase (Mosenfelder, 2000;Frigo et al, 2019). Indeed, our EMPA results demonstrate this, with nearly all conventional major elements in Earth sciences appearing in negligible amounts (Table 2).…”

“…Helium was used as a carrier gas to transport the ablated sample aerosol, and was mixed with argon and nitrogen before flowing into the ICP-MS to enhance sensitivity. Similar to ablating quartz, ablating Coe was supposed to be pretty challenging due to the high purity and transparency of Coe (Flem and Müller, 2012), in which case the laser energy might not be easily transmitted to the sample (e.g., Frigo et al, 2019). In our first trials, the analyses were performed with a laser beam focused to a 43-μm spot at a pulse repetition rate of 8 Hz and with a rather high energy density of 16 J cm −2 on the sample.…”

“…However, it is element-specific and strongly matrix-dependent, and thus requires a large suite of highquality standards. Due to these limitations, SIMS has been only applied to quantify a few trace elements (e.g., H, B, Li, Na, Al and Ti) in Coe in a few studies (Koch-Müller et al, 2001, 2003Deon et al, 2009;Frigo et al, 2019;Osborne et al, 2019). Moreover, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been conventionally regarded as an incapable tool for it could not successfully ablate Coe (Frigo et al, 2019).…”

“…Due to these limitations, SIMS has been only applied to quantify a few trace elements (e.g., H, B, Li, Na, Al and Ti) in Coe in a few studies (Koch-Müller et al, 2001, 2003Deon et al, 2009;Frigo et al, 2019;Osborne et al, 2019). Moreover, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been conventionally regarded as an incapable tool for it could not successfully ablate Coe (Frigo et al, 2019). It has been sparsely applied to Coe, and often resulted in very limited data (Bulatov et al, 2014;Brey et al, 2015;Miao et al, 2019;Yan et al, 2021).…”