Hydrogen, trace, and ultra-trace element distribution in natural olivines

Abstract

We investigate the coupling between H, minor, trace, and ultra-trace element incorporations in 17 olivines from ten different locations covering various petrological origins: magmatic, hydrothermal and mantle-derived context. Concentrations in major element are determined by micro X-ray fluorescence. Minor, trace, and ultra-trace element are determined by laser ablation inductively coupled plasma mass spectrometry. Hydrogen concentrations are quantified using unpolarized and polarized Fourier transform infrared spectroscopy (FTIR). Forsterite contents (83.2 to 94.1 %) reflect the petrogenetic diversity. Hydrogen concentrations range from 0 to 54 ppm H2O wt. Minor element concentrations (Ni + Mn) range from 3072 to 4333 ppm, and impurities are dominated by Ni, Mn, Ca or B. Total trace element concentrations range from 8.2 to 1473 ppm. Total rare Earth and extended ultra-trace elements concentrations are very low (< 0.5 ppm). Magmatic and hydrothermal olivines show the most and least amount of impurities, respectively, and mantle-derived olivines have concentrations between these two extremes. Combined with minor, trace, and ultra-trace element concentrations, the hydrogen concentrations and FTIR OH bands reflect the point defect diversity imposed by different geological settings. Hydrogen concentrations are inversely correlated with divalent impurities,indicating their competition for vacancies. However, a broad positive correlation is also found between OH bands at 3575 and 3525 cm -1 and Ti, confirming the existence of Ti-clinohumitelike point defect in mantle olivines. Nonetheless, Ti does not exclusively control hydrogen incorporation in olivine due to the co-existence with other mechanisms, and its effect appears diluted. Our results confirm that hydrogen behaves as a peculiar incompatible element, and furthermore as an opportunistic impurity in olivine.

Defects in olivine

“…The most abundant monovalent cationic impurities in olivine are H + , Li + , and Na + (Kent and Rossman, 2002;Sykes et al, 1994;Grant and Wood, 2010;Demouchy and Alard, 2021), but Rb + and Cs + are also present as ultra-trace elements (Fig. 4).…”

“…Atomic impurities are also used as proxies for equilibrium temperature (Ca in olivine and co-existing orthopyroxene) and pressure (Al in olivine; e.g., Brey and Kohler, 1990;Witt-Eickschen and O'Neill, 2005;Coogan et al, 2014;D'Souza et al, 2020;Bussweiler et al, 2017). Thanks to recent technical advances, we can now measure a broad array of atomic impurities in olivine, from heavy (e.g., Th, U, at concentrations > 1 ppb) to light (e.g., Li) elements, by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS; e.g., Tollan et al, 2018;Bussweiler et al, 2019;Batanova et al, 2019;Demouchy and Alard, 2021). Other analytical methods are used to quantify volatile concentrations: H by Fourier transform infrared spectroscopy (FTIR; e.g., Beran and Zemann, 1969;Beran and Putnis, 1983;Rossman, 2006), He and Ar by noble gas mass spectrometry (e.g., Trull et al, 1991;Parman et al, 2005;Tolstikhin et al, 2010;Burnard et al, 2015), and C by secondary ion mass spectrometry (SIMS; e.g., Keppler et al, 2003).…”

“…Examples of atomic impurity concentrations in olivines from a spinel-bearing peridotite from the wellknown San Carlos locality (Arizona) and from a Patagonian garnet-bearing peridotite are provided in Fig. 4 (analytical methods and original data reported in Demouchy and Alard, 2021). The elements are ranked in order of decreasing concentration as in Bussweiler et al (2019; see their Fig.…”

“…The San Carlos olivine is used as a reference for ranking the elements, and the Patagonian olivine is overlain to highlight slight enrichments or depletions observed in deeper olivines from garnet-bearing peridotites. Elements at concentrations below the typical detection limits (grey symbols; see details in Demouchy and Alard, 2021) are included at the far right.…”

“…Many texts and reviews are available in the materials science literature, mostly addressing a given type of defect, for example, point defects (e.g., Crawford and Slifkin, 1975;Schock, 1985), dislocations (e.g., Hirth and Lothe, 1968), or grain and interface boundaries (e.g., Sutton and Balluffi, 1995;Han et al, 2018). Other studies focus on physical processes and/or properties involving these defects in Earth minerals, for example ionic diffusion (e.g., Zhang and Cherniak, 2010), plastic deformation (e.g., Poirier, 1976Poirier, , 1985Karato and Wenk, 2002), electrical conductivity (e.g., Shankland, 1975;Yoshino et al, 2012), or metasomatic changes in the chemical composition of olivine (e.g., O'Reilly et al, 1997;Foley et al, 2013;O'Reilly and Griffin, 2013;Demouchy and Alard, 2021). These processes and properties notably involve hydrogen, by far the most popular atomic impurity in olivine (e.g., Beran and Zemann, 1969;Beran and Putnis, 1983;Kohlstedt et al, 1996;Mosenfelder et al, 2006;Férot and Bolfan-Casanova, 2012;Demouchy and Bolfan-Casanova, 2016;Blanchard et al, 2017;Demouchy and Alard, 2021).…”