Serpentine–Hisingerite Solid Solution in Altered Ferroan Peridotite and Olivine Gabbro

Benjamin Tutolo,Scott Kuehner,Bernard Evans

doi:10.3390/min9010047

Benjamin Tutolo, Scott Kuehner + Show 1 more

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https://doi.org/10.3390/min9010047

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Abstract

We present microanalyses of secondary phyllosilicates in altered ferroan metaperidotite, containing approximately equal amounts of end-members serpentine ((Mg,Fe2+)3Si2O5(OH)4) and hisingerite (□Fe3+2Si2O5(OH)4·nH2O). These analyses suggest that all intermediate compositions can exist stably, a proposal that was heretofore impossible because phyllosilicate with the compositions reported here have not been previously observed. In samples from the Duluth Complex (Minnesota, USA) containing igneous olivine Fa36–44, a continuous range in phyllosilicate compositions is associated with hydrothermal Mg extraction from the system and consequent relative enrichments in Fe2+, Fe3+ (hisingerite), Si, and Mn. Altered ferroan–olivine-bearing samples from the Laramie Complex (Wyoming, USA) show a compositional variability of secondary FeMg–phyllosilicate (e.g., Mg–hisingerite) that is discontinuous and likely the result of differing igneous olivine compositions and local equilibration during alteration. Together, these examples demonstrate that the products of serpentinization of ferroan peridotite include phyllosilicate with iron contents proportionally larger than the reactant olivine, in contrast to the common observation of Mg-enriched serpentine in “traditional” alpine and seafloor serpentinites. To augment and contextualize our analyses, we additionally compiled greenalite and hisingerite analyses from the literature. These data show that greenalite in metamorphosed banded iron formation contains progressively more octahedral-site vacancies (larger apfu of Si) in higher XFe samples, a consequence of both increased hisingerite substitution and structure modulation (sheet inversions). Some high-Si greenalite remains ferroan and seems to be a structural analogue of the highly modulated sheet silicate caryopilite. Using a thermodynamic model of hydrothermal alteration in the Fe–silicate system, we show that the formation of secondary hydrothermal olivine and serpentine–hisingerite solid solutions after primary olivine may be attributed to appropriate values of thermodynamic parameters such as elevated a S i O 2 ( a q ) and decreased a H 2 ( a q ) at low temperatures (~200 °C). Importantly, recent observations of Martian rocks have indicated that they are evolved magmatically like the ferroan peridotites analyzed here, which, in turn, suggests that the processes and phyllosilicate assemblages recorded here are more directly relevant to those occurring on Mars than are traditional terrestrial serpentinites.

Highlights

The process of Fe oxidation and coupled H2 production during serpentine formation has attracted sustained research interest across the geological, mineralogical, biological, and planetary sciences over the past few decades (e.g., References [1,2,3,4,5,6]), the presence of oxidized iron in serpentine-group minerals has been acknowledged since at least the late 1950s [7,8,9,10]
Using a thermodynamic model of hydrothermal alteration in the Fe–silicate system, we show that the formation of secondary hydrothermal olivine and serpentine–hisingerite solid solutions after primary olivine may be attributed to appropriate values of thermodynamic parameters such as elevated aSiO2 and decreased a H2 at low temperatures
Recent observations of Martian rocks have indicated that they are evolved magmatically like the ferroan peridotites analyzed here, which, in turn, suggests that the processes and phyllosilicate assemblages recorded here are more directly relevant to those occurring on Mars than are traditional terrestrial serpentinites

Summary

Introduction

The process of Fe oxidation and coupled H2 production during serpentine formation has attracted sustained research interest across the geological, mineralogical, biological, and planetary sciences over the past few decades (e.g., References [1,2,3,4,5,6]), the presence of oxidized iron in serpentine-group minerals has been acknowledged since at least the late 1950s [7,8,9,10]. Minerals 2019, 9, 47 the recent interest in Fe3+ -bearing serpentine has been instigated by the growing recognition that magnetite-free serpentinites in many cases still contain abundant Fe3+ , in the form of Fe3+ -bearing serpentine and, like their magnetite-bearing counterparts, produce H2 , a principal feedstock for chemolithoautrophic organisms (e.g., Reference [11]), during their formation [12,13,14] This observation has in turn led to a proliferation of geochemical models seeking to constrain the formation and stability of Fe3+ -bearing serpentine solid solutions during hydrothermal alteration of olivine (e.g., References [4,15,16,17]). We address this knowledge gap by presenting new and compiled analyses of Fe-rich serpentines formed in mafic igneous intrusions, ore deposits, Martian nakhlite meteorites, and banded iron formations Together, these analyses demonstrate complete miscibility between the end-members greenalite (Fe3 Si2 O5 (OH)4 ), Mg–serpentine (Mg3 Si2 O5 (OH)4 ), and hisingerite (Fe2 Si2 O5 (OH)4 ·nH2 O). These observations, in turn, can form the basis of more robust constraints on geochemical models of Fe3+ incorporation into serpentine and accompanying

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