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The Effects of Metamorphism on Iron Mineralogy and the Iron Speciation Redox Proxy

Slotznick, Sarah P. and Eiler, John M. and Fischer, Woodward W. (2018) The Effects of Metamorphism on Iron Mineralogy and the Iron Speciation Redox Proxy. Geochimica et Cosmochimica Acta, 224 . pp. 96-115. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20171212-102252703

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Abstract

As the most abundant transition metal in the Earth’s crust, iron is a key player in the planetary redox budget. Observations of iron minerals in the sedimentary record have been used to describe atmospheric and aqueous redox environments over the evolution of our planet; the most common method applied is iron speciation, a geochemical sequential extraction method in which proportions of different iron minerals are compared to calibrations from modern sediments to determine water-column redox state. Less is known about how this proxy records information through post-depositional processes, including diagenesis and metamorphism. To get insight into this, we examined how the iron mineral groups/pools (silicates, oxides, sulfides, etc.) and paleoredox proxy interpretations can be affected by known metamorphic processes. Well-known metamorphic reactions occurring in sub-chlorite to kyanite rocks are able to move iron between different iron pools along a range of proxy vectors, potentially affecting paleoredox results. To quantify the effect strength of these reactions, we examined mineralogical and geochemical data from two classic localities where Silurian-Devonian shales, sandstones, and carbonates deposited in a marine sedimentary basin with oxygenated seawater (based on global and local biological constraints) have been regionally metamorphosed from lower-greenschist facies to granulite facies: Waits River and Gile Mountain Formations, Vermont, USA and the Waterville and Sangerville-Vassalboro Formations, Maine, USA. Plotting iron speciation ratios determined for samples from these localities revealed apparent paleoredox conditions of the depositional water column spanning the entire range from oxic to ferruginous (anoxic) to euxinic (anoxic and sulfidic). Pyrrhotite formation in samples highlighted problems within the proxy as iron pool assignment required assumptions about metamorphic reactions and pyrrhotite’s identification depended on the extraction techniques utilized. The presence of diagenetic iron carbonates in many samples severely affected the proxy even at low grade, engendering an interpretation of ferruginous conditions in all lithologies, but particularly in carbonate-bearing rocks. Increasing metamorphic grades transformed iron in carbonates into iron in silicate minerals, which when combined with a slight increase in the amount of pyrrhotite, drove the proxy toward more oxic and more euxinic conditions. Broad-classes of metamorphic reactions (e.g. decarbonation, silicate formation) occurred at distinct temperatures-pressures in carbonates versus siliciclastics, and could be either abrupt between metamorphic facies or more gradual in nature. Notably, these analyses highlighted the importance of trace iron in phases like calcite, which otherwise might not be included in iron-focused research i.e. ore-system petrogenesis, metamorphic evolution, or normative calculations of mineral abundance. The observations show that iron is mobile and reactive during diagenesis and metamorphism, and these post-depositional processes can readily overprint primary redox information held by iron speciation. However, in principle, additional mineralogical and petrographic approaches can be combined with iron speciation data to help untangle many of these post-depositional processes and arrive at more accurate estimates of paleoenvironmental redox conditions and processes, even for metamorphosed samples.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/j.gca.2017.12.003DOIArticle
http://www.sciencedirect.com/science/article/pii/S0016703717307652PublisherArticle
ORCID:
AuthorORCID
Fischer, Woodward W.0000-0002-8836-3054
Additional Information:© 2017 Elsevier Ltd. Received 30 January 2017, Accepted 4 December 2017, Available online 10 December 2017. We thank associate editor Jan Wiederhold, Clark Johnson, Peter Kraal, and one anonymous reviewer for their comments that greatly improved the manuscript. Joseph Kirschvink, Alex Sessions, and John Grotzinger also provided helpful suggestions on an early version of this manuscript. Support for this work was provided by the Agouron Institute (W.W.F.), NSF Graduate Research Fellowship program (S.P.S.) and NASA Earth and Space Fellowship (S.P.S.).
Funders:
Funding AgencyGrant Number
Agouron InstituteUNSPECIFIED
NSF Graduate Research FellowshipUNSPECIFIED
NASA Earth and Space Science FellowshipUNSPECIFIED
Record Number:CaltechAUTHORS:20171212-102252703
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20171212-102252703
Official Citation:Sarah P. Slotznick, John M. Eiler, Woodward W. Fischer. The effects of metamorphism on iron mineralogy and the iron speciation redox proxy, Geochimica et Cosmochimica Acta, Volume 224, 2018, Pages 96-115, ISSN 0016-7037, https://doi.org/10.1016/j.gca.2017.12.003. (http://www.sciencedirect.com/science/article/pii/S0016703717307652)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:83822
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:12 Dec 2017 21:42
Last Modified:31 Jan 2018 00:04

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