The effects of metamorphism on iron mineralogy and the iron speciation redox proxy

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 surface 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 SilurianDevonian 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 2 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, a...

Mid‐Proterozoic Ferruginous Conditions Reflect Postdepositional Processes

“…Multiple sulfide phases often occur within the same sedimentary nodule emphasizing that recrystallization of pyrite incorporated trace metals. The movement of iron from pyrite to other phases-or out of the system entirely-will affect iron speciation data and complicate interpretations of paleoredox (Slotznick, Eiler, & Fischer, 2018). To the east of the pyrrhotite-siderite isograd, rock magnetic experiments identified nanophase pyrrhotite (i.e., pyrrhotite below the micron-size-threshold for ferromagnetism) in almost all lower Belt samples (Figure 4a) suggesting metamorphic pyrrhotite formation had begun in small domains within pyrite.…”

“…To the east of the pyrrhotite-siderite isograd, rock magnetic experiments identified nanophase pyrrhotite (i.e., pyrrhotite below the micron-size-threshold for ferromagnetism) in almost all lower Belt samples (Figure 4a) suggesting metamorphic pyrrhotite formation had begun in small domains within pyrite. The movement of iron from pyrite to other phases-or out of the system entirely-will affect iron speciation data and complicate interpretations of paleoredox (Slotznick, Eiler, & Fischer, 2018).…”

“…Therefore, we interpreted the Fe-bearing carbonates in the best-preserved lower Belt samples as indicative of a suite of postdepositional processes-there are no textural observations that indicate the formation and transport of these phases in the water column. Their presence skews iron speciation results toward incorrectly implying ferruginous conditions (Slotznick, Eiler, & Fischer, 2018).…”

“…More recently, the bulk sequential extraction technique of iron speciation has dramatically altered views on Proterozoic geochemistry, revealing a complex picture of predominately anoxic iron-rich (i.e., ferruginous) conditions with additional oxic and anoxic H 2 S-rich (i.e., euxinic) environments varying in time, geography, and water depth (Poulton & Canfield, 2011;Sperling et al, 2015). However, postdepositional processes especially diagenetic and metamorphic formation of carbonates and iron sulfides can affect iron speciation proxy data (Slotznick, Eiler, & Fischer, 2018). In order to address these issues, we took an approach that is sensitive to the redox state of iron within samples while also preserving textural data to understand how and when the iron-bearing minerals formedchemical imaging techniques including energy dispersive spectroscopy (EDS) and synchrotron X-ray absorption spectroscopy paired with sensitive bulk rock magnetics.…”

Sulfides in the marbles of spurrite-merwinite facies: the Kochumdek River, East Siberia

“…В регионально-метаморфических комплексах минералогия сульфидов изучена достаточно глубоко, что обусловлено необходимостью оценки их рудного потенциала и реконструкции обстановок древней седиментации [1]. В низкобарических метаморфических породах сульфиды главным образом изучаются в связи с проблематикой скарновых месторождений [2].…”