Elucidating Igneous and Ore-Forming Processes with Iron Isotopes by using Experimental and Field-Based Methods.

Iron (Fe) is a vital resource and the fourth most common element in the Earth’s crust, but variations in the Fe isotope composition of igneous rocks were only recently identified. This dissertation uses experimental and field-based methods to demonstrate the utility of Fe isotopes in tracking igneous and ore-forming processes. Chapter II presents the first experimental data that measure directly Fe isotope fractionation among phases in a fluid-bearing magmatic assemblage. The results, some of which contradict theoretical predictions, indicate that Fe isotopes fractionate during crystallization of magnetite from a melt and that Fe isotope fractionation between melt—fluid is influenced by the Cl content of the fluid. This is important considering the frequent extrapolation of data obtained from Fe-Cl complexes that are unrealistic for magmatic systems. Chapter III applies Fe isotopes to natural ore samples since Fe is globally mined from the rocks of iron oxide—apatite (IOA) deposits, which are a globally important source of Fe and other elements such as the rare earths but lack a genetic model. I focus on the world-class Los Colorados IOA, Chile as a case study and combine the Fe and O isotope composition of magnetite to investigate their formation. The data are consistent with a high-temperature (i.e., magmatic/magmatic-hydrothermal) origin for IOA deposits, and contributed to the development of a published novel IOA model. Iron is also abundant in layered mafic intrusions, and Chapter IV focuses on the uppermost portion of the world’s largest exposed mafic magma chamber, the Bushveld Complex, South Africa. These Fe isotope data demonstrate that fractional crystallization is reflected in the Fe isotope signature of the uppermost Bushveld. Stratigraphically, over the top ~2.5 km of this 9 km-thick intrusion, there is little variation in both whole rock and magnetite Fe isotope compositions, revealing that, despite theoretical predictions for the crystallization of magnetite to shift the isotopic composition of the whole rock, the presence of other Fe-bearing phases can buffer that effect. By incorporating published fractionation factors to model the measured data, this study provides the first benchmark for Fe isotope evolution during the crystallization of a large magma chamber.PhDEarth and Environmental SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113582/1/bilenker_1.pd

Magmatism, serpentinization and life: Insights through drilling the Atlantis Massif (IODP Expedition 357)

IODP Expedition 357 used two seabed drills to core 17 shallow holes at 9 sites across Atlantis Massif ocean core complex (Mid-Atlantic Ridge 30°N). The goals of this expedition were to investigate serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration in one borehole. The cores show highly heterogeneous rock types and alteration associated with changes in bulk rock chemistry that reflect multiple phases of magmatism, fluid-rock interaction and mass transfer within the detachment fault zone. Recovered ultramafic rocks are dominated by pervasively serpentinized harzburgite with intervals of serpentinized dunite and minor pyroxenite veins; gabbroic rocks occur as melt impregnations and veins. Dolerite intrusions and basaltic rocks represent the latest magmatic activity. The proportion of mafic rocks is volumetrically less than the amount of mafic rocks recovered previously by drilling the central dome of Atlantis Massif at IODP Site U1309. This suggests a different mode of melt accumulation in the mantle peridotites at the ridge-transform intersection and/or a tectonic transposition of rock types within a complex detachment fault zone. The cores revealed a high degree of serpentinization and metasomatic alteration dominated by talc-amphibole-chlorite overprinting. Metasomatism is most prevalent at contacts between ultramafic and mafic domains (gabbroic and/or doleritic intrusions) and points to channeled fluid flow and silica mobility during exhumation along the detachment fault. The presence of the mafic lenses within the serpentinites and their alteration to mechanically weak talc, serpentine and chlorite may also be critical in the development of the detachment fault zone and may aid in continued unroofing of the upper mantle peridotite/gabbro sequences. New technologies were also developed for the seabed drills to enable biogeochemical and microbiological characterization of the environment. An in situ sensor package and water sampling system recorded real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential (Eh), and temperature and during drilling and sampled bottom water after drilling. Systematic excursions in these parameters together with elevated hydrogen and methane concentrations in post-drilling fluids provide evidence for active serpentinization at all sites. In addition, chemical tracers were delivered into the drilling fluids for contamination testing, and a borehole plug system was successfully deployed at some sites for future fluid sampling. A major achievement of IODP Expedition 357 was to obtain microbiological samples along a west–east profile, which will provide a better understanding of how microbial communities evolve as ultramafic and mafic rocks are altered and emplaced on the seafloor. Strict sampling handling protocols allowed for very low limits of microbial cell detection, and our results show that the Atlantis Massif subsurface contains a relatively low density of microbial life

Abiotic Oxidation Rate of Chalcopyrite: Implications for Seafloor Mining

In situ mining of seafloor massive sulfide (SMS) deposits will have consequences thus far not quantified. On land, interaction of mined sulfide minerals with surface and groundwaters yields acid mine drainage. Pulverization of SMS on the ocean floors will produce highly reactive sulfide mineral surface areas, leading to the localized potential for seafloor acid generation. Chalcopyrite (CuFeS2) is one of several ore minerals found in SMS deposits whose oxidation kinetics need to be quantified to estimate the significance of acid production. To constrain the oxidation rate of chalcopyrite in seawater, the initial rate experimental method was employed and combined with the isolation method to derive a rate law. Data collected from batch reactor experiments without abundant precipitates (pH <4.5), between 7°C and 25°C, and PO2 from 0.10 to 0.995 atm were incorporated into the rate law. The molal specific rate law is:Rsp = - 10-9.38(PO2)1.22(H+)0.36Chalcopyrite oxidizes slowly in seawater relative to other sulfide minerals like pyrrhotite (Fe1-xS), so data from this study establishes a minimum rate of abiotic SMS weathering by oxidation. The slow rate of oxidation of chalcopyrite observed here has positive implications for seafloor mining. Not only will this sulfide not be the main culprit for acid production, but the copper ore will arrive at the surface with minimal dissolution and loss of metal value. Constraining the oxidation rates of individual sulfide mineral species will be useful in modeling SMS mining repercussions, as well as quantifying rates of natural chemical weathering in the oceans over geologic time. This information will be applicable to interpreting the Cu/Fe ratios of VMS deposits.The potential for local acid generation can be viewed as a microcosm of the global problem of ocean acidification caused by dissolution of anthropogenic atmospheric CO2. Data show sulfide mineral oxidation rates increase with lower pH, implying that a worldwide drop in ocean pH may amplify the dissolution of SMS deposits, changing the marine ecosystem

Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes

International audienceInternational Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites distributed ~10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N). Mantle exposed in the footwall of the Atlantis Massif oceanic core complex is predominantly nearly wholly serpentinized harzburgite with subordinate dunite. Altered peridotites are subdivided into three types: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatic serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases on the basis of macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (˂0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, ‘cryptic’ melt-impregnation is inferred on the basis of incompatible element enrichments. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, and by anomalously high anhydrous SiO2 concentrations (59–61 wt%) and low MgO/SiO2 values (0.48–0.52). Although many chondrite-normalized rare earth element (REE) and primitive mantle-normalized incompatible trace element anomalies, e.g., negative Ce-anomalies, are attributable to serpentinization, other compositional heterogeneities are due to melt-impregnation. On the basis of whole rock incompatible trace elements, a dominant mechanism of melt-impregnation is distinguished in the central and eastern serpentinites from fluid-rock alteration (mostly serpentinization) in the western serpentinites, with increasing melt-impregnation manifest as a west to east increase in enrichment in high-field strength elements and light REE. High degrees of melt extraction are evident in low whole-rock Al2O3/SiO2 values and low concentrations of Al2O3, CaO and incompatible elements. Estimates of the degree of melt extraction based on whole rock REE patterns suggest a maximum of ~20% non-modal fractional melting, with little variation between sites. As some serpentinite samples are ex situ rubble, the magmatic histories observed at each site are consistent with a local source (from the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models. Alternatively, sites may signify heterogeneities in these processes at spatial scales of a few km

Iron isotopic evolution during fractional crystallization of the uppermost Bushveld Complex layered mafic intrusion

We present δ56Fe (56Fe/54Fe relative to standard IRMM‐014) data from whole rock and magnetite of the Upper and Upper Main Zones (UUMZ) of the Bushveld Complex. With it, we assess the role of fractional crystallization in controlling the Fe isotopic evolution of a mafic magma. The UUMZ evolved by fractional crystallization of a dry tholeiitic magma to produce gabbros and diorites with cumulus magnetite and fayalitic olivine. Despite previous experimental work indicating a potential for magnetite crystallization to drastically change magma δ56Fe, we observe no change in whole rock δ56Fe above and below magnetite saturation. We also observe no systematic change in whole rock δ56Fe with increasing stratigraphic height, and only a small variation in δ56Fe in magnetite separates above magnetite saturation. Whole rock δ56Fe (errors twice standard deviation, ±2σ) throughout the UUMZ ranges from −0.01 ±0.03‰ to 0.21 ±0.09‰ (δ56FeaverageWR = 0.10 ±0.09‰; n = 21, isotopically light outlier: δ56FeWR = −0.15‰), and magnetites range from 0.28 ±0.04‰ to 0.86 ±0.07‰ (δ56FeaverageMgt = 0.50 ±0.15‰; n = 20), similar to values previously reported for other layered intrusions. We compare our measured δ56FeWR to a model that incorporates the changing normative mineralogy, calculated temperatures, and published fractionation factors of Fe‐bearing phases throughout the UUMZ and produces δ56FeWR values that evolve only in response to fractional crystallization. Our results show that the Fe isotopic composition of a multiply saturated (multiple phases on the liquidus) magma is unlikely to change significantly during fractional crystallization of magnetite due to the competing fractionation of other Fe‐bearing cumulus phases.Key PointsWhole rock and magnetite separates from the uppermost portion of the Bushveld Complex were analyzed for their Fe isotope compositionsWe find no systematic variation in whole rock or magnetite Fe isotope ratios with stratigraphic height85% crystallization of a dry tholeiitic multiply‐saturated magma does not significantly fractionate Fe isotopesPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136675/1/ggge21257_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136675/2/ggge21257.pd