The behavior of chalcophile elements during magmatic differentiation as observed in the Kilauea Iki lava lake, HawaiiMolybdenum is often considered chalcophile in low temperature hydrothermal and sedimentary environments, yet its geochemical behavior during high T igneous differentiation has been relatively unknown. The behavior of other ore-forming chalcophile elements like Cu, Ga, Ge, As, Ag, Cd, In, Sn, Sb, W, Tl, Pb, Bi during magmatic differentiation is equally unstudied - while some seminal work has been done to infer their behavior given whole rock or glass data, few studies have sought to understand their partitioning behavior using in-situ methods. In understanding how these elements partition in igneous systems like the Kilauea Iki lava lake, we can test proposed ideas of crustal evolution like the development of a theorized sulfide layer at the base of the crust (Lee et al., 2012) as well as test mixing models for the origin of the Hawaiian mantle.
My collaborators and I present a case study of the behavior of these elements during the evolution of Kilauea Iki Lava Lake, an amazing natural laboratory for studying magmatic differentiation in Greaney et al., 2017 (GCA). Crustal sources of molybdenumAs a redox sensitive trace element, Mo is an incredibly valuable tracer of oxidation in the early Earth - it's specifically used for pinpointing the initial rise of atmospheric oxygen at 2.4 Ga (The Great Oxidation Event). Studies have used Mo abundances in Precambrian ocean sediments (black shales) to ascertain the redox state of the atmosphere and ocean basin at the time of deposition. This method is dependent on the assumption that Mo is weathered from sulfides in the continental crust after the GOE because sulfides will break down and release oxidized, soluble Mo in the presence of atmospheric O2. My work tests this assumption by evaluating all mineral phases found in common upper crustal rocks to determine which are the predominate hosts of Mo. This study also explores the behavior of Mo during igneous differentiation, pluton crystallization, and crustal weathering. Using the data collected and experimental results obtained by my collaborators at ASU, weathering models can be built to determine just how much Mo could have been weathered from the crust both with and without the presence of atmospheric O2. Results were presented at Goldschmidt 2016 (abs#1754) and are in revision at GCA. Mo isotopes reveal oxidation of Earth's continental crust during the 2.4 Ga Great Oxidation Event
It is inferred that Mo is removed from the upper continental crust (UCC) during oxidative weathering and subsequently deposited in marine black shales. However, to fully interpret the Mo abundances and isotope fractionations recorded in shales, we must quantify how much Mo was being removed and fractionated from the continents. Fortunately, we can use glacial diamictites (see Gaschnig et al., 2014, EPSL) to track the weathering signature of the now inaccessible Precambrian atmosphere. As glaciers traverse a continent, they scrape off the uppermost weathered regolith of the UCC, and deposit those sediments in diamictite deposits. These diamictites thus record the weathering signature of the UCC at the time of glaciation. Gaschnig et al. (2014) analzyed diamictites that formed between 2900 and 300 Ma, and span four modern continents. Here, we analyze those same deposits for Mo isotope fractionation (measured as d98/95) to determine if Mo isotopes fractionate during continental weathering, and if that fractionation varies under oxidized and reduced atmospheres. Results were presented at AGU 2017 and are in prep for publication.
Mo isotope fractionation in subduction zonesGenerally, Mo in arc magmas and the UCC is isotopically heavier than the primitive mantle and MORB. This fractionation may occur during igneous differentiation or during slab dehydration and melting in the subduction zone. This project seeks to address this question by analyzing Mo isotopes in rutile separates from eclogites that are thought to represent subducted and partially melted oceanic crust. Preliminary results were presented at Goldschmidt 2018.
Identifying chalcophile elements in the mantleGiven the results of the Kilauea Iki study, we next look towards the mantle to determine if the same suite of chalcophile and lithophile elements exhibit similar geochemical behavior in the lithospheric mantle. Using peridotite xenoliths from the Hannuoba basalts in the North China Craton, I use in-situ laser ablation ICP-MS to determine the mineralogical hosts of Ga, Ge, As, Mo, Ag, Cd, In, Sn, Sb, W, Tl, Pb, and Bi. These peridotites are unique amongst xenoliths because they preserve abundant sulfides that are normally broken down during mantle metasomatism or alteration during eruption and weathering on the Earth's surface.
Mo isotope fractionation during oxidative continental weatheringA comparative study of weathering profiles is being undertaken to asses differences in Mo isotope fractionation during continental weathering in Fe-oxide-rich soils and oxide-poor soils.
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Running the Element2 at the University of Maryland
A hand sample of molybdenite, MoS2. FOV = 5cm
A photomicrograph of an immiscible sulfide bleb in Kilauea Iki Lava Lake, FOV = 200 um
Glacial diamictite from the Huronian Glaciation. Photo: Rich Gaschnig
Eclogite that is thought to represent subducted and partially melted Archean oceanic crust.
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Top photo: Photomicrograph of a basalt from the Kilauea Iki Lava Lake, xpl, field of view = 1 cm