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This study evaluates changes in copper (Cu) speciation that occur in sulfate-dominated basaltic and andesitic magmas equilibrated at oxygen fugacities (fO2’s) above the nickel-nickel oxide (NNO) buffer. Cu K-edge microfocused X-ray absorption fine structure spectroscopy (XAFS) data are presented from both natural and synthetic silicate glasses. Natural samples analyzed include olivine-hosted melt inclusions from tephra of mafic cinder cones in the Lassen segment of the Cascade arc (USA) and from the Michoacan-Guanajuato volcanic field (Mexico) as representative samples from melts equilibrated at fO2 > NNO. A comparison with melts equilibrated at fO2 < NNO is provided by analysis of olivine-hosted melt inclusions from Kīlauea Volcano. Data are also presented from copper- and sulfur-bearing synthetic hydrous andesitic glasses synthesized over a range of fO2, from roughly NNO-2 to NNO+2. The Cu spectroscopy data from the natural and synthetic glasses show two dominant Cu species, Cu1+ oxides (referred to here as Cu-O) and Cu1+ sulfides (referred to here broadly as Cu-S, but not precluding Cu-Fe-S species). The relative proportion of each species present correlates with the relative concentration of dissolved sulfide in the melt. Synthetic sulfur-bearing glasses equilibrated at NNO-1.2 were found to contain exclusively Cu-S species. Sulfur-bearing experimental glasses equilibrated at NNO-0.5 give calculated Cu-O/(Cu-O + Cu-S), defined here as the “Cu-O fraction”, of < 0.10, whereas sulfur-bearing glasses synthesized at NNO+0.6 and NNO+1.8 give calculated Cu-O fraction > 0.96. Natural melt inclusions from Lassen and Kīlauea show a bimodal distribution in Cu-O fraction, with overlapping ranges, of 0.14-0.77 for Lassen and 0.18- 0.78 for Kīlauea. Michoacan-Guanajuato inclusions yield Cu-O fractions of 0.68-0.91. The difference in the calculated proportions of Cu-O to Cu-S species appear correlated with available sulfide in the melt. As relative S2- concentrations decrease, the dissolved Cu species in the melt evolves from dominantly Cu-S to Cu-O. This includes melts equilibrated at fO2’s where S6+ is the dominant S species. At intermediate sulfide abundances both species appear to coexist. Thermodynamic modeling of the Cu speciation in these silicate melts suggests that speciation of Cu as a CuFeS2 melt species (akin to chalcopyrite or intermediate solid solution) most accurately predicts the measured Cu species. The modeling suggests that aFeO in the silicate melt, fO2 and melt S2- (expressed as fS2) are the most important parameters controlling the proportions of Cu-O vs. Cu-S species. Our results provide a new perspective for understanding Cu solubility, transport, and partitioning in magmatic systems.
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Genesis of Fe–Ti Oxide-Bearing Ultramafic Intrusions in the Duluth Complex, Minnesota, USA
Abstract The Duluth Complex is a large mafic intrusive system located in northeastern Minnesota emplaced as part of the 1.1-Ga Midcontinent Rift. Several Fe–Ti oxide-bearing ultramafic intrusions are hosted along the Western Margin of the Duluth Complex, and are discordant bodies present in a variety of geometries, hosted in multiple rock types, and dominated by peridotite, pyroxenite, and semi-massive to massive Fe–Ti oxide rock types. Their origin has been debated, and here we present geochemical evidence and modeling that supports a purely magmatic origin for the Titac and Longnose Fe–Ti oxide-bearing ultramafic intrusions. Ilmenite and titanomagnetite textures indicate a protracted cooling process, and δ34S values of sulfides reveal little assimilation of the footwall Virginia Formation, a fine-grained pelitic unit that contains sulfide-rich bands. We model the crystallization of a hypothetical parental magma composition to the host intrusion of Longnose using Rhyolite-MELTS and demonstrate that the accumulation of Fe–Ti oxides in the discordant intrusions cannot be explained by density-driven segregation of crystallized Fe–Ti oxides. Instead, we show that the development of silicate liquid immiscibility, occurring by the unmixing of the silicate melt into conjugate Si- and Fe-rich melts, can result in the effective segregation and transportation of the Fe-rich melt. The Fe-rich melt is ~2 orders of magnitude less viscous than the Si-rich melt, allowing the Fe-rich melt to be more effectively segregated and transported in the mush regime (crystallinities >50%). This suggests that viscosity, in addition to density, plays a significant role in forming the discordant Fe–Ti oxide-bearing ultramafic intrusions. We propose a genetic model that could also be responsible for the Fe–Ti oxide-rich layers or bands that are hosted within the igneous stratigraphy of mafic intrusions of the Duluth Complex.
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- PAR ID:
- 10505785
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Journal of Petrology
- Volume:
- 65
- Issue:
- 5
- ISSN:
- 0022-3530
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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