Oxygen fugacity at the base of the Talkeetna arc, Alaska

Bucholz, Claire E.; Kelemen, P. B.

doi:10.1007/s00410-019-1609-z

Cited by 37 publications

(21 citation statements)

References 129 publications

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“…The elevated magmatic fO 2 values of the Haib magmas relative to mid-oceanic ridge basalts (MORB; ΔFMQ − 2-0) 32 are strongly reminiscent of Phanerozoic oxidized island-arc magmas that are generally thought to originate from the oxidized sub-arc mantle 3,4,11,12,[33][34][35][36][37][38] . Deep crustal garnet fractionation has recently been hypothesized to auto-oxidize magma for porphyry Cu deposit formation 39,40 .…”

Section: Discussionmentioning

confidence: 99%

Oxidized sulfur-rich arc magmas formed porphyry Cu deposits by 1.88 Ga

et al. 2021

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Most known porphyry Cu deposits formed in the Phanerozoic and are exclusively associated with moderately oxidized, sulfur-rich, hydrous arc-related magmas derived from partial melting of the asthenospheric mantle metasomatized by slab-derived fluids. Yet, whether similar metallogenic processes also operated in the Precambrian remains obscure. Here we address the issue by investigating the origin, fO2, and S contents of calc-alkaline plutonic rocks associated with the Haib porphyry Cu deposit in the Paleoproterozoic Richtersveld Magmatic Arc (southern Namibia), an interpreted mature island-arc setting. We show that the ca. 1886–1881 Ma ore-forming magmas, originated from a mantle-dominated source with minor crustal contributions, were relatively oxidized (1‒2 log units above the fayalite-magnetite-quartz redox buffer) and sulfur-rich. These results indicate that moderately oxidized, sulfur-rich arc magma associated with porphyry Cu mineralization already existed in the late Paleoproterozoic, probably as a result of recycling of sulfate-rich seawater or sediments from the subducted oceanic lithosphere at that time.

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Section: Discussionmentioning

confidence: 99%

Oxidized sulfur-rich arc magmas formed porphyry Cu deposits by 1.88 Ga

et al. 2021

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“…Calculated log 10 fO 2 in the models varies between~FMQ +1 and FMQ +3 , where FMQ is the fayalite-magnetite-quartz buffer and the subscript indicates logarithmic deviations from the buffer. Such values are relatively common in metasedimentary granulites (e.g., Boger et al, 2012;Yakymchuk et al, 2019) and typical of many lower-crustal rocks in general (Bucholz & Kelemen, 2019). No explicit fO 2 buffer was imposed on the calculations, as that would require that O concentrations vary as a function of T, P, and the compositions of Fe 2+ -Fe 3+ solid solutions to maintain the buffer (i.e., not a closed system).…”

Section: 1029/2020gc009052mentioning

confidence: 99%

Accessory Mineral Eu Anomalies in Suprasolidus Rocks: Beyond Feldspar

Holder

Yakymchuk

Viete

2020

Geochem Geophys Geosyst

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Accessory mineral Eu anomalies (Eu/Eu*) are routinely measured to infer changes in the amount of feldspar over time, allowing accessory mineral U‐Pb dates to be linked to the progressive crystallization of igneous and metamorphic rocks and, by extension, geodynamic processes. However, changes in Eu/Eu* can reflect any process that changes the relative availability of Eu2+ and Eu3+. We constructed partitioning budgets for Sm, Eu2+, Eu3+, and Gd in suprasolidus metasedimentary rocks to investigate processes that can influence accessory mineral Eu anomalies. We modeled three scenarios: (1) closed‐system, equilibrium crystallization; (2) fractionation of Eu by feldspar growth during melt crystallization; and (3) removal of Eu by melt extraction. In the closed‐system equilibrium model, accessory mineral Eu/Eu* changes as a function of fO2 and monazite stability; Eu/Eu* changes up to 0.3 over a pressure‐temperature range of 4–12 kbar and 700–950°C. Fractionation of Eu by feldspar growth is modeled to decrease accessory mineral Eu/Eu* by ~0.05–0.15 per 10 wt% feldspar crystallized. Melt extraction has a smaller effect; removal of 10% melt decreases accessory mineral Eu/Eu* in the residue by ≤0.05. Although these models demonstrate that fractionation of Eu by feldspar growth can be a dominant control on a rocks u budget, they also show that the common interpretation that Eu/Eu* only records feldspar growth and breakdown is an oversimplification that could lead to incorrect interpretation about the duration and rates of tectonic processes. Consideration of other processes that influence Eu anomalies will allow for a broader range of geological processes to be investigated by petrochronology.

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“…Subsolidus equilibration between olivine and spinel was first considered by Irvine (1965), who described Mg 2+ diffusion from spinel to olivine and Fe 2+ diffusion from olivine to spinel. Bucholz and Kelemen (2019) found that subsolidus exchange reactions increased calculated fO 2 by 0.3-0.35 log units over 300℃ of cooling.…”

Section: Calculation Of the Oxygen Fugacitymentioning

confidence: 91%

“…A difficulty sometimes encountered when calculating fO 2 is lacking an (1991) simplified the equation used to calculate fO 2 by assuming that the effect of ferrosilite activity in orthopyroxene was canceled by the effect of fayalite activity in olivine for samples with high Mg#. In this case, the oxybarometer can give reasonable results for orthopyroxene-undersaturated ultramafic rocks because the corrections rarely exceed a shift in fO 2 of -0.2 log units (Bucholz and Kelemen, 2019). Except for gabbro, all the studied samples contain olivine, orthopyroxene, and Cr-spinel, and thus are suitable for the olivine-spinel oxybarometer (Ballbaus et al, 1991;Davis et al, 2017).…”

Section: Calculation Of the Oxygen Fugacitymentioning

confidence: 99%

Oxygen-fugacity evolution of magmatic Ni-Cu sulfide deposits in East Kunlun: Insights from Cr-spinel composition

Jia

Chen

et al. 2022

American Mineralogist

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In this study, we use Cr-spinel as an efficient indicator to evaluate the oxygen fugacity evolution of the Xiarihamu Ni-Cu deposit and the Shitoukengde non-mineralized intrusion. Oxygen fugacity is calculated using an olivine-spinel oxybarometer, with spinel Fe3+/ΣFe ratios determined by a secondary standard calibration method using an electron microprobe. Cr-spinel Fe3+/ΣFe ratios of the Xiarihamu Ni-Cu deposit vary from 0.32 ± 0.09 to 0.12 ± 0.01, corresponding to magma fO2 values ranging from ΔQFM+2.2 ± 1.0 to ΔQFM-0.6 ± 0.2. By contrast, those of the Shitoukengde mafic-ultramafic intrusion increase from 0.07 ± 0.02 to 0.23 ± 0.04, corresponding to magma fO2 varying from ΔQFM-1.3 ± 0.3 to ΔQFM+1.0 ± 0.5. A positive correlation between fO2 and Cr-spinel Fe3+/ΣFe ratios suggests that the Cr-spinel Fe3+/ΣFe ratios can be used as an indicator for magma fO2. The high fO2 (QFM+2.2) of the harzburgite in the Xiarihamu Ni-Cu deposit suggests that the most primitive magma was characterized by relatively oxidized conditions, and then became reduced during magmatic evolution, causing S saturation and sulfide segregation to form the Xiarihamu Ni-Cu deposit. The evolution trend of the magma fO2 can be reasonably explained by metasomatism in mantle source by subduction-related fluid and addition of external reduced sulfur from country gneisses (1.08–1.14 wt% S) during crustal processes. Conversely, the primitive magma of the Shitoukengde intrusion was reduced and gradually became oxidized (from QFM-1.3 to QFM+1.0) during crystallization. Fractional crystallization of large amounts of Cr-spinel can reasonably explain the increasing magma fO2 during magmatic evolution, which would hamper sulfide precipitation in the Shitoukengde intrusion. We propose that the temporal evolution of oxygen fugacity of the mantle-derived magma can be used as one of the indicators for evaluating metallogenic potential of Ni-Cu sulfide deposits and the reduction processes from mantle source to shallow crust play an important role in the genesis of magmatic Ni-Cu sulfide deposits.

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Oxygen fugacity at the base of the Talkeetna arc, Alaska

Cited by 37 publications

References 129 publications

Oxidized sulfur-rich arc magmas formed porphyry Cu deposits by 1.88 Ga

Oxidized sulfur-rich arc magmas formed porphyry Cu deposits by 1.88 Ga

Accessory Mineral Eu Anomalies in Suprasolidus Rocks: Beyond Feldspar

Oxygen-fugacity evolution of magmatic Ni-Cu sulfide deposits in East Kunlun: Insights from Cr-spinel composition

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