New Experimental Constraints for the Evolution and Thermobarometry of Alkali Ultrabasic to Intermediate Igneous Rocks

Salazar-Naranjo, Andrés Fabián; Vlach, Silvio Roberto Farias

doi:10.1093/petrology/egad078

Cited by 3 publications

(4 citation statements)

References 119 publications

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“…Nonetheless, K cpx−liq D,Fe−Mg values calculated assuming that all Fe is present as Fe 2+ are generally within the (large) ±0.08 uncertainty of the general predictive model provided by Equation 35of Putirka (2008). Once the effects of disequilibrium crystallisation are accounted for, our observations from natural OIB samples suggest that K cpx−liq D,Fe 2+ −Mg values lie close to 0.28 and are broadly consistent with experimentally derived K cpx−liq D,Fe 2+ −Mg values of 0.25-0.26 (Sisson and Grove, 1993;Salazar-Naranjo and Vlach, 2023). Moreover, different clinopyroxene populations within a single sample (PI-041 from Pico) record different clinopyroxene-liquid Fe 2+ -Mg systematics, potentially mirroring variations in f O2 conditions reported from other OIB system that are thought to result from SO 2 degassing during ascent (Moussallam et al, 2016;Helz et al, 2017;Taracsak et al, 2022).…”

Section: Iron-magnesium Exchange Equilibriasupporting

confidence: 84%

“…While our findings are consistent with the few experimental determinations of D cpx−liq Fe2O3 currently available, new experiments are required to disentangle the effects of system composition and disequilibrium crystallisation on clinopyroxeneliquid Fe 3+ partitioning. Extending our evaluation to clinopyroxene-liquid Fe 2+ -Mg exchange equilibria suggests that K cpx−liq D,Fe 2+ −Mg values (as opposed to K cpx−liq D,Fe−Mg values) lie close to experimental estimates (0.25-0.26; Sisson and Grove, 1993;Salazar-Naranjo and Vlach, 2023). Differences in apparent K cpx−liq D,Fe 2+ −Mg values within and between the populations of clinopyroxene crystals we analysed reflect the same disequilibrium processes recorded in IV Al-Fe 3+ systematics, and highlight the importance of evaluating whether phases are in chemical as well as textural equilibrium before interpreting the meaning of Fe 2+ -Mg exchange equilibria.…”

Section: Discussionsupporting

confidence: 56%

“…Equation 35of Putirka (2008) provides the most recent model capable of predicting K cpx−liq D,Fe−Mg values that is feasibly applicable to OIB samples. Although this model incorporates the effect of temperature on K cpx−liq D,Fe−Mg , and returns a value of ∼0.28 at temperatures relevant to OIB evolution (∼1200 • C) that is close to values from experiments on mafic compositions (0.25-0.26; Sisson and Grove, 1993;Salazar-Naranjo and Vlach, 2023), it assumes that all Fe occurs as Fe 2+ in both clinopyroxene crystals and their host glasses and thus strictly describes Fe-Mg exchange rather than Fe 2+ − −M g exchange specifically. This assumption is clearly incorrect for basaltic liquids characterised by Fe 3+ /ΣFe contents that range from ∼0.1 to ∼0.3 across a naturally relevant range of f O2 conditions (FMQ−1 to FMQ+2; Kress and Carmichael, 1991;Borisov et al, 2018;Cottrell et al, 2022).…”

Section: Iron-magnesium Exchange Equilibriamentioning

confidence: 56%

“…If f O2 were closer to FMQ+3, then the K cpx−liq D,Fe 2+ −Mg value would be closer to a nominal value of ∼0.28. Recent experiments reported by Salazar-Naranjo and Vlach (2023) suggest that equilibrium K cpx−liq D,Fe 2+ −Mg values for alkali systems may actually lie closer to ∼0.25 than to the ∼0.28 value calculated from Equation 35of Putirka (2008). Discordance between rims and microcrysts nonetheless remains, with the lowest-Mg# microcrysts implying a disequilibrium K cpx−liq D,Fe 2+ −Mg value greater than 0.36 at ∼FMQ+2.5.…”

Section: Iron-magnesium Exchange Equilibriamentioning

confidence: 85%

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Iron valence systematics in clinopyroxene crystals from ocean island basalts

Neave¹,

Stewart²,

Hartley³

et al. 2024

Preprint

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systems, which is commonly expressed in terms of oxygen fugacity (fO2). Magmatic fO2 varies between different tectonic settings and controls the trajectories along which magmas evolve and the properties of the volcanic gases they release into the atmosphere. Ocean island basalts (OIBs) derived at least in part from recycled mantle sources are increasingly thought to evolve under more oxidising conditions than midocean ridge basalts (MORBs) from relatively depleted sources. However, the oxybarometeric tools currently at our disposal are subject to a range of limitations, meaning that there are relatively few internally consistent estimates of fO2 across multiple settings. Our understanding of the nature and causes of fO2 variability within and between magmatic systems therefore remains limited. In Part I we demonstrated that previously discredited approaches for estimating clinopyroxene ferric iron contents can return comparably precise results to Mössbauer spectroscopy by optimising electron probe microanalysis techniques, raising the prospect of developing clinopyroxene-based oxybarometers. Here we present high-quality analyses of clinopyroxene crystals from OIBs erupted in Iceland and the Azores to determine their iron valence systematics and explore their potential use for oxybarometry. Although many studies assume that all iron in magmatic clinopyroxene crystals occurs as ferrous iron, we find that up to 50% of the total iron in magmatic clinopyroxene crystals may be ferric iron, with crystals from alkali systems typically containing more ferric iron than those from tholeiitic systems. Most ferric iron is hosted within esseneite component (CaFe3+AlSiO6), though some may also be hosted in aegirine component (NaFe3+Si2O6) in the most alkali crystals. Iron–magnesium exchange equilibria between clinopyroxene crystals and their host glasses return plausible fO2 estimates for a tholeiitic basalt from Holuhraun in Iceland and an alkali basalt from Pico in the Azores but are subject to considerable uncertainties because the influence of ferric iron on clinopyroxene-liquid equilibria are poorly understood. There is no current advantage in accounting for ferric iron when evaluating iron-magnesium exchange between clinopyroxene–glass pairs. Magmatic fO2 was also estimated by applying a recent parametrisation of ferric iron partitioning to determine the valence of iron in liquids in equilibrium with clinopyroxene crystals from Holuhraun and Pico. We infer that our samples from Holuhraun and Pico evolved at about one and half and two and a half log units above FMQ equilibrium, respectively. These values are in line with independent estimates from these eruptions and similar eruptions of OIB elsewhere, and demonstrate the considerable potential for further development of clinopyroxene-based oxybarometers.

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Section: Iron-magnesium Exchange Equilibriasupporting

confidence: 84%

Section: Discussionsupporting

confidence: 56%

Section: Iron-magnesium Exchange Equilibriamentioning

confidence: 56%

Section: Iron-magnesium Exchange Equilibriamentioning

confidence: 85%

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Iron valence systematics in clinopyroxene crystals from ocean island basalts

Neave¹,

Stewart²,

Hartley³

et al. 2024

Preprint

View full text Add to dashboard Cite

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Iron valence systematics in clinopyroxene crystals from ocean island basalts

Neave,

Stewart,

Hartley

et al. 2024

Contrib Mineral Petrol

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The valence state of Fe plays a vital role in setting and recording the oxidation state of magmas, commonly expressed in terms of oxygen fugacity ($$f_{\textrm{O}_{2}}$$ f O 2 ). However, our knowledge of how and why $$f_{\textrm{O}_{2}}$$ f O 2 varies within and between magmatic systems remains patchy because of diverse challenges associated with estimating the valence state of Fe in glasses and minerals routinely. Here we investigate Fe valence systematics in magmatic clinopyroxene crystals from ocean island basalts (OIBs) erupted in Iceland and the Azores to explore whether they record information about magma Fe$$^{3+}$$ 3 + contents and magmatic $$f_{\textrm{O}_{2}}$$ f O 2 conditions. Although many studies assume that all Fe in augitic clinopyroxene crystals from OIBs occurs as Fe$$^{2+}$$ 2 + , we find that up to half of the total Fe present can occur as Fe$$^{3+}$$ 3 + , with crystals from alkali systems typically containing more Fe$$^{3+}$$ 3 + than those from tholeiitic systems. Thus, Fe$$^{3+}$$ 3 + is a major if under-appreciated constituent of augitic clinopyroxene crystals erupted from ocean island volcanoes. Most Fe$$^{3+}$$ 3 + in these crystals is hosted within esseneite component (CaFe$$^{3+}$$ 3 + AlSiO$$_{6}$$ 6 ), though some may be hosted in aegirine component (NaFe$$^{3+}$$ 3 + Si$$_{2}$$ 2 O$$_{6}$$ 6 ) in crystals from alkali systems. Observations from samples containing quenched matrix glasses suggest that the incorporation of Fe$$^{3+}$$ 3 + is related to the abundance of tetrahedrally coordinated Al ($$\mathrm {^{IV}}$$ IV Al), implying some steric constraints over Fe$$^{3+}$$ 3 + partitioning between clinopyroxene and liquid (i.e., $$D\mathrm {^{{cpx-liq}}_{{Fe_{2}O_{3}}}}$$ D Fe 2 O 3 cpx - liq values), though this may not be an equilibrium relationship. For example, $$\mathrm {^{IV}}$$ IV Al-rich $$\{hk0\}$$ { h k 0 } prism sectors of sector-zoned crystals contain more Fe$$^{3+}$$ 3 + than $$\mathrm {^{IV}}$$ IV Al-poor $$\{\bar{1}11\}$$ { 1 ¯ 11 } hourglass sectors. Moreover, $$\mathrm {^{IV}}$$ IV Al-rich compositions formed during disequilibrium crystallisation are enriched in Fe$$^{3+}$$ 3 + . Apparent clinopyroxene-liquid Fe$$^{2+}$$ 2 + –Mg exchange equilibria (i.e., $$K\mathrm{{_{D, {Fe^{2+}-Mg}}^{cpx-liq}}}$$ K D , Fe 2 + - Mg cpx - liq values) are similarly affected by disequilibrium crystallisation in our samples. Nonetheless, it is possible to reconcile our observed clinopyroxene compositions with glass Fe valence systematics estimated from olivine-liquid equilibria if we assume that $$K\mathrm{{_{D, {Fe^{2+}-Mg}}^{cpx-liq}}}$$ K D , Fe 2 + - Mg cpx - liq values lies closer to experimentally reported values of 0.24$$-$$ - 0.26 than values of $$\sim$$ ∼ 0.28 returned from a general model. In this case, olivine-liquid and clinopyroxene-liquid equilibria record equivalent narratives, with one of our glassy samples from Iceland recording evolution under $$f_{\textrm{O}_{2}}$$ f O 2 conditions about one log unit above fayalite-magnetite-quartz (FMQ) equilibrium (i.e., $$\sim$$ ∼ FMQ+1) and our glassy Azorean sample recording evolution under significantly more oxidising conditions ($$\ge$$ ≥ FMQ+2.5) before experiencing syn-eruptive reduction, likely as a result of SO$$_{2}$$ 2 degassing; our other glassy sample from Iceland was also affected by reductive SO$$_{2}$$ 2 degassing. Overall, our findings demonstrate that the Fe valence systematics of clinopyroxene crystals can record information about the conditions under which OIBs evolve, but that further experimental work is required to properly disentangle the effects of magma composition, disequilibrium and $$f_{\textrm{O}_{2}}$$ f O 2 conditions on clinopyroxene-liquid equilibria involving Fe$$^{2+}$$ 2 + and Fe$$^{3+}$$ 3 + .

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