Europium and barium enrichments in compositionally zoned felsic tuffs : a smoking gun for the origin of chemical and physical gradients by cumulate melting

Felsic pyroclastic deposits of overall low crystallinity erupted from caldera volcanoes frequently display internal gradients in composition and crystal content. The chemical gradients in the melt phase are consistent with differentiation paths and mineral/melt element partitioning predicted from th...

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Main Authors: Wolff, John A., Forni, Francesca, Ellis, Ben S., Szymanowski, Dawid
Other Authors: Asian School of the Environment
Format: Article
Language:English
Published: 2021
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Online Access:https://hdl.handle.net/10356/146341
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spelling sg-ntu-dr.10356-1463412021-02-10T03:02:26Z Europium and barium enrichments in compositionally zoned felsic tuffs : a smoking gun for the origin of chemical and physical gradients by cumulate melting Wolff, John A. Forni, Francesca Ellis, Ben S. Szymanowski, Dawid Asian School of the Environment Science::Geology Crystal Mush Cumulate Melting Felsic pyroclastic deposits of overall low crystallinity erupted from caldera volcanoes frequently display internal gradients in composition and crystal content. The chemical gradients in the melt phase are consistent with differentiation paths and mineral/melt element partitioning predicted from the observed phenocryst assemblage. The same units typically show crystal-scale evidence for generation of eruptible magma by thermal rejuvenation of initially static, high-crystallinity (>50%) mush. Deposit-scale zoning can be reconciled with system rejuvenation by a model in which a high-crystallinity cumulate mush beneath its cognate supernatant liquid is melted by more mafic recharge to produce denser, remobilized magma of accumulative composition. In this model, zoning is a symptom of thermal rejuvenation, but requires a fusible cumulate, as provided by alkali feldspar-rich assemblages. We review existing data and present new whole-rock, glass and mineral compositions for nine examples of zoned felsic units, across the spectrum of alumina- and silica-saturation. We show that whole rocks and glasses in the late-erupted or least-evolved parts of these examples are strongly enriched in Ba (and, less consistently, Sr), and exhibit positive Eu anomalies, providing unequivocal evidence for the production of eruptible magma through melting of alkali feldspar cumulate mush. Hence, felsic volcanic rocks of low to moderate crystallinity may nonetheless have chemical signatures characteristic of cumulates. However, many other zoned tuffs show more subdued Ba-Eu enrichment and lack positive Eu anomalies. Enrichment of Eu and Ba in the melt is modulated by the extent and degree of equilibration during melting, and may be a transient signal that is quickly suppressed by new feldspar growth occurring between melting and eruption. In cases of very highly evolved rhyolites, the prior depletion of Ba, Sr and Eu by extensive alkali feldspar fractionation precludes development of any strong enrichment, although the patterns of relative variation are similar to those in the examples discussed here. We thank Will Starkel, Chris Henry, Paul Olin, and Joe Boro for discussion. Samples from Tenerife and McDermitt were collected under projects funded by NSF EAR-0001013 and the USGS EDMAP program respectively. BSE gratefully acknowledges support from the Swiss National Science Foundation (200021_166281). The paper was improved by helpful comments from Rebecca Williams and John C. White 2021-02-10T03:02:26Z 2021-02-10T03:02:26Z 2020 Journal Article Wolff, J. A., Forni, F., Ellis, B. S., & Szymanowski, D. (2020). Europium and barium enrichments in compositionally zoned felsic tuffs : a smoking gun for the origin of chemical and physical gradients by cumulate melting. Earth and Planetary Science Letters, 540, 116251-. doi:10.1016/j.epsl.2020.116251 0012-821X https://hdl.handle.net/10356/146341 10.1016/j.epsl.2020.116251 540 en Earth and Planetary Science Letters © 2020 Elsevier B.V. All rights reserved.
institution Nanyang Technological University
building NTU Library
continent Asia
country Singapore
Singapore
content_provider NTU Library
collection DR-NTU
language English
topic Science::Geology
Crystal Mush
Cumulate Melting
spellingShingle Science::Geology
Crystal Mush
Cumulate Melting
Wolff, John A.
Forni, Francesca
Ellis, Ben S.
Szymanowski, Dawid
Europium and barium enrichments in compositionally zoned felsic tuffs : a smoking gun for the origin of chemical and physical gradients by cumulate melting
description Felsic pyroclastic deposits of overall low crystallinity erupted from caldera volcanoes frequently display internal gradients in composition and crystal content. The chemical gradients in the melt phase are consistent with differentiation paths and mineral/melt element partitioning predicted from the observed phenocryst assemblage. The same units typically show crystal-scale evidence for generation of eruptible magma by thermal rejuvenation of initially static, high-crystallinity (>50%) mush. Deposit-scale zoning can be reconciled with system rejuvenation by a model in which a high-crystallinity cumulate mush beneath its cognate supernatant liquid is melted by more mafic recharge to produce denser, remobilized magma of accumulative composition. In this model, zoning is a symptom of thermal rejuvenation, but requires a fusible cumulate, as provided by alkali feldspar-rich assemblages. We review existing data and present new whole-rock, glass and mineral compositions for nine examples of zoned felsic units, across the spectrum of alumina- and silica-saturation. We show that whole rocks and glasses in the late-erupted or least-evolved parts of these examples are strongly enriched in Ba (and, less consistently, Sr), and exhibit positive Eu anomalies, providing unequivocal evidence for the production of eruptible magma through melting of alkali feldspar cumulate mush. Hence, felsic volcanic rocks of low to moderate crystallinity may nonetheless have chemical signatures characteristic of cumulates. However, many other zoned tuffs show more subdued Ba-Eu enrichment and lack positive Eu anomalies. Enrichment of Eu and Ba in the melt is modulated by the extent and degree of equilibration during melting, and may be a transient signal that is quickly suppressed by new feldspar growth occurring between melting and eruption. In cases of very highly evolved rhyolites, the prior depletion of Ba, Sr and Eu by extensive alkali feldspar fractionation precludes development of any strong enrichment, although the patterns of relative variation are similar to those in the examples discussed here.
author2 Asian School of the Environment
author_facet Asian School of the Environment
Wolff, John A.
Forni, Francesca
Ellis, Ben S.
Szymanowski, Dawid
format Article
author Wolff, John A.
Forni, Francesca
Ellis, Ben S.
Szymanowski, Dawid
author_sort Wolff, John A.
title Europium and barium enrichments in compositionally zoned felsic tuffs : a smoking gun for the origin of chemical and physical gradients by cumulate melting
title_short Europium and barium enrichments in compositionally zoned felsic tuffs : a smoking gun for the origin of chemical and physical gradients by cumulate melting
title_full Europium and barium enrichments in compositionally zoned felsic tuffs : a smoking gun for the origin of chemical and physical gradients by cumulate melting
title_fullStr Europium and barium enrichments in compositionally zoned felsic tuffs : a smoking gun for the origin of chemical and physical gradients by cumulate melting
title_full_unstemmed Europium and barium enrichments in compositionally zoned felsic tuffs : a smoking gun for the origin of chemical and physical gradients by cumulate melting
title_sort europium and barium enrichments in compositionally zoned felsic tuffs : a smoking gun for the origin of chemical and physical gradients by cumulate melting
publishDate 2021
url https://hdl.handle.net/10356/146341
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