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Several times in the past 60 ka, Mount Etna has erupted lavas with variable alkaline character. The most recent chemical excursion began in 1971, accompanied by an increase in explosivity and eruption frequency. The origin of the alkaline signature remains enigmatic, with endmember hypotheses involving dominant contributions from mantle vs. crust. For lavas that erupted between 1329 and 2016, we used thermodynamic modeling to test if post-1971 anomalous alkalinity is dominated by mantle processes. First, we assessed mantle melting conditions required to reproduce the chemistry of potential parental magmas. Second, we examined the differentiation of partial melts as they underwent closed-system crustal storage and ascent. The mantle melting conditions explored via ~300 models include source compositions (peridotite + pyroxenite ± metasomatic phases), pressure, extent of melting, and fO2. Best-fit models that reproduce the major element chemistry of volcanics interpreted as parental magmas involve 20 to 30% melting of a peridotite-pyroxenite mantle source that contained phlogopite, pargasite, and CO2 (~1 wt.%) between ~1-1.5 GPa (~30-45 km) along the QFM+1 buffer. Subsequent isentropic decompression (adiabatic and reversible) of mantle partial melts + crystallization at shallower pressures (0.8-0.2 GPa) were modeled to test the effects of closed system ascent and storage. Isentropic decompression models yield no crystallization although temperature decreases ~3°C/0.1 GPa. Decompression + closed system crystallization fail to replicate post-1971 glass samples and do not explain observed post-1971 alkali enrichments. We conclude that partial melting of a metasomatized source produced Etna parental magmas, but closed system crustal ascent and storage cannot fully account for alkali enrichment highlighted in post-1971 products at Etna. Open system modeling suggests that assimilation and crystallization (e.g., Takach et al. 2024) play a critical role, and ongoing modeling is testing the contributions of recharge (magma replenishment) and entrainment of previously formed mushes to the high alkalinity excursions.
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This content will become publicly available on May 1, 2026
Formation of Spongy Clinopyroxene: Insights from Eclogitic Inclusions in Diamonds
Abstract Spongy clinopyroxene is common in most mantle-derived xenoliths and megacrysts of eclogitic and peridotitic parageneses. Its formation is commonly attributed to the partial melting of a primary clinopyroxene in response to various factors, including changes in pressure and temperature or infiltration of external melts or fluids. In order to study the mechanism of spongy clinopyroxene formation in detail, we selected six eclogitic clinopyroxene inclusions in diamonds with varying amounts of spongy clinopyroxene (from ~10 to 100%). We employed computed tomography, electron microprobe analysis, and Raman spectroscopy to study the textural characteristics, major element concentrations, and the types of volatiles present in both phases. We also used pMELTS to model the compositions of spongy clinopyroxene and associated melts produced by the melting of primary clinopyroxene over a range of pressures and temperatures. We compare these results with estimates from major element thermobarometry of the spongy clinopyroxene. We conclude that the studied spongy clinopyroxene is the solid product of partial melting that occurs upon decompression of the primary clinopyroxene within the diamond in a near-closed system. Melting of the primary clinopyroxene occurred continuously or in pulses at different depths during the diamond’s ascent to Earth’s surface and produced variable spongy clinopyroxene and melt compositions even within the same inclusion. This is possible due to relatively rapid kimberlite ascent. The degrees of melting are various and unexpectedly high for mantle melting (between <10 and 60% with an average of ~20–30%). The produced melts are highly silicic, phonolitic, and alkali-rich. pMELTS modelling shows the spongy clinopyroxene compositions can be reproduced at pressures between 0.5–2.7 GPa and temperatures of 850–1300°C, with the majority of them satisfying the P–T conditions of 1–2 GPa and 1100–1300°C. This indicates decompression melting of primary clinopyroxene at shallow upper mantle or lower crustal conditions.
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- Award ID(s):
- 2320473
- PAR ID:
- 10627644
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Journal of Petrology
- Volume:
- 66
- Issue:
- 5
- ISSN:
- 0022-3530
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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