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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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Origin and Evolution of Miocene Basanite to Comendite Lavas at the Mount Overlord Volcanic Field, Northern Victoria Land, Antarctica
Continental alkaline magmatism produces a wide variety of igneous rock types because of varying degrees of partial melting of heterogenous mantle sources, fractional crystallization, and magma contamination during transit through the continental crust. The Mount Overlord Volcanic Field (MOVF) is a continental alkaline volcanic province in northern Victoria Land, Antarctica. Mount Overlord and the associated vents that make up the volcanic field are some of the least-explored volcanic rocks in the western Ross Sea. The MOVF sits within the Transantarctic Mountains, which form the rift shoulder of the extensive West Antarctic Rift System. The compositions of volcanic rocks in the MOVF range widely from basanite to evolved trachyte and comendite with a suite of intermediate rock types. Here we present 40Ar/39Ar ages, petrography, and whole-rock and mineral geochemistry to establish the temporal and magmatic evolution of the magmatic system. Volcanic activity occurred from 21.2 to 6.9 Ma, making it one of the longest records of volcanism in the western Ross Sea area. Mount Rittmann, an active volcano that is part of the MOVF, is not discussed here but extends the timing of volcanism of the MOVF into the Holocene. At Mount Overlord and surrounding areas, there were eruptions of lava flows, domes, and pyroclastic rocks. Localized deposits of hyaloclastites formed by magma-ice interactions provide an insight into former ice levels. Geochemically the MOVF shows a single magma differentiation trend except for Navigator Nunatak lavas which have a potassic affinity rarely seen in northern Victoria Land. Partial melting of an amphibole-bearing mantle lithology at or near the base of the continental lithospheric mantle (CLM) was the main source of the parental basaltic magmas. Polybaric crystal fractionation of the primary basaltic magmas mainly occurred at lower crustal depths and involved fractionation of clinopyroxene, olivine, kaersutite, feldspars, biotite, Fe–Ti oxides, apatite, and sodalite. Crustal assimilation of c. 10% granite harbor igneous complex granitoids was important in the evolution of intermediate composition magmas. Trachyte, phonolite, and comendite magmas stagnated and evolved at shallow crustal depths (c. <8 km). Over 95% crystal fractionation was required to generate the comendites. Extraction of the comendite melt from a felsic crystal mush was an important process. The potassic Navigator Nunatak magma required partial melting of phlogopite-bearing metasomatized CLM. The metasomes had ‘HIMU-like’ or FOZO isotopic compositions that ultimately originated from recycling of materials in the mantle. The MOVF displays a stronger affinity toward FOZO than other northern Victoria Land basaltic rocks. This suggests that the interaction between parental melt and juvenile CLM was limited, which is similar to volcanic rocks from the oceanic Adare Basin seamounts. Our result emphasizes the critical importance of a thick CLM for the genesis of diverse alkaline magma compositions in a continental rift system.
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- Award ID(s):
- 2137467
- PAR ID:
- 10593298
- Editor(s):
- Troll, Valentin
- Publisher / Repository:
- Journal of Petrology
- Date Published:
- Journal Name:
- Journal of Petrology
- Volume:
- 66
- Issue:
- 3
- ISSN:
- 0022-3530
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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