Noble gas geochemistry to explore the sources of magma feeding the Antarctic submarine volcanism in Bransfield Strait (NW Antarctica)

Bransfield Strait, located NW Antarctica, is one of the scarce accessible places to study Antarctic submarine volcanism. It is a back-arc basin hosting several submarine volcanoes (e.g. Edifice A, Three Sisters, Orca, Hook Ridge) along its main spreading axis, and two subaerial island volcanoes: Bridgeman and Deception. The submarine ones seem to be active: Orca suffered a seismic unrest in 2020 associated to magma intrusion; and Three Sisters and Hook Ridge registered recent hydrothermal episodes linked to past activity.

Understanding the primary sources of magmas depth and the processes that control its chemical composition within the volcano plumbing system is essential to advance in the knowledge of how magmas may develop, ascent and consequently of what kind of volcanic activity may occur at the surface. Noble gases, due to their inertness, represent a key tool completing these tasks. In this work, we studied eight samples dredged on the submarine volcanoes of Three Sisters and Orca, as well as on two adjacent seamounts.

In these samples, we measured the isotopic signatures of He, Ne and Ar of the magmatic gas trapped within the inclusions hosted by olivine phenocrysts and glass shards. He isotopic ratio (³He/⁴He), ranges from 4.6 to 6.5 R_a (R_a is the ³He/⁴He in atmosphere) along a 70 km length axis. These values are in line with the same ratio in oceanic water along Bransfield Strait and they are lower than both those associated to MORB convective mantle reservoir (8±1 R_a) and at Deception Island. Argon isotopic ratios (⁴⁰Ar/³⁶Ar < 310) are close to the atmospheric value (298.6±0.3). The Ar isotopic ratio in hosted olivine inclusions (up to 310) is slightly higher than those within the glasses.

The relatively low isotopic values of He can be explained by: (i) magma ageing in the chamber, (ii) crustal assimilation, or (iii) interaction of subducted metasomatic fluids at the magma source. The first two possibilities are not plausible at the studied volcanic suites as the different volcanoes are expected to record different evolutions and hence also more scattered values range including MORB. However, a common process such as subduction metasomatism, is in agreement with previous studies. Subduction metasomatism releases fluids rich in U and Th that produce ⁴He by radioactive decay, thus lowering the resulting ³He/⁴He.

The atmospheric-like Ar isotopic ratio is related to air contamination. This may have occurred in the shallow layers of the plumbing system; by seawater interaction with magma either during the eruption, or throughout cracks after cooling.

This study was funded by the research projects ERUPTING (PID2021-127189OB-I00) MCIN/AEI/10.13039/501100011033, HYDROCAL (PID2020-114876GB-I00) MCIN/AEI/10.13039/501100011033 and VOLGASDEC (PGC2018-095693-B-I00) (AEI/FEDER, UE); and is part of the CSIC Interdisciplinary Thematic Platform (PTI) Polar zone observatory. A. P. S acknowledges his grant “Programa Propio III USAL 2021 co-funded with Banco de Santander” and his joint COMNAP-IAATO Antarctic Fellowship 2022. A.C acknowledges the grant RYC2021‐033270‐I funded by MCIN/AEI/10.13039/501100011033 and by the EU “Next Generation EU/PRTR". Samples were provided by the Polar Rock Repository (https://prr.osu.edu) with support from the National Science Foundation, under Cooperative Agreement OPP-1643713.