Preliminary results on the composition of the Antarctic mantle below the Terror Rift, Western Ross Sea

A recent expedition aboard R/V Nathaniel B. Palmer (NBP25-01, 2025) sampled submarine seamounts across six regions of the Terror Rift, western Ross Sea, Antarctica. Volcanism in this area is dominated by explosive mafic alkaline magmas forming monogenetic and polygenetic seamounts (Tominaga et al., 2025). Of the 50 dredges collected, nearly half recovered mantle xenoliths, offering a rare opportunity to constrain the composition, thermal structure and evolution of the mantle beneath an active Antarctic rift.

The xenolith suite is dominated by peridotites (dunite to lherzolite), with subordinate pyroxenite and hornblendite. We present mineral chemistry and thermobarometric data from these lithologies to constrain their P-T-fluid history, and potential spatial heterogeneities.

Preliminary results from peridotite xenoliths sampled at Squid Ridge, a seamount located close to the rift axis, reveal evidence for two distinct melt–peridotite interaction events. The first event occurred at high temperature and is marked by the formation of interstitial diopside and Cr-rich spinel. Melt percolation was coeval with viscous deformation, recorded by olivine subgrain development and dynamic recrystallization of orthopyroxene when present. Pyroxene thermobarometry yields equilibrium conditions of 1065 ± 5 °C and 1.0 ± 0.2 GPa, corresponding to depths of ~30 km. The second event is characterized by brittle fracturing of the peridotites and the formation of alkali-rich glass, amphibole, augite and Mg-poor olivine in fractures. It is interpreted to reflect xenolith entrainment during magma ascent.

These results indicate a deep lithospheric mantle origin for the studied xenoliths, consistent with previous estimates from Franklin Island peridotite xenoliths located farther from the rift axis (Martin et al., 2023). Together, these observations suggest that rift-related fault systems efficiently channel deep-sourced melts to the surface and support the presence of a relatively cold geotherm beneath the Terror Rift, consistent with an idealized dynamic rift.

 

Martin et al. (2023). A review of mantle xenoliths in volcanic rocks from southern Victoria Land, Antarctica.

Tominaga, M. et al. (2025). Subglacial explosive volcanism in the Ross Sea of Antarctica. Communications Earth & Environment, 6(1), 921.