With nearly 98% of the continent covered by ice, the geologic history of Antarctica remains enigmatic. Much of our understanding comes from limited rock exposures, seismologic studies, and sedimentary deposits. This study explored an additional record of the geological evolution of Antarctica, namely ‘mantle xenoliths’, samples of the Earth’s mantle beneath the Antarctic continent that are brought to the Earth’s surface by volcanic eruptions. We focus on the part of the mantle within the West Antarctic Rift System (WARS; a rift is a linear zone where a plate is pulled apart by tectonic forces) and reconstruct a timeline of geologic events that shaped this part of Antarctica.

The ‘asthenospheric’ mantle below the ‘subcontinental lithospheric mantle’ (SCLM; sampled by the xenoliths) is hot and deformable, in contrast to the rigid SCLM. When asthenosphere is partially melted, the magma may become part of the continental crust and the solid ‘melt-depleted’ residue remains in the mantle but is buoyant and becomes part of the SCLM. The time when this occurred can be determined using the rhenium-osmium (Re-Os) isotope system. Previous work has shown that when the mantle melts, Re preferentially enters the melt whereas Os is left behind in the residue. Re naturally decays to form Os, but in melt-depleted SCLM that has lost its Re by melt extraction, no more Os is formed, and the Os isotopic composition from the time of melt extraction is preserved, allowing this event to be dated.

In this study mantle xenoliths, brought to the surface by recent volcanism, were collected along a transect that crosses the rift in the West Antarctic Rift System, thus providing a recent snapshot of the SCLM after major episodes of rifting. Os isotopes, combined with chemical compositions, reveal widespread Paleoproterozoic (1.7-2.4 Ga) formation of the lithospheric mantle, which is much older than the <100 Ma-old age of the WARS, thus indicating that the old (Paleoproterozoic) lithosphere has thinned in response to rifting. This study also revealed the oldest lithospheric mantle age thus far recorded in Antarctica (3.3 Ga). In addition, our new data show that the SCLM experienced multiple episodes of melt depletion that coincide with major tectonic events documented in the regional continental crust including the ~400-550 Ma Ross Orogeny, ~1.1 Ga Grenville Orogeny, ~1.7 Ga Nimrod Orogeny and ~2.4 Ga Nimrod Group. With the same ages recorded in both the SCLM and the overlying continental crust, our data confirm that the Antarctic lithospheric mantle and continental crust were formed simultaneously. A Grenville depletion event is further supported by Hf isotope sytematics.

In addition to melt extraction events that lead to the stabilization of SCLM, there are geologic events that lead to melt addition to the SCLM. Magmas generated below the SCLM must pass through it on their way to the surface and can leave behind geochemical imprints of the migrating magma, a process called ‘metasomatism’. In this study we show that a suite of mantle xenolith samples from a location within the WARS was metasomatized to varying degrees, providing the rare opportunity to date the age of the metasomatic process. A metasomatic enrichment age of 130 Ma appears to be linked to the subduction along the paleo-Pacific margin of Gondwana between 550 and 110 Ma ago. The metasomatic re-enrichment of the SCLM precedes WARS rifting, which started about 95 Ma ago. The metasomatized xenoliths show overlap in their Nd-Sr-Hf isotopic compositions with widespread rift-related volcanism that has occurred since about 50 Ma, providing new strong evidence that melting was induced by extension and resulted in preferential melting of SCLM that included the easily fusible hydrous phases amphibole and phlogopite.

The data resulting from...

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