A geochemical record of the magmatic activity at Bouvet Island

Sitting atop old oceanic crust of 7 Ma, Bouvet (Lat. 54°26’S, long. 3°24’E) is a solitary, ice-capped and pint sized (55 km2) volcanic island in the Southernmost Atlantic Ocean. It is located off-axis the Southwest Indian Ridge in the vicinity of the triple point joining the African, South American, and Antarctic plates. As early as the seventies, this island has been regarded as being the surface expression of a mantle plume, which triggered the break-up of the Gondwanaland supercontinent. However, the detailed lower mantle structure of the Bouvet hotspot is largely unknown, because seismic ray coverage of the lowermost mantle under the southwestern Indian Ocean is weak. Surface wave tomography lacks the lateral resolution to image small-scale structures such as hotspots, expected to generate anomalies only a few hundred kilometers wide. The geology of the island is also poorly known because of the presence of a permanent ice cap. Previous studies (e.g. Verwoerd et al., 1990) have established the presence of solely two geological formations: a lower hydrothermally altered sequence of mainly pyroclastic rocks and an upper formation of predominantly subaerial lava flows. This latter unit consists of mainly mildly alkaline basalts, and carries minor amounts of intermediate and acid lavas, ranging in age from 1.39 Ma to < 0.1 Ma. The early magmatic evolution of the island is virtually unknown. The geochemical data presented here are based on a suite of about 25 samples collected from the coastlines of Bouvet Island. The serie extends from basic (mildly alkaline basalts) to intermediate (benmoreites) and acid rocks (rhyolites). None of our samples (MgO < 4.84 wt. %) can be regarded as a primary product of mantle melting. The alkali basalt lavas are chemically homogeneous in spanning a narrow range of major and trace element variability. These lavas all show strong light Rare Earth Element (REE) enrichment typical of ocean island basalts and the overall parallel REE trends show increasing enrichment in all the REE from hawaiite through to rhyolite. The similarity of their trace element ratios and parallelism of their rare earth element patterns indicate: (1) a mantle source homogeneity; (2) an uniformity of the melting conditions (i.e. degree of melting and residual mineralogy) during most of the sub-aerial eruptive history. Their trace element systematics are not typical of a HIMU-like mantle source. Their homogeneous Sr-Nd-Pb isotopic composition occupy an intermediate position among the isotopic variability spectrum defined by Earth's oceanic island basalts, encompassing the isotopic variability range of the common component “C” representing the intersection point of the isotopic arrays of ocean island and mid-ocean ridge suites in two or in three dimensional isotopic spaces.