Geochemistry of Foundation Seamount Chain Eruptives: Effects of Lithospheric Thickness and Ridge-Hotspot Distance on Magma Generation.

From experiments and theory, the thickness of the lithosphere overlying a melting column in the mantle should control many of the characteristics of the erupted magmas. Thick lithosphere should lead to eruption of low-degree melts with a relatively large average depth of melting, silica undersaturation and strong incompatible-element enrichment. Hotspots migrating towards or away from spreading axes could provide a natural laboratory to test this model, as the oceanic lithosphere thickens progressively with age. Most examples of hotspot-ridge interaction are characterised, however, by a monotonic increase or decrease in hotspot-ridge separation (and hence age of the plate overlying the active hotspot) through time, making it impossible to separate the influences of melt column capping and hotspot-ridge magma mixing on magma chemistry. In the case of the Foundation Chain, which lies on the Pacific plate west of the Pacific-Antarctic spreading axis, an easterly, 380 km ridge jump at ca. 11Ma lead to the creation of the Selkirk Microplate and subsequently a situation where, for ca. 5Ma, the hotspot approached the spreading axis but lay under progressively thicker lithosphere. We present major and trace element compositions of lavas collected along the whole length of the Foundation chain. Combined with published information on the age of the seamounts and the underlying plate, we can examine the magma compositions in terms of ridge-hotspot separation and thickness of lithosphere over the last 21Ma. MgO and distance to spreading axis are negatively correlated for all samples. Lithospheric thickness, on the other hand, is positively correlated with MgO for the magmas erupted on the Selkirk microplate, but negatively for the other Foundation seamounts. This suggests that distance to axis rather than lithospheric thickness is controlling the MgO contents of the hotspot magmas. Exactly the opposite effect is seen when examining Dy/Yb (theoretically high when residual garnet is left after melting, low in ridge basalts) - the Selkirk samples show similar behaviour to the rest of the chain when plotted against lithospheric thickness (a positive correlation) but behave differently when distance to the spreading axis is considered (Selkirk negatively correlated, rest of chain positive). These results suggest that, over hotspot-ridge distances of up to 700km, the effects of the spreading axis dominate the MgO contents of the hotspot magmas. Lithospheric thickness on the other hand plays a major role in controlling trace element patterns.