Evidence for broad hotspot melting anomalies

Mantle plumes are widely portrayed as mushroom-like `head` and thin `tail` structures that rise from a deep thermal boundary layer, generally depicted as the core-mantle boundary. This `classic` plume model has been highly successful in explaining age-progressive seamount chains as a reflection of lithospheric plate motion over thin plume `tails`. Much effort has also been spent examining evidence that may link age-progressive seamount chains to Large Igneous Provinces (LIPs), the latter being interpreted as relic plume 'head' structures. However, recent numerical modeling of thermo-chemical plumes indicates that not all plumes have simple `head` and `tail` structures (e.g., Farnetani and Samuel, 2006; Lin and van Keken, 2004, 2006). Plumes may be impinging against the base of the lithosphere in a variety of shapes and sizes, possibly episodically. Similarly, our direct age dating of the Foundation Seamount Chain, SE Pacific, suggests that the narrowness of seamount chains might mask far broader underlying hotspots. Furthermore, our geochronological data show that the Galapagos Volcanic Province (GVP) developed via the progression of broad regions of concurrent dispersed volcanism that we link to a correspondingly broad mantle melting anomaly. Evidence from numerical modelling and direct dating of the volcanic record is therefore suggesting that hotspot melting anomalies might be much broader than commonly inferred from seamount chains. Thus, the criteria for sampling the volcanic record as a test of the plume hypothesis may require modification. We present a revised approach based on multiple seamount chains that stretch across broad regions of seafloor. These investigations test 1) the new thinking that plumes differ from the classic `head-tail` structure and 2) the inference from recent dating of Pacific seamount chains that hotspot melting anomalies are much broader than suggested by the dimensions of individual chains of seamounts and ridges.