High 3He/4He plumes are hotter and melt more: evidence from the petrology and geochemistry of ocean island basalts

High ³He/⁴He (i.e., ³He/⁴He above the convecting upper mantle mid-ocean ridge basalt range (8 ± 2 R_A)) is a rare component identified in hotspots that sample an early-formed, less-degassed domain in the planet. Relationships between the maximum ³He/⁴He and high hotspot buoyancy flux, and between maximum ³He/⁴He and low seismic shear-wave velocity anomalies, suggest that high ³He/⁴He is entrained only by the hottest and most buoyant mantle plumes. A prediction of this model is that the high ³He/⁴He component is sampled by high-degree melting. We use a new geochemical database of ocean island lavas to demonstrate that high ³He/⁴He is found only in tholeiitic (low alkali index) and mildly alkalic (intermediate alkali index) lavas, which result from high and moderate degrees of melting, respectively. Highly alkalic (high alkali index) lavas that are generated by low degrees of melting are not observed to have very high ³He/⁴He. These observations support a model where the highest ³He/⁴He plumes are the hottest and therefore melt to high degrees, which results in generation of tholeiites and mildly alkalic lavas. However, our observations suggest that high degrees of melting of hot plumes is a necessary but insufficient condition for generation of high ³He/⁴He lavas. High degrees of melting of mantle domains with a high fraction of recycled material and/or depleted mantle material—both of which have low ³He/⁴He—will generate low ³He/⁴He lavas. Thus, only hot melting of mantle domains that have relatively pure high ³He/⁴He source material (i.e., little or no recycled material or depleted low ³He/⁴He mantle material) will result in generation of high ³He/⁴He lavas. The conclusion that high ³He/⁴He plumes melt more is supported by geophysical observations that suggest high ³He/⁴He plumes are hotter—they have higher buoyancy flux and lower shear-wave velocity anomalies in the upper mantle—and hotter plumes should melt more than cooler plumes.