Titanium, tantalum, and niobium (TITAN) enrichment in high 3He/4He ocean island basalts.

Ocean island basalts (OIB) with high 3He/4He ratios are thought to be derived from an ancient, deep mantle reservoir. Global OIBs with high 3He/4He ratios are found to exhibit positive Ti, Ta, and Nb (TITAN) anomalies (on a Primitive Mantle normalized basis) relative to elements of similar compatibility in peridotite. High 3He/4He lavas also exhibit 187Os/188Os ratios (greater than 0.135) that are more radiogenic than putative Primitive Mantle values. Thus, TITAN-enrichment and radiogenic Os-isotopes are signatures associated with the high 3He/4He mantle. However, TITAN enrichment (and radiogenic Os-isotopes) are not unique to the high 3He/4He mantle: plots of 3He/4He versus TITAN-anomalies (or 187Os/188Os) reveal a "wedge- shaped" pattern. For example, Cape Verde lavas exhibit positive Nb anomalies, yet they have low 3He/4He. Similarly, lavas with low 3He/4He ratios can exhibit radiogenic Os-isotopes (e.g., Pitcairn, Mangaia,Tubuaii, etc.). Radiogenic 187Os/188Os and TITAN-enrichment in high 3He/4He OIBs indicate that the high 3He/4He OIB reservoir in the mantle hosts a non-primitive (non-chondritic) component. However, the origin of this non- primitive component, and how it became it associated with high 3He/4He ratios, remains unknown. One possible model is that recycled, refractory, rutile-bearing oceanic crust could contribute radiogenic Os- isotopes and TITAN-enrichment to the high 3He/4He reservoir in the mantle. Alternatively, extant partitioning data suggests that melts in equilibrium with Ca-perovskite could host positive TITAN anomalies; we cannot rule out the possibility that trace element partitioning between deep mantle melts and Ca-perovskite may be responsible for the TITAN enrichment in high 3He/4He lavas. These two models are highly dependant on uncertain partitioning data. Understanding the origin of the TITAN enrichment and radiogenic Os-isotopes in high 3He/4He lavas will be important for constraining the generation and evolution of the high 3He/4He reservoir in the mantle.