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

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

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v33c2239b&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #V33C-2239

Physics

1000 Geochemistry, 1025 Composition Of The Mantle, 1038 Mantle Processes (3621), 1040 Radiogenic Isotope Geochemistry, 1065 Major And Trace Element Geochemistry

Scientific paper

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.

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