No abstract

An in‐depth Sr‐Nd‐Pb‐He‐Os isotope and trace element study of the EMII‐defining Samoan hot spot lavas leads to a new working hypothesis for the origin of this high 87Sr/86Sr mantle end‐member. Systematics of the Samoan fingerprint include (1) increasing 206Pb/204Pb with time ‐ from 18.6 at the older, western volcanoes to 19.4 at the present‐day hot spot center, Vailulu'u Seamount, (2) en‐echelon arrays in 206Pb/204Pb – 208Pb/204Pb space which correspond to the two topographic lineaments of the 375 km long volcanic chain – this is much like the Kea and Loa Trends in Hawai'i, (3) the highest 87Sr/86Sr (0.7089) of all oceanic basalts, (4) an asymptotic decrease in 3He/4He from 24 RA [Farley et al., 1992] to the MORB value of 8 RA with increasing 87Sr/86Sr, and (5) mixing among four components which are best described as the “enriched mantle”, the depleted FOZO mantle, the (even more depleted) MORB Mantle, and a mild HIMU (high 238U/204Pb) mantle component. A theoretical, “pure” EMII lava composition has been calculated and indicates an extremely smooth trace element pattern of this end‐member mantle reservoir. The standard recycling model (of ocean crust/sediment) fails as an explanation for producing Samoan EM2, due to these smooth spidergrams for EM2 lavas, low 187Os/188Os ratios and high 3He/4He (>8 RA). Instead, the origin of EM2 has been modeled with the ancient formation of metasomatised oceanic lithosphere, followed by storage in the deep mantle and return to the surface in the Samoan plume.

No abstract

We report volatile (H 2 O, CO 2 , F, S, Cl) and trace element data for submarine alkalic basalt glasses from the three youngest Samoan volcanoes, Ta'u, Malumalu and Vailulu'u. Most samples are visibly sulfide saturated, so have likely lost some S during fractionation. Cl/K ratios (0.04 -0.15) extend to higher values than pristine MORBs, but are suspected to be partly due to source differences since Cl/K roughly varies as a function of 87 Sr/ 86 Sr. There are no resolvable differences in the relative enrichment of F among sources, and compatibility of F during mantle melting is established to be nearly identical to Nd. Shallow degassing has affected CO 2 in all samples, and H 2 O only in the most shallowly erupted samples from Vailulu'u. Absolute water contents are high for Samoa (0.63 -1.50 wt%), but relative enrichment of water compared to equally incompatible trace elements (Ce, La) is low and falls entirely below normal MORB values. H 2 O/Ce (58 -157) and H 2 O/La (120 -350) correlate inversely with 87 Sr/ 86 Sr compositions (0.7045 -0.7089). This leads us to believe that, because of very fast diffusion of hydrogen in mantle minerals, recycled lithospheric material with high initial water and trace element content will lose water to the drier ambient mantle during storage within the inner Earth. The net result is the counter-intuitive appearance of greater dehydration with greater mantle enrichment. We expect that subducted slabs will experience a two-stage dehydration history, first within subduction zones and then in the ambient mantle during long-term convective mixing.

New measurements of high 3 He/ 4 He ratios in Samoan lavas from Ofu Island (19.5-33.8 times atmospheric) extend the known range for 3 He/ 4 He in the southern hemisphere mantle. The Ofu data suggest that the high 3 He/ 4 He mantle component thought to be common to all oceanic hotspots, called FOZO (Focus Zone), is not homogeneous. Sr, Nd and Pb isotopes in Ofu lavas indicate that the Samoan high 3 He/ 4 He component is isotopically distinct from the high 3 He/ 4 He lavas from Hawaii, Iceland and Galapagos. Along with Samoa, the highest 3 He/ 4 He sample from each southern hemisphere high 3 He/ 4 He hotspot exhibits lower 143 Nd/ 144 Nd ratios than their counterparts in the northern hemisphere (excluding lavas erupted in continental, back-arc, and submarine ridge environments). The observation of a large-scale isotopic enrichment (generally higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd) in the FOZO-A (austral) high 3 He/ 4 He mantle compared to the FOZO-B (boreal) high 3 He/ 4 He mantle is similar to the DUPAL anomaly, a globe-encircling feature of isotopic enrichment observed primarily in southern hemisphere ocean island basalts. Additionally, the recent discovery that terrestrial samples have 142 Nd/ 144 Nd ratios higher than chrondrites has potentially important implications for the origin of the FOZO reservoirs, and suggest that the high 3 He/ 4 He mantle has been re-enriched.