Workman, R. K. and Hart, S. R. and Jackson, M. and Regelous, M. and Farley, K. A. and Blusztajn, J. and Kurz, M. and Staudigel, H. (2004) Recycled metasomatized lithosphere as the origin of the Enriched Mantle II (EM2) end-member: Evidence from the Samoan Volcanic Chain. Geochemistry, Geophysics, Geosystems, 5 (4). 2003GC000623. ISSN 1525-2027. http://resolver.caltech.edu/CaltechAUTHORS:WORggg04
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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.
|Additional Information:||Copyright 2004 by the American Geophysical Union. Received: 21 August 2003; Revised: 7 January 2004; Accepted: 22 January 2004; Published: 27 April 2004 Without Alberto Saal, we likely would have done none of this. He once printed an earthquake map that identified activity in a place that shouldn't have any. This led to the 1999 AVON 3 cruise, and discovery of active volcanism at Vailulu'u volcano. It also led to the dredging of tons of basalts that are the backbone of this paper. We thank Alberto for all of this, especially his help on the AVON 3 cruise and his help during fieldwork on Ta'u Island. Anthony Koppers was enormously helpful with data and sample processing during and following the AVON 3 cruise. We are also grateful to the captain and crew of the R/V Melville, along with the student volunteers, for catching and processing these tons of rocks. We thank Francis Albarède for access to the Lyon ICP/MS, and to Ken Sims and Sylvain Pichatt for their 24/7 efforts there in running many Pb isotope analyses. We thank Steve Galer and Wafa Abouchami for many tutorials in implementing the Mainz Pb chemistry at WHOI. The output of high-precision Pb data from the WHOI NEPTUNE is due largely to Lary Ball's skill and tenacity; our many thanks. Megan Coetzee's work on the Western Samoan seamounts, during a summer internship at WHOI, provided enthusiastic counterpoint to our work on the eastern volcanoes. We acknowledge NSF support, through grant OCE-9819038 (SRH and HS), and EAR-0125917 (SRH). Very constructive reviews were provided by Yaoling Niu and Rick Carlson. Finally, our gratitude to Jim Natland, for his generosity in sharing Samoan rocks, data and ideas, and for being an enduringly collegial provocateur. M. Regelous thanks A. Greig, Y. Niu, J. I. Wendt, W. Abouchami, S. Galer, C. Coath and C. Counsell for help and advice with the analytical measurements, and Warren Jopling of Safua Hotel for sharing his geological knowledge of Savai'i.|
|Subject Keywords:||EM2; Samoa; metasomatized lithosphere; Sr-Nd-Pb-He-Os isotopes; hot spot chain; Vailulu'u Volcano; 1040 Geochemistry: Isotopic composition/chemistry; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; 5480 Planetology: Solid Surface Planets: Volcanism|
|Official Citation:||Workman, R. K., S. R. Hart, M. Jackson, M. Regelous, K. A. Farley, J. Blusztajn, M. Kurz, and H. Staudigel (2004), Recycled metasomatized lithosphere as the origin of the Enriched Mantle II (EM2) end-member: Evidence from the Samoan Volcanic Chain, Geochem. Geophys. Geosyst., 5, Q04008, doi:10.1029/2003GC000623.|
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|Deposited On:||19 Dec 2005|
|Last Modified:||26 Dec 2012 08:42|
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