Steven B Shirey

 
 

The Earth’s mid-ocean ridge system is the largest active volcanic chain. It encircles the globe, erupts under water and is continuously active. The linearly upwelling mantle which feeds it is derived from shallower depths (<300 km) than the more centrally upwelling mantle that feeds melting at ocean islands and oceanic plateaus. 

Jackson M.G., Shirey S.B., 2011, Re–Os isotope systematics in Samoan shield lavas and the use of Os-isotopes in olivine phenocrysts to determine primary magmatic compositions. Earth and Planetary Science Letters v. 312, p. 91-101.

Class, C., Goldstein, S.L., and Shirey, S.B., 2009, Osmium isotopes in Grande Comore lavas: A new extreme among a spectrum of EM-type mantle endmembers: Earth and Planetary Science Letters, v. 284, p. 219-227.

Tomascak, P.B., Langmuir, C.H., le Roux, P.J., and Shirey, S.B., 2008, Lithium isotopes in global mid-ocean ridge basalts: Geochimica et Cosmochimica Acta, v. 72, p. 1626-1637.

le Roux, P.J., Shirey, S.B., Hauri, E.H., Perfit, M.R., and Bender, J.F., 2006, The effects of variable sources, processes and contaminants on the composition of northern EPR MORB (8-10oN and 12-14oN): Evidence from volatiles (H2O, CO2, S) and halogens (F, Cl): Earth and Planetary Science Letters, v. 251, p. 209-231.

Castillo, P.R., Klein, E., Bender, J., Langmuir, C., Shirey, S.B., Batiza, R., and White, W., 2000, Petrology and Sr, Nd, and Pb isotope geochemistry of mid-ocean ridge basalt glasses from the 11 degrees 45'N to 15 degrees 00'N segment of the East Pacific Rise: Geochemistry, Geophysics, Geosystems - G 3, v. 1, p. 1999GC000024.

Widom, E., Hoernle, K.A., Shirey, S.B., and Schmincke, H.E., 1999, Os isotope systematics in the Canary Islands and Madeira; lithospheric contamination and mantle plume signatures: Journal of Petrology, v. 40, p. 279-296.

Widom, E., and Shirey, S.B., 1996, Os isotope systematics in the Azores; implications for mantle plume sources: Earth and Planetary Science Letters, v. 142, p. 451-465.

Walker, R.J., Echeverria, L.M., Shirey, S.B., and Horan, M.F., 1991, Re-Os isotopic constraints on the origin of volcanic rocks, Gorgona Island, Colombia; Os isotopic evidence for ancient heterogeneities in the mantle: Contributions to Mineralogy and Petrology, v. 107, p. 150-162.

Shirey, S.B., Bender, J.F., and Langmuir, C.H., 1987, Three-component isotopic heterogeneity near the Oceanographer Transform, Mid-Atlantic Ridge: Nature, v. 325, p. 217-223.

Relevant Publications

Bathymetric map of the East Pacific Rise (EPR) from 15o to 18.5o N made during the 1997 Panorama Leg I cruise. Colored areas have been mapped bathymetrically from the surface by side-scanning sonar at high resolution. Grey area has never been mapped. Most of the ocean floor away from the ridge axis is grey and known more poorly than the surfaces of the other terrestrial planets. The EPR ridge axis is shallower and colored light brown to light greenish-yellow. The black dots are dredge sample stations along the ridge axis and at the tops of off-axis seamounts.

187Os/188Os versus (a) 87Sr/86Sr, and (b) 206Pb/204Pb in OIB, where only samples with Os N 40 pg and for Pitcairn Os N 50 pg (Eisele et al., 2002) are plotted to exclude samples with shallow contamination for Os. EM-type ocean island endmembers are highlighted to illustrate that each individual island seems to trend towards an individual composition rather than the mantle endmembers represented by Samoa (EMII), Pitcairn (EMI) and HIMU islands. Inset highlights peridotitic versus possible oceanic crust (OC) and upper continental crust (UCC) contribution. Data sources (Hauri and Hart, 1993; Marcantonio et al., 1995; Roy-Barman and Allègre, 1995; Bennett et al., 1996; Hauri et al., 1996; Widom and Shirey, 1996; Lassiter and Hauri, 1998; Yang et al., 1998; Brandon et al., 1999; Skovgaard et al., 2001; Eisele et al., 2002; Doucelance et al., 2003; Workman et al., 2004; Bryce et al., 2005; Escrig et al., 2005; Gaffney et al., 2005). Data compiled with GEOROC. Figure 4 from Class et al., EPSL, 2009.

Mantle heterogeneity and oceanic endmembers at ocean islands

Abyssal peridotites and mid-ocean ridge heterogeneity

New focus in the marine geology community on slow-spreading ridges has led to cruises that have been able to recover more abyssal peridotite than ever before. These samples enable the mantle to be sampled directly for the mineralogical, small scale basis for isotopic compositional variability.  With Stanford professor Jessica Warren (former DTM postdoc), we are combining the microscale analytical capability for work on sulfide inclusions in diamonds (see diamond pages above) to look at the relationship between Os and Pb isotopic composition in peridotite sulfides to large scale mantle heterogeneity in those systems.

The grouping of isotopic compositional extremes of ocean island basalts allowed Zindler and Hart (1986) to propose mantle endmembers that had specific geological history and age implications. It is now recognized that these endmembers show up in MORB and that there is a continuum between the endmembers (see right). Ongoing work with DTM postdoc Matt Jackson (now at BU) is examining the formation of these endmembers at Samoa, where some of the greatest heterogeneity and having the closest continental affinity has been found.

 

 
 

Two slow spreading ridges with a high proportion of recovered peridotite. The top map is from the Southwest Indian Ridge and the bottom map is from the Gakkel Ridge. Dredges with peridotite recovery are shown with white circles.