2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 7
Presentation Time: 3:00 PM

Lithophile and Siderophile Element Systematics and Isotope Chronology Associated with Melt Depletion and Metasomatism of a Relict Peridotite

LAPEN, Thomas J., Geosciences, University of Houston, 312 Science and Research 1, University of Houston, Hosuton, TX 77204-5007, BRANDON, Alan, NASA Johnson Space Center, Houston, TX 77058 and MEDARIS Jr, Gordon, Dept. of Geology and Geophysics, Univ of Wisconsin, Madison, WI 53706, tjlapen@uh.edu

Lu-Hf, Sm-Nd, and Re-Os isotope data of garnet peridotite from the Sandvik ultramafic body, Western Gneiss Region (WGR), Norway, records an exceptionally long history spanning 3 billion years. This long history affords a unique view into the response of lithospheric mantle to melting and metasomatic events as expressed in their petrologic and geochemical record. Lu-Hf and Re-Os whole rock isotope data of garnet lherzolite yields model mantle extraction (MA) ages that are 3.311 ± 0.024 Ga (Lu-Hf; εHf(0) = +2165) and 3.363 ± 0.057 and 3.329 ± 0.055 Ga (Re-Os; γOs(0) = – 17.5), indicating that the two isotope systems are very well correlated. The weighted mean melt extraction age is 3.320 ± 0.021 Ga (2ó) and agrees well with melt extraction ages observed in other peridotite bodies of the WGR [1] but is much older than protolith ages recorded in the surrounding host crustal rocks.

Sm-Nd whole rock isotope data of garnet lherzolite yields εNd(0) = -8.8, indicating that the Sm-Nd isotope system is strongly decoupled from the Re-Os and Lu-Hf systems. Although melt extraction would result in correlated depletions in Sm/Nd, Lu/Hf, and Re/Os ratios, the Sm-Nd systematics have been subsequently modified in a way that left the Lu-Hf and Re-Os systems unaffected. Sm-Nd isotope systematics of olivine-garnet pyroxenite and garnet periditite as well as the associated garnet lherzolite indicate that these rocks were metasomatized by LREE-rich fluids during the Sveconorwegian orogeny at 1149 ± 11 Ma [2]. On the basis of a Sm-Nd isochron age of low-pressure kelyphitic rims on garnet, emplacement of these mantle rocks into the crust likely started with Late Proterozoic (~600 Ma) rifting and was complete by the Scandian phase of the Caledonian orogeny (~400 Ma).

[1] Beyer et al. (2004) Geology, 32, 609-612. [2] Lapen et al. (2005) CMP 150, 131-145.