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Tollstrup, DL, Xie LW, Wimpenny JB, Chin E, Lee CT, Yin QZ.  2012.  A trio of laser ablation in concert with two ICP-MSs: Simultaneous, pulse-by-pulse determination of U-Pb discordant ages and a single spot Hf isotope ratio analysis in complex zircons from petrographic thin sections. Geochemistry Geophysics Geosystems. 13   Artn Q0301710.1029/2011gc004027   AbstractWebsite

We have developed a technique for the simultaneous in situ determination of U-Pb ages and Hf isotope ratios from a single spot in complex, discordant zircons by combining both a single-collector and a multicollector sector field inductively coupled plasma-mass spectrometry (ICP-MS) with a 193 nm excimer laser ablation system. With a suite of zircon standards of various ages, we first show that U-Pb ages can be determined accurately to within 0.3-2.5% (2 sigma) compared to the nominal value, while the internal errors are better than 0.4-0.7%; hafnium isotope ratios are accurate, relative to solution analyses, within one epsilon unit, and internal errors are typically <0.008%. We then apply the technique to complex, discordant zircons with variable Pb-206/U-238 and Pb-207/U-235 ratios, commonly discarded previously as "un-reducible data," to construct a Discordia in U-Pb Concordia plot, using every scan, every laser pulse as individual data points from a single laser ablation spot (typically > 200-250 data points). We show that the upper and lower intercept ages from the Discordia, augmented by high precision Hf isotope data obtained on the same spot, reveal invaluable information that permit unique insight to geological processes not available by other means. We demonstrate that our technique is useful for provenance studies of small, complex detrital zircons in sedimentary and high-grade metamorphic rocks, in relation to crustal growth and evolution.

Chin, EJ, Lee CTA, Luffi P, Tice M.  2012.  Deep Lithospheric Thickening and Refertilization beneath Continental Arcs: Case Study of the P, T and Compositional Evolution of Peridotite Xenoliths from the Sierra Nevada, California. Journal of Petrology. 53:477-511.   10.1093/petrology/egr069   AbstractWebsite

Thickening of arc lithosphere influences the extent of magmatic differentiation and is thereby important for the evolution of juvenile arcs into mature continental crust. Here, we use mantle xenoliths from the late Mesozoic Sierra Nevada continental arc in California (USA) to constrain the pressure, temperature, and compositional evolution of the deep lithosphere beneath a mature arc. These xenoliths consist of spinel peridotites and garnet-bearing spinel peridotites. The former are characterized by coarse-grained protogranular textures having bulk compositions indicative of high-degree melting. The latter are characterized by porphyroclastic textures, garnet coronas around spinels, garnet exsolution lamellae in pyroxenes, and pyroxenes with high-Al cores and low-Al rims. The garnet-bearing spinel peridotites range from depleted to fertile compositions, but the high Cr-numbers [molar Cr/(Cr + Al)] of spinel cores reflect high-degree melting. These observations suggest that the protoliths of the garnet-bearing spinel peridotites were melt-depleted spinel peridotites. Constraints from geothermobarometry and bulk compositions coupled with mantle melting models suggest that the protoliths underwent shallow melt depletion (1-2 GPa, 1300-1400 degrees C), followed by compression, cooling, and final equilibration within the garnet stability field (similar to 3 GPa, < 800 degrees C). The deepest equilibrated samples are the most refertilized, suggesting that refertilization occurred during compression. We interpret this P-T-composition path to reflect progressive thickening of the Sierran arc lithosphere perhaps as a result of magmatic inflation or tectonic thickening. We hypothesize that newly formed arc lithospheric mantle thickens enough to pinch out the asthenospheric wedge, juxtaposing Sierran arc lithosphere against the subducting oceanic plate. This could have terminated arc magmatism and initiated cooling of the deep Sierran lithosphere.