Mei Fei Chu

Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet

Adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones. Here we report the first example of such magmas from southern Tibet in an active continental collision environment. The Tibetan adakites were emplaced from ca. 26 to 10 Ma, and their overall geochemical characteristics suggest an origin by melting of eclogites and/or garnet amphibolites in the lower part (≥50 km) of thickened Tibetan crust. This lower-crustal melting required a significantly elevated geotherm, which we attribute to removal of the tectonically thickened lithospheric mantle in late Oligocene time. The identification of collision-type adakites from southern Tibet lends new constraints to not only the Himalayan-Tibetan orogenesis—how and when the Indian lithosphere started underthrusting Asia can be depicted—but also the growth of the early continental crust on Earth that consists dominantly of the tonalite-trondhjemite-granodiorite suites marked by adakitic geochemical affinities.

Zircon U-Pb and Hf isotope constraints on the Mesozoic tectonics and crustal evolution of southern Tibet

The first in situ Hf and U-Pb isotope analyses of zircon separates from Mesozoic granites in southern Tibet identify a significant, previously unknown stage of magmatism. Igneous zircons (n = 34) from a granite within the Gangdese batholith show a weighted mean 206 Pb/ 238 U age of 188.1 ± 1.4 Ma and e Hf (T) (the parts in 10 4 deviation of initial Hf isotope ratios between the zircon sample and the chondritic reservoir) values between +10.4 and +16.8, suggesting predominantly Early Jurassic intrusive activity with a juvenile mantle contribution. Of 40 inherited zircons from two Cretaceous S-type granites in the northern magmatic belt, 23 delineate a slightly older 206 Pb/ 238 U age cluster between 188 and 210 Ma. These zircons have e Hf (T) values from −3.9 to −13.7, yielding crustal Hf model ages from ca. 1.4 to 2.1 Ga, suggesting a major episode of crustal growth in Proterozoic time and remelting of this crust in Early Jurassic time. Combining these with literature data, we interpret the Jurassic Gangdese magmatism as an early product of the Neo-Tethyan subduction that played a long-lasting role in the tectonic evolution of southern Tibet prior to the India-Asia collision.

Measurements of natural uranium concentration and isotopic composition with permil‐level precision by inductively coupled plasma–quadrupole mass spectrometry

A new analytical technique using inductively coupled plasma–quadrupole mass spectrometry (ICP-QMS) has been developed that produces permil-level precision in the measurement of uranium concentration ([U]) and isotopic composition (δ234U) in natural materials. A 233U-236U double spike method was used to correct for mass fractionation during analysis. To correct for ratio drifting, samples were bracketed by uranium standard measurements. A sensitivity of 6–7 × 108 cps/ppm was generated with a sample solution uptake rate of 30 μL/min. With a measurement time of 15–20 min, standards of 30-ng uranium produced a within-run precision better than 3‰ (±2 R.S.D.) for δ234U and better than 2‰ for [U]. Replicate measurements made on standards show that a between-run reproducibility of 3.5‰ for δ234U and 2‰ for [U] can be achieved. ICP-QMS data of δ234U and [U] in seawater, coral, and speleothem materials are consistent with the data measured by other ICP-MS and TIMS techniques. Advantages of the ICP-QMS method include low cost, easy maintenance, simple instrumental operation, and few sample preparation steps. Sample size requirements are small, such as 10–14 mg of coral material. The results demonstrate that this technique can be applied to natural samples with various matrices.

Extreme lithium isotopic fractionation in three zircon standards (Plešovice, Qinghu and Temora)

To understand the behavior of Li in zircon, we have analyzed the abundance and isotopic composition of Li in three zircon standards (Plešovice, Qinghu and Temora) widely used for microbeam analysis of U-Pb ages and O-Hf isotopes. We have mapped Li concentration ([Li]) on large grains, using a Cameca 1280HR Secondary Ion Mass Spectrometer (SIMS). All zircons have a rim 5–20 μm wide in which [Li] is 5 to 20 times higher than in the core. Up to ~20‰ isotopic fractionation is observed on a small scale in the rims of a single zircon grain. The measured δ7Li values range from –14.3 to 3.7‰ for Plešovice, –22.8 to 1.4‰ for Qinghu and –4.7 to 16.1‰ for Temora zircon. The [Li] and δ7Li are highly variable at the rims, but relatively homogenous in the cores of the grains. From zircon rim to core, [Li] decreases rapidly, while δ7Li increases, suggesting that the large isotopic variation of Li in zircons could be caused by diffusion. Our data demonstrate that homogeneous δ7Li in the cores of zircon can retain the original isotopic signatures of the magmas, while the bulk analysis of Li isotopes in mineral separates and in bulk-rock samples may produce misleading data.