Plesovice zircon : A new natural reference material for U-Pb and Hf isotopic microanalysis

We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt^lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4^2·5 Ga, 1·6^2·2 Ga and 0·6^1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt^peridotite interaction and subsequent high-pressure metamorphism.The 2·5 Ga zircons have positive eHf(t) values (2·9^10·6), whereas the younger Precambrian zircons are dominated by negative eHf(t) values, indicating an ancient continental crustal origin.These observations suggest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 3 Ma (2 ), 80^170 Ma and 48^64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt^peridotite interactions in the central zone of the NCC. The combination of the positive eHf(t) values (2·91^24·6) of the 315 Ma zircons with the rare occurrence of 302^324 Ma subduction-related diorite^granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt^peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80^170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies.The eHf(t) values of these zircons vary greatly from ^47·6 to 24·6.The 170^110 Ma zircons are generally characterized by negative eHf(t) values, whereas the 110^100 Ma zircons have positive eHf(t) values.These observations suggest that melt^peridotite interactions at 80^170 Ma were induced by partial melting of recycled continental crust. The 48^64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive eHf(t) values.These features imply that the melt^peridotite interactions at 48^64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.

Continental and Oceanic Crust Recycling-induced Melt^Peridotite Interactions in the Trans-North China Orogen: U^Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths

Hf isotopic compositions of the standard zircons and baddeleyites used in U-Pb geochronology

Nature and significance of the Early Cretaceous giant igneous event in eastern China

A protocol was established for simultaneous measurements of zircon U-Pb ages and trace elements by LA-ICP-MS at spot sizes of 16–32 ?m. This was accomplished by introducing N2 into ICP to increase the sensitivity. The obtained U-Pb ages for zircon standards GJ-1, TEMORA and SK10-2 are consistent with the preferred values within about 1% uncertainty (2 σ ) by simple external calibration against zircon standard 91500. Different data reduction softwares could yield different uncertainties for calculation of U-Pb ages. The commercially available program GLITTER4.4 could apply an improper uncertainty calculation strategy, but it may yield artificial high precisions for single analyses. Our trace element analyses indicate that Si is not an ideal internal standard for zircon when calibrated against the NIST glasses. Calibration against the NIST glasses using Si as an internal standard, a systematic deviation of 10%–30% was found for most trace elements including Zr. However, the trace element compositions of zircon can be accurately measured by calibration against multiple reference materials with natural compositions (e.g., BCR-2G, BHVO-2G and BIR-1G), or calibration against NIST SRM 610 and using Zr as an internal standard. Analyses of two pieces of GJ-1 demonstrate that it is relatively homogenous for most trace elements (except for Ti).

Reappraisement and refinement of zircon U-Pb isotope and trace element analyses by LA-ICP-MS

Both concentration and isotope composition of bulk carbon in Phanerozoic granites from Southeast China have been determined by EA-MS online techniques. The results show that the carbon content ranges from 0.04 to 0.79wt% and δ 13 C from −37.0 to −7.5‰. Apatite has uniformly low δ 13 C values of −28.9 to −18.7‰ with contents of 0.04 to 0.19wt%. Some of the granites were re-analyzed after HCl leaching, and the results show the presence of the different forms of carbon in granites (at least carbonate and noncarbonate carbons). There is a correlation in δ 13 C between apatite and host granite, indicating that the carbon isotope composition of the granitic magmas was not significantly affected by magma degassing, crustal contamination and hydrothermal alteration during their emplacement and subsolidus crystallization. Therefore, the isotope composition of bulk carbon in the granites reflects the isotopic fingerprint of their sources. While the low δ 13 C values for the apatite are interpreted to be produced by the oxidation of organic and/or elemental carbon during magma generation, the higher δ 13 C values for the granites suggest the carbon isotope heterogeneity in their source survived after magma generation. The concentration and isotope composition of bulk carbon exhibit a log-normal distribution and two-peaks normal distribution, respectively, pointing to mixing processes between different carbon reservoirs. Along with Nd and Sr isotope data for the granites, a two end-member mixing model is suggested between the isotope composition and content of bulk carbon in the granites as well as between Nd, Sr and C isotopes. A mantle-like end-member can be represented by a single δ 13 C value of −5‰ which corresponds to a 87 Sr/ 86 Sr ratio of 0.704 and a e Nd (t) of 0 like the PREMA, whereas the crustal end-member requires variable δ 13 C values of −35 to −25‰ which correspond to a large variation in initial 87 Sr/ 86 Sr ratios of 0.715 to 0.740 but a single e Nd (t) value of −18. Although the crust-mantle mixing can explain their Nd and Sr isotopic patterns, the mantle-like δ 13 C value cannot be simply ascribed to a primary mantle source because such a δ 13 C value may be produced by the mass-balanced homogenization of sedimentary carbonate and organic carbon in the granite precursors. Some of the granite of igneous origin with the initial 87 Sr/ 86 Sr ratios less than 0.710 also have the low δ 13 C values of −30 to −20‰, implying that their igneous protoliths would be exposed to surface and suffered the contamination by organic carbon before remelting. Therefore, the carbon isotope study reveals significant input of surface organic carbon into the granite sources prior to magma generation.

Carbon concentration and isotope composition of granites from Southeast China

This is a report of carbon isotope anomaly in marbles associated with the UHP eclogites from the Dabie Mountains. The results place constraints on the degree of crust-mantle interaction during the UHP metamorphism.

Carbon isotope anomaly in marbles associated with eclogites from the Dabie Mountains

The composition of garnet in granite and pegmatite from the Gangdese orogen in southeastern Tibet: Constraints on pegmatite petrogenesis

Response to the Comment by J. Horita and R.N. Clayton on “The studies of oxygen isotope fractionation between calcium carbonates and water at low temperatures”

Oxygen isotope fractionation in the zinc oxides has been calculated by means of the modified increment method. The results suggest that zincite is slightly enriched in 18O relative to the franklinite of the spinel-type structure but considerably depleted in 18O relative to the franklinite of the inverse spinel-type structure. The zinc oxides are significantly depleted in 18O relative to water under hydrothermal and metamorphic conditions. The oxygen isotope analyses of mineral pairs including the zinc oxides and the common gangue minerals such as calcite and quartz can constitute a sensitive isotope geothermometer. Application of oxygen isotope geothermometry to natural assemblages is attempted for the calcite-zinc ore mineral pairs from the Sterling Hill deposit in USA. The results indicate that the temperature of the zinc mineralization may be in the range from 410° to 630 °C and thus lower than the metamorphic temperatures of granulite facies. A metamorphic fluid could have been involved in the formation of the zinc ore minerals. Franklinite would structurally be an inverse spinel in the infancy of its formation, and thus could have originally evolved from Zn2 + substitution to Fe2 + of magnetite at the high temperatures.

Yong-Fei Zheng

Papers

Carbon concentration and isotope composition of granites from Southeast China

Carbon isotope anomaly in marbles associated with eclogites from the Dabie Mountains

The composition of garnet in granite and pegmatite from the Gangdese orogen in southeastern Tibet: Constraints on pegmatite petrogenesis

Response to the Comment by J. Horita and R.N. Clayton on “The studies of oxygen isotope fractionation between calcium carbonates and water at low temperatures”

Oxygen isotope fractionation in zinc oxides and implications for zinc mineralization in the Sterling Hill deposit, USA