Chun-Sheng Wei

Oxygen and neodymium isotope evidence for recycling of juvenile crust in northeast China

It has been long recognized from Nd and Sr isotopes that depleted mantle sources consist of recycled oceanic materials, but difficulty was encountered in identifying this signature by means of oxygen isotopes because of significant postemplacement hydrother- mal alteration. Zircon is expected to preserve this signature because it is resistant to high- temperature hydrothermal alteration. This effect is illustrated by a combined Sm-Nd and oxygen isotope study of whole-rock and mineral samples from a Mesozoic A-type granite at Nianzishan in northeastern China. The Sm-Nd isotope results show positive «Nd(t) val- ues of 10.86 to 14.27 with young Nd model ages of 569-846 Ma, manifesting a significant input of newly mantle derived material. The zircon d 18 O values of 3.12‰-4.19‰ are significantly lower than the d 18 O value of 5.3‰ 6 0.3‰ for the normal mantle zircon and thus appear to require remelting of hydrothermally altered oceanic crust. The combined Nd-O isotope studies not only provide compelling evidence for geochemical recycling of young juvenile crust by plate subduction, but also demonstrate that the granitic magmas can result from partial melting of mantle-derived rocks that were subjected to seawater- hydrothermal alteration before magma generation. Disequilibrium oxygen isotope frac- tionations are observed between common rock-forming minerals with significantly lower d 18 O values for alkali feldspar than seawater, corresponding to meteoric-hydrothermal alteration after magma crystallization.

Oxygen isotope composition of quartz-vein in ultrahigh-pressure eclogite from Dabieshan and implications for transport of high-pressure metamorphic fluid.

Abstract The oxygen isotope composition of minerals from quartz-veins and host-eclogites in Dabieshan, China was measured in order to place geochemical constraints on the origin and transport of high-pressure metamorphic fluids. The results, along with structural and petrological relationships between vein and wallrock, show that the quartz veins are the high-pressure metamorphic and thus formed prior to eclogite-facies recrystallization when they were exhumated from mantle depths to deep crustal levels. Not only is the oxygen isotope composition of the vein-quartz identical to that of the host-eclogite, but in addition the oxygen isotope geothermometry of mineral-pairs from the quartz-veins yield temperatures that are close to the eclogite-facies temperatures. Therefore, the vein-forming fluid was likely derived from the local host-eclogites by the exsolution of dissolved hydroxyls within eclogite minerals due to significant pressure decrease. Local advective transport of fluid is the predominant mechanism in the processes of vein precipitation. Fluid flow prior to the eclogite-facies recrystallization may occur mainly along pressure gradients. The loss of the UHP or HP fluid at the different depths during exhumation may be the potential cause for concordant and discordant isotope temperatures between different mineral-pairs in the eclogites.

Oxygen isotope composition of granulites from Dabieshan in eastern China and its implications for geodynamics of Yangtze plate subduction.

Abstract The oxygen isotope composition of whole-rock and mineral separates was measured for granulite rocks from Dabieshan. According to their whole-rock δ18O values relative to the normal mantle δ18O values of 5.7±0.5‰, two groups are classified: (1) mafic granulite which exhibits lower δ18O values of 3.5 to 4.7‰, and (2) felsic granulite which shows higher δ18O values of 7.6 to 7.8‰. Consistent isotope temperatures of 800 to 900 °C are obtained for mineral pairs containing such refractory minerals as pyroxene, garnet, hornblende and iron oxides, suggesting the achievement and preservation of oxygen isotope equilibrium at the conditions of the peak granulite-facies metamorphism. This not only points to a rapid cooling and ascent for the granulite rocks in the early stage of exhumation, but also precludes the infiltration of external fluids during exhumation as the cause for the 18O-depletion in the mafic granulite. It is evident that the granulite rocks acquired the low δ18O values before the granulite-facies metamorphism by interaction with a certain 18O-depleted surface fluid. The surface fluid is assumed to exchange oxygen isotopes with the granulite protoliths prior to plate subduction. Fluid-absent metamorphism is suggested for the formation of the granulites on local scales. It is likely that the granulites together with the ultrahigh pressure eclogites and gneisses in Dabieshan were part of a single tectonic entity in the processes of subduction and Triassic metamorphism but experienced differential two-stage uplifts prior to amphibolite-facies retrogression.

Hydrogen and oxygen isotope geochemistry of A-type granites in the continental margins of eastern China

Hydrogen and oxygen isotope studies of both whole-rock and mineral separates were carried out for five representative Atype granites in eastern China. From North to South, the whole-rock d D and d 18 O values vary from 2145 to 282‰ and from 21.2 to 5.0‰ for Nianzishan pluton, from 2135 to 2110‰ and from 1.6 to 6.7‰ for Shanhaiguan pluton, and from 2145 to 2111‰ and from 1.5 to 9.1‰ for Laoshan massif, respectively. The highest whole-rock dD and d 18 O values are observed in Suzhou pluton and range from 281 to 259‰ and from 3.5 to 9.2‰, respectively. Homogeneous d D and d 18 O values are found in Kuiqi massif, ranging from 2113 to 299‰ and from 5.6 to 8.5‰, respectively. The results show that the large variabilities in the hydrogen and oxygen isotope ratios are mainly related to the diverse processes of magma evolution, such as magma degassing, fractional crystallization as well as post-magmatic alteration by local meteoric water under subsolidus conditions. The initial d D and d 18 O values of the A-type granites in eastern China are constrained to be about 250 ^ 5‰ and 7.5 ^ 1.0‰, respectively. This is consistent with mantle-like protoliths for granitoid rocks and thus precludes the possibility that the A-type granites is derived from the anatexis of the 18 O-rich upper crust. However, recycling of crustal materials by earlier subduction cannot be excluded in the processes of generating the A-type granitic magmas. The variation in the Ddepletion of the granites with latitude implies that the paleogeographic localities of the A-type granites have not changed significantly since the Mesozoic. q 2000 Elsevier Science B.V. All rights reserved.

Oxygen isotope evidence for two-stage water-rock interactions of the Nianzishan A-type granite in NE China

The oxygen isotope ratios of whole-rock, common rock-forming minerals and zircon from Mesozoic A-type granitic pluton at Nianzishan in northeastern China were analyzed by the conventional BrF5 method and the laser-probe technique, respectively. Both whole-rock and rock-forming minerals show large δ18O variations up to 5.5‰ with significant oxygen isotope disequilibrium between zircon and the other minerals, whereas the δ18O values of zircon are tightly clustered between 3.12‰ and 4.19‰ and thus lower than the normal-mantle δ18O values. These results indicate that the Nianzishan A-type granite experienced two-stage water-rock interactions subsequentially. The remarkably low zircon δ18O values are genetically due to seawater exchange with granite protolith in the first stage, and the oxygen isotope disequilibrium fractionations between zircon and rock-forming minerals are caused by meteoric-hydrothermal alteration in the second stage. It is inferred that the18O-depleted A-type granitic magma was derived from partial melting of subducted lower oceanic crust which was isotopically exchanged with seawater at high temperatures. In the process of granite emplacement into the upper crust, meteoric-hydrothermal circulation was triggered to overprint crystallizing granite under subsolidus conditions.

Carbon concentration and isotope composition of granites from Southeast China

Abstract 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.