Zi-Fu Zhao

Calculation of oxygen isotope fractionation in magmatic rocks

The increment method is applied to calculation of oxygen isotope fractionation factors for common magmatic rocks. The 18 O-enrichment degree of the different compositions of magmatic rocks is evaluated by the oxygen isotope indices of both CIPW normative minerals and normalized chemical composition. The consistent results are obtained from the two approaches, pointing to negligible oxygen isotope fractionation between rock and melt of the same compositions. The present calculations verify the following sequence of 18 O-enrichment in the magmatic rocks: felsic rocks>intermediate rocks>mafic rocks> ultramafic rocks. Two sets of internally consistent fractionation factors are acquired for phenocryst–lava systems at the temperatures above 1000 K and rock–water systems in the temperatures range of 0–1200 jC, respectively. The present calculations are consistent with existing data from experiments and/or empirical calibrations. The obtained results can be used to quantitatively determine the history of water–rock interaction and to serve geological thermometry for various types of magmatic rocks (especially extrusive rocks). D 2002 Elsevier Science B.V. All rights reserved.

Oxygen isotope equilibrium between eclogite minerals and its constraints on mineral Sm-Nd chronometer

Abstract Sm-Nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the Sulu terrane in eastern China. The results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and Sm-Nd isotope systems of eclogite minerals. The omphacite-garnet pairs of oxygen isotope equilibrium at eclogite-facies conditions yield meaningful Triassic Sm-Nd isochron ages, whereas those of oxygen isotope disequilibrium give non-Triassic ages of geological meaninglessness. This can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than, or close to, those of Nd diffusion, and thus attainment of isotopic equilibrium in the omphacite-garnet O system suggests achievement of Nd isotope equilibrium in the same mineral pairs. The presence or absence of fluid in the eclogite protoliths is a major rate-controlling factor for isotopic equilibration during high-grade metamorphism. It appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the Sm-Nd mineral chronometer. In addition, the exact timing of the ultrahigh pressure metamorphism in the Dabie-Sulu terranes is constrained at Early Triassic rather than Late Triassic.

Developing plate tectonics theory from oceanic subduction zones to collisional orogens

Crustal subduction and continental collision is the core of plate tectonics theory. Understanding the formation and evolution of continental collision orogens is a key to develop the theory of plate tectonics. Different types of subduction zones have been categorized based on the nature of subducted crust. Two types of collisional orogens, i.e. arc-continent and continent-continent collisional orogens, have been recognized based on the nature of collisional blocks and the composition of derivative rocks. Arc-continent collisional orogens contain both ancient and juvenile crustal rocks, and reworking of those rocks at the post-collisional stage generates magmatic rocks with different geochemical compositions. If an orogen is built by collision between two relatively old continental blocks, post-collisional magmatic rocks are only derived from reworking of the old crustal rocks. Collisional orogens undergo reactivation and reworking at action of lithosphere extension, with inheritance not only in the tectonic regime but also in the geochemical compositions of reworked products (i.e., magmatic rocks). In order to unravel basic principles for the evolution of continental tectonics at the post-collisional stages, it is necessary to investigate the reworking of orogenic belts in the post-collisional regime, to recognize physicochemical differences in deep continental collision zones, and to understand petrogenetic links between the nature of subducted crust and post-collisional magmatic rocks. Afterwards we are in a position to build the systematics of continental tectonics and thus to develop the plate tectonics theory.

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.

Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction.

Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins.

Two types of gneisses associated with eclogite at Shuanghe in the Dabie terrane: carbon isotope, zircon U-Pb dating and oxygen isotope

δ13C values of −28.6‰ to −22.3‰ and the carbon concentrations of 0.70–4.98 wt.% CO2 despite a large variation in δ18O from −4.3‰ to +10.6‰ for these gneisses. There is significant heterogeneity in both δ13C and δ18O within the gneisses on the scale of several tens meters, pointing to the presence of secondary processes after the UHP metamorphism. Considerable amounts of carbonate carbon occur in some of the gneisses that were also depleted in 13C primarily, but subjected to overprint of 13C-rich CO2-bearing fluid after the UHP metamorphism. The 13C-depleted carbon in the gneisses is interpreted to be inherited from their precursors that suffered meteoric–hydrothermal alteration before plate subduction. Both low δ13C values and structural carbonate in the apatite suggest the presence of 13C-poor CO2 in the UHP metamorphic fluid. The 13C-poor CO2 is undoubtedly derived from oxidation of organic matter in the subsurface fluid during the prograde UHP metamorphism. Zircons from two samples of the granitic orthogneiss exhibit low δ18O values of −4.1‰ to −1.1‰, demonstrating that its protolith was significantly depleted in 18O prior to magma crystallization. U–Pb discordia datings for the 18O-depleted zircons yield Neoproterozoic ages of 724–768 Ma for the protolith of the granitic orthogneiss, consistent with protolith ages of most eclogites and orthogneisses from the other regions in the Dabie–Sulu orogen. Therefore, the meteoric–hydrothermal alteration is directly dated to occur at mid-Neoproterozoic, and may be correlated with the Rodinia supercontinental breakup and the snowball Earth event. It is thus deduced that the igneous protolith of the granitic orthogneiss and some eclogites would intrude into the older sequences composing the sedimentary protoliths of the biotite paragneiss and some eclogites along the northern margin of the Yangtze plate at mid-Neoproterozoic, and drove local meteoric–hydrothermal circulation systems in which both 13C- and 18O-depleted fluid interacted with the protoliths of these UHP rocks now exposed in the Dabie terrane.

Zircon U-Pb age and o isotope evidence for neoproterozoic low-18O magmatism during supercontinental rifting in South China: Implications for the snowball earth event

Identification of Neoproterozoic low δ18O igneous rocks is a key to demonstrate that high-T water-rock interaction and low-18O magmatism in rift tectonic zones can serve as an efficient interface to transport heat and material from the Earths interior to exterior at the onset of the snowball Earth event. Low δ18O values of −10.9 to +4.4 permil characteristic of meteoric origin were observed, on an areal extent over 30,000 km2, for mid-Neoproterozoic metaigneous zircons from the ultrahigh-pressure eclogite-facies metamorphic belt of Triassic ages along the Dabie-Sulu orogenic belt, China. Although the zircon O isotope system could be reset to acquire its low δ18O values by exchange with matrix minerals during “wet” eclogite-facies metamorphism, the abnormal 18O depletion is confirmed by occurrence of Neoproterozoic low δ18O zircons in low-grade metaigneous rocks in the same belt. Cathodoluminscence imaging of zircons from low-grade metagranitoid and metavolcanic rocks shows preservation of the internal structures of oscillatory zoning typical of magmatic origin. Hornblende from two granites of low δ18O zircons gave Ar-Ar plateau ages of 747 ± 6 and 776 ± 12 Ma, ruling out that Phanerozoic metamorphism effected mineral O isotope values. Two groups of U-Pb ages were measured at 756 ± 5 Ma and 782 ± 5 Ma, respectively, for the low δ18O zircons. This indicates that low δ18O zircons crystallized during two episodes of low-18O magmatism in response to tectonic evolution from supercontinental rifting at ∼780 Ma to breakup at ∼750 Ma. The second episode of low-18O magmatism is contemporaneous with the continental Kaigas iceage. A plate-rift model is advanced to account for the tectonic connection between the low-18O magmatism, the continental glaciation and the supercontinental rifting during the Cryogenian period. High-T meteoric water-rock interaction occurred prior to low-18O magmatism in rift tectonic zones, and caldera collapse was responsible for melting of hydrothermally altered low δ18O rocks during rifting of the South China Block from the Rodinia supercontinent.

Oxygen isotope equilibrium between ultrahigh-pressure metamorphic minerals and its constraints on Sm-Nd and Rb-Sr chronometers

Abstract In the Sm-Nd and Rb-Sr isotopic geochronology of metamorphic rocks, an important question is whether radiometric systems of mineral isochrons have achieved isotopic equilibrium during a given metamorphic event and preserved the equilibrium afterwards. An analogue to mineral chronometry is O isotope geothermometry. Because the rates of Sm-Nd, Sr and O diffusion in metamorphic minerals are comparable in many cases, the state of O isotope equilibrium between metamorphic minerals can provide a test for the validity of mineral Sm-Nd and Rb-Sr chronometers. In order to illustrate this applicability, O isotope geothermometry was carried out for Sm-Nd and Rb-Sr isochron minerals from ultrahigh-pressure (UHP) eclogites and gneisses at Shuanghe in the Dabie terrane of east-central China. Although the Sm-Nd isochrons give consistent Triassic ages of 213 to 238 Ma for UHP metamorphism, the Rb-Sr isochrons give Jurassic ages of 171 to 174 Ma for the same samples. O isotope geothermometry of the gneiss, eclogite and amphibolite minerals yields two sets of temperatures of 600 to 720 °C and 420 to 550 °C, respectively, corresponding to cessation of isotopic exchange by diffusion at about 225 ± 5 Ma during high pressure eclogite-facies recrystallization and at about 175 ± 5 Ma during amphibolite-facies retrogression. The preservation of Triassic Sm-Nd isochron ages, but the occurrence of Jurassic Rb-Sr isochron ages and the regular O isotope temperatures for the same samples, suggest that rates of Sr and O diffusion in such hydroxyl-bearing minerals as biotite and hornblende are faster than rates of Nd diffusion in garnet and Sr diffusion in phengite on the scale of a hand-specimen during the amphibolite-facies retrogression. While the mineral with slow diffusivity has exerted the primary control on the homogenization rate of initial isotope ratios among isochron minerals during retrograde metamorphism, the mineral with high parent/daughter ratio has exerted the principal control on the initiation of the mineral isochron clock in response to retrogression. Valid mineral isochrons can be expected to date the timing of metamorphic resetting only if the mineral with high parent/daughter ratio has a fast rate of radiogenic isotope diffusion during the metamorphic resetting.

Relationships between O isotope equilibrium, mineral alteration and Rb–Sr chronometric validity in granitoids: implications for determination of cooling rate

A combined study of mineral O and Rb–Sr isotopes was carried out for a number of Mesozoic granitoids in China in order to compare the degree of O isotope equilibrium between coexisting minerals, with the validity of mineral Rb–Sr isochrons for granitoids. A scrutiny of both O isotope geothermometry and Rb–Sr internal isochron dating for corresponding minerals indicates that equilibrium O isotope fractionation between Rb–Sr isochron minerals corresponds to geologically meaningful isochron ages if the variation in 87Rb/86Sr ratio is big enough to provide reasonably small uncertainties in age. Significant deviation of the Rb–Sr isochron age from the actual age appears to depend on the difference in Sr isotopic composition between an external fluid and the igneous minerals. As a result, O isotope disequilibrium is often caused by interaction between the rock and the external fluid that results in mineral alteration. Post-magmatic alteration can cause isotope exchange between the minerals and an internally buffered fluid that is isotopically identical to the host rock. The O isotope composition of coexisting minerals in studied samples changed principally due to a decrease in temperature. Both Rb and Sr concentrations and the Sr isotope ratios of isochron minerals also changed due to the mixing of different Sr reservoirs. Nevertheless, the isochron age can remain unchanged if the mixing took place along the isochron chord between the internal fluid and the minerals from that newly altered minerals formed. This provides an insight into the effect of internal and external fluids on the validity of mineral Rb–Sr chronometry. In addition, an alternative approach is proposed to construct the cooling curve by a combined use of O isotope temperature and mineral isotope age for the granitoids of interest. Comparing with the traditional method using the empirical closure temperature for Rb–Sr chronometry, the proposed approach utilizes fewer variables with smaller uncertainties than the traditional way.

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.