CRPG-CNRS, BP20, 54501 Vandoeuvre Cedex, France. Abstract Oxygen ..... Taylor, H. P. Jr., Sheppard, S. M. F., Igneous rocks, m. Processes of isotopic ...
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Oxygen isotopic compositions of zircons from pyroxenite of Daoshichong, Dabieshan: Implications for crust-mantle interaction 1
2
1
XIA Qunke , E. Deloule , WU Yuanbao , 1 1 CHEN Daogong & CHENG Hao 1. Department of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; 2. CRPG-CNRS, BP20, 54501 Vandoeuvre Cedex, France
Abstract Oxygen isotopic compositions of zircons from pyroxenite (~145 Ma) of Daoshichong, Dabieshan have been measured by an ion microprobe. Both within the single grain and among different grains, oxygen isotopic ratios are homogeneous, δ 18O = (7.66‰B0.46)‰ (1 SD, 1 σ = 0.10, n = 22). High δ 18O values indicate that the mantle-derived parent magma of Daoshichong pyroxenite have undergone interaction with crustal materials. Combing with other geochemical constraints, the way of crust-mantle interaction is suggested to be source mixing other than crustal contamination. The time interval between crust-mantle interaction and formation of the parent magma of Daoshichong pyroxenite is less than several million years. The crustal component involving in crust-mantle interaction is mafic lower crust, and the parent magma of pyroxenite possibly contain large proportion (>37%) of such lower crust.
implications for crust-mantle interaction. 1 Geological background and samples Pyroxenite was collected from an outcrop in Daoshichong, ~45 km south of Huoshan County, Anhui Province. Similar to pyroxenites from other localities (such as Shacun, Zhujiapu), Daoshichong pyroxenite displays a cumulate texture and shows no sign of deformation and metamorphism. We selected about 100 zircon grains from ~30 kg pyroxenite using the conventional procedure. Zircons are large in size (150300 µm) and light brown in color, they generally show short prismatic or equiaxial shape. Several tens of zircons were picked up under a binocular microscope, and made into a polished mount. U-Pb ages of ~145 Ma were obtained by a Cameca 1270 ion microprobe in Nancy (CRPG-CNRS) in October 1999 and reported in Chen et al.[4]. 2 Analytical methods and results In order to investigate the internal structures of zircons, the mount is repolished and recleaned for cathodoluminescence (CL) imaging, we use Philips Oxford Mono CL in Microbeam Analytical Center of NancyUniversity. All grains show an oscillatory zoning structure, implying that they are typical magmatic zircons. No inherited cores, inclusions and cracks can be found. Representative CL image is shown in fig. 1.
Keywords: zircon, oxygen isotope, ion microprobe, Daoshichong, Dabieshan.
Mafic-ultramafic intrusions with variable dimensions are widely distributed in the northern part of Dabieshan. These rocks are generally undeformed, show no or little sign of metamorphism, and display cumulate textures and intrusive relations with country gneisses. U-Pb ages of zircons are 125145 Ma[1ü4], the parent magma of these intrusions are believed to be mantle-derived mafic melt based on the detailed geochemical studies[5, 6]. Some notable geochemical features are as follows[5ü7]: LILEs and LREEs are highly enriched and HFSEs (Nb, Zr, Ti, etc.) are depleted in the primitive mantle normalized spidergrams; the initial 87Sr/86Sr ratios are up to 0.710 and εNd (t) is down to –20. These continental geochemical signatures suggest that the parent magma should have undergone interaction with crustal materials. Due to the large difference between crust and mantle, oxygen isotope is an effective tracer of crust-mantle interaction[8]. But up to now, there is no report of oxygen isotopic study for Dabiehsan mafic-ultramafic intrusions. In this note we present the data on the oxygen isotopic compositions of zircons from pyroxenite of Daoshichong measured by an ion microprobe, and discuss the 1466
Fig. 1. Representative CL image of Daoshichong zircon.
After CL imaging, the mount is repolished and recleaned for oxygen isotopic measurements. We still use a Cameca 1270 ion microprobe in Nancy (CRPG-CNRS), the detailed analytical procedure can be found in Gurenko et al.[9]. During the experiment, we use Cs+ as primary ion and multicollection mode to collect 16O+ and 18O+ simultaneously. The intensities of 16O+ are (34)106 cps, and the total counting time is ~2 min. Large instrumental mass fractionation (IMF), which is the difference between the measured and the true value, is common for ion probe analysis of stable isotopic ratios[9,10]. In this note, we define IMF = δ 18OIMP − δ 18OTRUE, δ 18OIMP is the value measured by an ion microprobe, Chinese Science Bulletin Vol. 47 No. 17 September 2002
NOTES δ 18OTRUE is the true value. All δ 18O values are relative to SMOW. In order to determine IMF, we analyze eight and three standard (91500-zircon) points before and after Daoshichong zircons. δ 18OIMP of eleven standard points are 5.59‰7.17‰, the average value is (6.56 ± 0.49)‰ (1 SD, 1σ = 0.15). SD is the standard deviation of the average value, it predicts the reproducibility of repeated analysis for standard and the homogeneity for unknown sample; σ = SD / n (n is the number of analyzed points), it describes how well the average value is known[11]. δ18OTRUE of 91500-zircon is (10.290.25)‰ (http://www. crpg.cnrs-nancy.fr/SARM/Etalons.html), therefore the IMF of this set of experiment is –3.73‰. Gurenko et al.[9] reported the results of many different standards during different sets of experiments in Nancy from August 1998 to June 2001, 1 SD values ranged from 0.1‰ to 1.4‰ and most of them from 0.2‰ to 0.5‰, IMF ranged from –24.7‰to 0.9‰, it is evident that our results fall in their range. Table 1 Oxygen isotopic compositions of zircons from Daoshichong pyroxenite Grain
Point
δ 18OIMP
±
δ 18OTRUE
DSC1
1
3.29
0.26
7.02
2
3.84
0.27
7.57
3
3.68
0.27
average DSC2
7.41 7.33
4.02
0.22
7.75
2 3 4 average DSC3 1 2 3 4 5 6 average
4.13 4.29 4.15
0.20 0.32 0.33
4.36 4.48 4.01 4.31 4.72 4.09
0.29 0.24 0.40 0.34 0.31 0.29
7.86 8.02 7.88 7.88 8.09 8.21 7.74 8.04 8.45 7.82 8.06
DSC4
1
4.84
0.18
8.57
2 3 4 5 average
3.79 3.72 3.11 3.42
0.25 0.25 0.34 0.23
7.52 7.45 6.84 7.15 7.51
1
3.84
0.41
7.57
2 3.08 3 3.28 4 3.87 average average of all points
0.28 0.23 0.23
6.81 7.01 7.60 7.25 7.66
DSC5
1
SD
0.28
0.11
0.26
0.65
0.40 0.46
δ 18OIMP: Values measured by ion microprobe; δ 18OTRUE: values after instrumental fractionation correction; SD: standard deviation of the average value.
Chemical compositions of Daoshichong zircons are similar to that of 91500-zircon, thus avoiding the IMF shift due to different chemical compositions; the δ 18O values of analyzed points of 91500-zircon before and after Daoshichong samples are within the error, suggesting that the IMF is constant during this set of experiment. So, we apply the IMF obtained from 91500-zircon to Daoshichong zircons directly. The results of 22 analyzed points of five Daoshichong zircon grains by an ion microprobe are listed in table 1. The average values of five grains are as follows: (7.33 ± 0.28) ‰ (1SD) for DSC1; (7.88 ± 0.11) ‰ (1 SD) for DSC2; (8.06 ± 0.26) ‰ (1 SD) for DSC3; (7.51 ± 0.65) ‰ (1 SD) for DSC4 and (7.25 ± 0.40) ‰ (1 SD) for DSC5. Except DSC4, SD values of other four zircons are less than that of standard, indicating the homogeneities of oxygen isotopic compositions within the interior of these grains. Although the SD value (0.65‰) of five points is slightly higher than that of standard, DSC4 is reasonably believed to be homogeneous when its typical magmatic internal structure shown by CL image and possible instrumental instability are considered. δ 18OTRUE values of all 22 points are 6.81‰8.57‰, the average is (7.66 ± 0.46)‰ (1 SD, 1 σ = 0.10). SD is less than that of standard, indicating that oxygen isotopic ratios of all five grains are homogeneous; σ is close to the error of laser fluorination method (0.1‰0.2‰), indicating that the average value 7.66‰ can represent the δ 18O of Daoshichong zircons accurately. 3 Discussions () Do zircons preserve the primary δ 18O? Before discussing the implications of δ 18O values, we should confirm that zircons preserve their primary magmatic oxygen isotopic signatures. The results could not represent the primary magmatic δ 18O values if the following things happened[12ü16]: (1) zircon grains contain inherited cores with different δ 18O values and the cores were involved in the analysis; (2) zircons with high uranium contents have suffered radiation damage and metamictization, then forming microfractures through which zircons and the exotic fluids exchanged oxygen isotopes; (3) oxygen isotopic diffusion after the formation of zircons occurred; and (4) metamorphism after the formation of zircons occurred. CL images demonstrate that the Daoshichong zircons have not any inherited core, thus ruling out the first possibility. Chen et al.[4] reported the U-Pb results of ten points of Daoshichong zircons, all points have U contents less than 430 µg/g (eight points less than 200 µg/g) and display concordant 206Pb/238U-207Pb/235U ages; CL images do not show microfractures, so we do not think that these zircons have undergone metamictization and exclude the second possibility. All grains show homogeneous δ 18O values, implying that the oxygen isotopic diffusion is negligible for Daoshichong zircons. Zircons can preserve the primary magmatic δ 18O values even undergoing granu-
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NOTES lite-facies metamorphism[12,13,15], Daoshichong zircons are typical magmatic and do not show any sign of metamorphism, so the fourth possibility did not occur. From the above discussions, we believe that Daoshichong zircons preserve the primary magmatic δ 18O values. () Implications for crust-mantle interaction. δ 18O values of zircons from Daoshichong pyroxenite is 7.66‰, oxygen isotopic fractionation between zircon and mafic magma (δ 18Ozircon − δ 18Omagma) under magmatic temperature is about − 0.5‰[11,12], so δ 18O value of the parent magma of Daoshichong pyroxenite is ~ 8.2‰. From the data set of mantle peridotites and basalts, δ 18O value of the normal mantle is commonly accepted to be (5.7 ± 0.5)‰[17ü21]. Because mafic magma is typically mantlederived, high δ 18O value of the parent magma of Daoshichong pyroxenite should arise from the interaction with high δ 18O crustal materials. There are two ways for such interaction: source mixing and crustal contamination. Source mixing means the addition of crustal materials into the mantle source; crustal contamination means the assimilation of crustal wall rock during the emplacement of parent magma. Crustal contamination was ruled out by other geochemical studies[5ü7], so the high δ 18O feature of Daoshichong zircons should be due to source mixing. At present, there are two models about component, process and time of crust-mantle interaction inferred from studying Dabieshan mafic-ultramafic intrusions: (1) The part of subducted Yangtze lower crust interacted with asthenosphere and formed metasomatized mantle, this process lasted from late Triassic (~210 Ma, soon after peak metamorphism) to early Cretaceous (~130 Ma), at early Cretaceous mafic magma was formed by partial melting of such metasomatized mantle in response to a thermal pulse[6]. (2) At late Jurassic to early Cretaceous lithosphere was detached and induced the partial melting of upwelling asthenosphere and old enriched lithosphere, then such melt interacted with lower crust by underplating and got geochemical signatures of enriched lithosphere and lower crust1). The general agreement of the two models is that the crust component is lower crust other than upper crust. But four aspects are still in debate: (1) Is the time of crust-mantle interaction from late Triassic or at late Jurassic to early Cretaceous? (2) Is the lower crust component intermediate or mafic? (3) Is the mantle component peridotite or peridotite-derived melt? (4) Is the mantle component asthenosphere or asthenosphere + lithosphere? Oxygen isotopic compositions of lithosphere, asthenosphere and mantle melt are the same, oxygen isotope data itself cannot resolve the third and fourth aspects. But oxygen isotope data can provide some important constraints to the former two aspects, the detailed discussions are shown below.
High δ 18O value of Daoshichong zircons indicates that the time interval between crust-mantle interaction and the formation of parent magma of Daoshichong pyroxenite should be small, otherwise the high δ 18O signature would be erased due to the long-time interaction with ambient mantle. Exchange rate between mantle and lower crust is influenced by many factors such as chemical composition, mineral proportion, porosity and fluid content and is difficult to define accurately, but several hundreds to several thousands m/Ma is the reasonable estimation[22]. The scale of Dabieshan mafic-ultramafic intrusions is commonly small, ranging from 0.2 × 0.5 km2 to 2 × km2[23]. Daoshichong intrusion is about 0.3 × 0.3 km2, so the time interval between crust-mantle interaction and the formation of parent magma of pyroxenite is at most several million years. In other words, the time of crustmantle interaction is at late Jurassic to early Cretaceous other than from late Triassic. Representative rock of the lower crust is granulite. Unfortunately, no granulite is found in Daoshichong and other mafic-ultramafic intrusion localities (Shacun, Zhujiapu, etc.). Zheng et al.[24] analyzed seven granulites from Yanzihe, Huilanshan, Muzidian and Huangtuling and gave δ 18O values of whole rocks from 3.5‰ to 7.8‰. From these data we cannot find suitable crustal component contributing to the parent magma of Daoshichong pyroxenite, possibly due to the scarce of analysis of Dabieshan granulites. δ 18O values of global granulites range from 4.8‰ to 13.5‰[25ü28], the highest of mafic group is 12.5‰ and that of intermediate group is 13.5‰. We use 5.7‰ as mantle component and the highest δ 18O as crustal component, then the calculated proportion of crustal component is 37% for mafic lower crust and 33% for intermediate lower crust respectively in order to form the parent magma of Daoshichong pyroxenite with 8.2‰ δ 18O. The major element contents of the mantle-derived mafic magma should be shifted to more intermediate if involving such amount of intermediate crust, for example, the SiO2 content would have to be significantly increased, but this is not the case for Dabieshan mafic-ultramafic rocks[5ü7]. Therefore, we conclude that the crustal component involved in crust-mantle interaction should be mainly mafic lower crust. Addition of mafic lower crust did not change significantly the major element compositions of mafic mantle-derived magma, but imprinted the crustal signature of trace elements and isotopes on them. The studies of granulite xenoliths from Cenozoic basalts of Nushan and Hannuoba have demonstrated that the lowermost crust of eastern China is mafic[29,30]. So our conclusion is in accord with the geological model, because upwelling mantle materials interact with the lowermost crust first. Geochronology study of granulite
1) Huang, F., Li, S. G., Zhou, H. Y. et al., U-Pb isotope geochemistry of postcollisional mafic-ultramafic rocks: crust-mantle interaction and LOMU component, Sci. China, Ser. D, 2002 (in press).
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NOTES xenoliths from Hannuoba also suggested that underplating mafic magma had influenced lower crust of eastern China at about late Jurassic to early Cretaceous[31]. Our calculations from oxygen isotopic data indicate that the parent magma of Daoshichong pyroxenite could contain a large proportion (>37%) of lower crust. 4 Conclusions Oxygen isotopic compositions of zircons from pyroxenite of Daoshichong are homogeneous: δ 18O = (7.66 ± 0.46)‰ (1 SD, 1σ = 0.10, n = 22). High δ 18O values indicate that mantle-derived parent magma of Daoshichong pyroxenite have undergone interaction with crustal materials. Combining with the other geochemical constraints, the way of crust-mantle interaction is suggested to be source mixing other than crustal contamination. The time interval between the crust-mantle interaction and the formation of parent magma of Daoshichong pyroxenite is less than several million years. The crustal component involved in crust-mantle interaction is mafic lower crust, the parent magma of pyroxenite possibly contains a large proportion (>37%) of such lower crust. Acknowledgements We thank Profs. Zheng Yongfei, Chen Jiangfeng and Li Shuguang for critical reviews of the manuscript. This work was supported by the Chinese Academy of Sciences (CAS) (Grant No. KZCX2-107) and the CAS-CNRS cooperation project.
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