Burton, K. W.

Fitton, J. G.

Halliday, A. N.

Liang, Y.-H.

Siebert, Christopher

English

OceanRep

Goldschmidt2013 Conference Abstracts www.minersoc.org DOI:10.1180/minmag.2013.077.5.12  1607 Molybdenum isotope fractionation in the mantle Y.-H. LIANG1*, C. SIEBERT2, J. G. FITTON3, K. W. BURTON4, AND A. N. HALLIDAY1 1 Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, UK 2 GEOMAR, Helmholtz Center for Ocean Research, Wischhofstrasse 1-3, 24146 Kiel, Germany 3 School of GeoSciences, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK 4 Deparement of Earth Sciences, Durham University,Science Labs, Durham, DH1 3LE, UK  (*correspondence: Yu-Hsuan.Liang@earth.ox.ac.uk)  Molybdenum is a refractory and moderately siderophile element that is highly redox-sensitive and has mainly been used to study changes in marine sedimentary environments [1,2]. So far little is known about mass dependent isotope fractionation of Mo at high temperatures in the Earth, in particular during mantle processes. We have analyzed 42 mafic (mainly MORB and OIB) and 7 ultramafic rocks from diverse locations, using a double-spike technique and MC-ICPMS. The ! 98/95Mo values exhibit a significant range from +0.53 ± 0.21 to -0.56 ± 0.01‰ relative to NIST SRM 3134. The compositions of MORB (+0.03 ± 0.07‰, 2s.d.) and ultramafic rocks (+0.38 ± 0.15‰, 2s.d.) are each relatively uniform and distinct from each other, providing clear evidence of fractionation associated with partial melting. In contrast OIBs display significant variability within a single locality. The most extreme values measured are for nephelinites from the Cameroon Line and Trinidade, which also have anomalously high Ce/Pb [3] and low Mo/Pr relative to normal oceanic basalts [4]. The observed relationships between ! 98Mo and Ce/Pb, U/Pb, and Mo/Pr provide evidence that sulfide plays a critical role, not just in in fractionating chalcophile elements [5], but also in preferentially incorporating heavier Mo isotopes. Residual immiscible sulfide liquids lead to the eruption of magmas with relative light Mo isotopes and low Mo/Pr relative to the source composition. Therefore, the Mo isotopic compositon of the bulk silicate Earth and mantle will be best represented by values for peridotites.  [1] Colodner, D., et al.. (1995) Earth Planet. Sci. Lett. 131, 1-15. [2] Morford, J.L., and Emerson, S. (1999) Geochim. Cosmochim. Acta 63, 1735-1750. [3] Halliday, A.N., et al.. (1995) Earth Planet. Sci. Lett. 133, 379-395. [4] Newsom, H.E., et al.. (1986) Earth Planet. Sci. Lett. 80, 299-313.[5] Yi, W., et al.. (2000) J. Geophys. Res.-Solid Earth 105, 18927-18948. Tectonic control on the distribution of Paleogene deep-lake syn-rift deposits in Qikou sag, Bohaiwan Basin, Eastern China YUANTAO LIAO1,2,  HUA WANG1 AND WENLI XUI 1Faculty of Earth Resources, China University of Geosciences, Wuhan, China, lyt706@gmail.com 2Key Labortary of Tectonics and Petroleum Resources, Wuhan, China  The tectonic control on subaqueous sediment routes and stacking patterns is not well understood although the resulting deposits may form potentially important hydrocarbon reservoirs. In this study, an area of lacustrine syn-rift succession in the Qikou sag is examined in detail to unravel the interplay between fault geometries evolution  and sediment transport paths and infilling patters. Qikou sag is a tpyical superimposed rift basin at the center of Bohaiwan Basin, Eastern China. Structurally, it is characterized by a linked system deep NEE-SWW striking half-grabens. This study focuses on the eastern part of the Qikou sag and it is bounded to the north by the overlapping Gangdong, Gangxi and Binhai normal faults. Syn-rift succession of the Qikou sag comprises Oligence Shahejie Formation (EsF), Dongying Formation (EdF) and is mostly dark-grey mudstone interbedded with fine to coarse-grained sandstone deposited by large-scale turbidity currents in deep-lake. Using strata pattern, fault activity hsitory, seismic attribute and heavy mineral analysis based on three-dimensional seismic and well logging data, our study demonstrate that the growth and propagation of main border normal fault segments associated with paleogeomorphology changes (particularly subaqueous relay ramps) have a marked influence on dispersal and localization of the turbidity currents during the syn-rifting phase. The main sediment supply is interpreted to be sourced from the basin marginal Cangxian uplift and the significant coarse-grained materials were transported to the deep-lake over long distance. In subqueous settings, however, no clear evidence of channel incision on the relay ramp is obseved, and turbidity currents are likely to by-pass the relay ramp and flow down the inner fault slope directly into the basin. Therefore, Locating subqueous syn-rift sediment and relating them with evolutional relay ramps  need further investigation. Acknowledgements This work is supported by the National Natural Science Foundation of China (NSFC) No. 41102069 and Special Fund for Basic Scientific Research of Central Colleages, China University of Geosciences (Wuhan) No. CUGL110248. 

Molybdenum isotope fractionation in the mantle

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