Barling, J.

Halliday, A.

Liang, Y.-H.

Savage, P.

Siebert, Christopher

Yang, J.

English

OceanRep

Goldschmidt2013 Conference Abstracts www.minersoc.org DOI:10.1180/minmag.2013.077.5.25  2548 Molybdenum isotopes during magmatic differentiation J. YANG1, C. SIEBERT2, J. BARLING1, P. SAVAGE3,  Y.-H. LIANG1 AND A. N. HALLIDAY1 1 Department of Earth Sciences, University of Oxford, South Parks Road, OX1 3AN, UK (jiey@earth.ox.ac.uk) 2 GEOMAR, Helmholtz-Zentrum für Ozeanforschung, Wischhofstrasse 1-3, 24148 Kiel, Germany 3 Department of Earth and Planetary Sciences, Washington University in St. Louis  This study presents the first evidence for the behaviour of molybdenum isotopes during magmatic differentiation. Molybdenum isotopic compositions, as well as rare earth elements (REE), have been determined for a sequence of lavas from Hekla volcano, Iceland, covering a compositional range from basalt to rhyolite (from 46 to 72 wt.% SiO2), sequentially developed by magmatic differentiation from a cogenetic source. All samples are characterised by identical Mo isotopic composition with "98Mo approximating zero (with an average "98/95Mo of -0.02±0.1‰ (2s.d.) relative to an Alfa Aesar standard solution (lot 011895D) [1]). This study therefore reveals that resolvable Mo isotope fractionation is unlikely to occur during such magmatic differentiation. This is conistent with the fact that Mo is consistently incompatible in this suite of lavas; the lack of fractionating phases therefore preserves the original basaltic parent magma composition.   Previous work [2] suggests that Pr and Mo are equally incompatible in basaltic systems. However, in this study it can be seen that Mo becomes more incompatible than Pr in silicic systems.  Despite the fact that these are low 18O magmas, hydrothermal processes and assimilation have had no effect on the Mo isotopic compositions.    Two additional volcanic samples from nearby unrelated volcanos were analysed and are indistinguishable from the Hekla data. This is consistent with a previous study which also indicates a similarity of Mo isotopic signature ($98/95Mo~0.03±0.12‰) for riverine bedrocks from the vicinity of Vatnajökull glacier on Iceland [3]. Therefore, the Iceland crust appears to have a very homogeneous Mo isotopic signature ($98Mo~0±0.1‰). The insensitivity of the Mo isotope system to igneous differentiation means that it is especially well suited for studies of assimilation and water - rock interaction in magmatic systems.    [1] Greber et al. (2012) Geostand. Geoanalytical Res. 36, 291–300. [2] Newsom et al. (1986) Earth Planet. Sci. Lett. 80, 299–313. [3] Pearce et al. (2010) Earth Planet. Sci. Lett. 295, 104–114. Thallium isotope geochemistry of early Cambrian black shales and Ni-Mo sulfide ores in South China YANG JING-HONG1 , JIANG SHAO-YONG1, 2 ,  REHKAMPER M3, XUE XC3 , PI DH1,2 AND ZHU B1,4 1State Key Laboratory for Mineral Deposits, Department of Earth Sciences, Nanjing University, Nanjing 210093, China 2State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, Hubei, 430074, China 3Department of Earth Sciences & Engineering, Imperial College London, UK 4Institute of Isotope Hydrology, School of Earth Sciences and Engineering, Hohai University, Nanjing 210098, China  Thallium has two stable isotopes: 203Tl (29.5%) and 205Tl (70.5%). The present-day oceanic residence time of Tl is estimated to be ~20 kyr, significantly longer than the mixing time of the oceans (about 1 kyr). Natural Tl isotopic composition shows a variation of about 35 #205Tl units in terrestrial samples. In this study, we analyzed Tl isotopes for the early Cambrian Niutitang Formation black shales and the Ni-Mo sulfide ores from Zunyi, Guizhou province, south China. The Ni-Mo sulfide ore shows the highest Tl concentration (40.5 ppm) and the lowerest and negative #205Tl (-2.06), similar to seafloor hydrothermal fluid value (-1.9), but different from present-day seawater (-6.0). The black shales show smaller negative #205Tl near 0 (0 to -0.17). But the V-rich black shale has slightly higher Tl concentration (7.3 ppm) and lower #205Tl value of -0.6. The chert shows similar Tl concentration with the black shale, but significantly higher and positive #205Tl (-0.3 to 1.13). Good correlations are observed between #205Tl and Mo/TOC, Ni/TOC, and U/TOC. In summary, the new data of Tl isotopes in black shales are useful tools: 1) To distinguish between oxic and anoxic marine deposits; 2) To indentify hydrothermal input of Tl (and other metals) for the early Cambrian black shales and polymetallic Ni-Mo sulfide bed in south China. This study is supported by NSFC grant 41273009, 41230102.       

Molybdenum Isotopes during Magmatic Differentiation

Abstract

Similar works

Full text

Available Versions

OceanRep