Geochimica et Cosmochimica Acta 70 (2006) A379–A439 www.elsevier.com/locate/gca
Goldschmidt Conference Abstracts 2006 Behavior of major and trace elements in an extreme weathering profile on basalt in Hainan Island, South China J.L. MA, G.J. WEI, Y.G. XU, W.G. LONG Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China (
[email protected]; gjwei @gig.ac.cn;
[email protected];
[email protected]) Large amounts of laterite profiles have been developed on the Neogene basalts in Hainan Island, South China due to the high temperature and precipitation in this region. Major and trace elements of a lateritic profile in northern Hainan were studied to investigate the element mobility and re-distribution during extreme weathering. The profile is basically devided into two sections: The top 2–3 m comprises of homogeneous fine grain laterite, and the bottom 2–3 m is composed of yellowish fine laterite, containing un-weathered core stones. Ti, Zr, Hf, Nb, Ta, and Fe are conservative in this profile, and the ratios between each other remain those in the fresh basalts. Ca, Na, Sr, K and Mg are almost entirely removed from the profile, and losses of Rb and Ba are over 50 percent, whereas Cs is significantly enriched in the top section. Cs, K, Rb, Na, and Sr show an upward increasing trend in the top section, suggesting combination of extraneuos sources. Both Mn and P are removed significantly in the top section and enriched markedly in the bottom section, which may be the result of deposition of Mn oxides/hydroxides and secondary phosphate minerals. The distribution of REEs resembles Mn and P. Marked positive Ce and Gd anomalies occur in this profile, and the Ce enrichment is closely related to Mn, and other REEs seem largely associated secondary phosphate minerals and Mn oxides/hydroxides. Among the transition metals, Sc and V exhibit as conservative, and the re-distribution of Cr, Cu and Zn are closely related to P. However, Mn oxides/hydroxides prevent the enrichment of Cr because the oxic phase Cr(IV) is soluable and easily to be removed. Co is closely associated with Mn, and Ni seems not relate to both Mn and P. Th and U are basically conservative in this profile, but show increasing trend in the top section, which seem to combine extra materials in the top section after extreme weathering. doi:10.1016/j.gca.2006.06.766
Secondary U-phosphates—possible links to Quaternary pluvial periods in SE Australia R. MAAS1, S. MILLS2, W. BIRCH2, J. HELLSTROM1 1
School of Earth Sciences, University of Melbourne, Australia (
[email protected]) 2 Museum of Victoria, Melbourne, Australia (bbirch@ museum.vic.gov.au) Weathering-related minerals amenable to radiometric dating may be used to establish chronologies of weathering. These relate to climate and the history of water in the landscape (Vasconcelos, 1999; Osmond and Dabois, 2004). Here we present U–Th ages for secondary U-phosphates from the weathering zone of a 370 Ma granite (southern Murray Basin, SE Australia), obtained using MC-ICPMS (Hellstrom, 2003). Data for 18 samples indicate apparent ages from 115 to >550 ka, with initial d234U from 8 to 118 &. The significance of these ages depends on mineral growth mechanisms and crystal preservation. For example, it is unclear at present if the micro-crystal clusters represent discrete growth phases, mixtures of growth phases, or a growth phase that is long relative to the absolute age. Possible U loss/gain during repeated groundwater residence also needs to be evaluated. Present work thus focusses on age reproducibility and SEM imaging of microcrystallites, to detect growth discontinuities or resorption surfaces. Pending the results of this work, comparison of the available data with the global climate record (Karner et al., 2002) suggests a weak correlation of our apparent ages with periods of wet paleoclimate (interglacials), as expected if the growth of these weathering products relates to enhanced groundwater activity during humid (pluvial) periods. However, almost 40% of the ages nominally correlate with glacial periods and thus drier climate, weakening evidence for a link between pluvials and U-phosphate growth. More data are needed to establish clearly what controls secondary phosphate growth in this setting. For example, enhanced groundwater activity may be related to short-lived wet periods (interstadials), or to long-range recharge (old groundwater unrelated to ambient climate). Once this is clear, U–Th and U–Pb dating of secondary U phosphates may become a useful tool for improving existing (imprecise) chronologies of Quaternary pluvials in Australia.
References Hellstrom, J., 2003. JAAS 18, 1346–1352. Karner, D.B. et al., 2002. Paleoceanogr. 17, 1–16. Osmond, J.K., Dabois, A., 2004. Quaternary Res. 61, 85–94. Vasconcelos, P.M., 1999. Ann. Rev. Earth Planet Sci. 27, 183–229. doi:10.1016/j.gca.2006.06.767
