HR: 0800h
AN: V31C-1448    [Abstracts]
TI: MORB Mantle Hosts the Missing Eu in the Continental Crust
AU: * Niu, Y
EM: yniu@mail.uh.edu
AF: Department of Geosciences, University of Houston, Houston, TX 77204 United States
AU: O'Hara, M J
EM: mio@aber.ac.uk
AF: Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Aberystwyth, SY23 3DB United Kingdom
AB: The continental crust (CC) is an important geochemical reservoir. The origin and evolution of CC remains debated [1-13]. Estimation of bulk CC composition is model dependent [1-8,12]. A reliable upper CC composition has been obtained from shales and other fine-grained sedimentary rocks [1,2,15,16] due to the mixing/homogenization effects of sedimentary processes [14-16]. The upper CC has a negative Eu anomaly (NEuA or Eu/Eu* $<$ 1), 0.65, which points to a missing Eu storage. The upper CC is $<$10km thick and cannot represent the bulk CC [1-3], which has a mean thickness of $\sim$36km. To determine the composition of deep CC is required, but the task is hard [1-3]. The approach of combining geochemical data on deep crustal rocks (xenoliths and locally exposed amphibolites and granulites) with seismic and heat flow data to estimate the average deep CC composition is creative [1-4,12]. As some granulites show Eu/Eu* $>$ 1, the missing Eu in the upper CC may be in the deep CC. Indeed, the NEuA in the upper CC is largely reduced in model bulk CC compositions: 0.829 [4], 0.962 [1], 0.974 [3]. By assuming mantle derived igneous rocks display no Eu anomaly [1,12], the upper CC NEuA would come from intracrustal differentiation. For example, the granitic upper CC resulted from anatexis of mafic rocks in the deep CC with Plag as a residual phase (holding Eu) in the granulite residues [1-4,12]. This interpretation is sensible, but the protoliths of most of these granulites are found to be underplated mantle melts [1,17-19]. The NEuA ($\sim$0.8) and the more felsic nature of bulk CC composition in China encouraged the authors [12] to suggest that the more mafic lower CC rocks were tectonically removed. We show that high quality data on 306 fresh MORB glass samples (2-10 wt% MgO) from the Pacific and Atlantic [20-25] exhibit varying Eu/Eu* (0.68-1.18) and significant correlations: R[MgO-Eu/Eu*] = 0.876, R[MgO-Sr/Sr*] = 0.809 and R[Eu/Eu*-Sr/Sr*] = 0.875. 148 samples show Eu/Eu* $>$ 1. For Eu/Eu* $>$ 1, MgO $>$ 7.6 wt% or T[liquidus] $>$1185$\pm$10$\deg$C. For MORB, Plag begins to crystallize at $\sim$1200$\pm$10$\deg$C. These observations demonstrate that primitive MORB melts all have POSITIVE Eu and Sr anomalies. We interpret these anomalies as inherited from MORB source. Eu and Sr behave similarly because of the same charge [2+] and ionic radius (~1.31$\AA$ for CN=6). The larger radius may make Eu[2+] more incompatible than the smaller Sm and Gd, but the divalent Eu[2+] goes mostly with Sr into M2 site of Cpx, making it more compatible than the trivalent Sm and Gd. If the depleted MORB mantle indeed resulted from bulk CC extraction in the early Earth [9], then that event would have preferentially extracted the more incompatible trivalent REEs into CC, leaving the divalent Eu[2+] in the MORB mantle. Hence, we suggest MORB mantle host the missing Eu in CC. References: [1] Taylor, McLennan, RG, 1995; [2] Rudnick, Fountain, RG, 1995; [3] Rudnick, Nature, 1995; [4] Wedepohl, GCA, 1995; [5] Weaver, Tarney, Nature, 1984; [6] Bowring, Science, 1995; [7] McCulloch, Bennett, GCA, 1994; [8] Abbott et al., EPSL, 1997; [9] Hofmann, EPSL, 1988; [10] Condie, 282pp., 1997; [11] AlbarŠde, Tectonophys, 1998; [12] Gao et al., EPSL, 1998; [13] Niu et al., JPet, 2003; [14] Goldschmidt, 148pp, 1938; [15] Taylor, McLennan, 312 pp, 1985; [16] Goldstein, Jacobsen, EPSL, 1988; [17] Rudnick, Taylor, JGR, 1987; [18] Rudnick, GCA, 1992; [19] Rudnick, Nature, 1990; [20] Niu, H‚kinian, EPSL, 1997; [21] Niu, Batiza, EPSL, 1997; [22] Castillo et al., EPSL, 1998; [23] Niu et al., JGR, 1999; [24] Regelous et al., EPSL, 1999; [25] Niu et al., EPSL, 2001.
DE: 7220 Oceanic crust
DE: 7205 Continental crust (1242)
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting