HR: 1340h
AN: V33A-1167    [Abstracts]
TI: Chemical and Isotopic Heterogeneities in the Deep Earth:Importance of Lower Mantle Carbonate-rich Melts
AU: * Collerson, K D
EM: k.collerson@mailbox.uq.edu.au
AF: KDCollerson, ES UQ, Brisbane, Q 4072, Australia
AU: Williams, Q
AF: QWilliams, EPS UCSC, Santa Cruz, CA 95064, United States
AU: Murphy, D
AF: DMurphy, SNRS QUT, Brisbane, Q 4000, Australia
AB: Evolution of mantle chemical heterogeneity reflects a spectrum of processes. Nature of reservoirs has been inferred from radiogenic isotope and trace element systematics of mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) [1]. Carbonatites, kimberlites and lamproites [2-4] also sample depleted and enriched reservoirs, however, their origin remains equivocal. Secular decrease in Th/U ratio in MORB mantle (DMM), homogeneity of Th/U inferred from Pb-isotopic data, and systematic variation in Nb/Th and Nb/U ratios in MORBs [5], show that recycled components in DMM are well mixed. Thus isotopically hererogeneous domains in DMM must be transient features and are unlikely to yield HIMU and EM chemistries. Explanations for HIMU and EM OIB chemistries include involvement of: (1) subcontinental lithospheric mantle; (2) subducted oceanic lithosphere; (3) subducted sediment; or (4) an enigmatic lower mantle (LM) "plume component". Elevated ³He/⁴He in OIBs and kimberlites [6] and excess ¹²⁹Xe and high ⁴⁰Ar/³⁹Ar [e.g., 7-8] and solar ²⁰Ne/²²Ne [9] in carbonatites indicate that they were derived from a primitive, isolated, and less degassed source than MORB. Primordial compositions show that this reservoir escaped atmospheric contamination by Ar, Xe, and Ne and pollution by ⁴He-rich material (from recycled ²³⁸U) during subduction. This primitive reservoir likely exists below the depth subducted slabs obviously penetrate (ca. 1700 km) e.g., [10]. That kimberlites are deeply sourced is also shown by lower mantle inclusions in diamond, e.g., [11]. Importantly, Gp. 1 and 2 kimberlites are isotopically similar to HIMU and EM-1 OIBs [4]. We interpret Gp 1 kimberlites as mixtures of HIMU and EM sources, while Gp. 2 kimberlites (close to EM-1) are interpreted as melts of a Ca perovskite-rich reservoir, possibly from slabs in the LM. We model melting of LM phases to simulate evolution of EM1 and HIMU ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ¹⁷⁶Hf/¹⁷⁷Hf, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb isotopic compositions. HIMU appears to be the residue of LM melting, and thus is indicative of a buoyant deeply-derived component. Thus, U/Pb, Th/Pb, Rb/Sr, and Sm/Nd ratios of kimberlite and carbonatite likely reflect mode of the LM mineral assemblage being melted, as well as the buoyancy of melts and residues. The CO₂-rich nature of kimberlite and carbonatite magmas is clearly important for melt transport in LM- derived plumes. Carbonate phases are stable at high pressures [e.g., 12] and available elastic and density data on carbonate and C-bearing melts show that LM CO₂-rich melts are less dense than their source, and are able to migrate, in contrast to LM silicate melts. C-enrichment thus allows extraction of deep mantle melts. Escape of these LM-derived melts provides a mechanism to explain transfer of mantle isotope heterogeneity, as well as the rare gas isotope systematics of plume magmas. [1] Zindler, A. & Hart, S., 1986. AREPS. 14: 493-571. [2] Bell, K. & Tilton, G.R., 2001. J. Pet. 42: 1927-1945. [3] Murphy, D., Collerson, K.D. & Kamber, B.S., 2002. J. Pet. 43: 981-1001. [4] Smith, C.B., 1983. Nature, 304: 51-54. [5] Kamber, B.S. & Collerson, K.D., 1999. JGR, 104: 25479-25491. [6] Tachibana, Y. et al., 2006. Geology, 34:273-276. [7] Sasada, T. et al., 1997. GCA, 61: 4219-4228. [8] Tolstikhin, I.N. et al., 2002. GCA, 66: 881-901. [9] Yokochi, R. & Marty, B. 2004. EPSL, 225, 77-88. [10] Fukao, Y. et al., 2001. Rev. Geophys., 39: 291-323. [11] Hayman, P.C. et al., 2005. CMP. 149 430-445 [12] Isshiki M. et al., 2003. Nature, 427, 60-63.
DE: 1025 Composition of the mantle
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting