HR: 0800h
AN: U41A-0705    [Abstracts]
TI: A Lower Mantle Origin for Megacryst Suite Pyroxene-Ilmenite Xenoliths in Kimberlites: High-Pressure Experimental Constraints and Geodynamic Significance
AU: * Collerson, K D
EM: k.collerson@acquirelab.uq.edu.au
AF: ACQUIRE, Univ. Queensland, Brisbane, Q 4072 Australia
AU: Terasaki, H
AF: Faculty of Science, IMPEG, Tohoku University, Sendai, 980-8578 Japan
AU: Ohtani, E
AF: Faculty of Science, IMPEG, Tohoku University, Sendai, 980-8578 Japan
AU: Suzuki, A
AF: Faculty of Science, IMPEG, Tohoku University, Sendai, 980-8578 Japan
AU: Kondo, T
AF: Faculty of Science, IMPEG, Tohoku University, Sendai, 980-8578 Japan
AB: Megacryst suite xenoliths (MSX's) in kimberlites, alnoites and alkali basalts are an important and poorly understood association. MSX's comprise medium- to coarse-grained monomineralic, or rare, multi-grain aggregates of low Cr, high Ti-Na pyrope, Mg ilmenite, sub-calcic pyroxene, Fo85$\pm$3 olivine, orthopyroxene and zircon. Some MSX's exceed 30 cm in diameter. They are interpreted to form by fractional crystallization from their host magma, near the base of the lithosphere [1-2]. However, majorite, and other high-pressure phases in some garnetite MSX's, indicates a mantle transition zone (TZ) origin [3]. A sub-lithospheric, deeply subducted slab source is also supported by Hf isotopic data [4]. A common member of the MSX suite, are graphic intergrowths of pyroxene and Mg-ilmenite interpreted to reflect cotectic, or non-equilibrium crystallization [5-6] from the kimberlite magma. However, Pb isotopic data for Monastery [7], and Namibian [8] megacrysts shows that MSX's and their host magmas are unrelated. Thus the mineralogy of the Ti-rich px-ilm MSX's needs to be determined at TZ and higher P. We have conducted multi-anvil (MA) and diamond anvil (DA) experiments on natural px-ilm xenoliths from Monastery and Malaita with different TiO$_{2}$ contents (17% and 12%), in an attempt to synthesize the pre-exsolution phase. MA experiments were carried out on both starting compositions at 18 and 21 GPa, at 1800$\deg$C and 2100$\deg$C. None of the experiments yielded a single phase. Phases identified (EPMA, Raman & XRD) include: majorite, Si-rich ilmenite and Ca-Si-Ti Pv. At 21 GPa and 2100$\deg$C wadsleyite formed part of the assemblage, and melt was locally developed. Majorite is the most abundant phase in all experiments. Maximum majorite TiO$_{2}$ occurs at 18 GPa (i.e., 5.4% - Malaita and 6.2% - Monastery). In the lower Ti Malaita composition, at 25 GPa and 1800$\deg$C, the assemblage is dominated by almost equal amounts of majorite (TiO$_{2}$ 1.3% to 2.1%) and Ca-Si-Ti Pv, with a small amount of titaniferous Na$_{2}$O-bearing magnesiowustite. Conclusions from MA experiments are: (1) a single homogeneous Ti-rich silicate phase is not stable in the TZ; (2) closest approach to a homogeneous phase occurs in the lower TiO$_{2}$ Malaita composition at 25 GPa; (3) maximum solubility of TiO$_{2}$ in majorite is 5-6% at 18 GPa; (4) at 2100$\deg$C and 21 GPa, melting occurs, and majorite plus Si-ilmenite reacts to form Ti-Ca-Si Pv and wadsleyite. Furthermore, in a DA experiment at 30 GPa and 1800$\deg$C a multi-phase, Ti-Ca-Si Pv dominated assemblage still prevails. Pyroxene-Mg ilmenite MSX's are therefore interpreted to form from Ti-rich protoliths, possibly ilmenite-rich cumulates, in subducted slabs of oceanic lithosphere. The homogeneous phase from which the pyroxene-Mg-ilmenite intergrowths were exsolved, is thus interpreted to be Ti-Ca-Si Pv, not Ti-rich majorite [cf., 9-10]. Results confirm large-scale transport of oceanic lithosphere into the upper lower mantle during subduction. [1] Nixon, et al., (1963) {\it Am. Mineral.} 48, 1090. [2] Jones et al., (1987) In: {\it Mantle Xenoliths} 711. [3] Collerson et al., (2000) {\it Science}, 288, 1215. [4] Nowell et al., (2004) {\it J Petrol.} 45, 1583. [5] Wyatt (1977) {\it CMP} 61, 1-9. [6] Mitchell (2004) {\it Lithos} (in press) [7] Collerson et al., (2001) {\it EOS} 82. [8] Davies et al., (2001) {\it J Petrol.} 42, 159. [9] Ringwood & Lovering (1970) {\it EPSL,} 7, 371. [10] Zhang et al., (2003) {\it EPSL,} 216, 591.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8125 Evolution of the Earth
DE: 3630 Experimental mineralogy and petrology
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Union [U]
MN: 2004 AGU Fall Meeting