HR: 09:30h
AN: MR31D-07 INVITED    [Abstracts]
TI: Primary Ca-rich Carbonate Melts in the Transition Zone
AU: * Walter, M
EM: M.J.Walter@bristol.ac.uk
AF: University of Bristol, Queen's Rd, Bristol, BS8 1RJ, United Kingdom
AU: Bulanova, G
EM: galina_bulanova@hotmail.com
AF: University of Bristol, Queen's Rd, Bristol, BS8 1RJ, United Kingdom
AU: Armstrong, L
EM: L.Armstrong@bristol.ac.uk
AF: University of Bristol, Queen's Rd, Bristol, BS8 1RJ, United Kingdom
AU: Keshav, S
EM: Keshav@Uni-Bayreuth.DE
AF: Bayreuth Geoinstitut, Universitätsstraße 30, Bayreuth, D-95447, Germany
AU: Blundy, J
EM: Jon.Blundy@bristol.ac.uk
AF: University of Bristol, Queen's Rd, Bristol, BS8 1RJ, United Kingdom
AU: Hinton, R
EM: Richard.Hinton@ed.ac.uk
AF: University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, United Kingdom
AU: Lennie, A
EM: a.lennie@dl.ac.uk
AF: SRS, CCLRC Daresbury Laboratory, Warrington, WA4 4AD, United Kingdom
AB: We present new experimental and geochemical constraints on the origin of composite Ca(Ti,Si)O₃ and Ca- rich majorite garnet diamond inclusions from Juina kimberlite, Brazil. The evidence reveals that the inclusions did not form as subsolidus minerals, but instead crystallized directly from calcium-rich carbonate melts during crystallization of the host diamond. Subsolidus Phase Relations. We interpret composite CaSiO₃ + CaTiO₃ inclusions as exsolution products from a single-phase perovskite (Pv) in the transition zone¹. The MgSiO₃ component in the bulk CaTiSi-Pv is exceedingly low (<0.2 mol%), unlike experimental observations of Ca-Pv coexisting with either majorite-garnet or Mg-Pv (3-7 mol%) in peridotite or eclogite^2,3. Indeed, our new subsolidus phase relations show MgSiO₃ increasing substantially in Ca-Pv with increasing CaTiO₃- content (20-50 GPa, 2000 K). The Ca-content of the majoritic inclusions are exceptionally high (10-15 wt% CaO), also unlike in peridotite or eclogite (< 7%). Unless bizarre mantle lithologies are invoked, subsolidus paragenesis for these inclusions is effectively precluded. Melting Phase Relations. We present new experiments showing that at transition zone depths, primary melts from carbonated eclogite crystallize CaTi-rich perovskites with composition very like the inclusions, and with exceptionally low MgSiO₃ (<0.2 mol%). Liquidus majorite is very calcic (10-20 wt% CaO), spanning the range of garnet inclusions. This evidence indicates that the mineral inclusions crystallized from Ca-rich carbonate melts⁴. Trace Element Modeling. The trace element chemistry of the inclusions as determined using SIMS techniques support a model in which the inclusions equilibrated with small-degree melts. Overall the inclusions are massively enriched in a range of incompatible trace elements, (e.g. 10³ to 10⁴ x CI in perovskite). Based on experimental mineral-melt partitioning data, calculated coexisting melts have features inherited from subducted oceanic crustal materials. We suggest a process where small-degree carbonate melts are derived from subducted oceanic crust that has foundered in the transition zone. Primary Ca-rich melts evolve through reduction of the carbonate component^5,6 and crystallize diamond, which traps coexisting liquidus phases during growth. The carbonate melts are loaded with incompatible trace elements with crustally derived signatures, and may be agents of ancient, pervasive metasomatism in the transition zone and upper mantle. 1. Brenker et al., EPSL 236, 579-587 (2005). 2. Hirose et al, Nature 397, 53-56 (1999). 3. Irifune & Ringwood, EPSL 117, 101-110 (1993). Keshav et al,11th EMPG Abstracts, 36 (2006). 5. Gunn & Luth, Am. Min. 91, 1110- 1116 (2006). 6. Safonov et al, EPSL 253, 112-128 (2007).
DE: 1038 Mantle processes (3621)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting