HR: 09:45h
AN: DI41B-08    [Abstracts]
TI: Ca(Ti,Si)O3 Diamond Inclusions Crystallized From Carbonate Melts in the Transition Zone: Experimental Constraints
AU: * Armstrong, L S
EM: glxla@bristol.ac.uk
AF: U. Bristol, Dept. of Earth Sciences, Wills Memorial Building, Queens Road, Bristol, BS82SY, United Kingdom
AU: * Armstrong, L S
EM: glxla@bristol.ac.uk
AF: Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, D-95440, Germany
AU: Walter, M J
AF: U. Bristol, Dept. of Earth Sciences, Wills Memorial Building, Queens Road, Bristol, BS82SY, United Kingdom
AU: Keshav, S
AF: Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, D-95440, Germany
AU: Bulanova, G
AF: U. Bristol, Dept. of Earth Sciences, Wills Memorial Building, Queens Road, Bristol, BS82SY, United Kingdom
AU: Pickles, J
AF: U. Bristol, Dept. of Earth Sciences, Wills Memorial Building, Queens Road, Bristol, BS82SY, United Kingdom
AU: Lord, O T
AF: U. Bristol, Dept. of Earth Sciences, Wills Memorial Building, Queens Road, Bristol, BS82SY, United Kingdom
AU: Lennie, A
AF: Daresbury Laboratory, Keckwick Lane, Warrington, WA44AD, United Kingdom
AB: Composite diamond inclusions consisting of coexisting endmember CaSiO3 and CaTiO3 are rare but occur in diamond populations from Juina, Brazil^1-2. Phase relations show that above ~9 GPa (at 1500 K) a perovskite-structured solid solution exists between these endmembers, while at lower pressures intermediate compositions produce coexisting CaTiO3-perovskite and CaSiO3 in the walstromite structure³. Inclusions with `perovskite' stoichiometry are commonly interpreted as fragments of solid mantle from the transition zone or lower mantle^4-6. Here we report on two composite diamond inclusions from Juina kimberlite, and can effectively eliminate a subsolidus origin on the basis of experimental mineral phase relations. Instead, based on new melting experiments we find that the inclusions most likely crystallized directly from Ca-rich carbonate melts. Like other workers^1-2 we interpret the composite inclusions as exsolution products of a high-pressure Ca(Ti,Si)O3 perovskite stable in the transition zone. Our bulk inclusion compositions are estimated to contain 50- 65 mol% CaTiO3, and are remarkably low in MgSiO3 component at less than 0.2 mol%.
Experiments have shown that in peridotite or eclogite lithologies, Ca-rich perovskite in equilibrium with an MgSiO3-phase (majorite or Mg-perovskite) have about 3 to 7 mol% MgSiO3^7-8. Here we report on new subsolidus laser-heated diamond anvil cell experiments at 20-50 GPa in the ternary system CaSiO3-CaTiO3-MgSiO3 that bracket the CaTi-rich limb of the solvus between Ca- and Mg-rich perovskites. All experiments were made at 2000 (±200) K for 45-75 min, and were analysed using synchrotron micro-focus X-ray diffraction. We find that the solubility of MgSiO3 in CaTi-perovskite solid solutions increases significantly with increasing CaTiO3 component. Thus, Ti-rich calcium perovskite in peridotite or eclogite lithologies should have very high, not exceptionally low, MgSiO3 component. Accordingly, a subsolidus paragenesis is unlikely for the Juina inclusions, suggesting that melt may be involved in their formation.
The MgSiO3-content of calcium perovskite in nominally volatile-free melting experiments on natural peridotite and eclogite compositions^8-9 is also much higher than that of the Juina inclusions (>3 mol%). We have made melting experiments on model carbonated peridotite (CMS-Ti- CO2) and eclogite (CMAS-Ti-CO2) compositions at 20 GPa using multi-anvil techniques at the Bayerisches Geoinstitut. We find that liquidus Ca(Ti,Si)-perovskite coexisting with Ca-carbonatite melt is remarkably depleted in MgSiO3 component, and that the Ti-rich diamond inclusion compositions are well-matched by perovskite crystallized from melt derived from a model eclogite source. We suggest that diamond and CaTi-perovskite crystallized syngenetically from a melt derived from carbonated eclogite in the transition zone.

1. Hayman, Kopylova, & Kaminsky, CMP 149, 430-445 (2005). 2. Kaminsky et al., CMP 140, 734-753 (2001). 3. Kubo, Suzuki, & Akaogi, PCM 24, 488-494 (1997). 4. Brenker et al., EPSL 236, 579-587 (2005). Harte et al., GS Spec. Pub. #6, 125-153 (1999). 6. Stachel et al, CMP 140, 16-27 (2000). 7. Hirose et al.. Nature 397, 53-56 (1999). 8. Irifune & Ringwood, EPSL 117, 101-110 (1993). 9. Corgne et al., GCA 69, 485-496 (2005).
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3630 Experimental mineralogy and petrology
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting