HR: 1340h
AN: MR13A-0078    [Abstracts]
TI: Melting Interval of Natural Carbonated Lherzolite at 3 GPa and Genesis of Alkalic OIBs
AU: Smith, N D
EM: smit3394@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: * Dasgupta, R
EM: dasg0007@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Hirschmann, M M
EM: hirsc022@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AB: Alkalic lavas are characteristic of many intraplate magmatic provinces, both on continents and on many oceanic islands. Such lavas are generally thought to originate from small degrees of partial melting of fertile peridotite±ńCO₂¹. However, natural alkalic lavas have lower Al₂O₃, CaO and higher FeO* and TiO₂ than liquids likely to be descended from partial melts generated in previous experiments on peridotite+CO₂². We performed experiments at 3 GPa to investigate silicate melt compositions generated near the transition from near-solidus carbonate-rich to high temperature silicate-rich melts in the melting interval of carbonated lherzolite. Experiments were conducted in Pt/C capsules with a fertile carbonated lherzolite (MixKLB-1+2.5 wt.% CO₂) at 3 GPa from 1075 to 1600 °C. Below the solidus, CO₂ is stored in dolomite_ss, which disappears between 1075-1105 °C, generating ~6 wt.% carbonate-rich melt. Carbonatitic melt coexists with the four-phase lherzolite residue up to 1325 °C. Carbonated silicate melt is observed beginning 1350 °C. Cpx, garnet and opx disappear at 1350-1375 °C, 1425-1450 °C and 1550-1575 °C respectively. Melt compositions analysed with a defocused microprobe beam increase sharply in SiO₂ (~7 to 21 wt.%) and Al₂O₃ (~3 to 7 wt.%) between 1325 and 1350 °C, reflecting a transition from carbonatite to melilitite. From 1350 to 1600 °C (9 to 51 % melt), melts (on a CO₂-free basis) increase in SiO₂ (~29 to 46 wt.%), MgO (~20 to 29 wt.%), and decrease in TiO₂ (1.6 to 0.5 wt.%), CaO (~26 to 7 wt.%), and Na₂O (~2 to 0.6 wt.%). Al₂O₃ increases from ~9 to 12 wt.% from 1350 to 1450 °C and then decreases to ~7 wt.% at 1600 °C and FeO* varies between 12-10 wt.%. Experimental melt fraction versus composition trends suggest that small-degree partial melts of carbonated peridotite could evolve to match the TiO₂, FeO*, and Al₂O₃ of natural alkalic lavas, but these low-degree melts will have much higher CaO and CaO/Al₂O₃ ratios than those found in alkalic OIB lavas. Thus, although alkalic OIB magmatism may derive in part from partial melts of carbonated lherzolite similar to KLB-1, more complex scenarios are likely required to account for all of their compositional attributes. ¹Hémond, C. et al. 1994, Chem Geol 115, 7-45. ²Hirose, K. 1997, GRL 24, 2837-2840.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005