HR: 1340h
AN: V33E-1499    [Abstracts]
TI: Mantle metasomatism to form kosmochlor-bearing diopside in peridotite xenoliths from North Island, New Zealand
AU: * Ikehata, K
EM: ikkei@eqchem.s.u-tokyo.ac.jp
AF: Laboratory for Earthquake Chemistry,The University of Tokyo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Arai, S
EM: ultrasa@kenroku.kanazawa-u.ac.jp
AF: Department of Earth Sciences,Kanazawa University, Kakuma, Kanazawa, 920-1192 Japan
AU: Ishida, Y
AF: Department of Earth Sciences,Kanazawa University, Kakuma, Kanazawa, 920-1192 Japan
AU: Notsu, K
AF: Laboratory for Earthquake Chemistry,The University of Tokyo, Bunkyo-ku, Tokyo, 113-0033 Japan
AB: Pale green Cr- and Na-rich diopside (kosmochlor-bearing diopside) was observed in anhydrous Group I mantle xenoliths (dunite, wehrlite, harzburgite, clinopyroxenite) hosted by Pliocene-Quaternary hawaiite from the Ngatutura volcanic field of the North Island, New Zealand. The diopside is characterized by high Cr2O3 (up to 5.87 wt%) and Na2O (up to 3.31 wt%) and low Al2O3 (1.09 wt%, on average), and contains the highest kosmochlor component (ca. 18 mol%) known for diopsides in spinel peridotite xenoliths (Ikehata and Arai, Am Min. in press). It commonly rims around partly dissolved chromian spinel and replaces secondary Al-poor (less than 0.57 wt% Al2O3) and Si-rich (around 58.1 wt% SiO2) orthopyroxene crystallized from slab-derived fluid/melt at the expense of olivine in dunite. It also occurs as a marginal part of zoned diopside in a wehrlite xenolith. The mode of occurrence indicates a metasomatic origin for the Ko-bearing diopside. The kosmochlor-bearing diopside was analyzed for 33 trace elements by LA-ICP-MS. In all rock types it shows high REE concentrations, is slightly enriched in LREE relative to HREE, and is strongly depleted in Hf, Zr, Ta, Nb, and Ti relative to REE. These characteristics are similar to those of diopsides produced by carbonatite metasomatism in the upper mantle (e.g. Neumann et al. 2002). The Ko-poor diopside part of the zoned diopside in wehrlite is similar in trace element concentrations to the Ko-bearing one. The kosmochlor-bearing diopside may have been formed through metasomatism by a Na-bearing carbonatite melt, which selectively reacts with orthopyroxene and can carry a large amount of REE. Cr-rich spinel in refractory peridotites that had been formed at an arc setting is prerequisite for genesis of this kind of kosmochlor-bearing diopside upon carbonatite metasomatism. The carbonatite melt impacted the mantle wedge at a supra-subduction zone setting until 15 Ma, from the deeper part due to the absence of slab cover at the intraplate setting (10 Ma-) in the North Island. Ongoing work examines the importance of carbonatite metasomatism within the mantle wedge.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3665 Mineral occurrences and deposits
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting