HR: 0800h
AN: V41C-0724    [Abstracts]
TI: Boron-cycling by subducted lithosphere; insights from boron-isotope compositions of the Kokchetav tourmalines
AU: * Ota, T
EM: tsutom@pheasant.misasa.okayama-u.ac.jp
AF: Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori, 682-0193, Japan
AU: Kobayashi, K
EM: katsura@pheasant.misasa.okayama-u.ac.jp
AF: Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori, 682-0193, Japan
AU: Moriguti, T
EM: moriguti@misasa.okayama-u.ac.jp
AF: Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori, 682-0193, Japan
AU: Nakamura, E
EM: eizonak@misasa.okayama-u.ac.jp
AF: Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori, 682-0193, Japan
AB: For understanding the Earth's chemical evolution, the role of subducting plates has long been focused on. Stable isotopes can provide critical evidence to identify materials experienced geological processes near the surface. In this study, we examined B-isotope compositions of tourmalines from the Kokchetav diamondiferous UHP metamorphic belt, particularly a recently discovered high-K tourmaline from Kumdy-kol^[1]. The high-K tourmaline occurs in Qtz-Kfs layers, alternating with Grt-Cpx-Bt-Qtz rocks. It has microdiamond- bearing and K-rich (K₂O=~2.38 wt.%) cores, which yield heavy B-isotope ratios (δ¹¹B=+3.2~+7.7, analyzed by SIMS). Our results suggest that the high-K tourmaline would be crystallized under high-pressure within the diamond stability from fluids or melts with the surficial B-isotope signature, which is clearly different from that of ordinary tourmalines (δ¹¹B=-16.6~-2.3) experienced the isotope fractionation through subduction-related dehydration reactions. Tourmalines with heavy B-isotope ratios have been described from marine evaporites and carbonates^[2]. The presence of silicate-carbonate melt inclusions with microdiamonds in metacarbonate rocks from Kumdy- kol^[3] implies that the heavy B-isotope in the high-K tourmaline might have derived from the melt preserved in the metacarbonate rocks. However, recrystallized carbonates can yield light B-isotope ratios (δ¹¹B=~-5^[4]), and it is doubtful that carbonates could have retained the heavy B-isotope signature during subduction to the depths. The alternative source of the heavy B-isotope is serpentinite in hydrated lithospheric mantle, because serpentinized peridotites are enriched in heavy B-isotope (δ¹¹B=+5.4~+25^[5]). Serpentinization of subducting lithospheric mantle, with enrichment of heavy B-isotope, can be realized by transform faulting near oceanic ridges and normal faulting at trench-outer rise regions, followed by penetration of seawater into the lithospheric mantle prior to subduction^[6]. As high- pressure stability limits of serpentines^[7] are comparable to the peak metamorphic pressure of the Kokchetav diamond-grade rocks, the fluids derived from serpentine-breakdown in the lithospheric mantle would ascend to accelerate the partial melting of overlying crustal rocks. The serpentine-breakdown is the first, major dehydration reaction in subducting lithospheric mantle, the fluids from serpentine-breakdown should have retained the heavy B-isotope ratios, differing from subducting crustal rocks that have already experienced the isotope fractionation through dehydration reactions. Consequently, we conclude that the heavy B-isotope signature would be inherited from serpentinites in subducted lithospheric mantle, hydrated prior to subduction. The subducted lithospheric mantle is also an essential reservoir for the geochemical recycling of surficial materials, as implied by recent studies with radiogenic and stable isotopes in oceanic island basalts. [1]Shimizu & Ogasawara (2005) Mitt Öterr Miner Ges 150:141 [2]Palmer & Slack (1989) 103:434-451 [3]Korsakov et al.(2004) Terra Nova 16:146-151; Korsakov & Hermann (2006) EPSL 241:104-118 [4]Spivack & You (1997) EPSL 152:113-122 [5]Spivack & Edmond (1987) GCA 51:1033-1043; Benton et al.(2001) EPSL 187:273-282 [6]Peacock (2001) Geology 29:299-302 [7]Ulmer & Trommsdroff (1995) Science 268:858-861; Wunder & Schreyer (1997) Lithos 41:213-227
UR: http://www.misasa.okayama- u.ac.jp/eng/
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3654 Ultra-high pressure metamorphism
DE: 9320 Asia
DE: 9621 Cambrian
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting