11:00 〜 11:15
[SCG41-07] Systematics of minor and trace element chemistry of olivine in peridotites
キーワード：かんらん石、マントル、かんらん岩、微量元素
Upper mantle consists of mostly peridotites consisting mostly of olivine. Minor and trace element chemistry of olivine has received attention to understand mantle heterogeneity (Sobolev et al., 2007) and element mobility in subducted oceanic mantle (Scambelluri et al., 2004). Therefore, minor and trace element chemistry of olivine has been lately investigated for volcanic rocks (Foley et al., 2013), garnet-peridotite (De Hoog et al., 2010), deserpentinized peridotite (Garrido et al., 2005). However, those of residual spinel-peridotite after partial melting have not been well studied compared to those peridotites and also systematics of olivine minor and trace element chemistry is not well established. Therefore, a comprehensive study on minor and trace element chemistry of olivine from various origins is important as an indicator of constraining the origin of peridotite and understanding elements input/output during the secondary process such as metasomatism and serpentinization/deserpentinization.
We make a systematic inquiry into olivine trace elements of residual peridotites, cumulus ultramafic rocks, metasomatized and deserpentinized ultramafic rocks. We used previously well-studied samples for the trace element analyses: abyssal peridotites from the Atlantis Massif, the Central Indian Ridge, and Southwest Indian Ridge (Tamura et al., 2008; Morishita et al., 2007, 2009) and forearc peridotite from Izu-Bonin-Mariana arc (Morishita et al., 2011) for residual peridotite samples, and ultramafic xenolith from Takashima in the southwest Japan arc (Muroi and Arai, 2014), banded-dunite-harzburgite from Horoman peridotite complex (Matsufuji et al., 2006), and hornblende peridotite from Hakusan area for cumulative samples, respectively. Minor and trace elements in olivine of other origins such as lower dunite recovered by Oman Drilling Project, and metasomatized and/or deserpentinized ultramafic rocks from SW Greenland were analyzed to understand the secondary process.
The minor and trace element abundance in olivine was obtained by LA-ICP-MS analysis. The analysis was performed by an ablating spot of 100 μm in diameter and a repetition rate of 6-10 Hz. Each analysis consisted of 30 s measurement of gas bland and 30 s ablation. The following isotopes were measured: 7Li, 11B, 23Na, 27Al, 29Si, 31P, 43Ca, 45Sc, 49Ti, 51V, 53Cr, 59Co, 62Ni, 63Cu, 66Zn, 75As, 85Rb, 88Sr, 89Y, 90Zr, 93Nb, 95Mo, 118Sn, 121Sb, 137Ba, 140Ce, 181Ta, 208Pb, 209Bi, 232Th, and 238U. Signals of certain elements such as B, Ba, Sr, Ba, Pb, Al, and Cr, which indicate the presence of cracks or inclusions such as serpentine and spinel, were carefully monitored. In this study, we did not obtain rare earth elements abundance because they are mostly the below detection limit.
The minor and trace element abundances in olivine primarily depend on the degree of depletion. However, the results suggest that Ca, Ti, Y, B, and Li can be used as proxies to discriminate the origin of ultramafic rocks. Those elements tend to be higher in olivine of cumulus, metasomatized and deserpentinized ultramafic rocks than in olivine of residual peridotites. Y concentration in olivine of residual peridotite presents good correlations with the degree of partial melting estimated by REE concentration in clinopyroxene and Cr# of spinel. We emphasize that the minor and trace element chemistry of olivine has the potential as an indicator of the mantle process.
We make a systematic inquiry into olivine trace elements of residual peridotites, cumulus ultramafic rocks, metasomatized and deserpentinized ultramafic rocks. We used previously well-studied samples for the trace element analyses: abyssal peridotites from the Atlantis Massif, the Central Indian Ridge, and Southwest Indian Ridge (Tamura et al., 2008; Morishita et al., 2007, 2009) and forearc peridotite from Izu-Bonin-Mariana arc (Morishita et al., 2011) for residual peridotite samples, and ultramafic xenolith from Takashima in the southwest Japan arc (Muroi and Arai, 2014), banded-dunite-harzburgite from Horoman peridotite complex (Matsufuji et al., 2006), and hornblende peridotite from Hakusan area for cumulative samples, respectively. Minor and trace elements in olivine of other origins such as lower dunite recovered by Oman Drilling Project, and metasomatized and/or deserpentinized ultramafic rocks from SW Greenland were analyzed to understand the secondary process.
The minor and trace element abundance in olivine was obtained by LA-ICP-MS analysis. The analysis was performed by an ablating spot of 100 μm in diameter and a repetition rate of 6-10 Hz. Each analysis consisted of 30 s measurement of gas bland and 30 s ablation. The following isotopes were measured: 7Li, 11B, 23Na, 27Al, 29Si, 31P, 43Ca, 45Sc, 49Ti, 51V, 53Cr, 59Co, 62Ni, 63Cu, 66Zn, 75As, 85Rb, 88Sr, 89Y, 90Zr, 93Nb, 95Mo, 118Sn, 121Sb, 137Ba, 140Ce, 181Ta, 208Pb, 209Bi, 232Th, and 238U. Signals of certain elements such as B, Ba, Sr, Ba, Pb, Al, and Cr, which indicate the presence of cracks or inclusions such as serpentine and spinel, were carefully monitored. In this study, we did not obtain rare earth elements abundance because they are mostly the below detection limit.
The minor and trace element abundances in olivine primarily depend on the degree of depletion. However, the results suggest that Ca, Ti, Y, B, and Li can be used as proxies to discriminate the origin of ultramafic rocks. Those elements tend to be higher in olivine of cumulus, metasomatized and deserpentinized ultramafic rocks than in olivine of residual peridotites. Y concentration in olivine of residual peridotite presents good correlations with the degree of partial melting estimated by REE concentration in clinopyroxene and Cr# of spinel. We emphasize that the minor and trace element chemistry of olivine has the potential as an indicator of the mantle process.