17:15 〜 18:30
[SCG41-P03] Sr isotopic and trace element variations of gabbros and peridotites at the CM1A drilling site of Wadi Tayin massif in the Oman ophiolite
キーワード：オマーン陸上掘削コア試料、Sr同位体比、微量元素組成、地殻‐マントル境界
Hydrothermal alteration processes of seawater occurring in oceanic crust impact the physical, chemical, and biological processes of the Earth system (Staudigel., 2003). Comprehensive research of hydrothermal circulation on direct oceanic crust is not sufficient due to the difficulty of obtaining samples. Petrological and geochemical characteristics caused by hydrothermal alteration have been studied from detailed observations of ophiolite, which is believed to be fragments of the oceanic lithosphere (e.g. Lanphere et al., 1986).
We analyzed Sr isotopic and trace element compositions of gabbros and peridotites (dunite and harzburgite) collected from the drilling CM1A site that penetrates the crustal-mantle boundary of the Wadi Tayin massif in the southern Oman ophiolite. The core sample is 404.15 m in length, composed mainly with olivine gabbro in the upper 160 m of the core (Layered Gabbro sequence: Sequence I), with massive dunite from 160 m to 310 m (Crust-Mantle Transition sequence) that is divided into the upper 90 m Dunite sequence (Sequence II) and the lower 60 m Dunite with Gabbro Sequence (Sequence III), and with deeper harzburgite (Mantle sequence; Sequence IV) (Tamura et al., 2018; Takazawa et al., 2019).
The Sr isotopic variations of eight gabbro samples in Sequence I is small (87Sr/86Sr = 0.7030 to 0.7033). The 87Sr/86Sr ratios of dunites are higher than those of the gabbros and decrease from 0.7072 in sequence I to 0.7039 in lower part of Sequence II and Sequence III through 0.7042 in upper part of Sequence II. The harzburgite in the Sequence IV shows slightly higher 87Sr/86Sr ratio (0.7040) than the dunites in Sequence II and III.
The Sr isotopic variations of the gabbros of Sequence I is consistent with those of fresh gabbros in the Ibra massif of the southern part of the Oman ophiolite (0.7028 – 0.7033; Lanphere et a., 1986; Zhilman et al., 2018), but the Sr isotopic ratio of the Sequence I dunite is higher than them. This difference might depend on degree of alteration (gabrros = 11 - 16 %, dunite = 80 %). The dunite of Sequence I shows extensive enrichment of Th and U compared than the gabbros. The estimated Sr isotope ratio of the solution circulated in the Sequence I dunite is 0.70821 using the equation (1) of Kawahata et al (2001). The Sr isotopic variations of dunites from Sequence II and III do not simply correlate with degree of serpentinization. Degree of serpentinization and multi-element primordial mantle (PM) normalized patterns of these dunites are similar to each other. These results suggest that the hydrothermal fluid circulating at different depths shows different Sr isotope ratios. The Sequence IV harzburgites show various PM-normalized patterns and slightly higher Sr isotopic ratios compared than upper sequence dunites. This feature may be due to the influence of fluids from the metamorphic sole.
References:
Kawahata et al., JGR, 206, 11083-11099, 2001.
Lanphere et al., JGR, 86, 2709-2720, 1986.
Staudigel, Treatise on Geochemistry, 3, 511–535, 2003.
Takazawa et al. Abs. of JpGU, SCG49-10, 2019.
Tamura et al., Abs. of JpGU, SCG54-P08, 2018.
Zhilman et al., Lithos, 323, 103–124, 2018.
We analyzed Sr isotopic and trace element compositions of gabbros and peridotites (dunite and harzburgite) collected from the drilling CM1A site that penetrates the crustal-mantle boundary of the Wadi Tayin massif in the southern Oman ophiolite. The core sample is 404.15 m in length, composed mainly with olivine gabbro in the upper 160 m of the core (Layered Gabbro sequence: Sequence I), with massive dunite from 160 m to 310 m (Crust-Mantle Transition sequence) that is divided into the upper 90 m Dunite sequence (Sequence II) and the lower 60 m Dunite with Gabbro Sequence (Sequence III), and with deeper harzburgite (Mantle sequence; Sequence IV) (Tamura et al., 2018; Takazawa et al., 2019).
The Sr isotopic variations of eight gabbro samples in Sequence I is small (87Sr/86Sr = 0.7030 to 0.7033). The 87Sr/86Sr ratios of dunites are higher than those of the gabbros and decrease from 0.7072 in sequence I to 0.7039 in lower part of Sequence II and Sequence III through 0.7042 in upper part of Sequence II. The harzburgite in the Sequence IV shows slightly higher 87Sr/86Sr ratio (0.7040) than the dunites in Sequence II and III.
The Sr isotopic variations of the gabbros of Sequence I is consistent with those of fresh gabbros in the Ibra massif of the southern part of the Oman ophiolite (0.7028 – 0.7033; Lanphere et a., 1986; Zhilman et al., 2018), but the Sr isotopic ratio of the Sequence I dunite is higher than them. This difference might depend on degree of alteration (gabrros = 11 - 16 %, dunite = 80 %). The dunite of Sequence I shows extensive enrichment of Th and U compared than the gabbros. The estimated Sr isotope ratio of the solution circulated in the Sequence I dunite is 0.70821 using the equation (1) of Kawahata et al (2001). The Sr isotopic variations of dunites from Sequence II and III do not simply correlate with degree of serpentinization. Degree of serpentinization and multi-element primordial mantle (PM) normalized patterns of these dunites are similar to each other. These results suggest that the hydrothermal fluid circulating at different depths shows different Sr isotope ratios. The Sequence IV harzburgites show various PM-normalized patterns and slightly higher Sr isotopic ratios compared than upper sequence dunites. This feature may be due to the influence of fluids from the metamorphic sole.
References:
Kawahata et al., JGR, 206, 11083-11099, 2001.
Lanphere et al., JGR, 86, 2709-2720, 1986.
Staudigel, Treatise on Geochemistry, 3, 511–535, 2003.
Takazawa et al. Abs. of JpGU, SCG49-10, 2019.
Tamura et al., Abs. of JpGU, SCG54-P08, 2018.
Zhilman et al., Lithos, 323, 103–124, 2018.