09:30 〜 09:45
[SCG42-03] オマーンオフィオライト，下部地殻ガブロの単斜輝石のセクターゾーニング：過冷却度と冷却速度の指標
キーワード：オマーンオフィオライト、下部地殻ガブロ、単斜輝石、セクターゾーニング、過冷却度
Wadi Jidyah in the southern Oman Ophiolite exposes a 3.5 km thick lower crustal section that formed in the N-S running paleoridge segment center (Nicolas and Boudier, 2015). Hole GT1A and GT2A drilled into the middle and lower stratigraphic levels in the lower crust in W. Jidyah. The lower crust is divided into three lithological units; the 580-m thick heterolithologic upper gabbro, the 2770-m thick foliated gabbro, and the 125-m thick layered gabbro, in descending order. The foliated gabbro poorly develops modal layering in most stratigraphic levels, which are variably deformed, showing tight folding, truncation, pinch and swell structures. The layered gabbro develops subparallel modal layering that continues tens of meters but occurs only in the lowermost a hundred meter above the Moho Transition Zone. Sector zoning and ophitic textures of clinopyroxene are observed through the lower crust gabbros except for the lowermost 300 m of the crust, which indicates a large degree of supercooling for their formation.
Sector zoning of clinopyroxene is common in volcanic rocks that crystallized under a large supercooling (Ubide et al., 2019). Some incompatible elements such as Ti, Al, REEs and HFSEs are strongly partitioned into prims sectors {100}, {110} and {010}, while Si and Mg are concentrated in hourglass basal sectors {-111}. The differences in partition coefficients (Ds) between prism and basal sectors are correlated with the cooling rate under which the clinopyroxene crystallizes (Lofgren et al., 2006). The experimentally determined Ds in the high-Ti sector divided by that in the low-Ti sector generally increase as the cooling rate increases from 1 to 1000 degrees C/hr. Given the partition coefficient of La in the high-Ti and low-Ti sectors Hi-TiDLa and Lo-TiDLa, respectively, the ratio R= Hi-TiDLa/Lo-TiDLa is equal to CHi-Ti/CL-Ti. The R for the cpx in W. Jidyah gabbro ranges from 0.90 to 1.52, corresponding to the cooling rate of more than 250 degrees to less than 1 degree C per hour. This suggests that most gabbros crystallized clinopyroxenes in the uppermost axial melt lens, which were subsequently transported downward within the subsiding crystal mush in the lower crust. The absence of ophitic texture and sector zoning of clinopyroxene is consistent with the warm lowermost crust, including layered gabbros.
The empirical An-in-plagioclase geothermometer (Muller et al., 2022) indicates 1200 and 1150 degrees C for the lower GT1A and the upper GT2A gabbros. This is consistent with the observation that sector zoning of cpx from deep crust is less distinct due to diffusion at higher temperatures. By using Ti-Lu diffusion profiles between cpx sectors, the cooling rates for a GT2A gabbro core is estimated to be 0.01-0.001 degree C per year, consistent with the estimates by Mg-in-plag geospeedometry (Faak et al., 2016).
Sector zoning of clinopyroxene is common in volcanic rocks that crystallized under a large supercooling (Ubide et al., 2019). Some incompatible elements such as Ti, Al, REEs and HFSEs are strongly partitioned into prims sectors {100}, {110} and {010}, while Si and Mg are concentrated in hourglass basal sectors {-111}. The differences in partition coefficients (Ds) between prism and basal sectors are correlated with the cooling rate under which the clinopyroxene crystallizes (Lofgren et al., 2006). The experimentally determined Ds in the high-Ti sector divided by that in the low-Ti sector generally increase as the cooling rate increases from 1 to 1000 degrees C/hr. Given the partition coefficient of La in the high-Ti and low-Ti sectors Hi-TiDLa and Lo-TiDLa, respectively, the ratio R= Hi-TiDLa/Lo-TiDLa is equal to CHi-Ti/CL-Ti. The R for the cpx in W. Jidyah gabbro ranges from 0.90 to 1.52, corresponding to the cooling rate of more than 250 degrees to less than 1 degree C per hour. This suggests that most gabbros crystallized clinopyroxenes in the uppermost axial melt lens, which were subsequently transported downward within the subsiding crystal mush in the lower crust. The absence of ophitic texture and sector zoning of clinopyroxene is consistent with the warm lowermost crust, including layered gabbros.
The empirical An-in-plagioclase geothermometer (Muller et al., 2022) indicates 1200 and 1150 degrees C for the lower GT1A and the upper GT2A gabbros. This is consistent with the observation that sector zoning of cpx from deep crust is less distinct due to diffusion at higher temperatures. By using Ti-Lu diffusion profiles between cpx sectors, the cooling rates for a GT2A gabbro core is estimated to be 0.01-0.001 degree C per year, consistent with the estimates by Mg-in-plag geospeedometry (Faak et al., 2016).