17:15 〜 18:30
[SCG42-P04] The origin of Indian Mantle inferred from Re-Os isotopes of chrome spinel
キーワード：Indian MORB mantle、Podiform chromitite、Zambales Ophiolite、Oman Ophiolite、Re-Os isotope
Highly variable isotopic nature of the Indian MORB mantle including Os isotopic ratios (187Os/188Os) inherited from long-term depletion and fertilization by convecting mantle since the Archean Eon. Highly depleted harzburgites with low Os isotopic ratios are known as xenoliths in plume-related magmas and the serpentinite seamounts in the Izu-Bonin-Mariana (IBM) forearc regions. Recently, boninites from the IBM forearc were demonstrated to have derived from such depleted harzburgite blocks, which were remnants of the subcontinental lithosphere dispersed beneath the Indian to western Pacific. Fertile component is suggested to have produced by recycled subducted oceanic crust.
Podiform chromitites from ophiolites are generated through interaction of the shallow mantle peridotite and ascending magmas including boninite. The Eocene Zambales Ophiolite in the western Philippines and the late Cretaceous Oman Ophiolite in the Arabian Peninsula yield boninites and podiform chromitites with high Cr# spinel, which are proposed to have crystallized from boninitic magmas. We report textures, mineralogy, and Re-Os isotopic compositions of 6 chromitite samples from the Acoje (AC30) and Coto (CO30) blocks in the Zambales Ophiolite and the Fizh block in the Oman Ophiolite (OM19-63A, OM19-63C, K1-25 and K1-19), and discuss the origin of high Os isotopic ratios of those chromitites.
Major elements were analyzed by EPMA (JEOL JXA-8800) and Re and Os isotopes by N-TIMS (TRITON PLUS, Thermo Fisher SCIENTIFIC) and ICP-MS (Agilent 7500cx), and Re and Os concentrations were calculated by isotope dilution method.
Chromitite samples have chrome spinel with Cr#=40~85 and Mg#=55~75 and olivine with Fo93~97 and NiO <0.9 wt%. High Fo and NiO in olivine resulted from re-equilibration with coexisting spinel at low temperatures (500-800℃). TiO2 against Cr# of chrome spinels indicates that CO30 of Zambales Ophiolite and OM19-63C and K1-19 of Oman ophiolite were crystallized from MORB-like magma. On the other hand, AC30 of Zambales Ophiolite, OM19-63A and K1-25 of Oman Ophiolite with high-Cr#s are considered to have crystallized from arc magma derived from the more depleted source mantle.
The chromitites with higher Cr# and Δlog(FMQ) >+3 are consistent with an arc environment, whereas the chromitites with lower Cr# and Δlog(FMQ) are more compatible with a back arc origin. It is concluded that CO30, OM19-63C and K1-25 with lower Cr# and Δlog(FMQ) formed in a back arc basin, while AC30, OM19-63A and K1-19 with higher Cr# and Δlog(FMQ) formed in an arc environment.
The Os isotopic ratios of chrome spinels in chromitites range from 0.127 to 0.130, higher than the depleted MORB mantle of 0.1259 and equal to or slightly higher than the primitive mantle of 0.1296 and chondritic value of 0.1276. These high Os isotopic ratios indicate the presence of materials with high Os isotopic ratios in the mantle. It is possible that recycling and assimilation of crustal material with higher Re contents into the shallow upper mantle yielded the enriched Os isotopic ratios through time, which became the sources of the Zambales and Oman chromitites.
Podiform chromitites from ophiolites are generated through interaction of the shallow mantle peridotite and ascending magmas including boninite. The Eocene Zambales Ophiolite in the western Philippines and the late Cretaceous Oman Ophiolite in the Arabian Peninsula yield boninites and podiform chromitites with high Cr# spinel, which are proposed to have crystallized from boninitic magmas. We report textures, mineralogy, and Re-Os isotopic compositions of 6 chromitite samples from the Acoje (AC30) and Coto (CO30) blocks in the Zambales Ophiolite and the Fizh block in the Oman Ophiolite (OM19-63A, OM19-63C, K1-25 and K1-19), and discuss the origin of high Os isotopic ratios of those chromitites.
Major elements were analyzed by EPMA (JEOL JXA-8800) and Re and Os isotopes by N-TIMS (TRITON PLUS, Thermo Fisher SCIENTIFIC) and ICP-MS (Agilent 7500cx), and Re and Os concentrations were calculated by isotope dilution method.
Chromitite samples have chrome spinel with Cr#=40~85 and Mg#=55~75 and olivine with Fo93~97 and NiO <0.9 wt%. High Fo and NiO in olivine resulted from re-equilibration with coexisting spinel at low temperatures (500-800℃). TiO2 against Cr# of chrome spinels indicates that CO30 of Zambales Ophiolite and OM19-63C and K1-19 of Oman ophiolite were crystallized from MORB-like magma. On the other hand, AC30 of Zambales Ophiolite, OM19-63A and K1-25 of Oman Ophiolite with high-Cr#s are considered to have crystallized from arc magma derived from the more depleted source mantle.
The chromitites with higher Cr# and Δlog(FMQ) >+3 are consistent with an arc environment, whereas the chromitites with lower Cr# and Δlog(FMQ) are more compatible with a back arc origin. It is concluded that CO30, OM19-63C and K1-25 with lower Cr# and Δlog(FMQ) formed in a back arc basin, while AC30, OM19-63A and K1-19 with higher Cr# and Δlog(FMQ) formed in an arc environment.
The Os isotopic ratios of chrome spinels in chromitites range from 0.127 to 0.130, higher than the depleted MORB mantle of 0.1259 and equal to or slightly higher than the primitive mantle of 0.1296 and chondritic value of 0.1276. These high Os isotopic ratios indicate the presence of materials with high Os isotopic ratios in the mantle. It is possible that recycling and assimilation of crustal material with higher Re contents into the shallow upper mantle yielded the enriched Os isotopic ratios through time, which became the sources of the Zambales and Oman chromitites.