17:15 〜 18:45
[SCG48-P16] Estimates of the temperature and melting conditions of the Conrad Rise upper mantle
キーワード：マントルポテンシャル温度、捕獲岩、コンラッドライズ、インド洋
Conrad Rise is one of the largest edifices in the southwestern Indian Ocean. It was previously thought to have been formed during the Cretaceous, similar to most other large igneous provinces. However, recent research by Sato et al. (2024 JGR Solid Earth 10.1029/2023JB027380) found that seamounts on the rise were active during the Eocene and Miocene. Although tectonic reconstruction suggests that volcanic activities at the seamounts occurred in intraplate settings, the driving force behind the magmatism remains unknown. Detailed constraints on the melting conditions and characteristics of the source region may help to resolve this question. In this study, we modeled the petrological and mineralogical data of alkali basalts and xenoliths from the seamounts on the Conrad Rise with newly reporting geochemistry of a small seamount west of the Ob seamount (Ob-W seamount).
To estimate mantle potential temperatures (TP) based on major element thermobarometry, we used a python package "meltPT" (McNab and Ball, 2023 Volcanica 10.30909/vol.06.01.6376), which contains the olivine fractionation backtracking method by Lee et al. (2009 EPSL 10.1016/j.epsl.2008.12.020) and parameterizations of source temperature and pressure using an extensive compilation of experimental melt equilibration data sets by Plank and Forsyth (2016 G3 10.1002/2015GC006205). Alkali basalt with MgO >9.0 wt.% was used for the calculation. We used adiabatic decompression melting paths based on the calculated equilibration temperatures and pressures to establish a correlation between the primary melt compositions and mantle potential temperature. Melting paths were determined using the anhydrous parameterization described by Katz et al. (2003 G3 10.1029/2002GC000433). Our findings indicate that the Eocene Ob seamount lavas have a TP of 1408+78/-80 C, while the late Miocene Ob-one seamount lavas have a TP of 1478+16/-13 C. We also calculated the primary melt composition and TP using PRIMELT3 (Herzberg and Asimow, 2015 G3 10.1002/2014GC005631). However, most lavas from Conrad Rise did not yield consistent results. PRIMELT3 suggests the necessity for volatile and/or pyroxenite sources or augite fractionation/accumulation.
TP can be estimated through inverse modeling of rare earth element (REE) concentrations. REEs are sensitive to both the depth and degree of melting during mantle melting. The INVMEL algorithm developed by McKenzie and O'Nions (1991 JPetrol 10.1093/petrology/32.5.1021) was used for adiabatic decompression of the peridotite. The REE inversion provided a good match to the lava compositions derived from the MORB source enriched by adding a small fraction (4%) of the melt. Melting started in the garnet-spinel transition zone (66-70 km), and the calculated melt fractions were approximately 3% for the Ob seamount and less than 2% for the Ob-one and Ob-W seamounts. The estimated depth of melting (approximately 2 GPa) suggests that the TP should be approximately 1300 C based on both the anhydrous and hydrous melting models of Katz et al. (2003).
Sato et al. (2024) reported that some lavas, specifically those from the Lena seamount, contain xenoliths and/or xenocrysts of up to 1.5 cm. Large clusters of amphibole-bearing grains have a poikilitic texture, with kaersutite as the oikocryst and clinopyroxene, iddingsite after olivine, apatite, and magnetite-ilmenite-biotite as the chadacrysts. Amphibole temperatures and pressures were calculated by iterating equation 6 of Putirka (2016 RevMinGeochem 10.2138/rmg.2008.69.3) with the barometer of Mutch et al. (2016 CMP 10.1007/s00410-016-1298-9) using the iterative algorithm implemented in the python package "Thermobar" (Wieser et al. 2021 Volcanica 10.30909/vol.05.02.349384). Kaersutite oikocrysts from Ob lavas yielded equilibrated temperatures ranging from 948 to 988 C and pressures from 6.50 to 7.48 kbar, while those from Lena lavas yielded temperatures from 972 to 1013 C and pressures from 7.14 to 8.98 kbar.
To estimate mantle potential temperatures (TP) based on major element thermobarometry, we used a python package "meltPT" (McNab and Ball, 2023 Volcanica 10.30909/vol.06.01.6376), which contains the olivine fractionation backtracking method by Lee et al. (2009 EPSL 10.1016/j.epsl.2008.12.020) and parameterizations of source temperature and pressure using an extensive compilation of experimental melt equilibration data sets by Plank and Forsyth (2016 G3 10.1002/2015GC006205). Alkali basalt with MgO >9.0 wt.% was used for the calculation. We used adiabatic decompression melting paths based on the calculated equilibration temperatures and pressures to establish a correlation between the primary melt compositions and mantle potential temperature. Melting paths were determined using the anhydrous parameterization described by Katz et al. (2003 G3 10.1029/2002GC000433). Our findings indicate that the Eocene Ob seamount lavas have a TP of 1408+78/-80 C, while the late Miocene Ob-one seamount lavas have a TP of 1478+16/-13 C. We also calculated the primary melt composition and TP using PRIMELT3 (Herzberg and Asimow, 2015 G3 10.1002/2014GC005631). However, most lavas from Conrad Rise did not yield consistent results. PRIMELT3 suggests the necessity for volatile and/or pyroxenite sources or augite fractionation/accumulation.
TP can be estimated through inverse modeling of rare earth element (REE) concentrations. REEs are sensitive to both the depth and degree of melting during mantle melting. The INVMEL algorithm developed by McKenzie and O'Nions (1991 JPetrol 10.1093/petrology/32.5.1021) was used for adiabatic decompression of the peridotite. The REE inversion provided a good match to the lava compositions derived from the MORB source enriched by adding a small fraction (4%) of the melt. Melting started in the garnet-spinel transition zone (66-70 km), and the calculated melt fractions were approximately 3% for the Ob seamount and less than 2% for the Ob-one and Ob-W seamounts. The estimated depth of melting (approximately 2 GPa) suggests that the TP should be approximately 1300 C based on both the anhydrous and hydrous melting models of Katz et al. (2003).
Sato et al. (2024) reported that some lavas, specifically those from the Lena seamount, contain xenoliths and/or xenocrysts of up to 1.5 cm. Large clusters of amphibole-bearing grains have a poikilitic texture, with kaersutite as the oikocryst and clinopyroxene, iddingsite after olivine, apatite, and magnetite-ilmenite-biotite as the chadacrysts. Amphibole temperatures and pressures were calculated by iterating equation 6 of Putirka (2016 RevMinGeochem 10.2138/rmg.2008.69.3) with the barometer of Mutch et al. (2016 CMP 10.1007/s00410-016-1298-9) using the iterative algorithm implemented in the python package "Thermobar" (Wieser et al. 2021 Volcanica 10.30909/vol.05.02.349384). Kaersutite oikocrysts from Ob lavas yielded equilibrated temperatures ranging from 948 to 988 C and pressures from 6.50 to 7.48 kbar, while those from Lena lavas yielded temperatures from 972 to 1013 C and pressures from 7.14 to 8.98 kbar.