15:00 〜 15:15
[SCG51-05] Formation of lower oceanic crust along Marie Celeste Transform Fault at the Réunion hotspot-influenced Central Indian Ridge
キーワード：斑れい岩、トランスフォーム断層、プルーム－海嶺相互作用、含水溶融、中央インド洋海嶺
Mantle plumes originating from hotspots interact with approximately 20% of the length of the global mid-ocean ridge systems, resulting in geochemical anomalies, increased mantle temperature, ridge topography, and thickened igneous crust. The interaction between the Réunion plume and the Central Indian Ridge has been extensively studied using mid-ocean ridge basalts (MORBs). However, MORB geochemistry alone may not fully capture plume–ridge interactions, since MORBs often homogenize via magma mixing. Therefore, it calls for the study to focus on the chemical composition of gabbros to better constrain the compositional heterogeneity of the upper mantle beneath the Central Indian Ridge.
In this study, we present major and trace element data from in-situ gabbros, and associated basalts and dolerites, recovered at the Marie Celeste Transform Fault, Central Indian Ridge. A total of 10 gabbroic rocks, 16 basalts and 5 dolerites collected during KH-15-5 cruise of R/V Hakuho-maru were analyzed. The gabbroic rocks range from coarse- to fine-grained gabbros and highly evolved oxide gabbros.
Based on bulk rock major-element compositions, gabbroic rocks can be divided into two groups: Group 1 (MgO=6.92–10.41 wt. %) and Group 2 (MgO=1.51–3.18 wt.%). Group 1 gabbros show a positive Eu anomaly and significant depletion in light rare earth elements (LREEs), whereas Group 2 gabbros show a negative Eu anomaly and relatively flat REE patterns. Most basalts and dolerites show negative Eu anomalies and depletion in LREEs. Trace element ratios of most gabbroic rocks, basalts and dolerites are similar to those of depleted MORB mantle.
Plagioclase in the gabbroic rocks shows higher anorthite contents at a given clinopyroxene Mg# [Mg/(Mg + FeTot)] than in other oceanic gabbroic sections. Additionally, the correlation between plagioclase anorthite content and clinopyroxene Mg# in these gabbroic rocks follows a much gentler trend than in other oceanic gabbroic sections. Thermodynamic modelling shows that this trend can be reproduced by fractional crystallization of melts containing ~0.3 wt. % H2O.
Our findings suggest that the influence of the Réunion plume on the chemical compositions of crustal rocks along the Marie Celeste Transform Fault may be significantly less than previously assumed. Furthermore, the Marie Celeste gabbros could have formed through hydrous melting of depleted MORB mantle. This hydrous melt origin has never been documented in any gabbros from transform faults such as Vema, Vernadsky, and Doldrums Transform Fault at the Mid-Atlantic Ridge. However, combined with previous studies, it suggests that the hydrous melting may contribute to the formation of lower oceanic crust at transform fault possibly associated with deep hydrothermal circulation.
In this study, we present major and trace element data from in-situ gabbros, and associated basalts and dolerites, recovered at the Marie Celeste Transform Fault, Central Indian Ridge. A total of 10 gabbroic rocks, 16 basalts and 5 dolerites collected during KH-15-5 cruise of R/V Hakuho-maru were analyzed. The gabbroic rocks range from coarse- to fine-grained gabbros and highly evolved oxide gabbros.
Based on bulk rock major-element compositions, gabbroic rocks can be divided into two groups: Group 1 (MgO=6.92–10.41 wt. %) and Group 2 (MgO=1.51–3.18 wt.%). Group 1 gabbros show a positive Eu anomaly and significant depletion in light rare earth elements (LREEs), whereas Group 2 gabbros show a negative Eu anomaly and relatively flat REE patterns. Most basalts and dolerites show negative Eu anomalies and depletion in LREEs. Trace element ratios of most gabbroic rocks, basalts and dolerites are similar to those of depleted MORB mantle.
Plagioclase in the gabbroic rocks shows higher anorthite contents at a given clinopyroxene Mg# [Mg/(Mg + FeTot)] than in other oceanic gabbroic sections. Additionally, the correlation between plagioclase anorthite content and clinopyroxene Mg# in these gabbroic rocks follows a much gentler trend than in other oceanic gabbroic sections. Thermodynamic modelling shows that this trend can be reproduced by fractional crystallization of melts containing ~0.3 wt. % H2O.
Our findings suggest that the influence of the Réunion plume on the chemical compositions of crustal rocks along the Marie Celeste Transform Fault may be significantly less than previously assumed. Furthermore, the Marie Celeste gabbros could have formed through hydrous melting of depleted MORB mantle. This hydrous melt origin has never been documented in any gabbros from transform faults such as Vema, Vernadsky, and Doldrums Transform Fault at the Mid-Atlantic Ridge. However, combined with previous studies, it suggests that the hydrous melting may contribute to the formation of lower oceanic crust at transform fault possibly associated with deep hydrothermal circulation.