17:15 〜 19:15
[SGC37-P14] Depth water variation in the mantle beneath Nushan, SE-edge of Sino-Korean craton
キーワード：マントル捕獲岩、リソスフェアアセノスフェア境界、FTIR、岩石学
The boundary between the lithosphere and asthenosphere, the Lithosphere-Asthenosphere Boundary (LAB), is the "bottom" of the plate in plate tectonics and represents a viscosity contrast in the upper mantle. The presence of water in the mantle is closely related to this viscosity contrast, affecting it in two ways: (1) directly reducing the viscosity of mantle olivine, and (2) lowering the temperature required for partial melting, which significantly changes the depth, temperature, and properties of the LAB. Subduction zones are crucial as they involve the release of fluids from hydrated slabs, leading to widespread hydration of the wedge mantle. However, it is unclear how far this water spreads horizontally. In this study, 41 mantle xenoliths collected in the Nushan Maar in Anhui Province, southeast China, were used to reconstruct the mantle structures and water distribution. Nushan maar is located at the southeastern edge of the Sino-Korean craton above the western end of a stagnant slab and near the edge of a big mantle wedge, with a plume rising from deeper levels. By comparing the structures and water distribution beneath Nushan, the influence of the water from the stagnant slab on the LAB is estimated.
The Nushan mantle xenoliths exhibit diverse petrological characteristics and are grouped into three types based on mineral and chemical compositions: (1) MORB-like lherzolites occasionally containing garnet or phlogopite, (2) metasomatised lherzolites containing amphibole and apatite, and (3) harzburgites. Based on the Ca-in-olivine and Ca-in-orthopyroxene geothermobarometry, the derivation depth of the xenoliths is estimated to be between 0.4 and 2.6 GPa, with temperatures ranging from 830 to 1150°C. These estimates are consistent with previous studies on granulite and garnet peridotites. The reconstructed mantle structure beneath Nüshan shows an upper layer with coarse-grained protogranular textures containing hydrous minerals, alternating between MORB-like and metasomatised lherzolites. The lower layer consists of fine-grained tabular granular and porphyroclastic textures composed of MORB-like lherzolite. The deepest layer is composed of fine-grained tabular granular harzburgite. The base of the lithosphere is marked by the depth at which deformation structures begin to develop, around 57 km (~1.7GPa), which is similar to the reported depth of the seismic LAB by receiver function. We infer the upper mantle layer (<60 km depth) as the lithospheric mantle and the lower layers (>60 km depth) as the transitional zone of LAB or the uppermost asthenosphere.
FTIR analysis of 13 Nüshan xenoliths reveals the estimated water contents in olivine, orthopyroxene, and clinopyroxene ranges from 0.8 to 10.5 ppm, 70 to 146 ppm, and 105 to 408 ppm, respectively. The distribution coefficient between orthopyroxene and clinopyroxene (DCPX/OPX) marks around 2; however, those between olivine and pyroxenes are very high and scattered (14-134 for DCPX/OPX, 29-139 for DCPX/OPX). This suggests partial water loss during xenolith transportation by magma, which will be examined by a diffusion profile of water using SIMS in a future study. If the water loss is neglected, the depth variation of the water content in the Nüshan mantle shows different trends in either mantle layer bounded at ~57 km depth; the water content in olivine decreases with depth from 6.8 ppm to 0.8 ppm in the lithospheric mantle, and that increases from 0.8 ppm to 10.5 ppm in the transitional LAB zone. We infer that the water-decrease trend in the lithospheric mantle might be caused by the past subduction mode of the paleo-Pacific slab. On the other hand, the water-increase trend in the transitional LAB zone might be derived from the underlying hydrated asthenosphere via the subduction fluid released from the stagnant slab.
The Nushan mantle xenoliths exhibit diverse petrological characteristics and are grouped into three types based on mineral and chemical compositions: (1) MORB-like lherzolites occasionally containing garnet or phlogopite, (2) metasomatised lherzolites containing amphibole and apatite, and (3) harzburgites. Based on the Ca-in-olivine and Ca-in-orthopyroxene geothermobarometry, the derivation depth of the xenoliths is estimated to be between 0.4 and 2.6 GPa, with temperatures ranging from 830 to 1150°C. These estimates are consistent with previous studies on granulite and garnet peridotites. The reconstructed mantle structure beneath Nüshan shows an upper layer with coarse-grained protogranular textures containing hydrous minerals, alternating between MORB-like and metasomatised lherzolites. The lower layer consists of fine-grained tabular granular and porphyroclastic textures composed of MORB-like lherzolite. The deepest layer is composed of fine-grained tabular granular harzburgite. The base of the lithosphere is marked by the depth at which deformation structures begin to develop, around 57 km (~1.7GPa), which is similar to the reported depth of the seismic LAB by receiver function. We infer the upper mantle layer (<60 km depth) as the lithospheric mantle and the lower layers (>60 km depth) as the transitional zone of LAB or the uppermost asthenosphere.
FTIR analysis of 13 Nüshan xenoliths reveals the estimated water contents in olivine, orthopyroxene, and clinopyroxene ranges from 0.8 to 10.5 ppm, 70 to 146 ppm, and 105 to 408 ppm, respectively. The distribution coefficient between orthopyroxene and clinopyroxene (DCPX/OPX) marks around 2; however, those between olivine and pyroxenes are very high and scattered (14-134 for DCPX/OPX, 29-139 for DCPX/OPX). This suggests partial water loss during xenolith transportation by magma, which will be examined by a diffusion profile of water using SIMS in a future study. If the water loss is neglected, the depth variation of the water content in the Nüshan mantle shows different trends in either mantle layer bounded at ~57 km depth; the water content in olivine decreases with depth from 6.8 ppm to 0.8 ppm in the lithospheric mantle, and that increases from 0.8 ppm to 10.5 ppm in the transitional LAB zone. We infer that the water-decrease trend in the lithospheric mantle might be caused by the past subduction mode of the paleo-Pacific slab. On the other hand, the water-increase trend in the transitional LAB zone might be derived from the underlying hydrated asthenosphere via the subduction fluid released from the stagnant slab.