17:15 〜 18:45
[SCG42-P07] Characteristic and origin of spongy texture in clinopyroxene from mantle xenoliths, Tariat volcanic field, Central Mongolia
キーワード：スポンジ状組織、減圧、部分融解、タリアット火山地域
Many monogenetic volcanic centers from Pleistocene to Holocene are distributed in the Horgo area, Tariat volcanic field (TVF), Central Mongolia. I have observed the petrological characteristics of spongy texture in clinopyroxene from mantle xenoliths obtained from a volcanic cone in the area to investigate the origin.
The well-developed spongy textures mainly occur in the clinopyroxene grains consisting of sharp boundaries with other minerals, and various thicknesses moreover cover the grains. The spongy textures also occur in the boundary between clinopyroxene grains and olivine or orthopyroxene grains. The boundaries between spongy-textured clinopyroxene and surrounding mineral grains are sharp and do not have a preferential development toward the melt pockets. The distributions of spongy texture are random and do not show any development apparent toward the host magma. The clinopyroxene have a compositional zoning between cores and the spongy rims (spongy domain). The spongy domains are composed of secondary clinopyroxene, trace olivines and orthopyroxenes, glasses, ilmenites, and vugs.
The spongy rims of clinopyroxenes are higher CaO, MgO, Mg#, and Cr# and lower Al2O3, Na2O, AlVI, and AlIV/AlVI than the core. The studied glasses within the spongy domains have low MgO and FeO contents (< 2 wt% and < 3 wt%, respectively) and enriched Na2O (up to 8.7 wt%), SiO2 (up to 63 wt%), and Al2O3 (up to 25 wt%). By the application of Cpx only geothermobarometry (Wang et al. 2021), the spongy rims of clinopyroxenes show slightly lower temperatures (1146–1168 ℃) and significantly lower pressure (9.7–11.4 kbar) than the cores. Although many previous studies stated that the mantle xenolith from Horgo area within TVF had experienced the metasomatism process evidenced by the occurrence of melt pocket, this study suggest that the spongy texture had occurred by decompression-induced low-degree partial melting prior to the formation of melt pocket.
The well-developed spongy textures mainly occur in the clinopyroxene grains consisting of sharp boundaries with other minerals, and various thicknesses moreover cover the grains. The spongy textures also occur in the boundary between clinopyroxene grains and olivine or orthopyroxene grains. The boundaries between spongy-textured clinopyroxene and surrounding mineral grains are sharp and do not have a preferential development toward the melt pockets. The distributions of spongy texture are random and do not show any development apparent toward the host magma. The clinopyroxene have a compositional zoning between cores and the spongy rims (spongy domain). The spongy domains are composed of secondary clinopyroxene, trace olivines and orthopyroxenes, glasses, ilmenites, and vugs.
The spongy rims of clinopyroxenes are higher CaO, MgO, Mg#, and Cr# and lower Al2O3, Na2O, AlVI, and AlIV/AlVI than the core. The studied glasses within the spongy domains have low MgO and FeO contents (< 2 wt% and < 3 wt%, respectively) and enriched Na2O (up to 8.7 wt%), SiO2 (up to 63 wt%), and Al2O3 (up to 25 wt%). By the application of Cpx only geothermobarometry (Wang et al. 2021), the spongy rims of clinopyroxenes show slightly lower temperatures (1146–1168 ℃) and significantly lower pressure (9.7–11.4 kbar) than the cores. Although many previous studies stated that the mantle xenolith from Horgo area within TVF had experienced the metasomatism process evidenced by the occurrence of melt pocket, this study suggest that the spongy texture had occurred by decompression-induced low-degree partial melting prior to the formation of melt pocket.