15:45 〜 16:00
[SVC30-08] 西南日本，第四紀横田アルカリ玄武岩マグマの成因
キーワード：アルカリ玄武岩、第四紀、西南日本、横田、中国地方、マグマ含水量
Upwelling of the asthenosphere associated with the opening of the Japan Sea in addition to the subduction of the Philippine Sea plate has been considered to be the origin of the late Cenozoic volcanism in the Chugoku district. The Yokota basalts erupted 2.17 - 0.97 Ma and temporally expanded outwards from the center of the volcanic cluster to the margin forming a volcanic cluster with a diameter of 40km over Shimane and Tottori prefectures. The Yokota basalts have been divided into two subtypes in terms of Nd-Sr isotopes: the ABL with lower 87Sr/86Sr and the ABH with higher 87Sr/86Sr for the same 143Nd/144Nd (Kimura et al., 2014). The ABH basalts are distributed at the periphery of the cluster in the late stage of the activity. Such concentric expansion of the volcanic activity may suggest that the magmas were produced by the diapiric mantle upwelling. However, petrogenesis of the Yokota basalts remains unclear because primary magma water content has not been petrologically constrained from natural samples. We analyzed whole-rock major and trace element and mineral compositions of the fresh basaltic samples newly collected from the Yokota basalts in order to determine the water content of the basalts and estimate the mantle melting conditions.
The Yokota samples have primitive and alkaline basaltic compositions, ranging from 47.7 to 54.2 wt% SiO2 and from 0.67 to 1.31 FeO*/MgO. There is no clear difference between ABL and ABH in major element compositions except that the ABL have systematically higher K2O content than the ABH. Whole-rock multi-element trace element patterns for the ABL and ABH normalized to the Primitive mantle composition are generally similar to those of typical arc-type alkaline volcanic rocks (Kimura et al., 2014). No significant Eu anomaly is observed in the REE concentration patterns of the ABH and ABL samples. The modal abundance of phenocrysts is typically less than 10 vol%. Olivine and clinopyroxene phenocrysts are common but olivine is the major phenocryst phase for all the samples. Most olivine and clinopyroxene phenocrysts are normally-zoned and euhedral to subhedral. Several samples of the ABL and ABH additionally contain small amount of normally-zoned euhedral plagioclase phenocrysts. The An content of the plagioclase cores is 66 - 71.8 and 74 - 82.4 for the ABL and ABH, respectively, and their distributions are unimodal.
We estimated the water content of the ABL and ABH magmas using plagioclase-melt hygrometer at an assumed pressure condition of 0.3 GPa. Estimated water content of the ABL and ABH magmas was 3.6 wt% and 4.6 wt%, respectively, which are the highest among the volcanic activities of the same period in the Chugoku region. The estimated melting conditions were 1330 °C and 2.4 GPa for the ABL and 1300 °C and 2.6 GPa for the ABH. The mantle potential temperature for the Yokota basalts was estimated to be ~1280 °C, which was as high as that of the mantle beneath normal mid-ocean ridges, suggesting that the upper mantle beneath Yokota was not hot.
If magmas were progressively generated and released from a single mantle diapir, water content of the magmas should decrease as the mantle diapir uprises due to progressive dehydration from the mantle. However, more hydrous ABH magma erupted at the later stage than less hydrous ABL, suggesting that simple mantle diapir model cannot reproduce spatiotemporal geochemical variation of the Yokota basalts. We propose that the following model. (1) Mantle metasomatized by the ancient sediment-derived fluid upwelled from the mantle transition zone. (2) Fluid derived from the sediment on the Philippine Sea slab mixed with the upwelled metasomatized mantle and melted to produce the ABL magma. (3) As the contact area between the upwelled metasomatized mantle and the Philippine Sea slab increases, more fluid were from the slab supplied to the upwelled metasomatized mantle and enhanced further melting to generate more hydrous ABH magma.
The Yokota samples have primitive and alkaline basaltic compositions, ranging from 47.7 to 54.2 wt% SiO2 and from 0.67 to 1.31 FeO*/MgO. There is no clear difference between ABL and ABH in major element compositions except that the ABL have systematically higher K2O content than the ABH. Whole-rock multi-element trace element patterns for the ABL and ABH normalized to the Primitive mantle composition are generally similar to those of typical arc-type alkaline volcanic rocks (Kimura et al., 2014). No significant Eu anomaly is observed in the REE concentration patterns of the ABH and ABL samples. The modal abundance of phenocrysts is typically less than 10 vol%. Olivine and clinopyroxene phenocrysts are common but olivine is the major phenocryst phase for all the samples. Most olivine and clinopyroxene phenocrysts are normally-zoned and euhedral to subhedral. Several samples of the ABL and ABH additionally contain small amount of normally-zoned euhedral plagioclase phenocrysts. The An content of the plagioclase cores is 66 - 71.8 and 74 - 82.4 for the ABL and ABH, respectively, and their distributions are unimodal.
We estimated the water content of the ABL and ABH magmas using plagioclase-melt hygrometer at an assumed pressure condition of 0.3 GPa. Estimated water content of the ABL and ABH magmas was 3.6 wt% and 4.6 wt%, respectively, which are the highest among the volcanic activities of the same period in the Chugoku region. The estimated melting conditions were 1330 °C and 2.4 GPa for the ABL and 1300 °C and 2.6 GPa for the ABH. The mantle potential temperature for the Yokota basalts was estimated to be ~1280 °C, which was as high as that of the mantle beneath normal mid-ocean ridges, suggesting that the upper mantle beneath Yokota was not hot.
If magmas were progressively generated and released from a single mantle diapir, water content of the magmas should decrease as the mantle diapir uprises due to progressive dehydration from the mantle. However, more hydrous ABH magma erupted at the later stage than less hydrous ABL, suggesting that simple mantle diapir model cannot reproduce spatiotemporal geochemical variation of the Yokota basalts. We propose that the following model. (1) Mantle metasomatized by the ancient sediment-derived fluid upwelled from the mantle transition zone. (2) Fluid derived from the sediment on the Philippine Sea slab mixed with the upwelled metasomatized mantle and melted to produce the ABL magma. (3) As the contact area between the upwelled metasomatized mantle and the Philippine Sea slab increases, more fluid were from the slab supplied to the upwelled metasomatized mantle and enhanced further melting to generate more hydrous ABH magma.