17:15 〜 18:30
[SGC32-P03] Noble gas isotopic composition of directly ascending petit-spot from the northwest Pacific Ocean
キーワード：プチスポット、希ガス、かんらん石、急冷ガラス
The petit-spot magmatism provides us with information about the mantle beneath old part of oceanic lithosphere prior to subduction where we have never seen besides the underlying mantle below hotspot volcanoes and mid-ocean ridges. The asthenospheric 3He/4He ratios are recognized only using the mid-ocean ridge basalts (MORBs) as shown by uniform values of 8.75 ± 2.14 Ra in the world MORBs (Graham, 2002). The petit-spot volcanoes on the northwestern Pacific Plate are classified into two types based on magmatic compositions and the difference in stress fields associated with plate flexure. One is erupted at the eastern slope of the outer rise (regular petit-spot). In this case, the differentiated melt without phenocrystic olivine does not ascend rectilinearly due to the compressional stress field of upper lithosphere, which may cause the metasomatic interaction with lithospheric mantle. The other is the directly ascending petit-spot (DAP) erupted at the top of outer rise where the tectonic stress-field is totally tensile in lithosphere. The DAP melt directly ascends to the surface without magma-stagnations in lithospheric mantle, uniquely including phenocrystic olivines. Therefore, DAPs likely have more primordial composition than regular petit-spots.
In this study, we measured the noble gas isotopic compositions of quenched glasses and phenocrystic olivines separated from DAP lavas using stepwise heating methods. The samples were both regular petit-spot and DAP, located at the Japan Trench axis and at the top of the outer-rise, respectively.
The 3He/4He ratios of phenocrystic olivines of DAPs, relatively young in eruption age, are similar to those of MORB (7.5- 9.7 Ra), and the 40Ar/36Ar ratios are 337- 3284 at 600℃ and 1800℃. The 3He/4He ratio of the quenched glasses of regular petit-spot is 3.1 Ra, and the 40Ar/36Ar ratio is as high as 7000 at 1500℃. The Ne isotope ratios of all the samples are similar to the MORB trend although the values are close to atmosphere. The data of regular petit-spots are explained by the three components of MORB source, crust or sediment (radiogenic) and atmospheric components. Most of DAPs, on the other hand, simply show the mixing of atmospheric components and MORB sources, designating more primary composition than those of regular petit-spots. An exceptional data of high 3He/4He ratio (13.5 Ra) of the DAP quenched glasses is enigmatic. As the phenocrystic olivines of same sample, initially crystallized from primary melts, show MORB value, it might be due to the influence of shallower lithosphere or marine sediments, in which cosmic dusts with high 3He/4He ratios and high He concentrations are contained.
In this study, we measured the noble gas isotopic compositions of quenched glasses and phenocrystic olivines separated from DAP lavas using stepwise heating methods. The samples were both regular petit-spot and DAP, located at the Japan Trench axis and at the top of the outer-rise, respectively.
The 3He/4He ratios of phenocrystic olivines of DAPs, relatively young in eruption age, are similar to those of MORB (7.5- 9.7 Ra), and the 40Ar/36Ar ratios are 337- 3284 at 600℃ and 1800℃. The 3He/4He ratio of the quenched glasses of regular petit-spot is 3.1 Ra, and the 40Ar/36Ar ratio is as high as 7000 at 1500℃. The Ne isotope ratios of all the samples are similar to the MORB trend although the values are close to atmosphere. The data of regular petit-spots are explained by the three components of MORB source, crust or sediment (radiogenic) and atmospheric components. Most of DAPs, on the other hand, simply show the mixing of atmospheric components and MORB sources, designating more primary composition than those of regular petit-spots. An exceptional data of high 3He/4He ratio (13.5 Ra) of the DAP quenched glasses is enigmatic. As the phenocrystic olivines of same sample, initially crystallized from primary melts, show MORB value, it might be due to the influence of shallower lithosphere or marine sediments, in which cosmic dusts with high 3He/4He ratios and high He concentrations are contained.