5:15 PM - 6:30 PM
[SGC33-P01] Origin of intra-plate magmatism in Kamchatka constrained by noble gas isotopes
Keywords:Noble gasses, kamchatka
Introduction: In Kamchatka, three volcanic chains (the first is volcanic front, the second is back-arc chain and the third is Sredinny Range, SR) are identified, and the former two are considered to result from the subduction of Pacific plate. On the other hand, several origins have been proposed for intra-plate magmatism in SR: ascent of deep mantle plumes, back-arc spreading in response to trench rollback, etc. The purpose of this study is to constrain the origin of magmatism in SR by using noble gas isotopes, which have different features in different geochemical reservoirs, such as convecting MORB-source mantle, deep-mantle-derived plume, and crust.
Samples and analysis: We extracted noble gases (He, Ne, Ar, Kr, and Xe) from fluid/melt inclusions in olivines and pyroxenes separated from basalts and andesites collected from SR by crushing in vacuum, and analyzed their abundances and noble isotope ratios. After that we heated in vacuum the recovered powders after crushing to melt the sample for complete noble gas extraction, and then analyzed in the same way.
Results and discussion: Helium isotope ratios (3He/4He ratios) obtained with the crushing of olivines were in general around 8 Ra, where Ra denotes atmospheric 3He/4He ratio. The values are in the range of MORB-source mantle (8±1 Ra), which is similar to He in mantle-derived xenoliths from Avacha volcano in the volcanic front [1]. On the other hand, crush-released 3He/4He ratios of pyroxenes were systematically lower than those of olivines in the same host rocks. Because pyroxene crystallization is generally later than that of olivine in magma, the lower 3He/4He ratios of pyroxenes than olivines would have resulted from interaction of magma with surrounding crustal rocks, which have low 3He/4He ratios, between olivine and pyroxene crystallization. A few olivines showed lower 3He/4He ratios than MORB value during the crushing experiment, which would be trace of the slab fluids enriched in U and Th. Higher 3He/4He ratios than the MORB value were also observed during crushing of another fer olivines samples Because He extracted by heating of crushed powder yielded much higher 3He/4He ratio (>100 Ra), the high 3He/4He ratios obtained with crushing would be due to a small release of cosmic-ray produced He having accumulated in the sample after the eruption.
Based on the above observations, we conclude that the origin of magma erupted in SR is the convecting mantle, and the possibility of plume’s involvement is low. We also conclude that there are significant traces of slab fluids in some regions in SR.
Reference: [1] Kobayashi et al., EPSL, 2017.
Samples and analysis: We extracted noble gases (He, Ne, Ar, Kr, and Xe) from fluid/melt inclusions in olivines and pyroxenes separated from basalts and andesites collected from SR by crushing in vacuum, and analyzed their abundances and noble isotope ratios. After that we heated in vacuum the recovered powders after crushing to melt the sample for complete noble gas extraction, and then analyzed in the same way.
Results and discussion: Helium isotope ratios (3He/4He ratios) obtained with the crushing of olivines were in general around 8 Ra, where Ra denotes atmospheric 3He/4He ratio. The values are in the range of MORB-source mantle (8±1 Ra), which is similar to He in mantle-derived xenoliths from Avacha volcano in the volcanic front [1]. On the other hand, crush-released 3He/4He ratios of pyroxenes were systematically lower than those of olivines in the same host rocks. Because pyroxene crystallization is generally later than that of olivine in magma, the lower 3He/4He ratios of pyroxenes than olivines would have resulted from interaction of magma with surrounding crustal rocks, which have low 3He/4He ratios, between olivine and pyroxene crystallization. A few olivines showed lower 3He/4He ratios than MORB value during the crushing experiment, which would be trace of the slab fluids enriched in U and Th. Higher 3He/4He ratios than the MORB value were also observed during crushing of another fer olivines samples Because He extracted by heating of crushed powder yielded much higher 3He/4He ratio (>100 Ra), the high 3He/4He ratios obtained with crushing would be due to a small release of cosmic-ray produced He having accumulated in the sample after the eruption.
Based on the above observations, we conclude that the origin of magma erupted in SR is the convecting mantle, and the possibility of plume’s involvement is low. We also conclude that there are significant traces of slab fluids in some regions in SR.
Reference: [1] Kobayashi et al., EPSL, 2017.