東北日本弧リソスフェア性マントルにおけるHFS元素の枯渇現象  [in Japanese] High field strength element depletions in the lithospheric mantle beneath the Northeast Japan arc  [in Japanese]

東北日本弧の漸新世以降の火山岩類のSrおよびNd同位体組成のデータに基づき, 東北日本弧の上部マントルには同位体的に2つの異なるソースが提唱されている。一つは同位体的に非枯渇的な島弧リソスフェア性マントルで, もう一つは同位体的に枯渇的したソースで15 Maあたりに背弧側へ上昇したアセノスフェア性マントルである。漸新世以降の玄武岩のHFS元素(Nb, Ti, Y, Zr)どうしの比の時間的変化を検討した。島弧リソスフェア性マントルに由来する玄武岩のZr/Y比は35 Ma以降時間とともに減少するのに対して, Ti/Zr比は増加傾向を示す。また, アセノスフェア性マントルに由来する玄武岩のこれらの比は上記玄武岩とは異なる変化パターンを示す。これらの事実に基づき, 島弧リソスフェア性マントルは, 漸新世以降, OIBソース的なものからN-MORBソース的なものを経て現在のHFS元素に枯渇したソースに変化してきたことを議論した。

On the basis of Sr and Nd isotopic data for Tertiary and Quaternary volcanic rocks from the NE Japan arc, two isotopically different mantle source have been proposed by several workers. One is an undepleted sub-island arc upper mantle from which 〜35 Ma to〜15 Ma basaltic magmas in the back-arc side and 〜21 Ma to the present basalts in the trench side and transitional zone were extracted. The other is depleted asthenosphere which upwelled and intruded the undepleted sub-island arc upper mantle beneath the back-arc side of the NE Japan arc at around 15 Ma. The temporal change in HFS element (high field strength element) character of two different parts of the mantle beneath the NE Japan arc was investigated using Nb, Ti, Y and Zr in 103 basaltic samples (MgO≧5%, SiO_2≦53%) from the trench side (including some samples from the transitional zone) and 127 samples from the back-arc side. Systematic differences are observed in the HFS element contents of oder basalts relative to younger ones. Basaltic rocks derived from the sub-island arc upper mantle exhibit a gradual decrease in Zr/Y and a gradual increase in Ti/Zr with decreasing age from 〜35 Ma to the present, whereas those from the up welled asthenospheric mantle show different HFS element variations with decreasing age from 〜15 Ma to the present. Basalts with the lowest Zr/Y ratios have been plotted on the 2Nb-Zr/4-Y diagram of Meschede (1986), which shows that the sub-island arc upper mantle of the NE Japan arc has gradually changed its HFS element character from a highly in compatible HFS element-enriched OIB source, to an N-MORB source, to a highly in compatible HFS element-depleted IAB source, with decreasing in age. The gradual depletion in HFS elements with greater in compatibility in the sub-island arc upper mantle may be due to the successive extraction of basaltic magmas from the source since 〜35 Ma ago. On Meschede's (1986) diagram basalts derived from the upwelled asthenospheric mantle show a change in the source from slightly depleted in Zr and slightly enriched in Nb compared to N-MORB, to a source slightly depleted both in Zr and Nb compared to N-MORB during the last 15 Ma. Variations of Ti/Zr and Zr/Nb ratios of basaltic rocks from the NE Japan arc are not compatible with the presence of residual sphene, rutile and amphibole in the source mantle, which would partition Ti and Nb.