五大连池和宽甸地幔包体的惰性气体同位素特征——MORB 型地幔和交代型地幔

Lai Y,Liu YL,Huang BL and Chen YJ.2005.The characteristics of noble gases in mantle-derived xenoliths in Wudalianchi and Kuandian,NE China:MORB-like mantle and metasomated mantle.

黑龙江五大连池科洛和辽宁宽甸的碱性玄武岩中地幔包体的惰性气体同位素的地球化学特征研究表明,东北地区表现了地幔的不均匀性。五大连池地区的地幔包体中的~3He/~4He 比为4.5～5.3R_4,明显低于 MORB 的值,具有被交代的大陆富集地幔特征。氩同位素^(40)Ar/^(36)Ar 比的变化在557～4005。结合玄武岩和地幔包体的矿物学特征和微量元素地球化学特征,显示五大连池地区的岩浆源区可能遭受过来自壳源物质或和古俯冲事件有关的富 H_2O-CO_2流体的交代作用,辽宁宽甸黄椅山的样品~3He/~4He 比为7.30～7.52R_A,氩同位素^(40)Ar/^(36)Ar 比的范围在1496～7677。宽甸黄椅山地区的则显示了具亏损地幔 MORB 的特点,反映了未经地壳组分改造的大陆地幔的特征。本文通过对两种不同类型的地幔岩的研究,认为东北地区大陆地幔存在 MORB 型亏损地幔和交代型地慢两种类型,反映了各自不同的演化机理。

Noble gas abundances and isotopic compositions in mantle-derived xenoliths were analyzed by crushing method, which from Keluo, Wudalianehi, Heilongjiang Province, Kuandian of Liaoning Province, Northeastern China. These results indicate the heterogeneity of subcontinental lithospheric mantle beneath northeastern China. ^3He/^4He ratios in the samples from Wudalianehi vary between 4.5 ～5.3RA, obviously lower than that of the MORB, with the signature of metasomated mantle. And their ^40Ar/^36Ar ratios change from 557 to 4005. Comparing the characteristics of noble gases with evidences from published Sr-Nd-Pb isotope data and alkali basalt petrology, it was estimated that there was a palaeo-subduetion event took place in Wudalianehi area, and the upper mantle was metasomated by subducted crustic components or H2O- CO2-rieh fluid which derived from subducted slab, and coursed forming phlogopite-bearing lherzolite. On the other hand, the samples from Kuandian show a depleted MORB-like ^3He/^4He ratio （7.30～7.52RA） , with ^40Ar/^36Ar ratio of 1496～7677. It implies that there are at least two type mantles beneath Northeastern China, the one is a metasomated subcontinental lithospheric mantle, and the other is a MORB reservoir-like mantle. They reflect different mantle evolution orocesses.