Re-Os同位素体系在蛇绿岩应用研究中的进展

Comments on the progress on the applications of Re-Os isotopic study on the ophiolites.

Re-Os不同于由亲石元素构成的同位素体系,在原始上地幔(PUM)部分熔融过程中,母体Re是中等不相容元素,优先进入熔体相,子体Os是强相容元素,富集在残留相中,是研究蛇绿岩的极好示踪剂。在蛇绿岩应用研究中已经取得了4个方面的进展:(1)明确了熔体相的Re/Os和^(187)Os/^(188)Os比值高,而残留相的低;(2)铬铁矿中铂族元素矿物(PGM)的Re亏损年龄(T_(RD))证实了蛇绿岩中复杂的超镁铁岩体是多阶段部分熔融的产物;(3)现代大洋橄榄岩和玄武岩的Re-Os同位素研究表明熔体相和残留相的^(187)Os/^(188)Os比值在高于亏损地幔值(DMM)的部分是一致的,而低于DMM的存在不一致性,为研究蛇绿岩中熔体相与残留相是否存在“耦合”关系提供了新的制约因素;(4)揭示了蛇绿岩地幔橄榄岩中含有古大陆岩石圈地幔,这是前所未知的。虽然取得了不少进展,但是由于Re-Os同位素体系用于蛇绿岩研究的时间较短,尚存在一些问题,如显生宙蛇绿岩地幔橄榄岩的定年问题,有待进一步深化研究。

Re-Os isotopic systematics is the best tracer on studying the ophiolitic peridotite because, in contrast with the geochemical properties of Sm-Nd, Rb-Sr, U-Th-Pb and Lu-Hf, all of which are incompatible elements, Os is a compatible element in most mantle melting processes, so that it generally remains in the mantle, whereas the much more incompatible Re is extracted and enriched in the melt. This system therefore provides information that is different from, and complementary to, what we can learn from Sm-Nd, Rb-Sr, U-Th-Pb and Lu-Hf. Four points of progress have been made on the application of Re-Os studying on the ophiolite. First, determining the Re-Os characteristics of the ophiolite in which peridotites are low Re/Os and ^187Os/^188Os ratios, whereas lavas are high Re/Os and ^187Os/^188Os ratios; Secondly, dating of PGM within the ophiolitic chromitite by Re depleted ages supports the idea that the complicated ultramafic rocks of ophiolite are produced by multistage partial melting of mantle; Thirdly, measuring ^187Os/^188Os of MORB and the sulfides in abyssal peridotites （AP） indicate that there is no gap between the MORB and the most AP with higher ^187Os/^188Os ratios than DMM（0. 1246）, and gap still occurring between MORB and some AP with lower ^187Os/^188Os ratios than DMM, namely, Re-Os study on the ophiolite provides a new constraining on the geochemical relations on the residual peridotites and lavas; Finally, revealing that the ophiolitic peridotites contain the older sub-continental lithospheric mantle （SCLM）, this case is not previously known before Re-Os analyzing. Although the Re-Os isotope made progresses on studying the ophiolite, more problems, such as dating the Phanerozoic ophiolitic peridotites, still want to be done further work due to the short time of application of Re-Os study on the ophiolite.