The characterisation and origin of graphite in cratonic lithospheric mantle: a petrological carbon isotope and Raman spectroscopic study
The characterisation and origin of graphite in cratonic lithospheric mantle: a petrological carbon isotope and Raman spectroscopic study
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DOI：
10.1007/bf00320978
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发表时间：
1994-02
影响因子：
3.5
通讯作者：
D. Pearson;F. R. Boyd;Stephen E. Haggerty;J. Pasteris;S. Field;P. H. Nixon;N. Pokhilenko
中科院分区：
文献类型：
--
作者：
D. Pearson;F. R. Boyd;Stephen E. Haggerty;J. Pasteris;S. Field;P. H. Nixon;N. Pokhilenko
对来自南部非洲Kaapvaal克拉通和俄罗斯西伯利亚克拉通的含石墨橄榄岩、辉石岩和榴辉岩包体进行了研究，目的是：1)更好地表征浅层大陆岩石圈地幔中元素碳的丰度和分布；(2)确定石墨的同位素组成；(3)利用矿物热压计测试地幔岩石中石墨的显著亚稳性。橄榄岩、辉石岩和榴辉岩包体中的石墨晶体具有高温成因的高结晶石墨的X射线衍射图和拉曼光谱特征，并被解释为在地幔内结晶。使用各种元素分离和配方对石墨-橄榄岩组合进行热压测量，可以得出比含钻石组合更低的温度和压力下的估计平衡条件。此外，估计的石墨橄榄岩的压力和温度几乎完全落在实验确定的石墨稳定场内，因此我们没有发现实质性的石墨亚稳的证据。橄榄岩中石墨的碳同位素组成在δ13CPDB=−12.3~−−3.8%o之间变化，平均值为-6.7CPDB=2.1(n=22)，模式在-7~-6‰，σ之间。这一平均值与来自橄榄岩包体的钻石所显示的-4‰平均值相差不到一个标准差，与含有橄榄岩包裹体的钻石的平均值相同。在橄榄岩中，石墨和钻石的碳同位素范围比在榴辉岩或辉石岩中观察到的任何一种相的碳同位素范围更有限。橄榄岩、石墨和钻石显示的同位素范围包括大洋中脊玄武岩(MORB)玻璃和洋岛玄武岩(OIB)中观察到的碳同位素范围。与克拉通橄榄岩伴生的碳的同位素组成与大洋岩浆中的碳(以CO2形式存在)的相似性(MORB/OIB)表明，在克拉通橄榄岩中沉积碳的流体以石墨或钻石的形式存在于岩石圈下方的对流地幔内。结构观察提供了证据，证明克拉通橄榄岩中的一些石墨是亚固相线交代成因的，可能是沿裂缝渗入岩石圈的冷却C-H-O流体相沉积的。从克拉通地区喷发的金伯利岩或玄武岩形成的地幔包体中尚未发现原生大晶石墨。因此，地幔来源的包体中的石墨似乎仅限于太古代克拉通，并且只出现在被认为是岩石圈地幔特征的低温、粗橄榄岩中。地幔内石墨的构造组合与金刚石的构造组合非常相似。这种限制的发生不太可能仅仅是由于克拉通岩石圈地幔中独特的氧逸度条件，因为来自克拉通以外地区的一些橄榄岩包体与来自克拉通内部的橄榄岩包体一样被还原。橄榄岩系列钻石包裹体的放射成因同位素系统学表明，钻石结晶与形成岩石圈橄榄岩的熔融事件没有直接关系。然而，一些钻石(和石墨？)南部非洲的结晶作用发生在与岩石圈地幔稳定相关的时间跨度内(Pearson等人)。1993年)。造成克拉通地幔根部碳局部化的过程的性质尚不清楚。
Graphite-bearing peridotites, pyroxenites and eclogite xenoliths from the Kaapvaal craton of southern Africa and the Siberian craton, Russia, have been studied with the aim of: 1) better characterising the abundance and distribution of elemental carbon in the shallow continental lithospheric mantle; (2) determining the isotopic composition of the graphite; (3) testing for significant metastability of graphite in mantle rocks using mineral thermobarometry. Graphite crystals in peridotie, pyroxenite and eclogite xenoliths have X-ray diffraction patterns and Raman spectra characteristic of highly crystalline graphite of high-temperature origin and are interpreted to have crystallised within the mantle. Thermobarometry on the graphite-peridotite assemblages using a variety of element partitions and formulations yield estimated equilibration conditions that plot at lower temperatures and pressures than diamondiferous assemblages. Moreover, estimated pressures and temperatures for the graphite-peridotites fall almost exclusively within the experimentally determined graphite stability field and thus we find no evidence for substantial graphite metastability. The carbon isotopic composition of graphite in peridotites from this and other studies varies from δ13CPDB= − 12.3 to − −3.8%o with a mean of-6.7‰, σ=2.1 (n=22) and a mode between-7 and-6‰. This mean is within one standard deviation of the-4‰ mean displayed by diamonds from peridotite xenoliths, and is identical to that of diamonds containing peridotite-suite inclusions. The carbon isotope range of graphite and diamonds in peridotites is more restricted than that observed for either phase in eclogites or pyroxenites. The isotopic range displayed by peridotite-suite graphite and diamond encompasses the carbon isotope range observed in mid-ocean-ridge-basalt (MORB) glasses and ocean-island basalts (OIB). Similarity between the isotopic compositions of carbon associated with cratonic peridotites and the carbon (as CO2) in oceanic magmas (MORB/OIB) indicates that the source of the fluids that deposited carbon, as graphite or diamond, in catonic peridotites lies within the convecting mantle, below the lithosphere. Textural observations provide evidence that some of graphite in cratonic peridotites is of sub-solidus metasomatic origin, probably deposited from a cooling C-H-O fluid phase permeating the lithosphere along fractures. Macrocrystalline graphite of primary appearance has not been found in mantle xenoliths from kimberlitic or basaltic rocks erupted away from cratonic areas. Hence, graphite in mantle-derived xenoliths appears to be restricted to Archaean cratons and occurs exclusively in low-temperature, coarse peridotites thought to be characteristic of the lithospheric mantle. The tectonic association of graphite within the mantle is very similar to that of diamond. It is unlikely that this restricted occurrence is due solely to unique conditions of oxygen fugacity in the cratonic lithospheric mantle because some peridotite xenoliths from off-craton localities are as reduced as those from within cratons. Radiogenic isotope systematics of peridotite-suite diamond inclusions suggest that diamond crystallisation was not directly related to the melting events that formed lithospheric peridotites. However, some diamond (and graphite?) crystallisation in southern Africa occurred within the time span associated with the stabilisation of the lithospheric mantle (Pearson et al. 1993). The nature of the process causing localisation of carbon in cratonic mantle roots is not yet clearly understood.