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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Noble gas and carbon isotopic signatures of Cape Verde oceanic carbonatites: Implications for carbon provenance
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Noble gas and carbon isotopic signatures of Cape Verde oceanic carbonatites: Implications for carbon provenance

机译：佛得角海洋碳酸盐岩的稀有气体和碳同位素特征：对碳源的影响

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摘要

We discuss noble gas (He, Ne, At, Kr, and Xe) and C isotope signatures of carbonatites from the Cape verae Archipelago. These are the first noble gas compositions ever reported for oceanic carbonatites. The noble gas analyses were performed by crushing the calcite and apatite separates. Some of the analyzed calcites present low He-4/He-3 ratios (down to 46,700; R/Ra up to 15.5) that cannot be explained by the addition of cosmogenic He-3, demonstrating that carbonatite magmas came from a reservoir characterised by low time-integrated (U + Th)/He-3. Such a reservoir is thought to be localised in the deep lower mantle, constraining the depth of origin of the Cape Verde plume. in contrast, apatite samples return highly radiogenic He-4/He-3 signatures due to their high Th and U contents. An in situ source for 4 He in these apatites is further supported by air-like or lower Ne-20/Ne-22 ratios and relatively high Ne-21/Ne-22 ratios (up to 0.0485), which result from Ne-21 (and Ne-22) formation by nucleogenic reactions. Some apatites plot to the left of the MORB line in the neon three-isotope diagram. This is explained by mass-fractionation processes since these apatites are also characterised by Ar-38/Ar-36 ratios lower than the air value or even than the range of values usually used to characterise the MORB-OIB field. Considering that carbonate recycling, which could be characterised by high levels of Te and Ba. would simultaneously increase with time the Xe-129 and Xe-130, the observed 129Xe anomalies (Xe-129/Xe-130 up to 6.84) cannot be explained by models calling upon crustal carbonate recycling. We attribute these anomalies to an ancient mantle origin by decay of the now extinct I-129. Moreover, experimental work has suggested that crustal carbonates are unlikely to be transported to deep lower mantle depths as a consequence of their removal by melting reactions at subduction. Thus, our noble gas data are indicative of a non-recycled origin for carbon, endorsing the role of primordial carbon in the genesis of Cape Verde carbonatites. This conclusion is supported by typical mantle delta C-13 values (-8.0 to -4.2 parts per thousand) that are lighter than those characterising crustal inorganic carbonates. The fact that He is the noble gas element characterised by the highest solubility in magmas, the faster diffusion and the lower mineral/melt partition coefficients is taken as an explanation for its decoupling from the other noble gases, with He being the one better retaining a deep lower mantle signal.
机译：我们讨论了来自佛得角群岛的碳酸盐岩的稀有气体（He，Ne，At，Kr和Xe）和C同位素特征。这些是海洋碳酸盐岩中首次报道的稀有气体成分。通过粉碎方解石和磷灰石分离物进行稀有气体分析。一些分析的方解石呈现低He-4 / He-3比率（低至46,700; R / Ra高达15.5），无法通过添加宇宙成因的He-3来解释，这表明碳酸盐岩浆来自于具有以下特征的储层：低时间积分（U + Th）/ He-3。这种储层被认为位于下地幔深处，从而限制了佛得角羽流的起源深度。相反，磷灰石样品由于其高的Th和U含量而返回高放射力的He-4 / He-3信号。这些空气磷灰石中4 He的原位来源进一步得到了类似气态的或更低的Ne-20 / Ne-22比值以及相对较高的Ne-21 / Ne-22比值（最高0.0485）的支持，这是由Ne-21导致的（和Ne-22）通过成核反应形成。在霓虹灯三同位素图中，一些磷灰石标绘在MORB线的左侧。这是通过质量分级过程来解释的，因为这些磷灰石的特征还在于Ar-38 / Ar-36的比值低于空气值，甚至低于通常用于表征MORB-OIB场的值范围。考虑到碳酸盐的再循环，其特征可能在于高含量的Te和Ba。随Xe-129和Xe-130同时增加，观测到的129Xe异常（Xe-129 / Xe-130最高为6.84）无法用要求地壳碳酸盐再循环的模型解释。我们将这些异常归因于现已灭绝的I-129的衰落，是一个古老的地幔起源。此外，实验工作表明，地壳碳酸盐不太可能被运输到较深的地幔深处，这是由于它们在俯冲时通过熔化反应而被除去。因此，我们的稀有气体数据表明碳的不可回收来源，这证明了原始碳在佛得角碳酸盐岩成因中的作用。该结论得到地幔δC-13值（-8.0至-4.2千分之一）的支持，该值比地壳无机碳酸盐的特征值轻。 He是一种稀有气体元素，其特征是在岩浆中具有最高的溶解度，扩散较快且矿物/熔体分配系数较低，这一事实可作为其与其他稀有气体解耦的一种解释，其中He是一种更好地保留一种惰性气体的元素。下地幔深部信号。

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