Primary Igneous Graphite in Ultramafic Xenoliths: II Isotopic Composition of the Carbonaceous Phases Present in Xenoliths and Host Lava at Tissemt (Eggéré, Alegerian Sahara)
Primary Igneous Graphite in Ultramafic Xenoliths: II Isotopic Composition of the Carbonaceous Phases Present in Xenoliths and Host Lava at Tissemt (Eggéré, Alegerian Sahara)
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超镁铁质包体中的原生火成石墨：Tissemt 包体和宿主熔岩中存在的碳质相的 II 同位素组成（艾格雷，阿尔及利亚撒哈拉）
DOI：
10.1093/petrology/28.2.313
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发表时间：
1987
影响因子：
3.9
通讯作者：
J. Kornprobst
中科院分区：
文献类型：
--
作者：
F. Pineau;M. Javoy;J. Kornprobst
对阿尔及利亚撒哈拉埃热雷地区Tissemt碱性玄武岩中一套超镁铁质捕虏体的橄榄石和斜方辉石伴生石墨堆晶进行了13 C/12 C比值分析。这些石墨的δ 13 C值在-24.6到-14.4之间变化，在一个给定的样品中，内部变化高达1.6/mil。碳酸盐，与火山相有关，在中等温度下沉淀，δ 13 C值为−0·8至+ 3·5，δ 18 O值为21·6至31·5。它可以分为三个部分：主峰在670 - 985 °C之间获得，并且也通过缓慢的酸侵蚀释放，δ 13 C高达+7标准3，代表钠沸石中捕获的CO2;高温峰（1160 °C以上，-25·8时为47 p. p. m）将代表硅酸盐中捕获的碳，并且存在从300 °C到高温提取的未很好确定的额外碳，其代表-23·0，可能的分数低至−68·0。石墨被解释为碳从间隙液体中沉淀的结果，该间隙液体先前被脱气，然后被正在进行的结晶过程过饱和。火山事件可以被认为是一个第一近似的碳从捕虏体氧化成CO2，随后部分还原冷却和沉淀的碳酸盐和捕获的残余CO2沸石。只有挥发性还原的物质才能被显著地脱气，因此δ 13 C的总量可能接近（或略高于）原始的δ 13 C。
13C/12C ratios have been analysed in graphite adcumulate crystals associated with the olivine and orthopyroxene of a suite of ultramafic xenoliths in the alkali basalts of Tissemt (Eggéré, Algerian Sahara). The δ13C of these graphites varies from −24·6 to − 14·4 per milk with internal variations of up to 1·6 per mille within a given sample. Carbonates, linked to the volcanic phase and precipitated at medium temperatures have δ13C values from −0·8 to + 3·5 and δ18O values from 21·6 to 31·5 The host lava has a high carbon content of 675 p.p.m. with a global δ13C of −4·2. It can be separated into three parts: a major peak is obtained between 670 and 985 °C and also liberated by slow acid attack with a δ13C of up to +7·3 and representing CO2trapped in the natrolite; a high temperature peak (above 1160 °C with 47 p.p.m at −25·8) will represent carbon trapped in silicates and there is an additional carbon not well determined extracted from 300 °C to high temperature which represent 215 p.p.m. at −23·0 with a possible fraction as low as −68·0. The graphite is interpreted as the result of the precipitation of carbon from an interstitial liquid previously outgassed then supersaturated by the ongoing crystallization process. The volcanic event can be considered to a first approximation as remobilizing the carbon from the xenoliths by oxidation to CO2, subsequent partial reduction upon cooling and precipitation of carbonate and trapping of residual CO2within zeolite. Only volatilereduced species can be significantly outgassed so that the total δ13C of −4·2 is probably close to (or a little higher than) the original δ13C.