Abstract Submarine methane emissions in the Tuscan Archipelago have been studied since the 1960s, both for economic and research purposes. Offshore gas seepage is mainly concentrated southward and westward of Elba island, along N-S faults related to recent extensional activity in the Tuscan shelf and N-S trending positive magnetic anomalies, which have been interpreted as serpentinites associated with ophiolitic rocks due to their very high magnetic susceptibility. This study focuses on the gas chemistry of a new emission site corresponding to a shallow water mud volcano in the Scoglio d’Affrica area. The Scoglio d’Affrica seep has a gas composition typical of mud volcanoes, with methane as the prevalent component (95 vol%) and minor gases which include carbon dioxide, nitrogen and trace amounts of helium. The combined stable C and H isotope composition of CH4 (δ13C and δ2H) and the enrichment in heavy carbon isotopes of CO2, highlight a prevalent secondary microbial origin for these fluids (δ13C∼−35.8 ‰ vs VPDB; δ2H∼−166 ‰ vs VSMOW; δ13CCO2 up to + 21.7 ‰ vs VPDB). Thus, in spite of the occurrence of positive magnetic anomalies, a possible abiotic origin of methane is excluded. Moreover, the gas from the mud volcano is extremely depleted in 3He and presents typical 3He/4He ratios of a geological setting in which radiogenic crustal helium is strongly predominant. A photo-mosaic of the mud volcano is also reported. A possible connection with other submarine methane emissions in the Tuscan Archipelago is limited to emissions located few kilometers from the Scoglio d’Affrica area. Recent emissions in the area suggest that gases similar in composition from distinct reservoirs, find their way to the surface from Eocene deposits in different time intervals and through different faults and fractures, placed along the Elba-Pianosa ridge.

中文翻译：

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意大利北第勒尼安海浅层海底泥火山

摘要 自 1960 年代以来，出于经济和研究目的，人们一直在研究托斯卡纳群岛的海底甲烷排放。海上天然气渗流主要集中在厄尔巴岛的南部和西部，沿与托斯卡纳大陆架最近伸展活动相关的 NS 断层和 NS 趋势的正磁异常，由于其非常高的磁化率，被解释为与蛇绿岩相关的蛇纹岩。这项研究的重点是与 Scoglio d'Africa 地区的浅水泥火山相对应的新排放地点的气体化学。Scoglio d'Africa 渗流具有泥火山的典型气体成分，主要成分为甲烷（95 vol%），少量气体包括二氧化碳、氮气和痕量氦气。CH4 的稳定 C 和 H 同位素组成（δ13C 和 δ2H）和 CO2 重碳同位素的富集，突出了这些流体的普遍次生微生物来源（δ13C∼−35.8 ‰ vs VPDB；δ2H∼−166 ‰ vs VSMOW ; δ13CCO2 高达 + 21.7 ‰ vs VPDB)。因此，尽管发生了正磁异常，但排除了甲烷的可能非生物来源。此外，来自泥火山的气体在 3He 中极度枯竭，呈现出典型的 3He/4He 比率，在其中放射性地壳氦强烈占主导地位的地质环境中。还报道了泥火山的照片马赛克。与托斯卡纳群岛其他海底甲烷排放的可能联系仅限于距离 Scoglio d'Africa 地区几公里处的排放。