Mantle-derived helium and carbon in groundwaters and gases of Mount Etna, Italy
Mantle-derived helium and carbon in groundwaters and gases of Mount Etna, Italy
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意大利埃特纳火山地下水和气体中源自地幔的氦和碳
DOI：
10.1016/s0012-821x(97)00052-6
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发表时间：
1997
影响因子：
5.3
通讯作者：
C. Fléhoc
中科院分区：
文献类型：
--
作者：
P. Allard;P. Jean;W. D’Alessandro;F. Parello;B. Parisi;C. Fléhoc
我们首次对埃特纳火山地下水和气体中的氦和碳同位素进行了详细的研究，为了解这座非常活跃的火山的岩浆气体释放的分布、起源和预算提供了新的见解。根据两种流体的位置和被空气（气体）或溶解空气与有机碳（水）的混合物稀释的程度，确定了地幔源岩浆成分，其最终3he4he比为6.9±0.2 Raand δ13C约为- 4‰。除峰顶区外，该岩浆成分优先集中在富含二氧化碳的地下水中，这些地下水来自火山西南南侧和东部两翼的两个偏远地区，其比例随着大气补给高度（或路径流长度）的增加而增加。这种模式表明，除了可能的局部气体输入外，地下水在渗透并流经火山堆中一个较高的气体涌出部分时，还收集了大量溶解的岩浆He和C，其中埃特纳的东南-南断裂带是最佳候选。这些观测结果为该火山的远程地球化学监测提供了一个新的框架。岩浆气体端元的3he4he比与埃特纳玄武岩富氦橄榄石晶体中氦的3he4he比一致（平均范围：6.7±0.4 Ra,[2,3]），表明其被地壳基底放射成因的He稀释可以忽略不计，进一步限制了现今埃特纳岩浆的3he4he比。虽然低于典型的8 Ra的MORB值，但对于欧洲大陆的活火山来说，这一比值是最高的，可能是该地区下方上涌的相对放射性成因的上地幔带。据估计，埃特纳火山地幔源co2和3he的排放量分别占全球陆基火山排放量的10%和15%。这种巨大的贡献是由于大部分未喷发的富含He和c的碱性玄武岩岩浆不断脱气，这种脱气主要发生在火山中央的开放管道中，其次发生在火山的侧翼。地下水只携带co2和3he排放总量的一小部分（≈3%）。
We report the first detailed investigation of both helium and carbon isotopes in groundwaters and gases of Mt. Etna, providing new insight into the distribution, origin and budget of magmatic gas release at this very active volcano [1]. A mantle-derived magmatic component, with ultimate3He4He ratio of 6.9 ± 0.2 Raand δ13C of about −4‰, is identified in both types of fluids, depending on their location and the extent of their dilution by either air (gases) or a mixture of dissolved air and organic carbon (waters). Apart from the summit zone, this magmatic component is preferentially concentrated in CO2-rich groundwaters that issue from two remote sectors of the south-southwest and eastern volcano flanks, where its proportion increases with the altitude of meteoric recharge (or the length of pathflow) of the waters. Such a pattern suggests that, in addition to possible local gas input, the groundwaters collect much of their dissolved magmatic He and C while they infiltrate and flow through one of the more elevated, gas-effusing parts of the volcanic pile, among which the south-southeast fracture zone of Etna is the best candidate. These observations provide a new framework for remote geochemical monitoring of the volcano. The3He4He ratio of the magmatic gas end-member coincides with that of helium trapped in the He-rich olivine crystals of Etna basalts (mean range: 6.7 ± 0.4 Ra, [2,3]), pointing to its negligible dilution by radiogenic He from the crustal basement and further constraining the3He4He ratio of the present-day Etna magma. While being lower than the typical MORB value of 8 Ra, this ratio is the highest for an active volcano in continental Europe and probably tracks a relatively radiogenic upper mantle zone that is upwelling beneath this region [4]. The estimated outputs of mantle-derived CO2and3He from Etna account for 10% and 15%, respectively, of estimates for global subaerial volcanic emissions. This huge contribution results from continuous degassing of mostly unerupted He- and C-rich alkaline basaltic magma, which occurs principally through the central open conduits and secondarily through the flanks of the volcano. Groundwaters carry only a minor fraction (≈ 3%) of total emitted CO2and3He.