Helium and carbon isotope systematics of cold "mazuku" CO2 vents and hydrothermal gases and fluids from Rungwe Volcanic Province, southern Tanzania
Helium and carbon isotope systematics of cold "mazuku" CO2 vents and hydrothermal gases and fluids from Rungwe Volcanic Province, southern Tanzania
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DOI：
10.1016/j.chemgeo.2012.07.003
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发表时间：
2013-02-15
期刊：
影响因子：
3.9
通讯作者：
Ramirez, C.
中科院分区：
文献类型：
--
作者：
Barry, P. H.;Hilton, D. R.;Ramirez, C.
我们报告了新的氦和碳同位素（He-3/He-4和δ C-13）和相对丰度一套20种气体和流体的（CO2/He-3）特征（冷马祖库一样的二氧化碳喷口，冒泡的泥盆，冷热泉），来自龙圭火山省（RVP）的11个不同地点，坦桑尼亚南部和坦桑尼亚北方另外3个地方（Oldoinyo Lengai火山和纳特龙湖）。在RVP，流体和气体的特征是大范围的He同位素组成（He-3/He-4）从0.97 R-A至7.18 R-A（其中R-A =空气He-3/He-4），δ C-13比率范围很窄，从-2.8到-65份/千（相对于VPDB），CO2/He-3值的范围很大，跨越近四个数量级（4x 10（9）至3.2x10（13））。Oldoinyo Lengai具有上地幔样的He-CO2特征，如先前所报道的（Fischer等人，2009年），而纳特龙湖的温泉具有低He-3/He-4（类似于0.6 R-A），CO2/He-3（类似于5- 15 x10（8））和中等Δ C-13（类似于-3.7至-4.9千分之一）。在RVP，流体相样品已被修改的水热相分离的复杂效应，产生CO2/He-3和δ C-13值高于假定的起始组合物。相比之下，气相样品没有受到类似的影响，因此保留了更多的地幔CO2/He-3和δ C-13值。然而，添加地壳挥发物，特别是来自富含He-4的储集岩的放射性成因氦，改变了除了三个冷CO2气体喷口（即，Mazuku）地区（Ikama村，Kibila冷喷口和Kiejo冷喷口），保留了原始的上地幔He同位素（类似于7 R-A）和He-CO2特征。地壳污染的程度取决于热液系统内部相互作用的程度，这种相互作用随着与每个主要火山中心的距离而增加。相比之下，我们建议，原始的冷CO2 Mazuku气体收集在地层接触的RVP火山的侧翼可能会挖掘孤立的气袋，形成在以前的喷发事件，并保持脱钩从当地的热液系统。此外，通过识别和利用未经修改的气体样品，我们确定地幔与地壳来源的二氧化碳，我们用它来估计幔源二氧化碳通量在Rungwe和纳特龙湖。最后，我们研究了RVP中流体/气体和镁铁质斑晶之间He同位素明显差异的起源（来自希尔顿等人，2011），并讨论构造（即，裂谷带动力学）和成岩条件，区分RVP从其他羽状有关陆上裂谷带。(C)2012爱思唯尔有限公司版权所有。
We report new helium and carbon isotope (He-3/He-4 and delta C-13) and relative abundance (CO2/He-3) characteristics of a suite of 20 gases and fluids (cold mazuku-like CO2 vents, bubbling mud-pots, hot and cold springs) from 11 different localities in Rungwe Volcanic Province (RVP), southern Tanzania and from 3 additional localities in northern Tanzania (Oldoinyo Lengai Volcano and Lake Natron). At RVP, fluids and gases are characterized by a large range in He-isotope compositions (He-3/He-4) from 0.97 R-A to 7.18 R-A (where R-A = air He-3/He-4), a narrow range in delta C-13 ratios from -2.8 to -65 parts per thousand (versus VPDB), and a large range in CO2/He-3 values spanning nearly four orders of magnitude (4x10(9) to 3.2x10(13)). Oldoinyo Lengai possesses upper-mantle-like He-CO2 characteristics, as reported previously (Fischer et al., 2009), whereas hot springs at Lake Natron have low He-3/He-4 (similar to 0.6 R-A), CO2/He-3 (similar to 5-15x10(8)) and intermediate delta C-13 (similar to-3.7 to -4.9 parts per thousand). At RVP, fluid phase samples have been modified by the complicating effects of hydrothermal phase-separation, producing CO2/He-3 and delta C-13 values higher than postulated starting compositions. In contrast, gas-phase samples have not been similarly affected and thus retain more mantle-like CO2/He-3 and delta C-13 values. However, the addition of crustal volatiles, particularly radiogenic helium from He-4-rich reservoir rocks, has modified He-3/He-4 values at all but the three cold CO2 gas vent (i.e., mazuku) localities (Ikama Village, Kibila Cold Vent and Kiejo Cold Vent) which retain pristine upper-mantle He-isotope (similar to 7 R-A) and He-CO2 characteristics. The extent of crustal contamination is controlled by the degree of interaction within the hydrothermal system, which increases with distance from each major volcanic center. In contrast, we propose that pristine cold CO2 mazuku gases collected at stratigraphic contacts on the flanks of RVP volcanoes may potentially tap isolated gas pockets, which formed during previous eruptive events and have remained decoupled from the local hydrothermal system. Furthermore, by identifying and utilizing unmodified gas samples, we determine mantle versus crustal provenance of the CO2, which we use to estimate mantle-derived CO2 fluxes at both Rungwe and Lake Natron. Finally, we investigate the origin of the apparent discrepancy in He isotopes between fluids/gases and mafic phenocrysts at RVP (from Hilton et al., 2011), and discuss the tectonic (i.e., rift zone dynamics) and petrogenic conditions that distinguish RVP from other plume-related subaerial rift zones. (C) 2012 Elsevier B.V. All rights reserved.