The Helium and Carbon Isotope Characteristics of the Andean Convergent Margin
The Helium and Carbon Isotope Characteristics of the Andean Convergent Margin
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DOI：
10.3389/feart.2022.897267
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发表时间：
2022-06-13
影响因子：
2.9
通讯作者：
Giovannelli, D.
中科院分区：
文献类型：
--
作者：
Barry, P. H.;De Moor, J. M.;Giovannelli, D.
俯冲带代表地球内部（地壳和地幔）和外部（大气和海洋）之间的界面，其中碳和其他挥发性元素通过板块构造在地球储层之间积极循环。氦是一种敏感的挥发性来源示踪剂，可用于弧中的地幔和地壳来源的退卷;然而，它被认为不会通过俯冲过程再循环到地幔中。相比之下，碳很容易再循环，主要是以富碳沉积物的形式，因此可以用来了解通过俯冲挥发物的输送。此外，碳是化学反应和同位素分馏可以用来确定控制电弧系统内的挥发性运动的主要过程。在这里，我们报告的氦同位素和丰度数据42深源流体和气体样品从中央火山区（CVZ）和南部火山区（SVZ）的安第斯会聚边缘（ACM）。数据用于评估俯冲参数的影响（例如，地壳厚度、俯冲输入和会聚速率）对地表火山流体和气体排放中挥发物成分的影响。He同位素从CVZ弧后的范围从0.1到2.6 R-A（n = 23），最高的值在普纳和最低的亚安第斯前陆褶皱冲断带。SVZ的大气校正He同位素范围为0.7至5.0 R-A（n = 19）。总而言之，这些数据显示He-3/He-4明显向东南增加，最高值（在SVZ中）低于与纯上地幔氦相关的标称范围（8 +/- 1 R-A），接近平均He弧气体的同位素值（5.4 +/- 1.9 R-A）。值得注意的是，在CVZ中发现了最低值，这表明更重要的地壳输入（即，He-4的同化）到氦预算。CVZ的地壳厚度（高达70公里）明显大于SVZ，SVZ的地壳厚度仅为40公里。我们认为，地壳厚度施加的流体地壳相互作用的程度上的主要控制，氦和其他挥发物上升通过上板在ACM。我们还报告了CVZ中（n = 11）个地点的碳同位素，其中δ C-13在千分之-15.3和千分之-1.2之间变化[与维也纳Pee Dee Belemnite（VPDB）相比]，CO2/He-3值变化超过两个数量级（6.9 x 10（8）-1.7 x 10（11））。在SVZ中，还报告了来自（n = 13）站点的碳同位素比率，其变化范围为-17.2 ppm至-4.1 ppm。CO2/He-3值变化超过四个数量级（4.7 x 10（7）-1.7 x 10（12））。低Δ C-13和CO2/He-3值与CO2去除一致（例如，方解石沉淀和气体溶解）。碳同位素分馏模型表明，方解石沉淀发生在与生命温度上限（122摄氏度）相一致的温度下，这表明生物学可能在与弧有关的火山流体和气体排放的C-He系统学中发挥作用。
Subduction zones represent the interface between Earth's interior (crust and mantle) and exterior (atmosphere and oceans), where carbon and other volatile elements are actively cycled between Earth reservoirs by plate tectonics. Helium is a sensitive tracer of volatile sources and can be used to deconvolute mantle and crustal sources in arcs; however it is not thought to be recycled into the mantle by subduction processes. In contrast, carbon is readily recycled, mostly in the form of carbon-rich sediments, and can thus be used to understand volatile delivery via subduction. Further, carbon is chemically-reactive and isotope fractionation can be used to determine the main processes controlling volatile movements within arc systems. Here, we report helium isotope and abundance data for 42 deeply-sourced fluid and gas samples from the Central Volcanic Zone (CVZ) and Southern Volcanic Zone (SVZ) of the Andean Convergent Margin (ACM). Data are used to assess the influence of subduction parameters (e.g., crustal thickness, subduction inputs, and convergence rate) on the composition of volatiles in surface volcanic fluid and gas emissions. He isotopes from the CVZ backarc range from 0.1 to 2.6 R-A (n = 23), with the highest values in the Puna and the lowest in the Sub-Andean foreland fold-and-thrust belt. Atmosphere-corrected He isotopes from the SVZ range from 0.7 to 5.0 R-A (n = 19). Taken together, these data reveal a clear southeastward increase in He-3/He-4, with the highest values (in the SVZ) falling below the nominal range associated with pure upper mantle helium (8 +/- 1 R-A), approaching the mean He isotope value for arc gases of (5.4 +/- 1.9 R-A). Notably, the lowest values are found in the CVZ, suggesting more significant crustal inputs (i.e., assimilation of He-4) to the helium budget. The crustal thickness in the CVZ (up to 70 km) is significantly larger than in the SVZ, where it is just similar to 40 km. We suggest that crustal thickness exerts a primary control on the extent of fluid-crust interaction, as helium and other volatiles rise through the upper plate in the ACM. We also report carbon isotopes from (n = 11) sites in the CVZ, where delta C-13 varies between -15.3 parts per thousand and -1.2 parts per thousand [vs. Vienna Pee Dee Belemnite (VPDB)] and CO2/He-3 values that vary by over two orders of magnitude (6.9 x 10(8)-1.7 x 10(11)). In the SVZ, carbon isotope ratios are also reported from (n = 13) sites and vary between -17.2 parts per thousand and -4.1 parts per thousand. CO2/He-3 values vary by over four orders of magnitude (4.7 x 10(7)-1.7 x 10(12)). Low delta C-13 and CO2/He-3 values are consistent with CO2 removal (e.g., calcite precipitation and gas dissolution) in shallow hydrothermal systems. Carbon isotope fractionation modeling suggests that calcite precipitation occurs at temperatures coincident with the upper temperature limit for life (122 degrees C), suggesting that biology may play a role in C-He systematics of arc-related volcanic fluid and gas emissions.