Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components
Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components
复制标题
中美洲地热环境中氮和甲烷的来源：地壳和岩浆挥发性成分的稀有气体和 129I 证据
DOI：
--
复制
发表时间：
2003
期刊：
影响因子：
--
通讯作者：
A. Hunt
中科院分区：
文献类型：
--
作者：
G. Snyder;R. Poreda;U. Fehn;A. Hunt
对中美洲四个地热田的气体和流体进行了氮、甲烷和氦浓度、同位素组成和 129I/I 比值分析，以确定这些系统中挥发物的来源。其中三个气田的气体比率和同位素组成结果与其他俯冲带的观测结果一致。 N2/3He 的比率仅略高于平均弧值 1 × 108，中美洲系统的 N2 火山通量估计在 1.6 × 108 至 3.2 × 108 摩尔/年之间。 129I/I 比值分析表明，整个研究区域存在俯冲有机成分（25-30 Ma）以及更古老的地壳成分（40-65 Ma）。然后推断中美洲氮气和稀有气体的岩浆通量，以确定岛弧系统中氮气循环的程度。全球 N2 通量估计为 2.7 × 109 至 5.4 × 109 mol/年，与全球洋中脊通量相当，占俯冲沉积物通量的 29-58%。这一通量估计在 N2-CO2-He 系统中是一致的，表明几乎所有供应到地幔楔的氮都在火山前缘下方脱挥发分。尼加拉瓜莫莫托姆博地热田的特点是氮和甲烷的过量值异常高，这两种气体的密切相关性表明它们具有共同的来源。虽然高热流沉积盆地中含有过量氮的情况并不罕见，但莫莫托姆博地热田的独特之处在于，高 N2/3He 气体基本上具有岩浆 3He/4He 比率。尼加拉瓜的高过量氮成分与较古老的碘端元有关，表明其起源于地壳。沿弧的尼加拉瓜部分的地壳氮贡献约为 2.2 × 108 mol N2/年，或大致等于沿整个中美洲火山弧的岩浆贡献。莫莫托博火山的结果表明，岛弧系统重组过程中氮的释放可能对火山氮的全球通量产生重大影响。
Gases and fluids from four geothermal fields of Central America were analyzed for nitrogen, methane, and helium concentrations, isotopic composition, and 129I/I ratios in order to determine the sources of volatiles in these systems. Results for gas ratios and isotopic compositions for three of the fields are consistent with observations from other subduction zones. Ratios of N2/3He are only slightly higher than average arc values of 1 × 108 and the volcanic flux of N2 for the Central American systems is estimated to be between 1.6 × 108 and 3.2 × 108 mol/yr. Analysis of 129I/I ratios indicates the presence of a subducted organic component (25–30 Ma) as well as of a much older crustal component (40–65 Ma) throughout the study area. The magmatic flux of nitrogen and noble gases in Central America was then extrapolated to determine the degree of nitrogen recycling in island arc systems. Global N2 flux is estimated at 2.7 × 109 to 5.4 × 109 mol/yr, which is comparable to the global mid‐ocean ridge flux, and represents 29–58% of the subducted sediment flux. This flux estimate is consistent across the N2–CO2–He systems and suggests that nearly all of the nitrogen supplied to the mantle wedge is devolatilized beneath the volcanic front. The Momotombo geothermal field of Nicaragua is characterized by exceptionally high excess nitrogen and methane values, and the close correlation of these two gases indicates a common source. While it is not uncommon for sedimentary basins with high heat flow to have excess nitrogen, the Momotombo geothermal field is unique in that the high N2/3He gases have essentially magmatic 3He/4He ratios. The high excess nitrogen component of Nicaragua is related to the older iodine end‐member, pointing to a crustal origin. The crustal nitrogen contributions along the Nicaraguan portion of the arc are on the order of 2.2 × 108 mol N2/yr or roughly equal to the magmatic contribution along the entire Central American volcanic arc. The results for Momotombo indicate that the release of nitrogen during reorganization of island arc systems may have a significant impact on the global flux of volcanic nitrogen.