A new view of the He–Ar–CO2 degassing at mid-ocean ridges: Homogeneous composition of magmas from the upper mantle
A new view of the He–Ar–CO2 degassing at mid-ocean ridges: Homogeneous composition of magmas from the upper mantle
复制标题
大洋中脊 He-Ar-CO2 脱气的新观点：上地幔岩浆的均质成分
DOI：
10.1016/j.gca.2006.12.019
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发表时间：
2007
影响因子：
5
通讯作者：
M. Martelli
中科院分区：
文献类型：
--
作者：
A. Paonita;M. Martelli
深海勘探正在迅速提高我们对洋中脊火成岩产品中挥发物地球化学的认识。它还对地球地幔的脱气过程施加了更大的限制，其结果是，基于蒸汽-熔融平衡的脱气模型不再能够解释越来越多的数据。事实上，这些模型强制假定上地幔在任何尺度上都是强烈非均匀的，并且不能解释回收的火成岩产物中广泛存在的碳过饱和。在这里，我们回顾了全球He-Ar-CO2数据集的流体包裹体在大洋中脊玻璃的框架下，先进的模拟多组分气泡生长的岩浆。我们显示，He，Ar和CO2之间的非平衡分馏，由它们在硅酸盐熔体中的不同扩散率驱动，是常见的，在大多数的自然条件下的岩浆减压和他们的签名强烈依赖于压力的脱气。由于Ar和CO2的扩散率相当，低压岩浆脱气作用对He/Ar和He/CO2比值的分馏程度相当，而高压CO2扩散较慢，导致Ar/CO2比值的早期动力学效应，从而显著改变了岩浆脱气途径。来自不同海脊段的一套数据之间的非常不同的地球化学特征主要取决于岩浆房的深度，储存熔体。此外，单个岩套内部的变化突出了侵位熔岩上升速度和冷却速率的变化。因此，来自大西洋中脊24-30°N段和Rodriguez三联点的玻璃在He/Ar几乎恒定的情况下，He/CO2和Ar/CO2比值的大幅度变化被解释为高压特征。相反，东太平洋海隆、皮托海山和东南印度洋海岭的He/CO2和He/Ar同时增加，表明低压分馏作用占主导地位，这意味着浅部岩浆房的深度低于大西洋中脊24-30°N和Rodriguez三联点。我们的结论支持存在的扩张速率和深度之间的关系的高温区脊下，并与地震研究所建议的岩浆房的深度是一致的。非平衡脱气解释了大洋中脊玄武岩的挥发性系统学，从一个单一的幔源岩浆开始，免除了假设需要在大洋中脊以下地幔中的挥发物丰度比的非均匀性。
Deep-sea exploration is rapidly improving our understanding of volatiles geochemistry in mid-ocean-ridge igneous products. It is also placing greater constraints on degassing processes of the Earth’s mantle, with the result that degassing models based on vapour–melt equilibrium are no longer able to explain the increasing number of data. In fact, such models force to postulate an upper mantle strongly heterogeneous at any scale, and cannot account for the widespread carbon supersaturation of the recovered igneous products. Here we review the global He–Ar–CO2dataset of fluid inclusions in mid-ocean-ridge glasses using the framework of advanced modelling of multicomponent bubble growth in magmas. We display that non-equilibrium fractionations among He, Ar and CO2, driven by their different diffusivities in silicate melts, are common in most of the natural conditions of magma decompression and their signature strongly depends on pressure of degassing. Due to the comparable Ar and CO2diffusivity, magma degassing at low pressure fractionates both the He/Ar and He/CO2ratio by a similar extent, while the slower CO2diffusion at high pressure causes early kinetic effects on Ar/CO2ratio and dramatically changes the degassing path. On this ground, the very different geochemical signatures among suites of data coming from different ridge segments mainly depend on the depth of the magma chamber where the melt was stored. Besides, the variations inside a single suite highlight variable ascent speed and cooling rate of the emplaced lava. The large variations in both the He/CO2and Ar/CO2ratios at almost constant He/Ar, displayed in glasses coming from the Mid-Atlantic Ridge 24–30°N segment and the Rodriguez Triple Junction, are therefore interpreted as a high-pressure signature. In contrast, the simultaneous increase in both He/CO2and He/Ar of the East Pacific Rise, Pito Seamount and South-East Indian Ridge data sets suggests the dominance of low-pressure fractionation, implying that the shallow magma chambers are at a lower depth than those of the Mid-Atlantic Ridge 24–30°N and Rodriguez Triple Junction. Our conclusions support the presence of a relationship between spreading rate and depth of high-temperature zones below ridges, and are consistent with the depth of magma chambers as suggested from seismic studies. Non-equilibrium degassing explains the volatile systematics of mid-ocean-ridge basalts by starting from a single mantle-derived magma, dispensing with the supposed need for heterogeneities in abundance ratios of volatiles in the mantle below oceanic ridges.