Testing the Processes of Melt Differentiation in the Mexican Volcanic Belt

该项目由岩石学和地球化学部与国际科学和工程学办公室共同支持。它的目的是以墨西哥弧为例，研究俯冲带岩浆形成(“分异”)的过程。在地球表面，俯冲带形成了与深海海沟(如喀斯喀德、阿留申群岛、墨西哥)平行的明显的火山链(“火山弧”)。自从俯冲带的作用被认识到以来，弧岩浆的形成就引起了地球科学家的兴趣，但对它的了解仍然很少。然而，了解弧岩浆的形成过程对于定量评估弧外通量(包括挥发分外流)对宜居地球的演化和维持的影响是至关重要的。该项目将测试最近发现的在墨西哥中部火山带原始碱性和钙碱性岩浆中普遍存在的不寻常的富镍橄榄石(高达0.35-0.53wt%Ni)是否会提供重要线索。这些富镍橄榄岩是如此不同寻常，因为它们排除了弧岩浆只是熔体或熔体的混合物，它们要么起源于(I)俯冲洋壳(板岩)，位于火山弧下方约110-120公里，(Ii)地球橄榄岩地幔，深度45-110公里，或(Iii)墨西哥火山所在的先前存在的地壳(0-45公里深度)。因此，必须应用其他过程。一个可能的过程可能是在弧状岩浆源区形成了一种新的辉石岩岩性。这种岩性可能是板岩富硅熔体与弧下橄榄岩地幔反应形成的，并通过熔融产生弧状岩浆。该研究计划对墨西哥弧型火山岩中的橄榄石斑晶进行了详细的研究，包括：(A)岩石结构，(B)激光烧蚀橄榄石中的微量元素(Co，Mn，Ca，Sc，V，Ni，Ti，Al，B和Li)，(C)挥发物(H2O，CO2，S，F，Cl)和B和B同位素在其原始熔体包裹体中，用SIMS方法，和(D)橄榄石分离的氦同位素比值。由于最近在其他弧区也发现了类似的富镍橄榄石，如喀斯喀特、日本、阿留申群岛和堪察加群岛，因此该项目的结果对全球弧火山活动具有更广泛的影响。该项目促进了美国三个机构之间以及美国、墨西哥和苏格兰之间的科学伙伴关系。它还支持一名残疾女性职业科学家。

This project was co-supported by the Petrology and Geochemistry with the Office of International Science and Engineering. It aims to study processes leading to magma formation ('differentiation') in subduction zones using the Mexican arc as an example. On the Earth's surface, subduction zones form conspicuous chains of volcanoes ('volcanic arcs') parallel to the deep-sea trenches (e.g. the Cascades, Aleutians, Mexico). The formation of arc magmas has intrigued earth scientists ever since the role of subduction zones was recognized, and yet it remains poorly understood. Understanding the process of arc magma formation, however, is essential to quantitatively assess the impact of the arc out flux (including the out flux of volatiles) on the evolution and maintenance of a habitable Earth. This project will test whether recently discovered unusual Ni-rich olivines (up to 0.35-0.53 wt% Ni) that are ubiquitous in primitive alkaline and calc-alkaline magmas of the central Mexican Volcanic Belt may provide a vital clue. These Ni-rich olivines are so unusual since they rule out that arc magmas are simply melts, or mixtures of melts, that either originate from (i) the subducting oceanic crust ('slab'), located approximately ~110-120 km below the volcanic arc, (ii) the Earth peridotite mantle, at 45-110 km depth, or (iii) the pre-existing crust (0-45 km depth) on which the Mexican volcanoes are constructed. Thus, other processes must apply. A possible process may be the formation of a new 'pyroxenite lithology' in the source region of arc magmas. This lithology may form by reaction of silica-rich melts from the slab and the subarc peridotite mantle and generate arc magmas by melting. The research plan involves a detailed study of olivine phenocrysts from Mexican arc volcanic rocks that include: (a) petrographic textures, (b) trace elements in olivines (Co, Mn, Ca, Sc, V, Ni, Ti, Al, B and Li) by laser-ablation methods, (c) volatiles (H2O, CO2, S, F, Cl) and B and B isotopes in their primary melt inclusions by SIMS methods, and (d) He-isotope ratios of olivine separates. Since similar Ni-rich olivines have been recently been found in other arcs, e.g. the Cascades, Japan, Aleutians and Kamchatka, the outcome of this project has broader implications for global arc volcanism. The project promotes scientific partnership between three institutions in the U.S., as well as between the U.S., Mexico and Scotland. It also supports a female career scientist with a disability.