Isotopic Diversity in Mangaia Melt Inclusions: Mantle Source or Crustal Assimilation?

Mangaia熔体包裹体的同位素多样性：地幔来源还是地壳同化？智力优势：橄榄石赋存的熔体包裹体(MIs)被深部岩浆管道中不断生长的斑晶捕获，并提供了？快照？在熔体完全聚集之前具有不同的熔体成分。MI揭示了仅从全岩熔岩分析中并不清楚的常量和微量元素多样性，这种多样性的来源是一个激烈辩论的来源。一种端元假说认为，MI多样性反映了地幔源区成分的不均一性。或者，MI多样性可能是岩浆作用的结果，包括地壳同化作用。在一个案例中，来自曼盖亚岛(库克-南极洲群岛)的单一熔岩中的橄榄石寄主熔岩显示出极端的铅同位素变异性，范围是所有洋岛和MORB熔岩中观察到的范围的一半。令人惊讶的是，其中一些错误含有的铅比在任何曼加亚熔岩中观察到的放射性要低得多。这些数据引发了许多根本性的问题。为什么包裹体不能代表整体岩浆的同位素特征？这是不是因为来自单一熔岩样本的熔岩是来自同位素不同的地幔来源的一系列熔体？或者，同位素多样性是否反映了岩浆上升过程中岩石圈的同化作用？建议对Mangaia的一组新样品中的Mis的微量元素和同位素组成进行测量，以便这些综合数据可以用于评估地壳同化作用和其他导致铅同位素多样性的过程。对来自Mangaia的14个Mis的初步研究暗示了地壳同化的可能影响，但数据有限，还需要额外的数据来最终描述和评估相互竞争的假设。特别是，如果Mangaia mis中的铅同位素多样性是由地壳同化过程引起的，则预计Pb同位素组成将与同化的示踪物(例如高Cl和B、Sr同位素)相关，而如果同位素多样性反映Mangaia下不同的地幔来源，情况就不太可能是这样。为了能够对这些替代品进行全面评价，需要资金收集新的高精度铅和锶同位素数据，以及扩大原始芒盖亚熔岩的管理信息收集所需的微量元素和挥发性数据。这项拟议的研究旨在限制岩浆作用和地幔来源在产生非均质包裹体方面的相对作用，并将为岩浆在岩石圈中的迁移和侵位过程提供见解。广泛的影响：拟议的研究将支持一名职业生涯早期的非终身教职员工和一名博士生。该项目还将涉及几名本科生顾问。2010年在库克群岛进行实地考察时，少年派在曼加亚高中教授学生有关当地火山活动的知识，并在拉罗通加接受了当地报纸的采访，最后发表了一篇文章描述了他在那里的研究。国际和平协会还会见了库克群岛总理，讨论了拟议工作的相关性。

Isotopic diversity in Mangaia melt inclusions: Mantle source or crustal assimilation?Intellectual Merit: Olivine-hosted melt inclusions (MIs) are trapped by growing phenocrysts in magma conduits at depth and provide ?snapshots? of diverse melt compositions before complete melt aggregation. MIs reveal major and trace element diversity that is not clear from analyses of whole rock lavas alone and the origin of this diversity is a source of vigorous debate. One end member hypothesis is that the MI diversity reflects heterogeneity in mantle source compositions. Alternatively, MI diversity may result from magmatic processes, including crustal assimilation. In one case, olivine-hosted MIs from a single lava from the island of Mangaia (Cook-Austral Islands) exhibit extreme Pb-isotopic variability spanning half the range observed in all ocean island and MORB lavas. Surprisingly, some of these MIs contain Pb that is significantly less radiogenic than observed in any Mangaian lavas. These data give rise to many fundamental questions. Why is it that the inclusions are not isotopically representative of the bulk magma? Is this because MIs from a single lava sample an array of melts derived from isotopically-diverse mantle sources? Or, does the isotopic diversity reflect assimilation of the lithosphere during magma ascent? It is proposed to measure trace element and isotopic compositions of MIs in a new set of samples from Mangaia so that these comprehensive data can be used to evaluate crustal assimilation and other processes responsible for the Pb isotopic diversity. Preliminary study of 14 MIs from Mangaia hint at a possible influence of crustal assimilation, but the data are limited additional data are required to conclusively describe and assess the competing hypotheses. In particular, if the Pb isotopic diversity in Mangaia MIs is caused by crustal assimilation processes, it is expected that Pb-isotopic compositions will correlate with tracers of assimilation (e.g., high Cl and B, Sr isotopes), whereas this is unlikely to be the case if the isotopic diversity reflects diverse mantle sources beneath Mangaia. To enable comprehensive evaluation of these alternatives, funds are requested to collect new high-precision Pb- and Sr-isotopic data as well as trace element and volatile data for an expanded collection of MIs from primitive Mangaia lavas. The proposed research aims to constrain the relative roles of magmatic processes and mantle sources in generating heterogeneous inclusions, and will provide insights into magma transport and emplacement processes in the lithosphere.Broader Impacts: The proposed study will support an early-career non-tenured faculty and one PhD student. The project will also involve several undergraduate advisees. While carrying out fieldwork in the Cook Islands in 2010, the PI taught students about local volcanism at the Mangaia high school and interviewed with the local newspaper on Rarotonga, resulting in an article that describes his research there. The PI also met with the Prime Minister of the Cook Islands to discuss the relevance of the proposed work.