Infrared spectroscopy on garnet of eclogite and associated ultramafic rocks: Importance of garnet for water transport into the mantle

榴辉岩和相关超镁铁岩石榴石的红外光谱：石榴石对于水输送到地幔的重要性

• 批准号: 201026938
• 负责人: Professorin Dr. Esther Schmädicke
• 金额: --
• 依托单位: Division of Nuclear Physics
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/201026938?language=en
• 关键词: Infrared; spectroscopy; garnet; eclogite; associated

This project is aimed at deciphering the water amount hosted in eclogite and transported into the mantle by subduction of oceanic plates, facilitating magma generation. At >100 km depth, water in eclogite is mainly stored in garnet and omphacite. Unfortunately, little is known about H2O in orogenic eclogite. To close this gap of knowledge, garnet was studied first to define its H2O-amount, the substitution mechanisms, and the influencing parameters (pressure, temperature, mineral composition). The initial, successful project stage revealed that garnet of orogenic eclogite is able to incorporate as much H2O as c. 3 vol.% amphibole and that H2O is positively correlated to Ca. The spurious correlation of H2O and pressure is ascribed to the pressure-enhanced Ca-content of garnet. Notably, the co-stability of OH-minerals is an important variable governing H2O in garnet. If OH-minerals are present, garnet is free of water. Another novel finding is that garnet contains up to three H2O species, two of them secondary. Thus, the quantification of primary H2O is challenging, but this problem was successfully solved by detailed analysis of infrared (IR) spectra. Unrecognized secondary water results in unrealistically high H2O contents (as in some literature examples). The most unexpected result of the second project stage was that the omphacite H2O content in quartz eclogite and coesite eclogite is identical. This implies that H2O incorporation does not depend on the PT-peak conditions, at least not up to 30 kbar and 900 ºC. Since no more H2O can be released from hydrous minerals beyond these conditions, the obtained H2O contents should reflect the amount being transported to >100 km depth. Despite this progress, it remains difficult to reliably decipher the H2O content in omphacite because, for omphacite, the H2O contents determined by IR spectroscopy strongly depend on the choice of the IR absorption coefficient. Therefore, an additional, independent method for H2O analysis has to be applied. The proponent is optimistic to solve this problem in a final project stage by utilizing a nuclear method, precisely a combination of Elastic Recoil Detection Analysis (ERDA) und Proton-Proton Scattering Analysis (PPSA). Our working group successfully applied PPSA for the analysis of H2O in olivine. The preliminary result of constant water storage in omphacite up to 30 kbar and 900 ºC, if confirmed, has far-reaching implications. Given that beyond these conditions no further dehydration reactions are expected, eclogite should preserve most of the water liberated by eclogite-facies hydrous minerals up to 100 km depth in its NAMs. During further subduction, this H2O volume should remain in the system and not be released to the mantle wedge, which, consequently, should apply also to fluid-mobile elements.

该项目的目的是破译榴辉岩中的水量，并通过大洋板块俯冲输送到地幔，促进岩浆生成。当深度大于100 km时，榴辉岩中的水主要储存在石榴子石和绿辉石中.然而，对于造山带榴辉岩中的H_2O却知之甚少。为了填补这一知识空白，首先研究石榴石，以确定其H2O量，取代机制和影响参数（压力，温度，矿物成分）。初步的，成功的项目阶段表明，造山榴辉岩的石榴石能够纳入尽可能多的水作为c。3体积% H_2O与Ca成正相关。H2O与压力的假相关性归因于压力增强的石榴子石Ca含量。值得注意的是，OH-矿物的共稳定性是控制石榴石中H2O的重要变量。如果存在OH-矿物质，石榴石不含水。另一个新的发现是石榴石含有多达三种H2O物质，其中两种是次生的。因此，初级H2O的定量是具有挑战性的，但通过详细分析红外（IR）光谱成功地解决了这个问题。未识别的二次水导致不切实际的高H2O含量（如在一些文献示例中）。项目第二阶段最出乎意料的结果是石英榴辉岩和柯石英榴辉岩中绿辉石H2O含量相同。这意味着H2O掺入不依赖于PT峰条件，至少不依赖于30 kbar和900 ºC。由于含水矿物在超过这些条件后不再释放出H2O，因此所获得的H2O含量应反映被输送到>100 km深度的量。尽管取得了这一进展，但仍然难以可靠地破译绿辉石中的H2O含量，因为对于绿辉石，通过IR光谱法测定的H2O含量强烈依赖于IR吸收系数的选择。因此，必须采用额外的独立H2O分析方法。支持者乐观地认为，通过利用核方法，确切地说是弹性反冲检测分析（ERDA）和质子-质子散射分析（PPSA）的组合，可以在项目的最后阶段解决这个问题。我们的工作组成功地将PPSA应用于橄榄石中H2O的分析。绿辉石在高达30千巴和900 ºC的温度下持续储水的初步结果如果得到证实，将产生深远的影响。考虑到超出这些条件，预计不会发生进一步的脱水反应，榴辉岩应将榴辉岩相含水矿物释放的大部分水保存在其NAM中长达100 km的深度。在进一步的俯冲过程中，这一H2O体积应保持在系统中，而不是释放到地幔楔，因此，这也应该适用于流体活动的元素。