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

该项目旨在破译榴辉岩中赋存的水的数量，并通过大洋板块俯冲进入地幔，促进岩浆的产生。在100 km深处，榴辉岩中的水主要赋存于石榴石和绿辉石中。不幸的是，人们对造山榴辉岩中的H2O知之甚少。为了弥合这一认识差距，首先对石榴石进行了研究，确定了其水含量、取代机理和影响因素(压力、温度、矿物组成)。最初成功的项目阶段表明，造山榴辉岩中的石榴石能够吸收相当于角闪石体积分数为3%的水，水与钙呈正相关。H2O与压力的虚假关联归因于压力增强的石榴石中的钙含量。值得注意的是，OH-矿物的共稳定性是控制石榴石中H2O的一个重要变量。如果存在羟基矿物，则石榴石不含水。另一项新发现是，石榴石含有多达三种H2O，其中两种是次要的。因此，初级H2O的定量是具有挑战性的，但通过对红外光谱的详细分析成功地解决了这一问题。未被识别的二次水导致H2O含量高得不切实际(如在一些文献中的例子)。二期工程最令人意想不到的结果是，石英榴辉岩和柯石英榴辉岩中的绿辉石H2O含量完全相同。这意味着H2O的加入不取决于PT峰的条件，至少不能超过30kbar和900℃。由于超过这些条件的含水矿物不能释放更多的H2O，所获得的H2O含量应该反映出输送到&gt；100公里深处的量。尽管取得了这些进展，但仍然很难可靠地破译绿辉石中的H2O含量，因为对于绿辉石来说，红外光谱测定的H2O含量强烈依赖于红外吸收系数的选择。因此，必须采用一种额外的、独立的H2O分析方法。倡导者乐观地认为，在项目的最后阶段，利用核方法来解决这个问题，准确地说是弹性反冲探测分析(ERDA)和质子-质子散射分析(PPSA)的组合。我们的工作组成功地将PPSA应用于橄榄石中的H2O分析。绿辉石中持续储水的初步结果如果得到证实，将具有深远的意义。考虑到在这些条件之外不会发生进一步的脱水反应，榴辉岩应该会在其NAMS中保存榴辉岩相含水矿物释放的大部分水，深度可达100公里。在进一步俯冲期间，这些H2O体积应该留在系统中，而不是释放到地幔楔形，因此，这也应该适用于流体活动的元素。

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.