Alteration and hydrothermal circulation in the lower oceanic crust (IODP Expedition 345 Hess Deep Plutonic Crust)

考虑到海洋地壳覆盖了我们地球的2/3，我们对它的了解仍然非常少！这是因为，除了地壳最上层的火山层以外，很难对任何其他地方进行取样，特别是在太平洋，那里的扩张速度很快（10-15厘米/年），地形平坦，断层陡坎中地壳较深部分的暴露很少。下地壳可以取样的一个地方是赫斯深海，这是一个通往太平洋海底的构造窗口，位于赤道附近，加拉帕戈斯群岛以西约800公里。IODP第345次考察的目的是钻探出露在断层间裂脊南坡的辉长岩下地壳的更深部分，这次考察的目标有很多，但这项提议的重点是下洋壳的蚀变、变质和热液冷却。水渗入下地壳的深度、时间、温度条件和程度都是未知的，可以通过使用薄片和化学分析的船上调查以及通过以后的陆上工作加以限制，这些工作包括：1）锶和氧同位素分析，2）使用电子显微镜和电子探针微量分析仪表征矿物结构和化学性质，3）矿物中流体包裹体的化学研究; 4）冷却速率的数值模拟。

Considering that oceanic crust covers about 2/3 of our planet, we still know remarkably little about it! This is because it is extremely difficult to sample anything other than the uppermost volcanic layer of the crust, especially in the Pacific where the spreading rate is fast (10-15 cm/yr) and the topography is smooth with few exposures of deeper parts of the crust in fault scarps. One place where the lower crust CAN be sampled is Hess Deep, a tectonic window into the Pacific ocean floor situated near the equator and about 800 km west of the Galapagos Islands. IODP expedition 345 aims to drill into the deeper part of the gabbroic lower crust exposed on the south slope of the fault-bounded interrift ridge.There are many objectives for the expedition, but this proposal focuses on alteration, metamorphism and hydrothermal cooling in the lower oceanic crust. The depth, timing, temperature conditions and extent of water penetration are all unknown in the lower crust, and can be constrained by shipboard investigation using thin sections and chemical analyses, and by later work onshore through: 1) strontium and oxygen isotope analysis, 2) characterisation of mineral textures and chemistry using the electron microscope and the electron probe microanalyser, 3) study of the chemistry of fluid inclusions trapped in minerals 4) numerical modelling of cooling rates.