Geochemistry of rock samples from Lau Basin

We use a combination of graphic projections for exploratory interpretation of natural phase equilibria and liquid lines of descent. The data used are original shipboard X-ray fluorescence (XRF) analyses, supplemented by additional XRF analyses completed on shore, microprobe analyses of minerals in key samples, and microprobe data for glasses. Good agreement exists in the data groupings and trends between the projections used (CMAS, Al-Fe, and Pearce plots). The natural phase assemblages in the rock samples are consistent in most cases with those predicted by the graphically projected phase boundaries and liquid lines of descent, suggesting a major role for crystal-melt equilibria in the evolution of these rocks. This assumption is further tested with materials balance calculations and by analogy to experimental phase equilibria studies on a sample from Hole 839B. Shifts in graphic projections of inferred phase boundaries resemble those obtained experimentally at elevated pressure water-saturated, as detailed elsewhere in this volume. Materials-balance calculations yield excellent fits for most major element oxides. This provides strong support for inferred fractionation models, which imply "wet" equilibria for several subsets of the data. However, trace element variations in some data subsets are difficult to equate with the models, based on major element data, and appear to require heterogeneity in melt sources and/or in the trace element signatures of possible xenocrystal phases. It is also possible that the distribution coefficients usually assumed for "dry" systems are significantly altered under the high water contents inferred for these magmas.

本研究采用图解投影相结合的方法，对天然相平衡与液相分异演化线开展探索性解译。所用数据以原始船上X射线荧光光谱（X-ray fluorescence, XRF）分析数据为基础，补充了实验室后续完成的额外XRF分析、关键样品中矿物的电子探针分析，以及岩浆玻璃的电子探针数据。所采用的CMAS图（CMAS）、Al-Fe图（Al-Fe）与Pearce图解（Pearce plots）在数据分组与演化趋势上展现出良好的一致性。
多数情况下，岩石样品中的天然矿物组合与图解投影得到的相边界及液相分异演化线的预测结果相符，表明晶体-熔体平衡在这类岩石的演化过程中发挥了主导作用。这一假设通过物质平衡计算，并类比839B钻孔（Hole 839B）样品的实验相平衡研究得到了进一步验证。推断相边界的图解投影偏移，与本卷其他章节详述的高压水饱和实验条件下得到的结果相似。
物质平衡计算对绝大多数主量元素氧化物均实现了极佳拟合，为所提出的分异演化模型提供了有力支撑，该模型暗示部分数据子集的岩浆体系处于“湿”平衡状态。然而，部分数据子集的微量元素变化难以通过基于主量元素数据构建的模型进行解释，这表明其熔体源区可能存在不均一性，或存在捕虏晶相的微量元素地球化学特征差异。此外，针对“干”体系所通常采用的分配系数，在本次研究推断的高水含量岩浆条件下，可能发生了显著改变。