Geochemistry of rock samples from Lau Basin@en

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.本研究采用多组图形投影方法，对天然相平衡与液相演化线（liquid lines of descent）开展探索性解译。所用数据包含原始船上X射线荧光光谱（XRF）测试分析结果，辅以岸基补充XRF测试分析、关键样品中矿物的电子探针（microprobe）测试分析结果，以及玻璃相的电子探针数据。所用的三类投影图（CMAS图、Al-Fe图与皮尔奇图解（Pearce plots））在数据分组与变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相组合与图形投影得到的相边界及液相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关键作用。本研究通过物质平衡计算，以及类比839B钻孔（Hole 839B）样品的实验相平衡研究，进一步验证了这一假设。推断相边界的图形投影偏移特征，与本卷其他章节详述的高压水饱和实验所得结果高度相似。物质平衡计算对多数主量元素氧化物的拟合效果极佳，这为推断的分异演化模型提供了有力支撑，该模型表明数据集的多个子集均处于"wet"（含水）平衡状态。然而，基于主量元素数据来看，部分数据子集的微量元素变化难以与该模型匹配，这似乎暗示熔体源区存在不均一性，或可能的捕虏晶相的微量元素地球化学特征存在不均一性。此外，针对"dry"（无水）体系通常采用的分配系数，在本研究推断的岩浆高含水量条件下可能发生显著改变。