(Appendix) Petrographic information, trace element and Sr isotope ratios of Pito Deep samples

Fluid flow through the axial hydrothermal system at fast spreading ridges is investigated using the Sr-isotopic composition of upper crustal samples recovered from a tectonic window at Pito Deep (NE Easter microplate). Samples from the sheeted dike complex collected away from macroscopic evidence of channelized fluid flow, such as faults and centimeter-scale hydrothermal veins, show a range of 87Sr/86Sr from 0.7025 to 0.7030 averaging 0.70276 relative to a protolith with 87Sr/86Sr of ~0.7024. There is no systematic variation in 87Sr/86Sr with depth in the sheeted dike complex. Comparison of these new data with the two other localities that similar data sets exist for (ODP Hole 504B and the Hess Deep tectonic window) reveals that the extent of Sr-isotope exchange is similar in all of these locations. Models that assume that fluid-rock reaction occurs during one-dimensional (recharge) flow lead to significant decreases in the predicted extent of isotopic modification of the rock with depth in the crust. These model results show systematic misfits when compared with the data that can only be avoided if the fluid flow is assumed to be focused in isolated channels with very slow fluid-rock exchange. In this scenario the fluid at the base of the crust is little modified in 87Sr/86Sr from seawater and thus unlike vent fluids. Additionally, this model predicts that some rocks should show no change from the fresh-rock 87Sr/86Sr, but this is not observed. Alternatively, models in which fluid-rock reaction occurs during upflow (discharge) as well as downflow, or in which fluids are recirculated within the hydrothermal system, can reproduce the observed lack of variation in 87Sr/86Sr with depth in the crust. Minimum time-integrated fluid fluxes, calculated from mass balance, are between 1.5 and 2.6 * 10**6 kg/m**2 for all areas studied to date. However, new evidence from both the rocks and a compilation of vent fluid compositions demonstrates that some Sr is leached from the crust. Because this leaching lowers the fluid 87Sr/86Sr without changing the rock 87Sr/86Sr, these mass balance models must underestimate the time-integrated fluid flux. Additionally, these values do not account for fluid flow that is channelized within the crust.本研究以复活节微板块东北部皮托深谷（Pito Deep）构造窗（tectonic window）采集的上地壳样品的锶同位素（Sr-isotopic）组成为研究对象，对快速扩张脊轴部热液系统内的流体流动特征展开探究。采集自席状岩墙杂岩（sheeted dike complex）、且未观测到断层、厘米级热液脉等通道化流体流动宏观证据的样品，其87Sr/86Sr比值介于0.7025至0.7030之间，平均值为0.70276；对应的原岩（protolith）87Sr/86Sr比值约为0.7024。席状岩墙杂岩的87Sr/86Sr比值未随深度呈现系统性变化。将本次新获得的数据与另外两个拥有同类数据集的研究区——大洋钻探计划（ODP）504B钻孔及赫斯深谷构造窗（Hess Deep tectonic window）——进行对比后发现，所有研究区域的锶同位素交换程度均较为相似。假设流体-岩石反应发生于一维（补给）流动过程的模型预测，地壳内岩石的同位素改造程度会随深度显著降低，但上述模型结果与实测数据存在系统性偏差；只有假设流体流动集中于流体-岩石交换速率极慢的孤立通道内，才能消除这种偏差。在此场景下，地壳底部的流体87Sr/86Sr比值几乎未受海水影响，因此与喷口流体存在显著差异。此外，该模型预测部分岩石的87Sr/86Sr比值应与新鲜原岩完全一致，但该现象并未被观测到。与之相对，若假设流体-岩石反应同时发生于上升（排泄）与下降（补给）流动过程，或热液系统内存在流体循环，则此类模型能够复现实测的87Sr/86Sr比值不随地壳深度变化的观测结果。基于质量平衡法计算得到的迄今所有研究区域的时间积分流体通量最小值介于1.5×10^6至2.6×10^6 kg/m²之间。然而，来自岩石样品及喷口流体组分汇编数据的新证据表明，地壳中有部分锶被淋滤出来。由于该淋滤过程会降低流体的87Sr/86Sr比值，但不会改变岩石的该同位素比值，因此上述质量平衡模型必然低估了时间积分流体通量。此外，该类通量值未考虑地壳内通道化的流体流动。