Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D
Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D
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DOI：
10.1016/j.epsl.2015.01.042
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发表时间：
2015-04
影响因子：
5.3
通讯作者：
M. Harris;R. Coggon;C. Smith-Duque;M. Cooper;J. Milton;D. Teagle
中科院分区：
文献类型：
--
作者：
M. Harris;R. Coggon;C. Smith-Duque;M. Cooper;J. Milton;D. Teagle
位于赤道东太平洋的ODP 1256 D孔是第一个穿透快速扩展洋壳至岩脉-辉长岩过渡区的完整剖面的钻孔，也是继DSDP 504 B孔之后对席状岩脉进行现场取样的第二个钻孔。在这里，1256号站点的全岩和矿物锶同位素组成的高空间分辨率记录与岩心观察和井下电缆地球物理测量相结合，以确定玄武岩的范围。1256站火山岩序列以席状熔岩流和块状熔岩流为主，但Sr同位素剖面显示，与海水的交换有限。然而，两个异常厚（>25米）的块状流序列的上边缘强烈的热液蚀变与锶同位素比值升高，似乎是管道的横向低温离轴流体流动。在熔岩的其他地方，高87 Sr/86 Sr仅限于角砾岩层位。在熔岩岩脉过渡的矿化的玻璃质角砾岩强烈改变镁皂石，二氧化硅和黄铁矿，表明由混合海水和冷却的热液流体蚀变。在席状岩墙复合体中，87 Sr/86 Sr比值普遍向热液流体值（0.705）偏移。岩脉冷缘显示出在轴向补给和排泄过程中形成的次生矿物组合，其87 Sr/86 Sr比岩脉核高，表明流体优先沿沿着岩脉边缘流动。岩墙-辉长岩过渡带中87 Sr/86 Sr的局部增加表明流体沿着25 - 50 m厚辉长岩侵入体的近水平侵入边界的通道，辉长岩体核部中87 Sr/86 Sr仅略有增加。与504 B孔枕状熔岩为主的剖面相比，1256站的Sr同位素测量结果表明，大洋上地壳中热液循环的程度可能强烈地依赖于喷发类型。席状和块状流为主的熔岩序列典型的快速扩张的山脊可能会经历相对有限的流通，但可能有更广泛的流通通过枕状熔岩为主的部分。此外，孔1256 D席状岩脉显示更大程度的锶同位素交换相比，从孔504 B岩脉。由于海水来源的热液流体在其演化为黑烟型流体的过程中必须经过岩脉，因此504 B孔和1256 D孔的不同Sr同位素剖面表明，在快速和中间扩张脊处的洋中脊热液系统存在显著变化，这可能会影响不同温度下流体-岩石交换所动员的元素的地球化学循环。
ODP Hole 1256D in the eastern equatorial Pacific is the first penetration of a complete section of fast spread ocean crust down to the dike–gabbro transition, and only the second borehole to sample in situ sheeted dikes after DSDP Hole 504B. Here a high spatial resolution record of whole rock and mineral strontium isotopic compositions from Site 1256 is combined with core observations and downhole wireline geophysical measurements to determine the extent of basalt–hydrothermal fluid reaction and to identify fluid pathways at different levels in the upper ocean crust.The volcanic sequence at Site 1256 is dominated by sheet and massive lava flows but the Sr isotope profile shows only limited exchange with seawater. However, the upper margins of two anomalously thick (>25 m) massive flow sequences are strongly hydrothermally altered with elevated Sr isotope ratios and appear to be conduits of lateral low-temperature off-axis fluid flow. Elsewhere in the lavas, high87Sr/86Sr are restricted to breccia horizons. Mineralised hyaloclastic breccias in the Lava–Dike Transition are strongly altered to Mg-saponite, silica and pyrite, indicating alteration by mixed seawater and cooled hydrothermal fluids. In the Sheeted Dike Complex87Sr/86Sr ratios are pervasively shifted towards hydrothermal fluid values (∼0.705). Dike chilled margins display secondary mineral assemblages formed during both axial recharge and discharge and have higher87Sr/86Sr than dike cores, indicating preferential fluid flow along dike margins. Localised increases in87Sr/86Sr in the Dike–Gabbro Transition indicates the channelling of fluids along the sub-horizontal intrusive boundaries of the 25 to 50 m-thick gabbroic intrusions, with only minor increases in87Sr/86Sr within the cores of the gabbro bodies.When compared to the pillow lava-dominated section from Hole 504B, the Sr isotope measurements from Site 1256 suggest that the extent of hydrothermal circulation in the upper ocean crust may be strongly dependent on the eruption style. Sheet and massive flow dominated lava sequences typical of fast spreading ridges may experience relatively restricted circulation, but there may be much more widespread circulation through pillow lava-dominated sections. In addition, the Hole 1256D sheeted dikes display a much greater extent of Sr-isotopic exchange compared to dikes from Hole 504B. Because seawater-derived hydrothermal fluids must transit the dikes during their evolution to black smoker-type fluids, the different Sr-isotope profiles for Holes 504B and 1256D suggest there are significant variations in mid-ocean ridge hydrothermal systems at fast and intermediate spreading ridges, which may impact geochemical cycles of elements mobilised by fluid–rock exchange at different temperatures.