Gabbros from IODP Site 1256, equatorial Pacific: Insight into axial magma chamber processes at fast spreading ocean ridges
Gabbros from IODP Site 1256, equatorial Pacific: Insight into axial magma chamber processes at fast spreading ocean ridges
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DOI：
10.1029/2011gc003655
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发表时间：
2011-09-22
影响因子：
3.5
通讯作者：
Ildefonse, B.
中科院分区：
文献类型：
--
作者：
Koepke, J.;France, L.;Ildefonse, B.
ODP/IODP 多段活动在 ODP 站点 1256（科科斯板块，赤道东太平洋）首次对快速扩张的洋壳进行连续原位采样，从喷出的熔岩穿过片状岩脉，向下进入最上面的辉长岩。本文重点对 IODP 312 探险期间钻探的辉长岩部分进行详细的岩相学和微观分析研究。在许多 1256D 孔辉长岩中可以观察到的明显的斑片状和点状特征主要是由于两个不同数量的不同岩性域的紧密关联：(1) 亚绿岩域和 (2) 粒状基质。主量和微量元素矿物成分、地温测量和岩石学模型表明，亚绿岩和粒状区域遵循原位分馏形成的单一岩浆演化趋势。次绿质区域对应于相对原始的高温端元，其成分与喷出单元剖面中的玄武岩和岩脉相似，而粒状区域则符合通过晶体分馏至较低温度的岩浆演化，最高结晶度接近 80%。我们的结果支持以下 ODP 站点 1256 轴向熔融透镜体化石化的情景：在接近液相线条件下相对原始的 MORB 熔融物填充熔融透镜体并供给上部的喷出地壳。在较低温度下，在熔融透镜片状岩脉边界附近，结晶从第一个斜长石开始，然后在糊状区域中的单斜辉石形成次绿晶域。当温度降低时，亚绿晶域继续结晶，最终形成一个连接良好的框架。演化后的残余熔体最终被困在亚绿网络内，在接近固相线条件下结晶成粒状基质。 1256D 洞辉长岩的另一个重要结构特征是存在微颗粒区域，这些区域被解释为停止/同化的片状岩脉的遗迹（通过接触变质作用转变为“花岗质岩脉”）。所有这些不同的区域都可以紧密关联地观察到，通常是在薄片尺度上，展示了轴向熔融透镜体中正在进行的组合结晶/同化过程的极其复杂的岩石学记录。与 1256 号站点观察到的非常相似的辉长岩具有明显的斑点/斑驳外观，并且具有与 1256 号站点观察到的相同的岩性域的密切关联，这些辉长岩在来自阿曼蛇绿岩的所谓“杂纹辉长岩”单元中被发现，位于相同的结构水平，位于堆积辉长岩和花岗质岩脉之间。 IODP 孔 1256D 和阿曼蛇绿岩之间的辉长岩/岩脉过渡的岩石学相似性表明，原位分异和岩脉同化/污染是控制快速扩张的洋脊处轴向熔融透镜体的动力学和化石化的主要岩浆过程。
The ODP/IODP multileg campaign at ODP Site 1256 (Cocos plate, eastern equatorial Pacific) provides the first continuous in situ sampling of fast spreading ocean crust from the extrusive lavas, through the sheeted dikes and down into the uppermost gabbros. This paper focuses on a detailed petrographic and microanalytical investigation of the gabbro section drilled during IODP Expedition 312. The marked patchy and spotty features that can be observed in many Hole 1256D gabbros is mostly due to a close association of two different lithological domains in variable amounts: (1) subophitic domains and (2) a granular matrix. Major and trace element mineral compositions, geothermometry, and petrological modeling suggest that subophitic and granular domains follow one single magma evolution trend formed by in situ fractionation. The subophitic domains correspond to the relative primitive, high-temperature end-member, compositionally similar to the basalts and dikes from the extrusive unit upsection, while the granular domains fit with a magma evolution by crystal fractionation to lower temperatures, up to a degree of crystallization of similar to 80%. Our results support the following scenario for the fossilization of the axial melt lens at ODP Site 1256: relatively primitive MORB melts under near-liquidus conditions fill the melt lens and feed the upper, extrusive crust. Near the melt lens-sheeted dike boundary at lower temperatures, crystallization starts with first plagioclase before clinopyroxene in a mushy zone forming the subophitic domains. At decreasing temperatures, the subophitic domains continue to crystallize, finally forming a well-connected framework. Evolved, residual melt is finally trapped within the subophitic network, crystallizing at near-solidus conditions to the granular matrix. Another important textural feature in Hole 1256D gabbros is the presence of microgranular domains which are interpreted as relics of stoped/assimilated sheeted dikes (transformed to "granoblastic dikes" by contact metamorphism). All these different domains can be observed in close association, often at the thin section scale, demonstrating the extremely complex petrological record of combined crystallization/assimilation processes ongoing in the axial melt lens. Very similar gabbros with a marked spotty/patchy appearance, and bearing the same close association of lithological domains as observed at Site 1256, are known in the so-called "varitextured gabbro" unit from the Oman Ophiolite located at the same structural level, between cumulate gabbros and granoblastic dikes. The close petrological similarity of the gabbro/dike transition between both IODP Hole 1256D and the Oman ophiolite suggests that in situ fractionation and dike assimilation/contamination are major magmatic processes controlling the dynamics and fossilization of the axial melt lens at fast spreading oceanic ridges.