Stacked gabbro units and intervening mantle: A detailed look at a section of IODP Leg 305, Hole U1309D
Stacked gabbro units and intervening mantle: A detailed look at a section of IODP Leg 305, Hole U1309D
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DOI：
10.1029/2008gc002012
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发表时间：
2008-10
期刊：
影响因子：
3.7
通讯作者：
G. Suhr;E. Hellebrand;K. Johnson;D. Brunelli
中科院分区：
文献类型：
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作者：
G. Suhr;E. Hellebrand;K. Johnson;D. Brunelli
孔U1309 D（综合大洋钻探计划（IODP）腿304/305）穿透1415米进入亚特兰蒂斯地块的海底，这是一个位于大西洋中脊30°N的海洋核心复合体。超过96%的回收岩石是辉长岩。在矿物化学概述的基础上，我们认为在海底以下≤800 ~ 1100 m之间存在一个岩浆单元，其范围从下部的橄榄辉长岩和橄榄长岩到上部的辉长岩和氧化辉长岩。在1235 mbsf以下，钻取了大量的辉长岩/氧化辉长岩，它们可能代表了下面岩浆单元的顶部。这里的重点是两个单元相互作用的区域，以及显微结构独特的、富含橄榄石的钙长岩的屏幕，总计50 m。我们认为，富橄榄石troctolite是前地幔岩石，它被转换为壳幔过渡带纯橄榄岩的基础上岩浆单位。后来，由于来自较低岩浆单元的熔体通过它，它被平衡到更进化的化学物质，并转化为细粒，富含橄榄石的troctolite。我们反对富橄榄石长角闪岩可能具有累积性质的主要论点是，富橄榄石长角闪岩中相容元素缺乏系统的井下趋势、其明显的细粒显微结构、cpx的高Cr含量及其富Ni橄榄石成分。富橄榄石troctolite中给定Mg/（Mg + Fe）的高NiO可以通过残留地幔橄榄石与温和演化熔体的简单平衡来模拟。演化熔体渗透通过富含橄榄石的长花岗岩的证据是富钛的间隙辉石，以及作为铬尖晶石中的包裹体的高度演化的角闪石和正辉石，加上毫米级的钠长岩脉的出现。演化熔体的渗透作用也是与蛇绿壳幔过渡带中概念上相似的岩石的主要区别。
Hole U1309D (Integrated Ocean Drilling Program (IODP) Legs 304/305) penetrated 1415 m into the seafloor of the Atlantis Massif, an oceanic core complex at 30°N, Mid‐Atlantic Ridge. More than 96% of all recovered rocks are gabbroic. On the basis of a mineral chemical overview, we suggest that between ≤800 and 1100 m below sea floor (mbsf), a magmatic unit occurs, ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part. Below 1235 mbsf, massive gabbronorites/oxide gabbros were drilled and they may represent the roof of an underlying magmatic unit. The focus here is on the zone where both units interact and screens, totaling 50 m, of a microstructurally distinct, olivine‐rich troctolite occur. We argue that the olivine‐rich troctolite is a former mantle rock which was converted to a crust‐mantle transition zone dunite at the base of the upper magmatic unit. Later, as melts derived from the lower magmatic unit percolated through it, it was equilibrated to a more evolved chemistry and transformed to a fine‐grained, olivine‐rich troctolite. Our main arguments against a possible cumulate nature of the olivine‐rich troctolite are the lack of a systematic downhole trend in compatible elements within the olivine‐rich troctolite, its distinctly fine‐grained microstructure, the high Cr content of cpx, and its Ni‐rich olivine composition. The high NiO for a given Mg/(Mg + Fe) in the olivine‐rich troctolite can be modeled by simple equilibration of relict mantle olivine with a mildly evolved melt. Evidence for the percolation of evolved melts through the olivine‐rich troctolites are Ti‐rich, interstitial pyroxenes and, as inclusions in Cr‐spinel, highly evolved amphiboles and orthopyroxenes plus the occurrence of millimeter‐scale noritic veins. The percolation by evolved melts would also be the major difference to otherwise conceptually similar rocks from the ophiolitic crust‐mantle transition zone.