Spatial variations in cooling rate in the mantle section of the Samail ophiolite in Oman: Implications for formation of lithosphere at mid-ocean ridges
Spatial variations in cooling rate in the mantle section of the Samail ophiolite in Oman: Implications for formation of lithosphere at mid-ocean ridges
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DOI：
10.1016/j.epsl.2017.02.038
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发表时间：
2017-05
影响因子：
5.3
通讯作者：
N. Dygert;P. Kelemen;Yan Liang
中科院分区：
文献类型：
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作者：
N. Dygert;P. Kelemen;Yan Liang
为了了解地幔如何冷却下大洋中脊扩张中心，我们应用了稀土在两辉石温度计和主要元素温度计橄榄岩从瓦迪Tayin Tayf在南部的Samail蛇绿岩在阿曼苏丹国，这代表了超过10公里的结构深度下的古莫霍面。从REE-双辉石温度计（T REE）推导出的辉石中稀土元素的闭合温度随剖面深度的增加而平稳而系统地降低，从地壳附近的> 1300° C到变质底部附近的< 1100° C，与先前观察到的矿物温度计中具有较低冷却温度的类似变化一致。估计的冷却速率从壳幔过渡带（MTZ）下方的0.3° C/y下降到MTZ下方6公里处的0.10 − 3° C/y。冷却速率来自钙橄榄石测温也降低移动到更深的部分。冷却速率的这些变化与上覆冷地壳下地幔的传导冷却最为一致。反过来，这表明热液循环延伸到快速扩张的山脊轴附近的MTZ，Samail蛇绿岩的火成地壳在那里形成。这些观测结果与关于下洋壳增生的席状岩床模型相一致，也与以前的工作相一致，表明Wadi Tayin Baghoff的地壳冷却速度非常快。我们的观察，结合以前的结果，表明在快速扩张中心下面的有效热液循环将最上地幔从岩浆温度冷却到< 1000° C，与无岩浆扩张中心的构造折返一样快。相比之下，对较低温度间隔的冷却敏感的温度计表明，Wadi Tayin橄榄岩冷却得比在海底附近沿沿着洋中脊取样的构造折返橄榄岩慢。地壳的热液冷却可能已经减弱，因此壳幔组合冷却得较慢，而深海橄榄岩在构造折返过程中的快速冷却继续到海底温度。
To understand how the mantle cools beneath mid-ocean ridge spreading centers, we applied a REE-in-two-pyroxene thermometer and major element thermometers to peridotites from the Wadi Tayin massif in the southern part of the Samail ophiolite in the Sultanate of Oman, which represent more than 10 km of structural depth beneath the paleo-Moho. Closure temperatures for REEs in pyroxenes deduced from the REE-in-two-pyroxene thermometer (T REE) decrease smoothly and systematically with depth in the section, from> 1300° C near the crust to< 1100° C near the metamorphic sole, consistent with previously observed, similar variations in mineral thermometers with lower cooling temperatures. Estimated cooling rates decrease from∼ 0.3° C/y just below the crust–mantle transition zone (MTZ) to∼ 10− 3° C/y at a depth of six km below the MTZ. Cooling rates derived from Ca-in-olivine thermometry also decrease moving deeper into the section. These variations in cooling rate are most consistent with conductive cooling of the mantle beneath a cold overlying crust. In turn, this suggests that hydrothermal circulation extended to the MTZ near the axis of the fast-spreading ridge where the igneous crust of the Samail ophiolite formed. These observations are consistent with the Sheeted Sills model for accretion of lower oceanic crust, and with previous work demonstrating very rapid cooling rates in the crust of the Wadi Tayin massif. Our observations, combined with previous results, suggest that efficient hydrothermal circulation beneath fast spreading centers cools the uppermost mantle from magmatic temperatures to< 1000° C as quickly as tectonic exhumation at amagmatic spreading centers. In contrast, thermometers sensitive to cooling over lower temperature intervals indicate that the Wadi Tayin peridotites cooled more slowly than tectonically exhumed peridotites sampled near the seafloor along mid-ocean ridges. Hydrothermal cooling of the crust may have waned, so that the crust–mantle package cooled more slowly, whereas rapid cooling of abyssal peridotites during tectonic exhumation continued to seafloor temperatures.