Roof assimilation at fast spreading ridges: An investigation combining geophysical, geochemical, and field evidence
Roof assimilation at fast spreading ridges: An investigation combining geophysical, geochemical, and field evidence
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DOI：
10.1029/2001jb001171
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发表时间：
2003-01-03
影响因子：
3.9
通讯作者：
O'Hara, MJ
中科院分区：
文献类型：
--
作者：
Coogan, LA;Mitchell, NC;O'Hara, MJ
同化作用的作用，在快速扩张的洋中脊的轴向岩浆房（AMC）的屋顶进行了研究，使用现场，地球物理和地球化学数据。从Ophiacia的实地观测表明，屋顶同化是一个共同的过程，但提供的材料同化量的限制。基于地球物理数据的参数，包括2A/2B层边界在AMC宽度上的沉降，以及大轴向峰槽的深度，需要至少类似于50 -100 m的同化。然而，东太平洋海隆（EPR）上AMC深度在短轴向距离上的显著变化表明，AMC深度在短时间尺度上的波动可能高达500 m。EPR玄武岩中氯过度富集的地球化学模拟表明，大约20%的洋壳可能经历了一个结晶、蚀变、同化的循环。这是远远超过可以解释的屋顶下沉，并表明，在屋顶的水平波动必须占同化的大部分。同化最有可能发生在一些不同的时间尺度上，从十年，由于在热制度的扰动有关的筑堤，从地幔岩浆供应控制的较长时期的波动。大规模的屋顶同化的影响下地壳的吸积由于热损失与同化，并解释海洋地壳的地球化学。
The role of assimilation at the roof of the axial magma chamber (AMC) at fast spreading mid-ocean ridges is investigated using field, geophysical, and geochemical data. Field observations from ophiolites indicate that roof assimilation is a common process, but provide little constraint on the amount of material assimilated. Arguments based on geophysical data that include the subsidence of the layer 2A/2B boundary across the width of the AMC, and the depth of large axial summit troughs, require at least similar to50-100 m of assimilation. However, significant variations in the depth of the AMC on the East Pacific Rise (EPR), over short along-axis distances, suggest up to similar to500 m fluctuations in the depth of the AMC may be common on short timescales. Geochemical modeling of the over-enrichment of chlorine in EPR basalts suggests that similar to20% of the oceanic crust may go through a cycle of crystallization, alteration, then assimilation. This is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation. Assimilation most likely occurs on a number of different timescales ranging from decadal, due to perturbations in the thermal regime related to diking, to longer-period fluctuations controlled by magma supply from the mantle. Large-scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation, and for the interpretation of the geochemistry of the oceanic crust.