Many geological and geodynamical studies of metamorphism in subduction zones have relied upon worldwide compilations of modelled slab‐top pressure–temperature (P–T) conditions, although recent evaluation of such data sets suggests that these predictions are ~100–300°C colder at any given pressure than the conditions recorded by exhumed metamorphic rocks. As such, geochemical, petrological and geophysical interpretations formulated using such ‘cold’ assumptions may be subject to error and uncertainty. Here, we apply thermodynamic phase equilibrium calculations to forward‐model how phase assemblages, the P–T conditions of key devolatilization reactions and the effect of densification with depth vary for typical mid‐ocean ridge basalt (MORB) along these newly defined ‘hotter’ subduction zone geotherms for cold, warm and average environments. The depth and extent of devolatilization of MORB is strongly dependent on the geotherm along which the oceanic crust subducts. At the onset of subduction along a warm geotherm, metabasites contain ~3 wt% H₂O and release ~45% of this fluid in a single pulse at ~20 km, correlating with chlorite and epidote breakdown. Below these depths, metamorphosed MORB will dehydrate incrementally due to gradual amphibole breakdown, becoming almost completely dehydrated at ~70 km. Oceanic crust subducting along an average geotherm will contain ~3.5 wt% of H₂O at the onset of subduction and will release ~40% of the bulk‐rock H₂O in two fluid pluses occurring at ~30 and 50 km, correlating with chlorite breakdown. Below these depths, gradual dehydration of ~50% of the bulk‐rock H₂O due to amphibole breakdown leads to near‐complete dehydration at ~80 km. By contrast, in cold subduction zones, metamorphosed MORB will typically be H₂O‐undersaturated and will dehydrate gradually at different depths, transporting ~0.6 wt% H₂O to sub‐arc depths. As the volume of fluid released via these dehydration reactions differs strongly between cold, average and warm scenarios, different degrees of serpentinization of the mantle forearc are expected worldwide and thus, the efficacy of buoyancy‐driven exhumation should vary strongly in space and time. Metabasites subducting along a warm and average geotherm will liberate most of the fluids at shallower depths, suggesting that these lithologies might preferentially exhume, yet MORB subducting along cold geotherms will not dehydrate until greater depths, inhibiting its return to the surface. Critically, while we show that metabasites formed along warmer geotherms are denser than metabasites from colder geotherms at any equivalent depth, buoyancy‐driven exhumation provoked by fluids plays a notably more important role in exhumation potential than the overall bulk‐rock ‘metamorphic’ density. Furthermore, we show that lawsonite does not stabilize in average and warmer subduction zones, which provides a simple but important solution to the mismatch between its predicted abundance in experiments and its rarity in nature and argues against its use as a reliable petrogenetic indicator of subduction throughout deep geological time, as has been suggested by some recent studies.

中文翻译：

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修正的俯冲洋壳岩石学模型：相平衡模型的启示

许多对俯冲带变质作用的地质和地球动力学研究都依赖于全球模拟的板顶压力-温度（PT）条件的汇编，尽管对这些数据集的最新评估表明，这些预测在大约100-300°C较冷。任何给定的压力都比发掘出的变质岩所记录的条件大。因此，使用这种“冷”假设制定的地球化学，岩石学和地球物理解释可能会存在误差和不确定性。在这里，我们应用热力学相平衡计算，正向模型是如何相组合时，对叔典型的洋中脊玄武岩（MORB）沿这些新定义的“较热”俯冲带地热，在寒冷，温暖和中等环境中，关键的脱挥发分反应的条件和深度致密化的影响会有所不同。MORB的脱挥发分的深度和程度在很大程度上取决于地壳俯冲的地热。在沿着温暖的地热发生俯冲时，变质岩含有约3  wt％的H ₂ O，并在约20 km的单个脉冲中释放出约45％的这种流体，这与绿泥石和埃迪奥德的分解有关。在这些深度以下，由于逐渐的闪石分解，变质的MORB将逐渐脱水，在〜70 km处几乎完全脱水。沿平均地热俯冲的大洋地壳将含有〜3.5 wt％的H ₂O在俯冲开始时，会在约30和50 km处发生的两个流体正中释放约40％的块状H ₂ O，这与亚氯酸盐分解有关。在这些深度以下，由于闪石分解而导致的大约50％的块状H ₂ O逐渐脱水，导致〜80 km处几乎完全脱水。相比之下，在寒冷的俯冲带中，变质的MORB通常会被H ₂ O欠饱和，并会在不同深度逐渐脱水，从而输送〜0.6 wt％H _2。O至弧深。由于通过这些脱水反应释放的流体量在冷，中，暖情况之间差异很大，因此预计全世界范围内地幔前臂的蛇形化程度不同，因此，浮力驱动的掘尸术的功效在时空上应有很大差异。沿着温暖而平均的地热俯冲的准成因将释放浅层深度中的大部分流体，这表明这些岩性可能优先开采，但是沿着寒冷的地热俯冲的MORB直到更大深度才会脱水，从而抑制了其返回地面。至关重要的是，尽管我们证明了在任何等效深度处，沿较暖地热形成的变质基质要比较冷地热形成的变质基质浓，流体引起的浮力驱动的发掘活动在发掘潜力方面比整体岩石“变质”密度更重要。此外，我们表明，钙钠榴石在平均俯冲带和较热俯冲带中不稳定，这为解决其在实验中预测的丰度与其在自然界中的稀有性之间的不匹配提供了一种简单而重要的解决方案，并反对将其用作可靠的整个俯冲的岩石成因指标正如最近的一些研究所建议的那样，地质时期很深。