Influence of magmatism on mantle cooling, surface heat flow and Urey ratio
Influence of magmatism on mantle cooling, surface heat flow and Urey ratio
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DOI：
10.1016/j.epsl.2012.02.011
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发表时间：
2012-04
影响因子：
5.3
通讯作者：
T. Nakagawa;P. Tackley
中科院分区：
文献类型：
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作者：
T. Nakagawa;P. Tackley
二维热化学地幔对流模拟研究了熔融诱导分异作用对地幔热演化的影响，特别关注了现今地表热流和Urey比的匹配。研究了内部加热速率、地幔初始温度和生热元素在玄武质地壳中的分配等因素的影响。高的初始地幔温度，这是预期以下地球的吸积，造成早期地幔热化学结构的重大差异，但由今天的表面热通量和内部结构是无法区分的情况下，低的初始温度。假设地幔产热的三个不同值变化超过两倍，则现今热流的差异很小，假设产热元素在地壳中的分配比例不同也是如此。事实上，这里提出的所有情况，不管确切的参数，都近似于地球现在的热流，其中相当大的部分来自地核和地幔冷却。由于模型现今的表面热流仅随参数略有变化，Urey比（总产热与总表面热流的比率）高度依赖于内部产热的量，并且由于其中的大不确定性，Urey比被认为是比热流差得多的热演化约束。在这里的模拟中观察到的现今Urey比的范围约为0.3至0.5，这与观测和地球化学约束一致（Jaupart等人，2007年）。岩浆热输运对地球现今热损失的贡献上限为9%，但在早期的时候，这一比例要高得多，通常超过对流热损失，因此忽略这一点会导致对尤里比的高估。岩浆热输运在地幔冷却中也起着重要作用。考虑到这些问题，在试图理解地球的热演化时，包括岩浆效应是很重要的。
Two-dimensional thermo-chemical mantle convection simulations are used to investigate the influence of melting-inducted differentiation on the thermal evolution of Earth's mantle, focussing in particular on matching the present-day surface heat flow and the ‘Urey ratio’. The influence of internal heating rate, initial mantle temperature and partitioning of heat-producing elements into basaltic crust are studied. High initial mantle temperatures, which are expected following Earth's accretion, cause major differences in early mantle thermo-chemical structures, but by the present-day surface heat flux and internal structures are indistinguishable from cases with a low initial temperature. Assuming three different values of mantle heat production that vary by more than a factor of two results in small differences in present-day heat flow, as does assuming different partitioning ratios of heat-producing elements into crust. Indeed, all of the cases presented here, regardless of exact parameters, have approximately Earth's present-day heat flow, with substantial fractions coming from the core and from mantle cooling. As a consequence of the model present-day surface heat flow varying only slightly with parameters, the Urey ratio (the ratio of total heat production to the total surface heat flow) is highly dependent on the amount of internal heat production, and due to the large uncertainty in this, the Urey ratio is considered to be a much poorer constraint on thermal evolution than the heat flow. The range of present-day Urey ratio observed in simulations here is about 0.3 to 0.5, which is consistent with observational and geochemical constraints (Jaupart et al., 2007). Magmatic heat transport contributes an upper bound of 9% to Earth's present-day heat loss but a much higher fraction at earlier times—often more than convective heat loss—so neglecting this causes an overestimation of the Urey ratio. Magmatic heat transport also plays an important role in mantle cooling. Considering these points, it is important to include magmatic effects when attempting to understand the thermal evolution of the Earth.