THE VOLUME AND COMPOSITION OF MELT GENERATED BY EXTENSION OF THE LITHOSPHERE
THE VOLUME AND COMPOSITION OF MELT GENERATED BY EXTENSION OF THE LITHOSPHERE
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DOI：
10.1093/petrology/29.3.625
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发表时间：
1988-06-01
影响因子：
3.9
通讯作者：
BICKLE, MJ
中科院分区：
文献类型：
--
作者：
MCKENZIE, D;BICKLE, MJ
要计算岩石圈伸展产生的岩浆的体积和成分，需要准确的缓和地温，并了解熔体分数作为压力和温度的函数的变化和组成。概述了相关的地球物理观测，并从参数化对流模型中获得了地热。用简单的经验函数描述了在不同压力下约束固相线和液相线的实验观测结果。熔体分数的变化被参数化，要求从固相线上的0到液体上的1变化。熔体的组成主要由熔体分数控制，尽管FeO、MgO和SiO2的组成也随压力而变化。另一种简单的参数化方法允许计算91个实验中观察到的主要元素的组成，平均误差为1.1%，而二氧化钛和Na2O的平均误差为0.3%。这些表达式然后被用来计算绝热上升流产生的岩浆的预期成分。最富镁的MORB玻璃的平均主元素组成类似于1280°C的地幔潜在温度计算的平均成分。在这个温度下，地幔上涌过程中的绝热熔化产生7公里的熔体厚度。观测到的大量MORB玻璃成分中MgO和TiO2浓度的变化表明，发生了广泛的低压分步析晶，但对SiO_2、Al_2O_3和CaO的浓度影响很小。没有证据表明正常的洋壳是由含氧化镁超过11%的岩浆产生的。热上升喷流内的地幔位温约为1480℃，可产生27千米的含17%镁的岩浆。大陆岩石圈的伸展不产生熔体，除非β&Gt；2和Tp&Gt；1380°C。β或Tp较大的熔体产生的熔体为碱性玄武岩，并随着熔融量的增加而转变为拉斑玄武岩。可能会产生大量的熔融，特别是在大陆边缘，从地壳厚度变化中获得的β估计值通常太小。
Calculation of the volume and composition of magma generated by lithospheric extension requires an accurate mitial geotherm, and knowledge of the variation and composition of the melt fraction as a function of pressure and temperature. The relevant geophysical observations are outlined, and geotherms then obtained from parameterized convective models. Experimental observations which constrain the solidus and liquidus at various pressures are described by simple empirical functions. The variation in melt fraction is then parameterized by requiring a variation from 0 on the solidus to 1 on the liquidus.The composition of the melts is principally controlled by the melt fraction, though those of FeO, MgO, and SiO2in addition vary with pressure. Another simple parameterization allows the observed compositions of major elements in 91 experiments to be calculated with a mean error of 1.1%, and those of TiO2and Na2O to 0.3%. These expressions are then used to calculate the expected compositions of magma produced by adiabatic upwelling. The mean major element composition of the most magnesium-rich MORB glasses resemble the mean composition calculated for a mantle potential temperatureTpof 1280°C. Adiabatic melting during upwelling of mantle of this temperature generates a melt thickness of 7 km. The observed variations of the MgO and TiO2concentrations in a large collection of MORB glass compositions suggest that extensive low pressure fractional crystallization occurs, but that its effect on the concentrations of SiO2, Al2O3, and CaO is small. There is no evidence that normal oceanic crust is produced from magmas containing more than 11% MgO. The mantle potential temperature within hot rising jets is about 1480°C and can generate 27 km of magma containing 17% MgO.Extension of the continental lithosphere generates little melt unless β> 2 andTp> 1380°C. The melts generated by larger values of β or ofTpare alkali basalts, and change to tholeiites as the amount of melting increases. Large quantities of melt can be generated, especially at continental margins, where estimates of β obtained from changes in crustal thickness will in general be too small.