Melting Behavior of Simplified Lherzolite in the System CaO-MgO-Al2O3-SiO2-Na2O from 7 to 35 kbar
Melting Behavior of Simplified Lherzolite in the System CaO-MgO-Al2O3-SiO2-Na2O from 7 to 35 kbar
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DOI：
10.1093/petrology/35.2.329
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发表时间：
1994-04
影响因子：
3.9
通讯作者：
M. Walter;D. C. Presnall
中科院分区：
文献类型：
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作者：
M. Walter;D. C. Presnall
本文收集了CaO-MgO-Al_2O_3-SiO_2-Na_2O系二辉橄榄岩简化成分在7-35kbar压力范围内等压单变量熔融的相平衡数据。这些数据使得可以用代数方法定量地确定各种模型二辉橄榄石的熔融行为。斜长石到尖晶石二辉橄榄岩和尖晶石到石榴石二辉橄榄岩的两个P-T单变量熔融反应被鉴定为包晶型PE转变，具有正的Clapeyron斜率。随着液体中Na2O含量的增加，单变量曲线向更高的压力和温度方向移动。单变曲线对熔融的影响是沿着二辉橄榄岩固溶体产生低温区和等压不变的熔融区间。在斜长二辉橄榄岩稳定场中，四相模型二辉橄榄岩的熔融是假侵入性的，发生在较小的温度区间(~5℃)，在8kbar时产生的液体为石英拉斑玄武岩。计算的斜长石-液体平衡的平衡常数既与温度有关，也与压力有关。在某些大洋玄武岩中观察到的斜长岩含量(AN)和90摩尔%的斜长石可以从含10%的液体中结晶出来。根据石榴石二辉橄榄岩领域有限的数据，在一定的熔融程度下，石榴石二辉橄榄岩的熔体比尖晶石二辉橄榄岩的熔体富含更多的二氧化硅，并且随着压力的增加，液体成分倾向于顽火辉石。源肥力(尤其是Na2O含量)对熔融温度和液体成分有很强的控制作用。肥力较低的来源产生的液体数量较少，富含标准二氧化硅。对于某些块体成分(高SiO_2和低Al_2O_3)，尖晶石不是沿二辉橄榄岩固相线的稳定相。
Phase equilibrium data have been collected for isobarically univariant melting of simplified lherzolite compositions in the system CaO-MgO-Al2O3-SiO2-Na2O over a pressure range of 7-35 kbar. These data permit the melting behavior of a wide variety of model lherzolite compositions to be determined quantitatively by algebraic methods. Two P-T univariant melting reactions, corresponding to plagioclase to spinel lherzolite and spinel to garnet lherzolite, are identified as peritectic-ty pe transitions and have positive Clapeyron slopes. The univariant curves move to higher pressures and temperatures with increasing Na2O in the liquid. The effect of the univariant curves on melting is to produce low-temperature regions and isobarically invariant melting intervals along lherzolite solidi. In the plagioclase lherzolite stability field, melting of four-phase model lherzolite is pseudo-invari ant, occurring over small temperature intervals (~ 5 °C) and producing liquids that are quartz tholeiites at 8 kbar. Calculated equilibrium constants for plagioclase-liquid equilibria show both temperature and pressure dependence. Plagioclase with anorthite content (AN) >90 mol%, as observed in some oceanic basalts, can crystallize from liquids with 10%). On the basis of limited data in the garnet lherzolite field, melts from garnet lherzolite are more silica rich for a given degree of melting than melts from spinel lherzolite, and liquid compositions trend toward enstatite with increase in pressure. Source fertility (especially Na2O content) has a strong control on the temperature of melting and liquid composition. Less fertile sources produce smaller amounts of liquids richer in normative silica. For certain bulk compositions (high SiO2 and low A12O3), spinel is not a stable phase along the lherzolite solidus.