Geochemistry of ODP Hole 135-839B basalts

Experimental phase relations were used to assess the role of volatiles and crustal level fractional crystallization in the petrogenesis of lavas from Hole 839B in the central Lau Basin. Melting experiments were performed on Sample 135-839B-15R-2, 63-67 cm, at 1 atm, anhydrous, and 2 kbar, H2O-saturated (~6 wt% H2O in the melt) to determine the influence of variable pressure and H2O content on phase appearances, mineral chemistry, and liquid line of descent followed during crystallization. The effects of H2O are to depress the liquidus by ~100°C, and to suppress crystallization of plagioclase and orthopyroxene relative to olivine and high-Ca clinopyroxene. At 1 atm, anhydrous, olivine and plagioclase coexist near the liquidus, whereas orthopyroxene and then clinopyroxene appear with decreasing temperature. Crystallization of 50 wt% produces a residual liquid that is rich in FeO* (10.8 wt%) and poor in Al2O3 (13.6 wt%). At 2 kbar, H2O-saturated, the liquidus phases are olivine and chromian spinel, with high-Ca clinopyroxene appearing after ~10% crystallization. Plagioclase saturation is suppressed until ~20% crystallization has occurred. The residual liquid from 35 wt% crystallization is rich in AI2O3 (17.4 wt%), and poor in MgO (4.82 wt%); it contains moderate FeO* (8.2 wt%), and resembles the low-MgO andesites recovered from Hole 839B. On the basis of these experiments we conclude that the primitive lavas recovered from Hole 839B have experienced crystallization along the Ol + Cpx saturation boundary, under hydrous conditions (an ankaramitic liquid line of descent), and variable amounts of olivine and chromian spinel accumulation. The low-MgO andesites from Hole 839B are the products of hydrous fractional crystallization, at crustal pressures, of a parent magma similar to basaltic andesite Sample 135-839B-15R-2, 63-67 cm.本研究借助实验相平衡关系，评估了挥发分（volatiles）与地壳级分离结晶（crustal level fractional crystallization）在劳盆地（Lau Basin）中部839B钻孔（Hole 839B）熔岩岩石成因（petrogenesis）中的作用。本研究针对样品135-839B-15R-2的63~67 cm段开展熔融实验，分别设置1个标准大气压（1 atm）无水体系与2千巴（2 kbar）水饱和（熔体中H₂O含量约6质量百分比）的实验条件，以探究压力与H₂O含量变化对结晶过程中矿物相出现规律、矿物化学特征及液相演化线（liquid line of descent）的影响。H₂O的作用为降低液相线温度约100℃，且相较于橄榄石（olivine）与高钙单斜辉石（high-Ca clinopyroxene），会抑制斜长石（plagioclase）与斜方辉石（orthopyroxene）的结晶。在1 atm无水体系下，橄榄石与斜长石在液相线附近共存，随着温度降低，会依次析出斜方辉石与单斜辉石。当结晶程度达到50质量百分比时，残余熔体的全铁以FeO计（FeO*）含量为10.8%，氧化铝（Al₂O₃）含量偏低，仅为13.6%。在2 kbar水饱和条件下，液相线矿物为橄榄石与铬尖晶石（chromian spinel），高钙单斜辉石在结晶程度约10%时开始析出。斜长石的饱和析出被抑制，直至结晶程度达到约20%时才会出现。结晶程度达35质量百分比时的残余熔体富含氧化铝（17.4%），氧化镁（MgO）含量偏低（4.82%），全铁以FeO计含量适中（8.2%），其地球化学特征与839B钻孔中采获的低MgO安山岩（andesites）一致。基于本实验结果，我们认为839B钻孔采获的原始熔岩曾在含水条件下沿橄榄石（Ol）+ 单斜辉石（Cpx）饱和边界发生结晶作用，演化轨迹符合苦橄玄武岩质（ankaramitic）液相演化特征，且伴随不同程度的橄榄石与铬尖晶石堆积作用。839B钻孔中的低MgO安山岩，是地壳压力条件下，源岩浆经含水分离结晶作用形成的产物，该源岩浆与样品135-839B-15R-2的63~67 cm段玄武安山岩（basaltic andesite）相似。