Oxygen isotope evidence for the formation of andesitic-dacitic magmas from the fast-spreading Pacific-Antarctic Rise by assimilation-fractional crystallisation
Oxygen isotope evidence for the formation of andesitic-dacitic magmas from the fast-spreading Pacific-Antarctic Rise by assimilation-fractional crystallisation
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DOI：
10.1016/j.chemgeo.2013.04.013
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发表时间：
2013-06
期刊：
影响因子：
3.9
通讯作者：
S. Freund;C. Beier;S. Krumm;K. Haase
中科院分区：
文献类型：
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作者：
S. Freund;C. Beier;S. Krumm;K. Haase
安山质到英安质熔岩沿着太平洋-南极海隆的300公里长的部分出现，靠近扩张轴与基金会海山链的交叉点。新鲜的玄武岩玻璃具有较低的δ 18 O同位素值，在5.6‰ ~ 5.1‰之间，而同一洋脊剖面的玄武岩玻璃具有正常的MORB δ 18 O值。此外，两种FeTi玄武岩玻璃和所有微晶玻璃都具有高Cl（高达1.1wt.%）和K2 O（最多1.6wt.%）内容，表明同化的热液蚀变材料。模拟表明，分馏岩浆同化高达30%的热液蚀变物质后，57%的玄武质岩浆在熔融透镜在不到2公里的深度的分馏结晶。相比之下，玄武质玻璃几乎没有同化和单斜辉石熔体气压计表明在更深的熔体岩床的晶体分馏。相对较低的H2O/Ce和Li/Ce比值，以及低δ 18 O值和高Cl和K浓度的玻璃中的蚀变地壳岩石，而不是盐水同化。虽然高度不相容元素比率的一些可变性最好由FeTi氧化物的晶体分馏过程解释（例如，降低Nb/U），其它则需要熔体与残余角闪石和单斜辉石（Cl/K，Tb/Yb，Hf/Sm，Ce/Yb）反应。FeTi氧化物结晶的初始发作与氧逸度降低有关，导致硫化物饱和和S，Cu和Co从演化熔体中的显著损失。这种变化，以更减少熔体作为氧化物结晶的结果是支持的强烈发展的负铕异常的安山岩和英安岩，表明更强的分配到斜长石的铕。演化熔岩中的反向矿物分带也表明，镁铁质熔体的补充发生在浅熔体透镜中。
Andesitic to dacitic lavas occur along a 300km long portion of the Pacific–Antarctic Rise close to the intersection of the spreading axis with the Foundation seamount chain. The fresh silicic glasses have low δ18O isotope values between 5.6‰ and 5.1‰ whereas basaltic glasses from the same ridge section have normal MORB δ18O values. Additionally, two FeTi basaltic and all silicic glasses have high Cl (up to 1.1wt.%) and K2O (up to 1.6wt.%) contents, indicating assimilation of hydrothermally altered material. Modelling suggests that the fractionating magma assimilated up to 30% of hydrothermally altered material after 57% fractional crystallisation of the basaltic magma in a melt lens at less than 2km depth. In contrast, the basaltic glasses show little assimilation and clinopyroxene-melt barometry indicates crystal fractionation in deeper melt sills. Relatively low H2O/Ce and Li/Ce ratios as well as the low δ18O values and high Cl and K concentrations in the silicic glasses suggest assimilation of altered crustal rock rather than a brine. While some of the variability in highly incompatible element ratios is best explained by crystal fractionation processes of FeTi-oxides (e.g., decreasing Nb/U) others require a reaction of the melt with residual amphibole and clinopyroxene (Cl/K, Tb/Yb, Hf/Sm, Ce/Yb). The initial onset of FeTi oxide crystallisation is associated with a reduced oxygen fugacity causing sulphide saturation and significant loss of S, Cu, and Co from the evolved melts. This change to more reducing melts as a result of oxide crystallisation is supported by the strong development of a negative Eu anomaly in the andesites and dacites indicating stronger partitioning of Eu into plagioclase. Reverse mineral zoning in the evolved lavas also indicates that replenishment by mafic melts occurs in the shallow melt lens.