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首页> 外文期刊>Chemical geology >Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers
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Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers

机译：大洋中脊上玄武岩至大山熔岩的挥发性丰度和氧同位素：在扩散中心的同化作用

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摘要

Most geochemical variability in MOR basalts is consistent with low- to moderate-pressure fractional crystallization of various mantle-derived parental melts. However, our geochemical data from MOR high-silica glasses, including new volatile and oxygen isotope data, suggest that assimilation of altered crustal material plays a significant role in the petrogenesis of dacites and may be important in the formation of basaltic lavas at MOR in general. MOR high-silica andesites and dacites from diverse areas show remarkably similar major element trends, incompatible trace element enrichments, and isotopic signatures suggesting similar processes control their chemistry. In particular, very high Cl and elevated H_2O concentrations and relatively light oxygen isotope ratios (~5.8‰ vs. expected values of ~6.8‰) in fresh dacite glasses can be explained by contamination of magmas from a component of ocean crust altered by hydrothermal fluids. Crystallization of silicate phases and Fe-oxides causes an increase in δ~(18)O in residual magma, but assimilation of material initially altered at high temperatures results in lower δ~(18)O values. The observed geochemical signatures can be explained by extreme fractional crystallization of a MOR basalt parent combined with partial melting and assimilation (AFC) of amphibole-bearing altered oceanic crust. The MOR dacitic lavas do not appear to be simply the extrusive equivalent of oceanic plagiogranites. The combination of partial melting and assimilation produces a distinct geochemical signature that includes higher incompatible trace element abundances and distinct trace element ratios relative to those observed in plagiogranites.
机译：MOR玄武岩中大多数地球化学变化与各种地幔衍生的母体熔体的低压至中压分步结晶相一致。但是，我们从MOR高硅玻璃获得的地球化学数据（包括新的挥发性和氧同位素数据）表明，改变后的地壳物质的同化作用在dacites的岩石成因中起着重要作用，并且可能对MOR的玄武岩熔岩的形成很重要。 。来自不同地区的MOR高硅质安山岩和辉石显示出非常相似的主要元素趋势，不相容的痕量元素富集和同位素特征，表明相似的过程控制着它们的化学性质。尤其是新鲜的达克特玻璃中很高的Cl和较高的H_2O浓度以及相对较轻的氧同位素比（〜5.8‰与〜6.8‰的预期值）可以解释为由热液引起的洋壳成分对岩浆的污染。硅酸盐相和Fe-氧化物的结晶导致残余岩浆中δ〜（18）O的增加，但是在高温下最初改变的材料的同化会导致较低的δ〜（18）O值。观察到的地球化学特征可以通过MOR玄武岩母体的极端分步结晶以及含角闪石蚀变海洋地壳的部分熔融和同化（AFC）来解释。 MOR的高铁熔岩似乎不仅仅是海洋斜长花岗岩的挤压等价物。部分熔化和同化的结合产生了独特的地球化学特征，与斜长花岗岩中观察到的特征相比，包括更高的不相容痕量元素丰度和不同的痕量元素比率。

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