Silica-rich lavas in the oceanic crust: experimental evidence for fractional crystallization under low water activity

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• 作者：Martin Erdmann ; Jürgen Koepke
• 关键词：Crystallization experiments ; Phase equilibria ; Differentiation ; Dacite ; Fast ; spreading mid ; ocean ridge ; Oceanic plagiogranite
• 刊名：Contributions to Mineralogy and Petrology
• 出版年：2016
• 出版时间：October 2016
• 年：2016
• 卷：171
• 期：10
• 全文大小：871 KB
• 刊物类别：Earth and Environmental Science
• 刊物主题：Earth sciences
  Geology
  Mineral Resources
  Mineralogy
  
• 出版者：Springer Berlin / Heidelberg
• ISSN：1432-0967
• 卷排序：171

文摘

We experimentally investigated phase relations and phase compositions as well as the influence of water activity (aH₂O) and redox conditions on the equilibrium crystallization path within an oceanic dacitic potassium-depleted system at shallow pressure (200 MPa). Moreover, we measured the partitioning of trace elements between melt and plagioclase via secondary ion mass spectrometry for a highly evolved experiment (SiO₂ = 74.6 wt%). As starting material, we used a dacitic glass dredged at the Pacific-Antarctic Rise. Phase assemblages in natural high-silica systems reported from different locations of fast-spreading oceanic crust could be experimentally reproduced only in a relatively small range of temperature and melt-water content (T ~950 °C; melt H₂O < 1.5 wt%) at redox conditions slightly below the quartz–fayalite–magnetite buffer. The relatively low water content is remarkable, because distinct hydrothermal influence is generally regarded as key for producing silica-rich rocks in an oceanic environment. However, our conclusion is also supported by mineral and melt chemistry of natural evolved rocks; these rocks are only congruent to the composition of those experimental phases that are produced under low aH₂O. Low FeO contents under water-saturated conditions and the characteristic enrichment of Al₂O₃ in high aH₂O experiments, in particular, contradict natural observations, while experiments with low aH₂O match the natural trend. Moreover, the observation that highly evolved experimental melts remain H₂O-poor while they are relatively enriched in chlorine implies a decoupling between these two volatiles during crustal contamination.