Silica-rich lavas in the oceanic crust: experimental evidence for fractional crystallization under low water activity
International audience We experimentally investigated phase relations and phase compositions as well as the influence of water activity ( aH 2 O) and redox conditions on the equilibrium crystallization path within an oceanic dacitic potassium-depleted system at shallow pressure (200 MPa). Moreover,...
Published in: | Contributions to Mineralogy and Petrology |
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Main Authors: | , |
Other Authors: | , , , , |
Format: | Article in Journal/Newspaper |
Language: | English |
Published: |
HAL CCSD
2016
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Subjects: | |
Online Access: | https://hal-insu.archives-ouvertes.fr/insu-03712931 https://doi.org/10.1007/s00410-016-1294-0 |
Summary: | International audience We experimentally investigated phase relations and phase compositions as well as the influence of water activity ( aH 2 O) 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 (SiO 2 = 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 H 2 O < 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 aH 2 O. Low FeO contents under water-saturated conditions and the characteristic enrichment of Al 2 O 3 in high aH 2 O experiments, in particular, contradict natural observations, while experiments with low aH 2 O match the natural trend. Moreover, the observation that highly evolved experimental melts remain H 2 O-poor while they are relatively enriched in chlorine implies a decoupling between these two volatiles during crustal contamination. |
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