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

• Published in: Contributions to Mineralogy and Petrology
• Main Authors: Erdmann, Martin, Koepke, Jürgen
• Other Authors: Centre de Recherches Pétrographiques et Géochimiques (CRPG), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), Institut für Mineralogie Hannover, Leibniz Universität Hannover=Leibniz University Hannover, German Research Foundation (DFG)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2016
• Subjects: Crystallization experiments; Phase equilibria; Differentiation; Dacite; Fast-spreading mid-ocean ridge; Oceanic plagiogranite; [SDU]Sciences of the Universe [physics]; Antarctic; Pacific; Antarc*
• Online Access: https://hal-insu.archives-ouvertes.fr/insu-03712931; https://doi.org/10.1007/s00410-016-1294-0

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.