TY - JOUR

T1 - Silica-rich lavas in the oceanic crust

T2 - experimental evidence for fractional crystallization under low water activity

AU - Erdmann, Martin

AU - Koepke, Jürgen

N1 - Funding Information: We thank Otto Dietrich and Julian Feige for their careful sample preparation. We also thank the Editor J. Hoefs and the reviewers M. Perfit and B. Scaillet for their constructive comments that considerably improved the quality of this work. We gratefully acknowledge the chief scientists of RV Sonne, especially K.M. Haase, for access to the sample 3DS1 used in this study. Funding for this research was provided by grants from the Deutsche Forschungsgemeinschaft (KO 1723/13). This is CRPG Contribution Number 2461.

PY - 2016/10/1

Y1 - 2016/10/1

N2 - 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.

AB - 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.

KW - Crystallization experiments

KW - Dacite

KW - Differentiation

KW - Fast-spreading mid-ocean ridge

KW - Oceanic plagiogranite

KW - Phase equilibria

UR - http://www.scopus.com/inward/record.url?scp=84987910507&partnerID=8YFLogxK

U2 - 10.1007/s00410-016-1294-0

DO - 10.1007/s00410-016-1294-0

M3 - Article

AN - SCOPUS:84987910507

VL - 171

JO - Contributions to Mineralogy and Petrology

JF - Contributions to Mineralogy and Petrology

SN - 0010-7999

IS - 10

M1 - 83

ER -