Phase Equilibrium Constraints on the Evolution of Transitional and Mildly Alkalic Fe-ti Basalts in the Rift Zones of Iceland

  1. John M. Sinton
  1. P. Thy

Published Online: 19 MAR 2013

DOI: 10.1029/GM057p0039

Evolution of Mid Ocean Ridges

Evolution of Mid Ocean Ridges

How to Cite

Thy, P. (1989) Phase Equilibrium Constraints on the Evolution of Transitional and Mildly Alkalic Fe-ti Basalts in the Rift Zones of Iceland, in Evolution of Mid Ocean Ridges (ed J. M. Sinton), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM057p0039

Author Information

  1. Nasa, Johnson Space Center, SN2, Houston, Texas 77058

Publication History

  1. Published Online: 19 MAR 2013
  2. Published Print: 1 JAN 1989

ISBN Information

Print ISBN: 9780875904580

Online ISBN: 9781118666449

SEARCH

Keywords:

  • Sea—floor spreading—Congresses;
  • Mid-ocean ridges—Congresses

Summary

Fe-Ti rich basalts from the Iceland and Galapagos propagating rifts show contrasting crystallization trends. Fe-Ti basalts occur in the western and south-eastern rift zones of Iceland and are plagioclase, olivine, and augite-phyric and span the range of compositions from quartz to mildly nepheline normative. Fe-Ti basalts from the Galapagos propagating rifts are quartz normative and are the product of advanced degrees of plagioclase, olivine, and augite cotectic crystallization relatively close to or at pigeonite saturation. Available melting experiments on tholeiitic to mildly alkalic basalts suggest that low-temperature liquids, coexisting with olivine, plagioclase, and augite, show a range in compositions which parallel the range observed in the natural Fe-Ti basalts. This range is mimicked by the augite compositions which become increasingly calcic going from tholeiitic, to transitional, and mildly alkalic basalts. The lack of correlation between normative composition and variables as Mg/(Mg+Fe2 +), TiO2, and crystallization temperature suggests that the experimental data, as well as the natural basalts, cannot be related to a unifying liquid line of descent. Considering the good correspondence between low-temperature, experimental liquids and the natural Icelandic Fe-Ti basalts, the latter must represent high degrees of crystallization and can be explained by three-phase cotectic crystallization without reaching low-Ca pyroxene saturation. The Fe-Ti basalts of the Galapagos propagating rifts are quartz normative and crystallize pigeonite after about 40–50 % crystallization. These differences are consistent with phase equilibria of a model basalt phase diagram. Magmas parental to the Galapagos Fe-Ti basalts are quartz normative and plot to the oversaturated side while the parental magmas to the Icelandic Fe-Ti basalts are olivine normative and plot to the saturated side of the relevant plagioclase-augite-pigeonite join. The former may evolve past pseudo-peritectic relations towards quartz saturation while the latter will, dependent on the amount of crystal fractionation, tend to be consumed at pseudo-invariant, pigeonite saturated relations. Basaltic andesites and andesites are the products of Fe-Ti oxide crystallization. Because of the low degree of liquid left and the appearance of Fe-Ti oxides, the Icelandic basalts may never reach invariant relations. This is in contrast to the Galapagos propagating rift basalts where Fe-Ti oxides appear after pigeonite. These fundamental differences between the Iceland and the Galapagos propagating rifts reflect primary melt compositions, which can be related to deep-seated melting processes and the hotspot influence on normal ridge segments.