Low-pressure differentiation of tholeiitic lavas as recorded in segregation veins from Reykjanes (Iceland), Lanzarote (Canary Islands) and Masaya (Nicaragua) |
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| Authors: | E Martin O Sigmarsson |
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| Institution: | (1) Laboratoire Magmas et Volcans, OPGC - Université Blaise Pascal – CNRS, 5, rue Kessler, 63038 Clermont-Ferrand Cedex, France;(2) Institute of Earth Sciences, University of Iceland, 101 Reykjavik, Iceland |
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| Abstract: | Segregation veins are common in lava sheets and result from internal differentiation during lava emplacement and degassing.
They consist of evolved liquid, most likely replaced by gas-filter pressing from a ∼50% crystallised host lava. Pairs of samples,
host lavas and associated segregation veins from the Reykjanes Peninsula (Iceland), Lanzarote (Canary Islands) and the Masaya
volcano (Nicaragua) show extreme mineralogical and compositional variations (MgO in host lava, segregation veins and interstitial
glass ranges from 8–10 wt%, 3–6 wt%, and to less than 0.01 wt%, respectively). These samples allow the assessment of the internal
lava flow differentiation mechanism, since both the parental and derived liquid are known in addition to the last magma drops
in the form of late interstitial glasses. The mineralogical variation, mass-balance calculated from major- and trace element
composition, and transitional metal partition between crystals and melts are all consistent with fractional crystallisation
as the dominant differentiation mechanism. The interstitial glasses are highly silicic (SiO2 = 70–80 wt%) and represent a final product of high-degree (75–97%) fractional crystallisation of olivine tholeiite at a pressure
close to one atmosphere. The tholeiitic liquid-line-of-decent and the composition of the residual melts are governed by the
K2O/Na2O of the initial basaltic magma. The granitic minimum is reached if the initial liquid has a high K2O/Na2O whereas trondhjemitic composition is the final product of magma with low initial K2O/Na2O.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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