Abstract
Two-thirds of the Earth is covered by mid-ocean ridge basalts, which form along a network of divergent plate margins. Basalts along these margins display a chemical diversity, which is consequent to a complex interplay of partial mantle melting in the upper mantle and magmatic differentiation processes in lower crustal levels. Igneous differentiation (crystal fractionation, partial melting) and source heterogeneity, in general, are key drivers creating variable chemistry in mid-ocean ridge basalts. This variability is reflected in iron isotope systematics (expressed as δ57Fe), showing a total range of 0.2 ‰ from δ57Fe = + 0.05 to + 0.25 ‰. Respective contributions of source heterogeneity and magma differentiation leading to this diversity, however, remain elusive. This study investigates the iron isotope systematics in basalts from the ultraslow spreading Gakkel Ridge in the Arctic Ocean and compares them to existing data from the fast spreading East Pacific Rise ridge. Results indicate that Gakkel lavas are driven to heavier iron isotope compositions through partial melting processes, whereas effects of igneous differentiation are minor. This is in stark contrast to fast spreading ridges showing reversed effects of near negligible partial melting effects followed by large isotope fractionation along the liquid line of descent. Gakkel lavas further reveal mantle heterogeneity that is superimposed on the igneous differentiation effects, showing that upper mantle Fe isotope heterogeneity can be transmitted into erupting basalts in the absence of homogenisation processes in sub-oceanic magma chambers.
Highlights
Two-thirds of the Earth is covered by mid-ocean ridge basalts, which form along a network of divergent plate margins
Thirty-five dredged, fresh basalts from three different tectonomagmatic segments, the Western Volcanic Zone (WVZ), the Sparsely Magmatic Zone (SMZ) and the Eastern Volcanic Zone (EVZ) from the Gakkel Ridge were selected for this study (Fig. 1)
The distinction of the Gakkel basalts is based on MgO value at 8.5 wt%, as this is the inception point of decreasing MgO content along the liquid line of descent. This value a larger diversity of trace elements occurs in a global mid-ocean ridge basalts (MORBs) compilation[57], indicating an onset of sub-surface magma lens activity
Summary
Two-thirds of the Earth is covered by mid-ocean ridge basalts, which form along a network of divergent plate margins. Igneous differentiation (crystal fractionation, partial melting) and source heterogeneity, in general, are key drivers creating variable chemistry in mid-ocean ridge basalts This variability is reflected in iron isotope systematics (expressed as δ57Fe), showing a total range of 0.2 ‰ from δ57Fe = + 0.05 to + 0.25 ‰. Olivine plays a critical role during magmatic differentiation processes, because when removed from the melt during crystal fractionation, it is expected to cause a shift in δ57Fe along the liquid line of descent This is convincingly illustrated in mid-ocean ridge basalts from fast spreading ridges[40,49]. It is, unclear to which extend, both, source heterogeneity and igneous differentiation, effect iron isotope composition, and if this can be linked to a difference in spreading rate
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