Richter Marianne, Nebel Oliver, Schwindinger Martin, Nebel-Jacobsen Yona, Dick Henry J B
Isotopia Laboratory, School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC, 3800, Australia.
Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543-1539, USA.
Sci Rep. 2021 Feb 18;11(1):4123. doi: 10.1038/s41598-021-83387-7.
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 δFe), showing a total range of 0.2 ‰ from δFe = + 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.
地球表面三分之二被大洋中脊玄武岩覆盖，这些玄武岩沿着离散板块边缘网络形成。沿着这些边缘的玄武岩呈现出化学多样性，这是上地幔部分地幔熔融与下地壳岩浆分异过程复杂相互作用的结果。一般来说，火成岩分异（晶体分馏、部分熔融）和源区不均一性是导致大洋中脊玄武岩化学成分变化的关键驱动因素。这种变化反映在铁同位素体系（以δFe表示）中，δFe的总范围为0.2‰，从δFe = +0.05到 +0.25‰。然而，导致这种多样性的源区不均一性和岩浆分异各自的贡献仍然难以捉摸。本研究调查了北冰洋超慢速扩张的加克尔洋中脊玄武岩的铁同位素体系，并将其与快速扩张的东太平洋海隆脊的现有数据进行比较。结果表明，加克尔熔岩通过部分熔融过程被驱动至更重的铁同位素组成，而火成岩分异的影响较小。这与快速扩张的洋中脊形成鲜明对比，快速扩张洋中脊显示出相反的效应，即部分熔融效应几乎可以忽略不计，随后沿着液相线下降有较大的同位素分馏。加克尔熔岩进一步揭示了叠加在火成岩分异效应之上的地幔不均一性，表明在洋下岩浆房不存在均质化过程的情况下，上地幔铁同位素不均一性可以传递到喷发的玄武岩中。
