A multi-proxy investigation of mantle oxygen fugacity along the Reykjanes Ridge
A multi-proxy investigation of mantle oxygen fugacity along the Reykjanes Ridge
复制标题
DOI：
10.1016/j.epsl.2019.115973
复制
发表时间：
2020-02
影响因子：
5.3
通讯作者：
D. Novella;J. Maclennan;O. Shorttle;J. Prytulak;B. Murton
中科院分区：
文献类型：
--
作者：
D. Novella;J. Maclennan;O. Shorttle;J. Prytulak;B. Murton
地幔氧逸度（fO 2）控制着地球的物理化学演化，但目前常用的玄武岩氧化还原代用指标的估计往往不一致。在这项研究中，我们比较了三个不同的潜在玄武岩fO 2代理：Fe 3+/Fe总，V/Sc和V同位素，确定了同一个海底熔岩从700公里的一段雷克雅未克海脊，冰岛附近。这些样品提供了一个有价值的测试fO 2代理玄武岩岩石成因的敏感性，因为它们形成于不同的熔融条件和地幔，对冰岛表现出越来越多的长期富集不相容的元素。对63个已知Fe 3+/Fetot的玄武岩进行了新的微量元素测定。一个子集的19个熔岩，覆盖地理分布的山脊样带，被选中进行钒同位素分析。钒是一种多价元素，其同位素分馏理论上易受氧化还原条件的影响。然而，沿着雷克雅内斯海岭玄武岩玻璃的δ 51 V AA组成仅覆盖一个狭窄的范围（δ 51 V AA=− 1.09至− 0.86‰; 1 SD = 0.02-0.09），且与分馏校正的Fe 3+/Fe tot不协变。（0.134-0.151; 1 SD = 0.005）或V/Sc（6.6-8.5; 1 SD = 0.1-1.3）比值。然而，在全球范围内，玄武岩δ 51 VAA可能受熔融程度的控制。原始玄武岩（MgO> 7.5wt%）的V/Sc组成在整个雷克雅内斯海脊没有表现出沿着系统的变化。典型的橄榄岩熔融模型（源Fe 3+/Fetot为5%）能较好地再现地幔柱中心附近地幔位温升高和V的fO 2分配。然而，虽然这些熔融模型预测玄武岩Fe 3+/Fe总比应该随着朝向冰岛的地幔位温的升高而降低，但雷克珍海脊熔岩的分馏校正Fe 3+/Fe总比在海脊长度上几乎保持不变。这种差异是由来源的异质性，其中氧化地幔辉石岩成分有助于熔融与冰岛越来越接近。Fe 3+/Fetot的观测值与模拟值的比较表明，在雷克雅内斯海脊下存在fO 2的源变化，在冰岛附近有较高的Fe 3+/Fetot。这种源的变化fO 2不能解决的V同位素和氧化还原敏感的微量元素的比例，而不是出现记录岩浆过程。
Mantle oxygen fugacity (fO 2) governs the physico-chemical evolution of the Earth, however current estimates from commonly used basalt redox proxies are often in disagreement. In this study we compare three different potential basalt fO 2 proxies: Fe 3+/Fe tot, V/Sc and V isotopes, determined on the same submarine lavas from a 700 km section of the Reykjanes Ridge, near Iceland. These samples provide a valuable test of the sensitivities of fO 2 proxies to basalt petrogenesis, as they formed at different melting conditions and from a mantle that towards Iceland exhibits increasing long-term enrichment of incompatible elements. New trace element data were determined for 63 basalts with known Fe 3+/Fe tot. A subset of 19 lavas, covering the geographical spread of the ridge transect, was selected for vanadium isotope analyses. Vanadium is a multi-valence element whose isotopic fractionation is theoretically susceptible to redox conditions. Yet, the δ 51 V AA composition of basaltic glasses along the Reykjanes Ridge covers only a narrow range (δ 51 V AA=− 1.09 to− 0.86‰; 1SD= 0.02–0.09) and does not co-vary with fractionation-corrected Fe 3+/Fe tot (0.134–0.151; 1SD= 0.005) or V/Sc (6.6–8.5; 1SD= 0.1-1.3) ratios. However, on a global scale, basaltic δ 51 V AA may be controlled by the extent of melting. The V/Sc compositions of primitive (MgO> 7.5 wt%) basalts show no systematic change along the entire length of the Reykjanes Ridge. Typical peridotite melting models in which source Fe 3+/Fe tot is constant at 5% and that account for the increased mantle potential temperature nearer the plume center and the fO 2 dependent partitioning of V, can reproduce the V/Sc data. However, while these melting models predict that basalt Fe 3+/Fe tot ratios should decrease with increasing mantle potential temperature towards Iceland, fractionation-corrected Fe 3+/Fe tot of Reykjanes Ridge lavas remain nearly constant over the ridge length. This discrepancy is explained by source heterogeneity, where an oxidized mantle pyroxenite component contributes to melting with increasing proximity to Iceland. Comparison of observed and modeled Fe 3+/Fe tot indicate that source variation in fO 2 is present under the Reykjanes Ridge, with higher Fe 3+/Fe tot closer to Iceland. This source variability in fO 2 cannot be resolved by V isotopes and redox-sensitive trace element ratios, which instead appear to record magmatic processes.