Extreme enriched and heterogeneous 87Sr/86Sr ratios recorded in magmatic plagioclase from the Samoan hotspot
Extreme enriched and heterogeneous 87Sr/86Sr ratios recorded in magmatic plagioclase from the Samoan hotspot
复制标题
DOI：
10.1016/j.epsl.2019.01.040
复制
发表时间：
2019-04-01
影响因子：
5.3
通讯作者：
Spera, F. J.
中科院分区：
文献类型：
--
作者：
Edwards, M. A.;Jackson, M. G.;Spera, F. J.
我们报告了来自两个萨摩亚熔岩的 6 个斜长石晶体的主量元素、微量元素和 Sr-87/Sr-86 组成，这些晶体具有极端的 EM2 同位素组成（ALIA-115-18 的全岩 Sr-87/Sr-86 为 0.718592，ALIA-115-21 的全岩 Sr-87/Sr-86 为 0.718592。 0.720469）。我们采用激光烧蚀分流质谱 (LASS) 同时测量同一斜长石晶体体积中的 Sr-87/Sr-86 比率、主量元素浓度和痕量元素浓度。我们发现两个斜长石晶体具有超过 5000 ppm 的极端 Sr-87/Sr-86 异质性（其中 Sr-87/Sr-86 变异性的 ppm = 10(6) 。[Sr-87/Sr-86(max) - Sr-87/Sr-86(min)]/Sr-87/Sr-86(avg)）。在两个斜长石晶体中，我们确定了在任何新鲜的、源自地幔的洋岛玄武岩 (OIB) 或 OIB 托管矿物相中测量到的最高的 Sr-87/Sr-86 比率 (0.7224)。我们发现，在 Sr-87/Sr-86 与 Sr 浓度空间中，六个斜长石晶体在“共同成分”区域重叠，具有更高的比值。 Sr-87/Sr-86 比之前在萨摩亚全岩熔岩或矿物分离物中发现的要多。我们利用一颗斜长石晶体的高 Sr-87/Sr-86 区域中出现的橄榄石矿物包裹体 (Fo = 74.5 +/- 0.8, 2 SD) 来推断橄榄石和高 Sr-87/Sr-86 斜长石的极端 EM2 岩浆的整体成分 (Mg# = 46.8 +/- 0.8, 2 SD)结晶。我们认为，相对演化的 EM2 端元岩浆与至少一种较低 Sr-87/Sr-86 熔体混合，产生了观察到的晶体内斜长石同位素异质性。通过推断俯冲陆源沉积物在萨摩亚熔岩中产生了 EM2 特征，我们估计了产生在萨摩亚斜长石与地幔源的 7% 相似。我们还估计，沉积物在地质时期俯冲到地幔中，产生了占地幔质量 0.02% 的沉积物域，远低于 EM2 地幔源所需的比例。即使俯冲沉积物集中在地幔底部的大片低剪切速度省（LLSVP）（占地幔质量的 7.7%），那么只有 0.25% 的 LLSVP 由沉积物组成。这就要求地幔中俯冲沉积物的分布是不均匀的，而萨摩亚EM2地幔中沉积物的相对丰度较高是远古俯冲在对流衰减中幸存下来的异常遗迹。 (C) 2019 Elsevier B.V. 保留所有权利。
We report the major-element, trace-element, and Sr-87/Sr-86 compositions of six plagioclase crystals from two Samoan lavas with extreme EM2 isotopic compositions (ALIA-115-18 with whole-rock Sr-87/Sr-86 of 0.718592, and ALIA-115-21 with whole-rock Sr-87/Sr-86 of 0.720469). We employed laser-ablation split-stream mass spectrometry (LASS) to simultaneously measure Sr-87/Sr-86 ratios, major-element concentrations, and trace-element concentrations in the same plagioclase crystal volume. We find that two plagioclase crystals have extreme Sr-87/Sr-86 heterogeneity in excess of 5000 ppm (where ppm of Sr-87/Sr-86 variability = 10(6) . [Sr-87/Sr-86(max) - Sr-87/Sr-86(min)]/Sr-87/Sr-86(avg)). In two of the plagioclase crystals, we identify the highest Sr-87/Sr-86 ratios (0.7224) ever measured in any fresh, mantle-derived ocean island basalt (OIB) or OIB-hosted mineral phase.We find that in Sr-87/Sr-86-versus-Sr concentration space, the six plagioclase crystals overlap in a "common component" region with higher Sr-87/Sr-86 than has been previously identified in whole-rock Samoan lavas or mineral separates. We use the occurrence of olivine mineral inclusions (Fo = 74.5 +/- 0.8, 2 SD) in the high-Sr-87/Sr-86 zone of one plagioclase crystal to infer the bulk composition (Mg# = 46.8 +/- 0.8, 2 SD) of the extreme EM2 magma from which the olivine and high-Sr-87/Sr-86 plagioclase crystallized. We argue that a relatively evolved EM2 endmember magma mixed with at least one lower-Sr-87/Sr-86 melt to generate the observed intra-crystal plagioclase isotopic heterogeneity.By inferring that subducted terrigenous sediment gives rise to EM2 signatures in Samoan lavas, we estimate that the quantity of sediment necessary to generate the most-elevated Sr-87/Sr-86 ratios observed in the Samoan plagioclase is similar to 7% of the mantle source. We also estimate that sediment subduction into the mantle over geologic time has generated a sediment domain that constitutes 0.02% of the mass of the mantle, a much lower proportion than required in the EM2 mantle source. Even if subducted sediment is concentrated in large low-shear-velocity provinces (LLSVPs) at the base of the mantle (which constitute up to 7.7% of the mantle's mass), then only 0.25% of the LLSVPs are composed of sediment. This requires that the distribution of subducted sediment in the mantle is heterogeneous, and the high relative abundance of sediment in the Samoan EM2 mantle is an anomalous relic of ancient subduction that has survived convective attenuation. (C) 2019 Elsevier B.V. All rights reserved.