Nickel variability in Hawaiian olivine: Evaluating the relative contributions from mantle and crustal processes
Nickel variability in Hawaiian olivine: Evaluating the relative contributions from mantle and crustal processes
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DOI：
10.2138/am-2017-5763
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发表时间：
2017-03
影响因子：
3.1
通讯作者：
K. Lynn;T. Shea;Michael O. Garcia
中科院分区：
文献类型：
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作者：
K. Lynn;T. Shea;Michael O. Garcia
夏威夷拉斑岩熔岩中的橄榄石在给定的橄榄石（Fo）含量（例如，Fo88为0.25-0.60 wt%）下具有较高的NiO，而MORB（例如，Fo88为0.10-0.28 wt%）。这种差异通常与源变量有关，如熔融深度和温度和/或岩性。夏威夷橄榄石的NiO含量也变化很大，在给定的Fo下，可以在0.25-0.60 wt%之间变化。本文研究了地壳过程（分离结晶、岩浆混合、扩散再平衡）对夏威夷玄武岩橄榄石中镍含量的影响。夏威夷5座主要火山的橄榄石组成可被细分为≥Fo88的高镍（0.25-0.60 wt% NiO, Ko ' olau， Mauna Loa和Mauna Kea）和低镍（0.25-0.45 wt% NiO， Kllauea和lgi ' ihi），这些组与主要同位素趋势（例如Loa和Kea）无关。在每一组中，单个火山在给定的Fo中显示出高达2.5倍的橄榄石NiO含量变化。科奥劳火山、莫纳罗亚火山、莫纳克亚火山和克鲁瓦火山熔岩的全岩镍含量重叠明显，与橄榄石镍含量差异不相关。然而，火山间熔体聚合（NBO/T）和镍分配系数（DNiO1/melt） $(D^{\text O1/\text{melt}}_{\text{Ni}}) $的变化与观测到的Fo90橄榄石NiO差异相关，表明无橄榄石的源岩性不产生火山间群。此外，在熔岩SiO2和NBO/T变化很小的情况下，火山内橄榄石NiO可能发生较大的变化。母熔体NiO含量的微小变化（0.09-0.11 wt%）解释了≥Fo88橄榄石中NiO的观测范围。对Kīlauea火山喷发（1500-2010年）橄榄石的高精度电子探针分析表明，与较小的斑晶（沿c轴400 μm）相比，初级控制的橄榄石保留原始Xpx的可能性高出50%，后者很少（6%）恢复原始Xpx。平行或亚平行于c轴和/或通过晶体核心附近的部分最好地保存生长特征。因此，扩散再平衡、晶体大小和切片效应对夏威夷火山地幔源岩性的表征有很大影响。
Abstract Olivine in Hawaiian tholeiitic lavas have high NiO at given forsterite (Fo) contents (e.g., 0.25–0.60 wt% at Fo88) compared to MORB (e.g., 0.10–0.28 wt% at Fo88). This difference is commonly related to source variables such as depth and temperature of melting and/or lithology. Hawaiian olivine NiO contents are also highly variable and can range from 0.25–0.60 wt% at a given Fo. Here we examine the effects of crustal processes (fractional crystallization, magma mixing, diffusive re-equilibration) on the Ni content in olivine from Hawaiian basalts. Olivine compositions for five major Hawaiian volcanoes can be subdivided at ≥Fo88 into high-Ni (0.25–0.60 wt% NiO; Ko‘olau, Mauna Loa, and Mauna Kea) and low-Ni (0.25–0.45 wt% NiO; Kllauea and Lō‘ihi), groups that are unrelated to major isotopic trends (e.g., Loa and Kea). Within each group, individual volcanoes show up to 2.5× variation in olivine NiO contents at a given Fo. Whole-rock Ni contents from Ko‘olau, Mauna Loa, Mauna Kea, and Kīlauea lavas overlap significantly and do not correlate with differences in olivine NiO contents. However, inter-volcano variations in parental melt polymerization (NBO/T) and nickel partition coefficients (DNiO1/melt), $(D^{\text O1/\text{melt}}_{\text{Ni}}),$ caused by variable melt SiO2, correlate with observed differences in olivine NiO at Fo90, indicating that an olivine-free source lithology does not produce the inter-volcano groups. Additionally, large intra-volcano variations in olivine NiO can occur with minimal variation in lava SiO2 and NBO/T. Minor variations in parental melt NiO contents (0.09–0.11 wt%) account for the observed range of NiO in ≥Fo88 olivine. High-precision electron microprobe analyses of olivine from Kīlauea eruptions (1500–2010 C.E.) show that the primary controls on 50% more likely to preserve original Xpx compared to smaller phenocrysts (400 μm along c-axis) which rarely (6%) recover original Xpx. Sections that are parallel or sub-parallel to the c-axis and/or pass near the core of the crystal best preserve growth signatures. Thus, diffusive reequilibration, crystal size, and sectioning effects can strongly influence the characterization of mantle source lithologies for Hawaiian volcanoes.