Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones
Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones
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DOI：
10.1093/petrology/egu066
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发表时间：
2014-12
影响因子：
3.9
通讯作者：
B. Winpenny;J. Maclennan
中科院分区：
文献类型：
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作者：
B. Winpenny;J. Maclennan
使用一个新的高分辨率数据集，这项研究提出了证据的短长度尺度18 O/16 O的不均匀性在年轻的地幔源区（年龄12 ka bp）冰岛玄武岩。该数据集包括二次离子质谱测定的18 O/16 O在单成分区的斜长石晶体从原始Borgarhraun流在北方冰岛，沿着与微量元素和主要元素的数据从相同的区域。冰岛下地幔的δ 18 O值低于典型的洋中脊玄武岩（MORB）值15.5 ± 0.3 ‰（VSMOW），这一点以前一直存在争议，因为许多冰岛玄武岩中δ 18 O的变化也是由玄武岩熔体与地壳岩性的相互作用引起的，这些岩石已被低δ 18 O的大气降水改变。原始玄武岩流（如Borgarhraun）及其粗晶是最有可能保留地幔δ 18 O特征的候选者。然而，地壳过程在从这些流动中产生低δ 18 O的橄榄石晶体中的作用尚未明确排除。通过对Borgarhraun斜长石进行晶内分析，本研究可以获得斜长石带结晶熔体的化学和同位素组成的详细记录。在早期结晶的钙长石斜长石带（80·9 - 89·4摩尔%钙长石）的微量元素组成的变化，首先被证明是由熔体组成的变化，和平衡熔体浓度的Sr，La和Y，然后计算这些元素的晶体浓度使用精心选择的分配系数。不相容的微量元素比值（La/Y，Sr/Y）在这些平衡熔体的范围内反映了一系列的组成部分地幔熔体，结果是在协议与以前的建议的微量元素指数的变化的原因Borgarhraun橄榄石托管熔融包裹体和单斜辉石组成区。因此，在与Borgarhraun斜长石带平衡的熔体中观察到的La/Y和Sr/Y与这些带的δ 18O之间的相关性支持了冰岛下地幔18O/16O不均匀的假设。这种相关性以前在冰岛的晶体内数据中没有观察到，并提供了强有力的证据，表明具有异常低δ 18 O的地幔物质可能以易熔非均匀性的形式存在于具有MORB样δ 18 O（δ 18 O +5 ‰）的环境地幔旁边，其长度尺度<100 km。斜长石的δ 18O最低值为4.5 ± 0.4 ‰，相当于4.3 ± 0.5 ‰的熔体当量值或3.8 ± 0.5 ‰的橄榄石当量值。
Using a new high-resolution dataset, this study presents evidence for short length scale 18O/16O heterogeneity in the mantle source region of young (age ≲12 ka bp) Icelandic basalts. The dataset comprises secondary ion mass spectrometry determinations of 18O/16O in single compositional zones of plagioclase crystals from the primitive Borgarhraun flow in northern Iceland, along with trace and major element data from the same zones. The presence of mantle under Iceland with δ18O below typical mid-ocean ridge basalt (MORB) values of ∼5·5 ± 0·3‰ (VSMOW) has previously been disputed, because variability in δ18O in many Icelandic basalts is also known to be caused by the interaction of basaltic melts with crustal lithologies that have been altered by low-δ18O meteoric water. Primitive basalt flows, such as Borgarhraun, and their macrocrysts are the most likely candidates to retain a mantle δ18O signature. However, the role of crustal processes in generating the low δ18O in olivine crystals from these flows has not unequivocally been ruled out. By making intra-crystal analyses in Borgarhraun plagioclase it has been possible in this study to obtain a detailed record of the chemical and isotopic compositions of the melts that crystallized the plagioclase zones. The variability observed in trace element compositions of the early crystallized anorthitic plagioclase zones (80·9–89·4 mol % anorthite) is firstly shown to arise from melt compositional variability, and equilibrium melt concentrations of Sr, La and Y are then calculated from the crystal concentrations of these elements using carefully selected partition coefficients. The ranges of incompatible trace element ratios (La/Y, Sr/Y) in these equilibrium melts reflect a range of compositions of fractional mantle melts, a result that is in agreement with previous proposals for the cause of variability in trace element indices of Borgarhraun olivine-hosted melt inclusions and clinopyroxene compositional zones. Correlations observed between La/Y and Sr/Y in the melts in equilibrium with the Borgarhraun plagioclase zones and the δ18O of these zones therefore support the hypothesis that the mantle under Iceland is heterogeneous in 18O/16O. Such correlations have not previously been observed in intra-crystal data from Iceland, and provide strong evidence that mantle material with abnormally low δ18O may exist in the form of readily fusible heterogeneities alongside ambient mantle with MORB-like δ18O (≈+5·5‰) on a length scale of <100 km. The lowest δ18O of plagioclase that is attributed to a mantle origin in this study is 4·5 ± 0·4‰, equating to a melt equivalent value of 4·3 ± 0·5‰ or an olivine equivalent value of 3·8 ± 0·5‰.