Cryptic lower crustal signature in the source of the Ontong Java Plateau revealed by Os and Hf isotopes
Cryptic lower crustal signature in the source of the Ontong Java Plateau revealed by Os and Hf isotopes
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DOI：
10.1016/j.epsl.2013.07.022
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发表时间：
2013-09
影响因子：
5.3
通讯作者：
M. Tejada;Katsuhiko Suzuki;T. Hanyu;J. Mahoney;A. Ishikawa;Y. Tatsumi;Q. Chang;S. Nakai
中科院分区：
文献类型：
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作者：
M. Tejada;Katsuhiko Suzuki;T. Hanyu;J. Mahoney;A. Ishikawa;Y. Tatsumi;Q. Chang;S. Nakai
前人对安通爪哇高原玄武岩的研究表明，它们是20%-30%熔融的产物，具有海岛式的铅、钕、锶同位素特征，在有限的范围内，但确定了以克伦克型和奎姆拜塔型玄武岩和星盖洛型玄武岩为代表的两个不同的同位素群。这两个群的起源尚不清楚，尽管克伦克-奎姆拜塔型签名被认为代表了高原ʼS的主地幔来源，可能来自下地幔。在这项研究中，分析了马莱塔中部、所罗门群岛和大洋钻探计划(ODP)807、1185B和1187个地点的样品中的Os和Hf同位素组成，以进一步探讨OJP玄武岩的双峰同位素组成的起源。克伦克型玄武岩是昆拜塔型玄武岩的母岩，侵位后蚀变对其Os同位素影响最小，其εHf(T)=+10.8~+13.3，Os=118~174 ppt，Re=161~1111ppt，近球粒陨石(OS187/OS188)t=0.1322±0.0029[γOs(T)=2.7±2.2，4.3±2.3(n=6；1σ)，表明OJP为近原始地幔源区。侏罗纪太平洋MORB蚀变壳与分馏的克伦克型岩浆的同化作用高达25%，可以解释某些奎姆拜塔型玄武岩的组成：(OS187/OS188)t=0.1395±0.0020[γOs(T)=8.4±1.5和10.0±1.5(n=3；1σ)]。这些成分与εHf(T)=+10.2~+11.5和(OS187/OS188)t=0.3301±0.0175[γOs(T)=15 6±14和16 0±14(n=8，1σ)]和低含量(2 1~37ppt，Os)的下洋壳状单相玄武岩明显不同。尽管Re-Os同位素数据分散，但获得了有意义的等时线年龄和初始值。马来滩奎姆拜塔组的年龄为121.4±4.6 Ma。其中玄武岩的初始年龄为0.129±0.025 Ma，单星型玄武岩的初始年龄为123±24 Ma，初始年龄为0.325±0.098。Re-Os年龄与OJP的∼平均年龄122 Ma、40Ar-39Ar非常吻合。Re-Os和Lu-Hf结果加强了对OJP的两组分地幔来源的解释，其中包括一个主要的近原始地幔，类似于以Kroenke和Kwaimbaita类型玄武岩为代表的几个洋岛的来源，以及在Singgalo类型玄武岩的同位素特征中巧妙表达的古老的、再循环的下大陆地壳物质。下地壳的影响可以通过将古老的、拆离的镁铁质陆壳并入热化学热柱来解释；并入可能暂时涉及积累在下地幔中的致密的壳外物质或已停滞在中地幔深处的密度较低的物质。
Abstract Previous studies of Ontong Java Plateau (OJP) basalts reveal that they are products of 20–30% degree of melting and possess ocean-island-like Pb, Nd, and Sr isotopic signatures that fall within a limited range yet define two distinct isotopic groups represented by the Kroenke-and Kwaimbaita-type basalts and by the Singgalo-type basalts. The origin of the two groups is not clear, although the Kroenke–Kwaimbaita-type signature has been suggested to represent the plateauʼs main mantle source, probably originating from the lower mantle. In this study, samples from central Malaita, Solomon Islands and Ocean Drilling Program (ODP) Sites 807, 1185B, and 1187 were analyzed for Os and Hf isotopic composition to further investigate the origin of the bimodal isotopic composition of OJP basalts. The Kroenke-type basalts, which are parental to Kwaimbaita-type basalts, show the least effect of post-emplacement alteration on their Os isotopic composition and have ε Hf (t)=+ 10.8 to+ 13.3, Os= 118–174 ppt, Re= 161–1111 ppt, and near-chondritic (Os 187/Os 188) t= 0.1322±0.0029 [γ Os (t)= 2.7±2.2 and 4.3±2.3 (n= 6; 1σ) relative to primitive upper mantle (PUM) and average chondrite values, respectively], suggesting a near-primitive mantle source for the OJP. Up to 25% assimilation of altered Jurassic Pacific MORB crust into fractionating Kroenke-type magma could explain the lower, 11–51 ppt, Os contents and more radiogenic,(Os 187/Os 188) t= 0.1395±0.0020 [γ Os (t)= 8.4±1.5 and 10.0±1.5 (n= 3; 1σ)], composition of some Kwaimbaita-type basalts. These compositions are markedly different from those of the least altered Singgalo-type basalts with ε Hf (t)=+ 10.2 to+ 11.5 and (Os 187/Os 188) t= 0.3301±0.0175 [γ Os (t)= 156±14 and 160±14 (n= 8, 1σ)] and low, 21–37 ppt, Os contents that are lower oceanic crust-like. Despite the scatter in the Re–Os isotope data, meaningful isochron ages and initial values were obtained. These ages are 121.4±4.6 Ma for Malaitan Kwaimbaita Fm. basalts and 123±24 Ma for Singgalo-type basalts, with initial 187 Os/188 Os of 0.129±0.025 and 0.325±0.098, respectively. The Re–Os ages agree very well with the average∼ 122 Ma 40 Ar–39 Ar age for the OJP. The Re–Os and Lu–Hf results reinforce the interpretation of a two-component mantle source for the OJP consisting of a dominant near-primitive mantle similar to that involved in the sources of several ocean islands, represented by the Kroenke-and Kwaimbaita-type basalts, and old, recycled lower continental crustal material expressed subtly in the isotopic signature of the Singgalo-type basalts. A lower crustal influence may be explained by incorporation of ancient, delaminated mafic continental crust into a thermochemical plume; incorporation could provisionally involve dense ex-crustal material that had accumulated in the lower mantle or less dense material that had stagnated at mid-mantle depths.