Nd, Sr and Os isotope systematics in young, fertile spinel peridotite xenoliths from northern Queensland, Australia: A unique view of depleted MORB mantle?
Nd, Sr and Os isotope systematics in young, fertile spinel peridotite xenoliths from northern Queensland, Australia: A unique view of depleted MORB mantle?
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DOI：
10.1016/j.gca.2005.08.003
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发表时间：
2005-12
影响因子：
5
通讯作者：
M. Handler;V. Bennett;R. Carlson
中科院分区：
文献类型：
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作者：
M. Handler;V. Bennett;R. Carlson
昆士兰州东北部是澳大利亚东部显生宙复合塔斯曼褶皱带的一部分，其古生代至中生代历史以俯冲带过程为主。一组来自 <3 Ma 阿瑟顿高原火山省的 13 颗橄榄岩捕虏体，主要来自昆肯山，包含肥沃的尖晶石二辉橄榄岩（1.8–3.4 wt.% Al2O3 和 38.7–41.9 wt.% MgO），不含次生挥发物相，仅具有弱交代富集的不相容微量元素 (SmN/YbN= 0.23–1.1;LaN/YbN=0.11–4.9)。该套件具有同位素异质性，测得的 Sr (87Sr/86Sr = 0.7027–07047)、Nd (143Nd/144Nd = 0.51249–0.51362) 以及较小程度的 Os (187Os/188Os = 0.1228–0.1292) 成分与 MORB 源地幔大体重叠（DMM）并扩展到更多的枯竭成分，反映了时间积分枯竭储层的演化。主要和稀土元素系统学与地幔一致，地幔在低至中等程度的熔体提取后残留，主要存在于尖晶石相中，但在石榴石场或石榴石-尖晶石转变附近，有一些样品需要在更大压力下部分熔融。与大多数先前研究的大陆岩石圈地幔样品组相比，这些样品的不相容的微量元素含量和Sr和Nd同位素系统表明，交代过程对采样的岩石圈仅进行了最小程度的修改。六块昆肯山捕虏体中的五块保留了贫化中重稀土元素模式，形成了完整的岩石 Sm-Nd 等时线，年龄约为 275 Ma (εNdi = +9)，与上覆区域广泛的花岗岩就位一致。该等时线被解释为指示 DMM 类源部分熔化的时间。来自阿瑟顿其他地区的包体散布在等时线周围，这表明采样的地幔代表了二叠纪岩石圈中 DMM 地幔的添加，当时该地区可能大致处于俯冲带环境内。第六个中重稀土元素贫乏的昆坎山捕虏体具有独特的 Nd 和 Os 同位素组成，这与早期可能的前寒武纪熔体萃取事件一致，或者与二叠纪衍生自具有较少贫化（时间平均较低的 Sm/Nd）Nd 同位素组成但较贫化（低 Re/Os）Os 同位素组成的地幔源相一致。测量的全岩石锇同位素组成的范围不能仅仅是可变熔体提取的时间积分效应的结果，特别是考虑到该套件的相干钐-钕系统学。 Os异质性更可能反映了一种异质~275 Ma DMM源，其具有与深海橄榄岩和球粒陨石重叠的当今Os成分（187Os/188Os~0.1265-0.1287），或者在隐秘的亲铜矿富集事件中携带放射性Os的次生硫化物在岩石圈地幔内显着且可变的富集。无论 Os 同位素变异的起源如何，这些数据强调了地幔 Re-Os 同位素异质性，这种异质性可能存在于小长度尺度上，其中亲石 Sm-Nd 系统可能相对均匀。
Northeastern Queensland, a part of the Phanerozoic composite Tasman Fold Belt of eastern Australia, has a Paleozoic to Mesozoic history dominated by subduction zone processes. A suite of 13 peridotite xenoliths from the <3 Ma Atherton Tablelands Volcanic Province, predominantly from Mount Quincan, comprise fertile (1.8–3.4 wt.% Al2O3and 38.7–41.9 wt.% MgO) spinel lherzolites free from secondary volatile-bearing phases and with only weak metasomatic enrichment of incompatible trace elements (SmN/YbN= 0.23–1.1; LaN/YbN= 0.11–4.9). The suite is isotopically heterogeneous, with measured Sr (87Sr/86Sr = 0.7027–07047), Nd (143Nd/144Nd = 0.51249–0.51362), and to a lesser extent, Os (187Os/188Os = 0.1228–0.1292) compositions broadly overlapping MORB source mantle (DMM) and extending to more depleted compositions, reflecting evolution in a time-integrated depleted reservoir. Major and rare earth element systematics are consistent with mantle that is residual after low to moderate degrees of melt extraction predominantly in the spinel facies, but with a few samples requiring partial melting at greater pressures in the garnet field or near the garnet-spinel transition. In contrast to most previously studied suites of continental lithospheric mantle samples, the incompatible trace element contents and Sr and Nd isotopic systematics of these samples suggest only minimal modification of the sampled lithosphere by metasomatic processes. Five of six Mount Quincan xenoliths preserving depleted middle to heavy REE patterns form a whole rock Sm-Nd isochron with an age of ∼275 Ma (εNdi = +9), coincident with widespread granitoid emplacement in the overlying region. This isochron is interpreted to indicate the timing of partial melting of a DMM-like source. Xenoliths from other Atherton localities scatter about the isochron, suggesting that the sampled mantle represents addition of DMM mantle to the lithosphere in the Permian, when the region may have broadly been within a subduction zone setting. A sixth middle to heavy REE-depleted Mount Quincan xenolith has a distinct Nd and Os isotopic composition consistent either with an earlier, possibly Precambrian melt extraction event, or with Permian derivation from a mantle source with a less depleted (time-averaged lower Sm/Nd) Nd isotopic composition, but a more depleted (low Re/Os) Os isotopic composition. The range in measured whole rock Os isotopic compositions cannot solely be the result of time-integrated effects of variable melt extraction, especially considering the coherent Sm-Nd systematics of the suite. The Os heterogeneity more likely reflects either a heterogeneous ∼275 Ma DMM source that would have a present-day Os composition (187Os/188Os ∼ 0.1265–0.1287) overlapping both abyssal peridotites and chondrites, or significant and variable enrichment within the lithospheric mantle by secondary sulfides carrying radiogenic Os in a cryptic chalcophile enrichment event. Regardless of the origin of the Os isotopic variability, these data highlight the mantle Re-Os isotopic heterogeneity that may be present over small length scales where the lithophile Sm-Nd system may be relatively homogeneous.