On the equilibration timescales of isolated trace phases in mantle peridotites: Implications for the interpretation of grain-scale isotope heterogeneity in peridotitic sulfides
On the equilibration timescales of isolated trace phases in mantle peridotites: Implications for the interpretation of grain-scale isotope heterogeneity in peridotitic sulfides
复制标题
地幔橄榄岩中孤立痕量相的平衡时间尺度：对橄榄岩硫化物颗粒尺度同位素异质性解释的启示
DOI：
10.1016/j.epsl.2018.07.016
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发表时间：
2018
影响因子：
5.3
通讯作者：
Lassiter, John C.
中科院分区：
文献类型：
--
作者：
Lassiter, John C.
大洋中脊玄武岩（MORB）和地幔橄榄岩（例如深海橄榄岩）的地球化学研究为对流地幔的组成和演化提供了独立的约束。最近的研究揭示了 MORB 和深海橄榄岩的放射性同位素组成的系统差异，这使人们对这两个上地幔窗口的互补性质产生了疑问。这些差异的起源对于我们理解 MORB 岩石成因以及上地幔的组成和消耗历史至关重要。利用玄武岩中的同位素变化来探测地幔的组成和演化是基于地幔熔化过程中局部（即颗粒尺度）同位素平衡的假设。然而，一些研究报告了不同橄榄岩衍生硫化物群体中的锇和铅同位素不平衡，硅酸盐颗粒内的硫化物通常比间隙硫化物具有更多“贫化”同位素组成。原则上，晶粒尺度的同位素异质性可以反映具有可变母体/子体比例的古代硫化物中可变的放射成因向内生长，或者在地幔熔融过程中沿晶界具有更多放射成因硅酸盐相的低Re/Os和U/Pb硫化物的部分重新平衡。这要求硫化物在地质长（~Ga）时间尺度内无法与相邻相保持同位素平衡。一些研究将橄榄岩中锇同位素不平衡的保存归因于低[锇]硅酸盐的铠装效应，这限制了孤立的富锇硫化物之间的扩散交换。这提出了这样的前景：橄榄岩衍生的熔体可能不会继承其来源的锇（或铅）同位素组成，这可以解释最近记录的MORB和地幔橄榄岩的锇和铅同位素组成的系统差异。虽然地幔硫化物中同位素异质性的保存通常归因于上述“装甲效应”，但地幔中空间分离的硫化物的扩散平衡时间尺度此前尚未对橄榄岩进行过严格的估计。本研究研究了控制该平衡时间尺度的参数（平均硫化物尺寸和间距、铠装硅酸盐矿物中的 Os 和 Pb 扩散率以及硅酸盐和硫化物相之间的元素分配）。使用这些参数的可用约束进行的平衡时间尺度估计表明，在对流地幔温度下，大多数地幔硫化物预计将在不到几十 Myr 的时间内与邻近的硫化物实现同位素重新平衡。观察到的样品内 Os 和 Pb 同位素异质性表明，在 ~ Ga 时间尺度上维持同位素不平衡需要非常大的硫化物 (>100 μm)，其间隔为几毫米或扩散速率 (D <10−18m2/s)，比橄榄石中的大多数元素慢。结合样品内硫化物 Ni 和 Fe 丰度的主要元素异质性观察（它们应该在对流地幔中迅速平衡），这些结果表明观察到的同位素不平衡不太可能是地幔橄榄岩的古老特征。相反，最近的硫化物交代作用与来自同位素丰富的榴辉岩或辉石岩（或肥沃和/或交代橄榄岩）成分的熔体的相互作用有关，更可能解释橄榄岩中观察到的样品内同位素异质性。这种解释也与 MORB 和深海橄榄岩的 Nd 和 Hf 同位素组成之间的系统差异一致。这些结果进一步证实了 MORB 并未准确地...
Geochemical studies of mid-ocean-ridge basalts (MORB) and mantle peridotites (e.g., abyssal peridotites) provide independent constraints on the composition and evolution of the convecting mantle. Recent studies have revealed systematic differences in the radiogenic isotope compositions of MORB and abyssal peridotites that call into question the complementary nature of these two windows to the upper mantle. The origin of these differences is fundamental to our understanding of MORB petrogenesis and the composition and depletion history of the upper mantle. The use of isotope variations in basalts to probe the composition and evolution of the mantle is predicated on the assumption of local (i.e., grain-scale) isotopic equilibrium during mantle melting. However, several studies have reported Os- and Pb-isotope disequilibrium in distinct populations of peridotite-derived sulfides, with sulfides included within silicate grains typically possessing more “depleted” isotopic compositions than interstitial sulfides. In principle, grain-scale isotopic heterogeneity could reflect variable radiogenic ingrowth in ancient sulfides with variable parent/daughter ratios, or partial re-equilibration of low-Re/Os and U/Pb sulfides with more radiogenic silicate phases along grain boundaries during mantle melting. This would require that sulfides fail to maintain isotopic equilibrium with neighboring phases over geologically long (∼ Ga) time scales. The preservation of Os-isotope disequilibrium in peridotites has been ascribed in several studies to the armoring effect of low-[Os] silicates, which limits diffusive exchange between isolated Os-rich sulfides. This raises the prospect that peridotite-derived melts may not inherit the Os- (or Pb-) isotope composition of their source, which could account for the recently documented systematic differences in the Os- and Pb-isotope compositions of MORB and mantle peridotites.Although the preservation of isotopic heterogeneity in mantle sulfides is commonly ascribed to the above “armoring effect”, the diffusive equilibration timescale of spatially separated sulfides in mantle peridotites has not previously been rigorously estimated. This study examines the parameters that control this equilibration timescale (average sulfide size and spacing, Os and Pb diffusivity in armoring silicate minerals, and element partitioning between silicate and sulfide phases). Equilibration timescale estimates using available constraints on these parameters reveal that most mantle sulfides are expected to isotopically re-equilibrate with neighboring sulfides in less than a few 10 s of Myr at convecting mantle temperatures. Maintenance of isotopic disequilibrium over the ∼ Ga timescales suggested by observed intra-sample Os- and Pb-isotope heterogeneity requires very large sulfides (>100 μm) separated by several mm or diffusion rates (D <10−18m2/s), slower than for most elements in olivine. Combined with the observation of intra-sample major element heterogeneity in sulfide Ni and Fe abundances (which should equilibrate quickly in the convecting mantle), these results suggest that the observed isotopic disequilibrium is unlikely to be an ancient feature in mantle peridotites. Instead, recent sulfide metasomatism linked to interaction with melts derived from isotopically enriched eclogite or pyroxenite (or fertile and/or metasomatized peridotite) components is a more likely explanation for observed intra-sample isotopic heterogeneity in peridotites. This interpretation is also consistent with systematic differences between the Nd- and Hf-isotope compositions of MORB and abyssal peridotites. These results further strengthen the proposal that MORB do not accurately …
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