Spandler C, O'Neill H St C, Kamenetsky V S
Research School of Earth Sciences, Australian National University, Canberra 0200, Australia.
Nature. 2007 May 17;447(7142):303-6. doi: 10.1038/nature05759.
The chemical composition of basaltic magma erupted at the Earth's surface is the end product of a complex series of processes, beginning with partial melting and melt extraction from a mantle source and ending with fractional crystallization and crustal assimilation at lower pressures. It has been proposed that studying inclusions of melt trapped in early crystallizing phenocrysts such as Mg-rich olivine and chromite may help petrologists to see beyond the later-stage processes and back to the origin of the partial melts in the mantle. Melt inclusion suites often span a much greater compositional range than associated erupted lavas, and a significant minority of inclusions carry distinct compositions that have been claimed to sample melts from earlier stages of melt production, preserving separate contributions from mantle heterogeneities. This hypothesis is underpinned by the assumption that melt inclusions, once trapped, remain chemically isolated from the external magma for all elements except those that are compatible in the host minerals. Here we show that the fluxes of rare-earth elements through olivine and chromite by lattice diffusion are sufficiently rapid at magmatic temperatures to re-equilibrate completely the rare-earth-element patterns of trapped melt inclusions in times that are short compared to those estimated for the production and ascent of mantle-derived magma or for magma residence in the crust. Phenocryst-hosted melt inclusions with anomalous trace-element signatures must therefore form shortly before magma eruption and cooling. We conclude that the assumption of chemical isolation of incompatible elements in olivine- and chromite-hosted melt inclusions is not valid, and we call for re-evaluation of the popular interpretation that anomalous melt inclusions represent preserved samples of unmodified mantle melts.
在地球表面喷发的玄武质岩浆的化学成分是一系列复杂过程的最终产物，这些过程始于从地幔源区的部分熔融和熔体提取，终于在较低压力下的分离结晶和地壳同化作用。有人提出，研究被困在早期结晶的斑晶（如富镁橄榄石和铬铁矿）中的熔体包裹体，可能有助于岩石学家透过后期过程，追溯到地幔中部分熔体的起源。熔体包裹体组合的成分范围通常比相关的喷发熔岩大得多，而且有相当一部分包裹体具有独特的成分，据称这些成分代表了熔体生成早期阶段的熔体样本，保留了地幔不均一性的单独贡献。这一假设的依据是，熔体包裹体一旦被困住，除了在寄主矿物中相容的元素外，对于所有元素而言，都与外部岩浆保持化学隔离。在此我们表明，在岩浆温度下，稀土元素通过橄榄石和铬铁矿的晶格扩散通量足够快，能在与地幔源岩浆的生成和上升或岩浆在地壳中停留时间相比很短的时间内，使被困熔体包裹体的稀土元素模式完全重新平衡。因此，具有异常微量元素特征的斑晶寄主熔体包裹体必定在岩浆喷发和冷却前不久形成。我们得出结论，橄榄石和铬铁矿寄主熔体包裹体中不相容元素化学隔离的假设是无效的，我们呼吁重新评估一种普遍的解释，即异常熔体包裹体代表未改性地幔熔体的保留样本。
