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Survival times of anomalous melt inclusions from element diffusion in olivine and chromite

机译：元素在橄榄石和铬铁矿中扩散的异常熔体包裹体的生存时间

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摘要

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.
机译：地球表面喷出的玄武岩浆的化学成分是一系列复杂过程的最终产物，首先是部分熔融和从地幔源中提取熔体，最后是在较低压力下进行分步结晶和地壳同化。有人提出，研究早期结晶的表晶（例如富含镁的橄榄石和亚铬酸盐）中捕获的熔体包裹体，可能有助于岩石学家超越后期过程，回到地幔中部分熔体的起源。熔体夹杂物套件的成分范围通常比相关的喷发熔岩大得多，并且极少数的夹杂物带有不同的成分，据称可以对熔体生产早期阶段的熔体进行采样，从而保留了地幔非均质性的独立贡献。该假设由以下假设所支撑，即熔体夹杂物一旦被捕集，除与主体矿物中相容的那些元素外，所有元素仍与外部岩浆化学隔离。在这里，我们表明，在岩浆温度下，通过晶格扩散通过橄榄石和亚铬铁矿流过的稀土元素通量足够快，可以在短于与估算的熔体夹杂物相比更短的时间内完全重新平衡捕获的熔体夹杂物的稀土元素模式。来自地幔的岩浆的生产和上升，或者用于居住在地壳中的岩浆。因此，在岩浆喷发和冷却之前不久，必须形成具有异常痕量元素特征的，由隐晶系包裹的熔体包裹体。我们得出结论，认为在橄榄石和铬铁矿熔体夹杂物中化学分离不相容元素的假设是无效的，我们呼吁重新评估流行的解释，即异常熔体夹杂物代表未改性地幔熔体的保存样品。

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  • 来源
    《Nature》 |2007年第7142期|p.303-306|共4页
  • 作者

    C. Spandler; H. St C. ONeill; V. S. Kamenetsky;

  • 作者单位

    Institute of Geological Sciences, University of Bern, Bern 3012, Switzerland;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 自然科学总论;
  • 关键词

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