Polyphase groundmasses (micro-scale minerals with or without glass) are generated from silicate liquids during the cooling of natural lavas often alongside larger minerals formed long before eruption. Many researchers have posited that compositions gleaned from the analysis of groundmasses closely approximate the compositions of the melts they were derived from, and these have been used frequently to model pre-eruptive magma conditions. However, it is difficult to confidently identify and sample these groundmasses once they are formed. Using a sample of lava that exhibits a wide degree of textural variation (ranging from holocrystalline to hypohyaline) we show that compositions of groundmasses sampled using defocused electron beams are significantly different from glass compositions in terms of mean composition and covariance. Despite this, several groundmass compositions qualify as ‘in equilibrium’ with matrix/rim olivine. When processed using available thermometers, however, modelled equilibrium temperatures are significantly higher than those produced using glass data, on average. Because of this, we prescribe caution in using polyphase groundmass data generated using defocused beam analysis even as a rudimentary approach.

中文翻译：

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通过散焦束电子探针微分析推断多相基质成分的岩石学效用的统计视角

多相基质（含有或不含玻璃的微型矿物）是在天然熔岩冷却过程中由硅酸盐液体产生的，通常与喷发前很久形成的较大矿物一起形成。许多研究人员认为，从基质分析中收集到的成分与它们所源自的熔体的成分非常接近，并且这些成分经常被用来模拟喷发前的岩浆条件。然而，一旦这些基质形成，就很难自信地识别它们并对其进行采样。使用表现出广泛结构变化（从全晶到次透明）的熔岩样本，我们发现使用散焦电子束采样的基质成分在平均成分和协方差方面与玻璃成分显着不同。尽管如此，一些基质成分仍符合基质/边缘橄榄石“平衡”的条件。然而，当使用现有温度计进行处理时，平均而言，模拟的平衡温度明显高于使用玻璃数据产生的平衡温度。因此，即使作为一种基本方法，我们也建议谨慎使用通过散焦光束分析生成的多相基质数据。