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首页> 外文期刊>Contributions to Mineralogy and Petrology >Zoning of phosphorus in igneous olivine
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Zoning of phosphorus in igneous olivine

机译：火成橄榄石中磷的分区

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We describe P zoning in olivines from terrestrial basalts, andesites, dacites, and komatiites and from a martian meteorite. P_2O_5 contents of olivines vary from below the detection limit (<0.01 wt percent) to 0.2-0.4 wt percent over a few microns, with no correlated variations in Fo content. Zoning patterns include P-rich crystal cores with skeletal, hopper, or euhedral shapes; oscillatory zoning; structures suggesting replacement of P-rich zones by P-poor olivine; and sector zoning. Melt inclusions in olivines are usually located near P-rich regions but in direct contact with low-P olivine. Crystallization experiments on basaltic compositions at constant cooling rates (15-30 deg C/h) reproduce many of these features. We infer that P-rich zones in experimental and natural olivines reflect incorporation of P in excess of equilibrium partitioning during rapid growth, and zoning patterns primarily record crystal-growth-rate variations. Occurrences of high-P phenocryst cores may reflect pulses of rapid crystal growth following delayed nucleation due to undercooling. Most cases of oscillatory zoning in P likely reflect internal factors whereby oscillating growth rates occur without external forcings, but some P zoning in natural olivines may reflect external forcings (e.g., magma mixing events, eruption) that result in variable crystal growth rates and/or P contents in the magma. In experimental and some natural olivines, Al, Cr, and P concentrations are roughly linearly and positively correlated, suggesting coupled substitutions, but in natural phenocrysts, Cr zoning is usually less intense than P zoning, and Al zoning weak to absent. We propose that olivines grow from basic and ultrabasic magmas with correlated zoning in P, Cr, and Al superimposed on normal zoning in Fe/Mg; rapidly diffusing divalent cations homogenize during residence in hot magma; Al and Cr only partially homogenize; and delicate P zoning is preserved because P diffuses very slowly. This interpretation is consistent with the fact that zoning is largely preserved not only in P but also in Al, Cr, and divalent cations in olivines with short residence times at high temperature (e.g., experimentally grown olivines, komatiitic olivines, groundmass olivines, and the rims of olivine phenocrysts grown during eruption). P zoning is widespread in magmatic olivine, revealing details of crystal growth and intra-crystal stratigraphy in what otherwise appear to be relatively featureless crystals. Since it is preserved in early-formed olivines with prolonged residence times in magmas at high temperatures, P zoning has promise as an archive of information about an otherwise largely inaccessible stage of a magma's history. Study of such features should be a valuable supplement to routine petrographic investigations of basic and ultrabasic rocks, especially because these features can be observed with standard electron microprobe techniques.
机译：我们描述了来自陆上玄武岩，安山岩，Dacites和Komatiite以及火星陨石的橄榄石中的P带。橄榄石中P_2O_5的含量在几微米范围内从检测限以下（<0.01 wt％）到0.2-0.4 wt％不等，Fo含量无相关变化。分区模式包括具有骨骼，漏斗形或自体形状的富含P的晶体核;振荡分区;表明贫磷橄榄石替代了富磷区的结构;和扇区分区。橄榄石中的熔体夹杂物通常位于富磷区域附近，但与低磷橄榄石直接接触。在恒定冷却速率（15-30℃/ h）下对玄武岩成分进行结晶实验，可以再现许多这些特征。我们推断实验和天然橄榄石中的P富集区反映了P的掺入超过了快速生长过程中的平衡分配，而分区模式主要记录了晶体生长速率的变化。高P隐晶核的出现可能反映了由于过冷导致的延迟成核后晶体快速生长的脉冲。 P的大多数振荡分区可能反映了内部因素，从而在没有外部强迫的情况下发生了振荡的生长速度，但是天然橄榄石中的某些P分区可能反映了外部强迫（例如，岩浆混合事件，喷发），从而导致了晶体生长速率的变化和/或岩浆中的P含量。在实验和一些天然橄榄石中，Al，Cr和P的浓度大致呈线性正相关，这表明偶联取代，但是在天然隐晶中，Cr分区通常不如P分区强，而Al分区则弱到不存在。我们认为，橄榄岩从基本岩浆和超碱性岩浆中生长，在P，Cr和Al中的相关分区与在Fe / Mg中的正常分区重叠。在热岩浆中停留期间，快速扩散的二价阳离子均质化; Al和Cr仅部分均质;由于P扩散非常缓慢，因此保留了精细的P分区。这种解释与以下事实一致：在高温下短停留时间的橄榄石中（例如，实验生长的橄榄石，科马提叶橄榄石，地层橄榄石和硅藻土），不仅在P中而且在Al，Cr和Al，Cr和二价阳离子中都保留了分区功能。火山喷发过程中生长的橄榄石表晶的边缘）。 P带在岩浆橄榄石中很普遍，揭示了晶体生长和晶体内地层学的细节，而在其他情况下这些晶体似乎是相对无特征的晶体。由于它被保存在早期形成的橄榄石中，并且在高温下在岩浆中的停留时间延长，因此P分区有望成为有关岩浆历史原本无法进入的阶段的信息的档案库。对这些特征的研究应该是对基础岩石和超基性岩石常规岩石学研究的宝贵补充，尤其是因为可以使用标准的电子微探针技术观察到这些特征。

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