The Effect of Postcumulus Reactions on Composition of Chrome-spinels from the Jimberlana Intrusion
The Effect of Postcumulus Reactions on Composition of Chrome-spinels from the Jimberlana Intrusion
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DOI：
10.1093/petrology/26.3.763
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发表时间：
1985-08
影响因子：
3.9
通讯作者：
P. Roeder;I. Campbell
中科院分区：
文献类型：
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作者：
P. Roeder;I. Campbell
对Jimberlana岩体钻孔岩芯中橄榄石堆晶、中堆晶和正堆晶中的铬铁矿进行了分析，并将其与堆晶类型和周围硅酸盐矿物的性质联系起来。堆晶和中堆晶中的铬铁矿显示出有限的成分范围，并且富含Mg、Al和Cr。正累积型铬铁矿的组成与累积型铬铁矿的组成不同，后者的Fe和Ti含量更高，Fe 3 3 3：Fe 2 3比例更高。正积云中铬铁矿的组成取决于包裹硅酸盐矿物的性质，这被认为反映了包裹矿物保护原积云铬铁矿不与积云间液体反应的能力。因此，铬铁矿在早期古铜岩oikocrysts保护反应，而在斜长石和金云母的保护在一个更晚的阶段，并具有较高的铁和钛含量的橄榄石内的铬铁矿似乎已经能够平衡与堆间液体，直到晚在岩浆的历史，除了在包围铬铁矿的橄榄石本身被古铜岩包围。铬铁矿通过橄榄石与堆间液进行元素交换。有两种可能性;或者诸如Cr，Al，Ti和Fe 3 3能够通过橄榄石结构扩散，或者表面上封闭的铬铁矿晶体能够沿着橄榄石和铬铁矿的差异热收缩产生的细裂缝与熔体保持沿着直接接触。正累积古铜岩和橄榄石中的铬铁矿晶体的平均直径分别为28和20 μ m，而斜长石和金云母中的铬铁矿晶体的平均直径分别为28和20 μ m。具有48和56微米的平均直径。对于在特定硅酸盐中发现的颗粒，铬铁矿的大小与其组成之间没有明显的相关性。任何大小的铬铁矿都能与液体平衡，除非它们被保护而不发生反应。反应产物的成核在决定任何特定铬铁矿晶体的最终组成方面起着重要作用。被认为是相对于主要的硅酸盐和最后的液体的位置的局部尺度上的毫米的反应和成核的意义。有人建议，最后的液体往往集中在口袋里的反应物晶体，产品晶体未能成核，直到晚在岩浆历史。据估计，在具有正积云结构的岩石中，积云间液体在高达300 °C的温度范围内结晶，并且在接近固相线温度时变得更加氧化。
Chromites in olivine adcumulates, mesocumulates and orthocumulates from drill core of the Jimberlana intrusion have been analysed and related to the cumulate type and to the nature of the surrounding silicate mineral. Chromites in adcumulates and mesocumulates show a restricted range of composition and are high in Mg, Al and Cr. The orthocumulate chromites vary in composition from that found in adcumulates to chromites which are much higher in Fe and Ti and with a higher Fe3³: Fe2³ratio. The chromites in orthocumulates vary in composition depending upon the nature of the enclosing silicate mineral. This is believed to reflect the ability of the enclosing mineral to protect the original cumulus chromite from reaction with the intercumulus liquid. Thus chromite within early bronzite oikocrysts was protected from reaction whereas that in plagioclase and phlogopite was protected at a much later stage and has a higher Fe and Ti content Chromite within olivine appears to have been able to equilibrate with intercumulus liquid until late in the magmatic history except where the olivine enclosing chromite has itself been surrounded by bronzite. It is suggested that chromite can exchange elements with intercumulus liquid through the olivine. There are two possibilities; either elements such as Cr, Al, Ti and Fe3 ³were able to diffuse through the olivine structure or the apparently enclosed chromite crystals were able to maintain direct contact with the melt along fine fractures produced by the differential thermal contraction of olivine and chromite.The average diameters of chromite crystals within orthocumulate bronzite and olivine are 28 and 20 microns respectively whereas chromites in plagioclase and phlogopite have average diameters of 48 and 56 microns. There is no obvious correlation between the size of the chromite and their composition for grains found within a particular silicate. Chromites of every size have been able to equilibrate with the liquid unless they were protected from reaction.Nucleation of reaction products played an important role in determining the final composition of any particular chromite crystal. The significance of reaction and nucleation on a local scale of millimetres is considered with respect to the major silicates and to the location of the last liquid. It is suggested that the last liquid tended to concentrate in pockets of reactant crystals, where product crystals failed to nucleate until late in the magmatic history. It is estimated that in rocks with an orthocumulate texture, the intercumulus liquid crystallized over a temperature range as large as 300 °C and that it became significantly more oxidizing near the solidus temperature.