To investigate the petrogenesis of cyclic units in layered intrusions, we examined chromitite, dunite, poikilitic harzburgite and bronzitite from the Ultramafic Zone of the Stillwater Complex and measured stable isotopes of Li and O in their major minerals. The Li isotopes in olivine range from 4 to 26‰ in δ 7 Li with uniform Li contents of 1–3 ppm, whereas orthopyroxene and clinopyroxene have Li contents of 0.5–5 ppm and 4–8 ppm, and δ 7 Li ranges of −13 to 7‰ and −14 to −6‰, respectively. The δ 18 O values vary from 4.91 to 5.72‰ in olivine, from 5.11 to 5.87‰ in orthopyroxene, and from 4.64 to 5.86‰ in clinopyroxene. For a given sample, olivine displays more variable and higher δ 7 Li but lower δ 18 O values than orthopyroxene, indicating that olivine experienced more extensive compositional modification after crystallization relative to orthopyroxene. The general Li and O isotopic compositions are interpreted as the result of re-equilibration between interstitial liquids, from which pyroxenes crystallized, and cumulus minerals. The inter-mineral and inter-sample isotopic variations correlate with mineral assemblages, crystal sizes and major and trace element compositions, revealing that the interstitial liquids varied compositionally mainly due to mixing between fractionated magma and newly injected primitive magma. Abrupt mineralogical and geochemical changes from silicate rocks to chromitites imply that hydrous fluids, which collected on chromite surfaces and were later released from chromite seams, played an additional, critical medium of chemical exchange between minerals in the chromitites.

中文翻译：

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北美斯蒂尔沃特杂岩体超镁铁带的岩石成因：矿物化学和锂和氧稳定同位素的限制

为了研究层状侵入体中循环单元的岩石成因，我们检测了来自 Stillwater 杂岩体超镁铁质带的铬铁矿、纯长岩、变质斜方辉石和青铜长石，并测量了它们主要矿物中锂和氧的稳定同位素。橄榄石中的锂同位素在 δ 7 Li 中的范围为 4 到 26‰，Li 含量均匀为 1-3 ppm，而斜方辉石和斜辉石的 Li 含量为 0.5-5 ppm 和 4-8 ppm，δ 7 Li 的范围为 -分别为 13 到 7‰ 和 -14 到 -6‰。δ 18 O 值在橄榄石中从 4.91 到 5.72 ‰、在斜辉石中从 5.11 到 5.87 ‰ 以及在斜辉石中从 4.64 到 5.86 ‰ 不等。对于给定的样品，橄榄石显示出比斜方辉石更多的可变性和更高的 δ 7 Li，但 δ 18 O 值更低，表明相对于斜方辉石，橄榄石在结晶后经历了更广泛的成分改变。一般的 Li 和 O 同位素组成被解释为间隙液体（从中结晶出辉石）和积云矿物之间重新平衡的结果。矿物间和样品间同位素变化与矿物组合、晶体大小以及主要和微量元素组成相关，表明间隙液体的成分变化主要是由于分馏岩浆和新注入的原始岩浆之间的混合。从硅酸盐岩石到铬铁矿的突然矿物学和地球化学变化意味着含水流体，收集在铬铁矿表面，后来从铬铁矿接缝中释放出来，起到了额外的作用，