One of the largest chromium deposits on Earth occurs in the Rustenburg Layered Suite (RLS) of the Bushveld Complex as laterally continuous chromitite layers. None of the hypotheses proposed for the origin of the chromitites can explain both the abundance of Cr in the RLS and the unusual enrichment in Cr and V over Ni, relative to typical depleted mantle values. This study investigates the possibility that the layering and chromitite formation are consequences of the entrainment of source components into the magmas that formed the RLS. Thermodynamic modelling results reveal a wedge-shaped domain in pressure-temperature space in the subcratonic mantle within which Cr-bearing orthopyroxene forms as a peritectic product of incongruent melting. Entrainment of this orthopyroxene produces magmas that crystallise peritectic olivine and chromite on ascent, due to the consumption of orthopyroxene by melt. The chromite- and olivine-bearing magmas intrude as sills and can produce chromite and dunite layers by density separation. This model, which interprets the RLS Sr-isotopic composition to reflect prior mantle metasomatism by crustal fluids (ideally ancient and of low volume), readily explains the formation of chromitite layers from relatively thin sills, as well as the very high ratios of Cr and V to other compatible elements relative to typical mantle compositions. The special circumstances required to produce the RLS chromitites do not relate to some oddity of repetitive crustal assimilation or magma compositions that allow chromite-only saturation. Rather, they relate to speed of melting and magma extraction which enabled peritectic orthopyroxene entrainment to the magmas.

地球上最大的铬矿床之一出现在布什维尔德杂岩的勒斯滕堡层状套件 (RLS) 中，呈横向连续的铬铁矿层。相对于典型的贫化地幔值，针对铬铁矿起源提出的假设都无法解释 RLS 中 Cr 的丰度以及 Cr 和 V 相对于 Ni 的异常富集。这项研究调查了分层和铬铁矿形成是源成分夹带进入形成 RLS 的岩浆的结果的可能性。热力学模拟结果揭示了克拉通下地幔压力-温度空间中的楔形区域，其中含铬斜方辉石作为不均匀熔融的包晶产物形成。由于斜方辉石被熔体消耗，这种斜方辉石的夹带产生岩浆，在上升时使包晶橄榄石和铬铁矿结晶。含铬铁矿和橄榄石的岩浆作为基岩侵入，通过密度分离可产生铬铁矿和纯橄榄岩层。该模型解释了 RLS Sr 同位素组成，以反映地壳流体（理想情况下古老且体积较小）先前的地幔交代作用，很容易解释从相对较薄的基岩中形成铬铁矿层，以及非常高的 Cr 和 Cr 比例。 V 相对于典型地幔成分的其他相容元素。产生 RLS 铬铁矿所需的特殊环境与重复地壳同化或仅允许铬铁矿饱和的岩浆成分的某些奇怪现象无关。相反，它们与熔化和岩浆提取的速度有关，这使得包晶斜方辉石夹带到岩浆中。