Clinopyroxene and orthopyroxene are the two major repositories of rare-earth elements (REE) in spinel peridotites. Most geochemical studies of REE in mantle samples focus on clinopyroxene. Recent advances in in situ trace element analysis have made it possible to measure REE abundance in orthopyroxene. The purpose of this study is to determine what additional information one can learn about mantle processes from REE abundances in orthopyroxene coexisting with clinopyroxene in residual spinel peridotites. To address this question, we select a group of spinel peridotite xenoliths (9 samples) and a group of abyssal peridotites (12 samples) that are considered residues of mantle melting and that have major element and REE compositions in the two pyroxenes reported in the literature. We use a disequilibrium double-porosity melting model and the Markov chain Monte Carlo method to invert melting parameters from REE abundance in the bulk sample. We then use a subsolidus reequilibration model to calculate REE redistribution between clinopyroxene and orthopyroxene at the extent of melting inferred from the bulk REE data and at the closure temperature of REE in the two pyroxenes. We compare the calculated results with those observed in clinopyroxene and orthopyroxene in the selected peridotitic samples. Results from our two-step melting followed by subsolidus reequilibration modeling show that it is more reliable to deduce melting parameters from REE abundance in the bulk peridotite than in clinopyroxene. We do not recommend the use of REE in clinopyroxene alone to infer the degree of melting experienced by the mantle xenolith. In general, HREE in clinopyroxene and LREE in orthopyroxene are more susceptible to subsolidus redistribution. The extent of redistribution depends on the modes of clinopyroxene and orthopyroxene in the sample and thermal history experienced by the peridotite. By modeling subsolidus redistribution of REE between orthopyroxene and clinopyroxene after melting, we show that it is possible to discriminate mineral mode of the starting mantle and cooling rate experienced by the peridotitic sample. We conclude that endmembers of the depleted MORB mantle and the primitive mantle are not homogeneous in mineral mode. A modally heterogeneous peridotitic starting mantle provides a simple explanation for the large variations of mineral mode observed in mantle xenoliths and abyssal peridotites. Finally, using different starting mantle compositions in our simulations, we show that composition of the primitive mantle is more suitable for modeling REE depletion in cratonic mantle xenoliths than the composition of the depleted MORB mantle.

Clinopyroxene和Orthopyroxene是尖晶石橄榄岩中稀土元素（REE）的两个主要储层。地幔样品中稀土元素的大多数地球化学研究都集中在斜柏基。原位痕量元素分析的最新进展使得测量邻位邻苯二茂铁中的稀土元素丰度成为可能。这项研究的目的是确定人们可以从邻苯二茂铁中的REE富集与残留尖晶石橄榄岩中的斜柏石中的REE丰度中了解到有关地幔过程的哪些其他信息。为了解决这个问题，我们选择了一组尖晶石橄榄石异岩（9个样品）和一组深渊橄榄石（12个样品），它们被认为是地幔融化的残留物，并且在文献中报道的两种辉石中具有主要元素和稀土元素。我们使用不平衡双孔熔融模型和马尔可夫链蒙特卡洛方法，从大体积样品中的稀土元素丰度中转化出熔融参数。然后，我们使用亚固相线再平衡模型，计算从大量REE数据推断出的熔融程度以及两种辉石中REE的封闭温度下，斜茂铁和邻苯二茂之间的REE再分布。我们将计算结果与在选定的橄榄岩样品中的斜柏和正柏中观察到的结果进行了比较。我们的两步熔融然后进行亚固相线再平衡建模的结果表明，从块状橄榄岩中的稀土元素丰度推导熔融参数比在斜ino石中更可靠。我们不建议仅在斜ino中使用REE来推断地幔异种岩经历的熔化程度。总的来说，斜发py中的HREE和邻苯二茂中的LREE更容易发生亚固相线的重新分布。重新分布的程度取决于样品中的亚次氯铵和邻苯二酚的模式以及橄榄石经历的热历史。通过模拟熔融后邻苯二茂基和斜茂铁之间的REE固相线再分布，我们表明可以区分起始地幔的矿物模式和橄榄岩样品所​​经历的冷却速率。我们得出结论，在矿物模式下，耗尽的MORB地幔和原始地幔的末端成员不是均质的。模态不均匀的橄榄岩化起始地幔为在地幔异种岩和深渊橄榄岩中观察到的矿物模式的巨大变化提供了简单的解释。最后，