Bonadiman Costanza, Brombin Valentina, Andreozzi Giovanni B, Benna Piera, Coltorti Massimo, Curetti Nadia, Faccini Barbara, Merli Marcello, Pelorosso Beatrice, Stagno Vincenzo, Tesauro Magdala, Pavese Alessandro
Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Via Saragat 1, 44122, Ferrara, Italy.
Istituto di Geologia Ambientale e Geoingegneria del Consiglio Nazionale delle Ricerche (IGAG-CNR), Via Salaria km 29, 300, 00015, Montelibretti, Italy.
Sci Rep. 2021 Jun 4;11(1):11829. doi: 10.1038/s41598-021-90844-w.
The occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensively studied. In this work, we discuss the geochemical conditions at which the pargasite-phlogopite assemblage becomes stable, through the investigation of two mantle xenoliths from Mount Leura (Victoria State, Australia) that bear phlogopite and the phlogopite + amphibole (pargasite) pair disseminated in a harzburgite matrix. Combining a mineralogical study and thermodynamic modelling, we predict that the P-T locus of the equilibrium reaction pargasite + forsterite = Na-phlogopite + 2 diopside + spinel, over the range 1.3-3.0 GPa/540-1500 K, yields a negative Clapeyron slope of -0.003 GPa K (on average). The intersection of the P-T locus of supposed equilibrium with the new mantle geotherm calculated in this work allowed us to state that the Mount Leura xenoliths achieved equilibrium at 2.3 GPa /1190 K, that represents a plausible depth of ~ 70 km. Metasomatic K-Na-OH rich fluids stabilize hydrous phases. This has been modelled by the following equilibrium equation: 2 (K,Na)-phlogopite + forsterite = 7/2 enstatite + spinel + fluid (components: NaO,KO,HO). Using quantum-mechanics, semi-empirical potentials, lattice dynamics and observed thermo-elastic data, we concluded that K-Na-OH rich fluids are not effective metasomatic agents to convey alkali species across the upper mantle, as the fluids are highly reactive with the ultramafic system and favour the rapid formation of phlogopite and amphibole. In addition, oxygen fugacity estimates of the Mount Leura mantle xenoliths [Δ(FMQ) = -1.97 ± 0.35; -1.83 ± 0.36] indicate a more reducing mantle environment than what is expected from the occurrence of phlogopite and amphibole in spinel-bearing peridotites. This is accounted for by our model of full molecular dissociation of the fluid and incorporation of the O-H-K-Na species into (OH)-K-Na-bearing mineral phases (phlogopite and amphibole), that leads to a peridotite metasomatized ambient characterized by reduced oxygen fugacity.
金云母和角闪石在地幔超镁铁质岩石中的出现被广泛认为是上地幔交代作用的典型效应。然而，它们在交代作用过程中的同时形成以及与橄榄岩相关的亚固相线平衡尚未得到广泛研究。在这项工作中，我们通过对来自澳大利亚维多利亚州勒拉山的两块地幔捕虏体进行研究，探讨了镁金云母 - 金云母组合变得稳定的地球化学条件，这些捕虏体含有金云母以及散布在方辉橄榄岩基质中的金云母 + 角闪石（镁金云母）对。结合矿物学研究和热力学建模，我们预测在1.3 - 3.0 GPa / 540 - 1500 K范围内，平衡反应镁金云母 + 镁橄榄石 = 钠金云母 + 2透辉石 + 尖晶石的P - T轨迹平均产生 -0.003 GPa K的负克拉佩龙斜率。假设平衡的P - T轨迹与本工作中计算的新地幔地热曲线的交点使我们能够指出，勒拉山捕虏体在2.3 GPa / 1190 K达到平衡，这代表了约70 km的合理深度。富含K - Na - OH的交代流体使含水相稳定。这已通过以下平衡方程进行建模：2（K，Na） - 金云母 + 镁橄榄石 = 7/2顽火辉石 + 尖晶石 + 流体（组分：NaO、KO、HO）。利用量子力学、半经验势、晶格动力学和观测到的热弹性数据，我们得出结论，富含K - Na - OH的流体不是在上地幔中输送碱金属物种的有效交代剂，因为这些流体与超镁铁质系统高度反应，并有利于金云母和角闪石的快速形成。此外，勒拉山地幔捕虏体的氧逸度估计值[Δ（FMQ）= -1.97 ± 0.35； -1.83 ± 0.36]表明，其地幔环境比含尖晶石橄榄岩中出现金云母和角闪石所预期的环境更具还原性。这是由我们的流体全分子解离模型以及O - H - K - Na物种掺入含（OH） - K - Na的矿物相（金云母和角闪石）来解释的，这导致了以降低的氧逸度为特征的橄榄岩交代环境。
