Sheared peridotites: linking deformation and metasomatism contributing to the onset of craton destabilization

地幔柱代表最古老的大陆地壳，有一个厚的岩石圈地幔根（~180- 250公里）。这个根形成于太古代（3.5-2.5 Ga），它的存在直接归因于古龙的长寿。一个重要的因素是根部和周围软流圈之间的粘度差异。这种对比的一个解释是岩石圈的基础是H2O贫乏，橄榄石和辉石中的低OH含量使橄榄岩的流变性更硬（更高的粘度）。这意味着粘度对比度可以通过（重新）引入挥发物和其他化学组分的交代活动而降低，从而导致岩石块的不稳定和潜在的破裂。失稳将通过剪切区中剪应力的集中来引导。这样的区域可以促进和局部化熔体和流体迁移。交代作用和变形之间的反馈机制还不清楚，需要多学科的研究来将地球化学修饰与变形联系起来。“剪切”橄榄岩具有指示一个或多个强烈变形事件的纹理，并反映上地幔条件下的流变行为。剪切橄榄岩通常记录最深的深度为一个给定的捕虏体套件，这样的样品应该作为韧性剪切过程的代理或附近的岩石圈的底部，因此有关的了解岩石圈地幔可能变得不稳定。我们的目标是调查之间的关系在地幔岩石圈的变形过程和地球化学变化的交代作用。将对莱索托和南非金伯利的剪切橄榄岩捕虏体进行研究，这两个参数分别来自不同的深度和温度，因此可以评估这两个参数对结构和矿物地球化学的影响。这是一项多方面的研究，涉及：i）微观结构分析，包括通过EBSD对橄榄石和斜方辉石进行粒度和晶格择优取向测量，ii）通过EPMA，LA-ICP-MS和穆斯堡尔光谱进行化学表征，以确定强度参数（P，T，fO 2）并评估交代作用，iii）使用FTIR和西姆斯测量橄榄石和辉石中的OH浓度。表征将取决于晶粒尺寸，以检测变形引起的变化。我们将比较40-50微米以下谷物中的微量元素特征，并通过西姆斯分析30微米以下新变晶谷物中的OH浓度。参考材料用于保持FTIR和西姆斯之间的一致性。结构数据将产生关于差应力和变形机制的信息，从而允许计算粘度。这些结果将与地球化学数据一起进行评估，以确定交代作用可能对变形行为产生的影响，以及这些过程对岩石圈根的不稳定性的影响。

Cratons represent pieces of the oldest continental crust that have a thick lithospheric mantle root (~180-250km). This root formed during the Archean (3.5-2.5 Ga) and its presence is directly attributable to the longevity of cratons. An important factor is a viscosity contrast between the root and the surrounding asthenosphere. One explanation for this contrast is that the base of the lithosphere is H2O-poor, and low OH contents in olivine and pyroxene make the peridotite rheologically stiffer (higher viscosity). This means that the viscosity contrast can be lowered by metasomatic activity that (re)-introduces volatiles and other chemical components, thereby leading to destabilization and potential breakup of cratonic blocks. Destabilization will be guided by focussing of shear stresses in shear zones. Such zones can facilitate and localize melt and fluid migration. Feedback mechanisms between metasomatism and deformation are not well known and require multidisciplinary studies to correlate geochemical modifications with deformation. "Sheared" peridotites have textures indicative of one or more episodes of strong deformation and reflect rheological behavior under upper mantle conditions. As sheared peridotites generally record the deepest depths for a given xenolith suite, such samples should serve as proxies for ductile shearing processes at or near the base of the cratonic lithosphere and are thus relevant to understanding how cratonic mantle may become destabilized.Our goal is to investigate the relationship between deformation processes in the mantle lithosphere and geochemical changes driven by metasomatism. Sheared peridotite xenoliths will be studied from Lesotho and Kimberley, S.A., that originated from different depths and temperatures, allowing the influence of these two parameters on textures and mineral geochemistry to be assessed. This is a multifaceted study involving: i) microstructural analysis, including grain size and lattice preferred orientation measurements on olivine and orthopyroxene via EBSD, ii) chemical characterization via EPMA, LA-ICP-MS and Mössbauer spectroscopy to determine intensive parameters (P,T,fO2) and assess metasomatic interactions, and iii) the measurement of OH concentrations in olivine and pyroxene using FTIR and SIMS. Characterization will be grain size dependent to detect changes due to deformation. We will compare trace element signatures in grains down to ~40-50 micrometres and OH concentrations will be analysed by SIMS in neoblastic grains down to ~30 micrometres. Reference materials are used to maintain consistency between FTIR and SIMS. The structural data will yield information on differential stress and deformation mechanism(s), allowing the viscosity to be calculated. These results will be assessed together with the geochemical data to identify the influence that metasomatism may have had on deformation behavior, and the implications these processes have for the destabilization of cratonic lithospheric roots.