Sodium in olivine as a potential pressure indicator for orogenic dunite

橄榄石Na含量:造山带纯橄岩潜在的压力指示计

Orogenic mantle peridotite,mainly derived from the lithospheric mantle wedge,was initially incorporated into the subduction channel at depths of mostly 30–120 km and then underwent ultradeep(>150 km)subduction and crustal metasomatism[1].Therefore,orogenic peridotite provides a window into subduction-zone mantle geodynamics.Compared with garnet lherzolite showing abundant metamorphic and metasomatic evidence,orogenic dunite can better preserve their initial compositions and thus sheds light on their origins and mantle evolution prior to subduction[2].Given that dunite has different chemical and physical properties(e.g.,wave velocity,viscosity and magnetic conductivity)from garnet lherzolite[3],constraining the distribution and evolution of orogenic dunite in subduction zones is critical to our understanding of mantle wedge heterogeneities and geodynamic processes.However,it is still a challenge to obtain the subsolidus pressure of dunite because of its simple mineral assemblage and the lack of suitable barometers.

造山带纯橄岩主要来源于俯冲板片上盘40~200km深度的地幔楔底部,广泛分布在各个高压-超高压变质带中,是认识地幔楔演化和俯冲带动力学的重要窗口.纯橄岩矿物组合简单,普遍缺失石榴子石,常规的地质压力计已不再适用,导致其变质峰期压力很难限定.为了寻找能够限定造山带纯橄岩压力的方法,本文选取大别-苏鲁超高压变质带和挪威西部片麻岩区的尖晶石相纯橄岩、尖晶石-石榴石相纯橄岩和石榴石相纯橄岩为研究对象,对比分析发现橄榄石Na含量具有显著的压力效应:尖晶石相纯橄岩橄榄石Na<18 ppm,石榴石相纯橄岩橄榄石Na>18 ppm,尖晶石-石榴石相纯橄岩橄榄石Na含量范围较大,涵盖前两者.本文提出橄榄石Na含量可作为造山带纯橄岩的压力指示计,对未来研究蛇绿橄榄岩和蛇纹岩的变质演化也具有重要启示意义.