Trace and Rare Earth Element Compositions of Lawsonite as a Chemical Tracer of Metamorphic Processes in Subduction Zones
Trace and Rare Earth Element Compositions of Lawsonite as a Chemical Tracer of Metamorphic Processes in Subduction Zones
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硬铝石矿的痕量和稀土元素组成作为俯冲带变质过程的化学示踪剂
DOI：
10.1093/petrology/egac065
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发表时间：
2022
影响因子：
3.9
通讯作者：
Fornash, Katherine F
中科院分区：
文献类型：
--
作者：
Kang, Patricia;Whitney, Donna L;Martin, Laure A;Fornash, Katherine F
滑石是俯冲带各种岩石类型（变质岩、变质沉积岩、变质岩）中广泛存在的微量元素（稀土、锶、铅、铀、钍等）和水的主要寄主矿物。因此，在高压/低温（HP/LT）地层中，lawsonite的成分是追踪俯冲和/或挖掘过程中发生的化学交换的有用档案。本研究提供了一个广泛的数据集，分析了来自两个不同地区（Franciscan/美国、里约热内卢San Juan/多米尼加共和国）、两个结构一致的地体（tav<s:1> anllu /土耳其、阿尔卑斯科西嘉/法国）和Pinchi湖/加拿大杂岩榴辉岩块体的HP/LT岩石中榴辉岩的主要元素和TE组成。同时测定了含lawsonite寄主岩石的Bulk major和TE组成，以了解岩石成因和评价成分演化。大多数分析的混匀地元玄武岩具有正常的洋中脊/弧后盆地玄武岩特征，它们保留了支持与（元）沉积物±元长岩/蛇纹岩相互作用的成分证据（如LILE/LREE富集；Ni/Cr富集）。大多数被分析的lawsonite颗粒在组成上以过渡金属元素（Fe, Ti, Cr），其他TEs（例如Sr， Pb）和/或REE分带，一些颗粒显示出与分带模式相关的成分变化（例如Ti-扇形分带，Fe的核心到边缘分带，Cr-振荡分带）。我们的研究结果表明，钙辉石的组成变化是由于晶体学控制（钛扇区分带）、流体-寄主岩石相互作用、矿物的模态变化和/或与共存的矿物（如绿帘石、钛矿）竞争te的元素分选而形成的。长石的Cr/V和Sr/Pb比值可用于跟踪蛇纹岩/变长岩（高Cr/V）和富石英（元）沉积物（低Sr/Pb）的组成影响。因此，长石微量元素和稀土元素组成有效地记录了发生在俯冲体系中的变质反应和流体-岩石相互作用驱动的元素再分配。
Lawsonite is a major host mineral of trace elements (TEs; e.g. REE, Sr, Pb, U, Th) and H2O in various rock types (metabasite, metasediment, metasomatite) over a wide range of depths in subduction zones. Consequently, the composition of lawsonite is a useful archive to track chemical exchanges that occurred during subduction and/or exhumation, as recorded in high-pressure/low-temperature (HP/LT) terranes. This study provides an extensive dataset of major element and TE compositions of lawsonite in HP/LT rocks from two mélanges (Franciscan/USA; Rio San Juan/Dominican Republic), two structurally coherent terranes (Tavşanlı/Turkey; Alpine Corsica/France), and the eclogite blocks of the Pinchi Lake/Canada complex. Bulk major and TE compositions were also determined for lawsonite-bearing host rocks to understand petrogenesis and assess compositional evolution. Most analyzed mélange and coherent-terrane metabasalts have normal mid-ocean ridge/back-arc basin basalt signatures and they preserve compositional evidence supporting interactions with (meta)sediment ± metagabbro/serpentinite (e.g. LILE/LREE enrichments; Ni/Cr enrichments). Most lawsonite grains analyzed are compositionally zoned in transition-metal elements (Fe, Ti, Cr), other TEs (e.g. Sr, Pb), and/or REE, with some grains showing compositional variations that correlate with zoning patterns (e.g. Ti-sector zoning, core-to-rim zoning in Fe, Cr-oscillatory zoning). Our results suggest that compositional variations in lawsonite formed in response to crystallographic control (in Ti-sector zoning), fluid–host rock interactions, modal changes in minerals, and/or element fractionation with coexisting minerals that compete for TEs (e.g. epidote, titanite). The Cr/V and Sr/Pb ratios of lawsonite are useful to track the compositional influence of serpentinite/metagabbro (high Cr/V) and quartz-rich (meta)sediment (low Sr/Pb). Therefore, lawsonite trace and rare earth element compositions effectively record element redistribution driven by metamorphic reactions and fluid–rock interactions that occurred in subduction systems.