基于电子探针分析的辉石分解出溶及初始组分再整合研究  

Electron Microprobe Reintegration Method for Clinopyroxene Breakdown with Lamellae Exsolution

作　　者：李小犁 张立飞[1] LI Xiaoli;ZHANG Lifei(Key Laboratory of Orogenic Belts and Crustal Evolution,Ministry of Education,School of Earth and Space Sciences,Peking University,Beijing 100871,China)

机构地区：[1]北京大学地球与空间科学学院,造山带与地壳演化重点实验室,北京100871

出　　处：《岩矿测试》2025年第2期201-213,共13页Rock and Mineral Analysis

基　　金：国家自然科学基金面上项目(41872190)。

摘　　要：辉石是一种重要的造岩矿物,在温度和压力发生变化的条件下常常发生分解,形成出溶结构。在榴辉岩中,绿辉石在退变质过程中经常会发生不同类型的晶片出溶,包括石英和/或角闪石、斜方辉石以及斜长石晶片的析出,而后者还常常会进一步演化形成单斜辉石-斜长石的后成合晶聚合体。为了研究辉石分解的化学反应,厘定分解前的初始辉石组分,进而探讨母体岩石的温度和压力条件变化及变质演化过程,亟需对其进行组分恢复实验。目前,主要有两种技术方法用于辉石分解出溶前的初始组分再整合分析,包括(1)间接计算法和(2)直接测量法,两者均以电子探针(EPMA)定量分析技术为基础。其中,方法(1)主要依赖于主晶和客晶的化学成分以及相应的组分占比,而方法(2)则主要取决于EPMA分析条件,包括须采用网格分析选项,调节束斑光栅为正方形模式,以及选用堆积计算优化的设置,同时,还需要注意标准样品类型和基体效应对实验结果的影响。本文以基性榴辉岩和退变榴辉岩为研究对象,对其中绿辉石出溶斜方辉石结构(ⅱ类型)和单斜辉石-斜长石后成合晶聚合体(ⅲ类型)分别进行了两种再整合方法的实验分析。结果显示,两种类型的辉石出溶分解情况在很大程度上是一个近似等化学的过程,符合再整合方法的应用前提。在ⅱ类型实验中,方法(1)下获得的初始辉石与样品中的未分解绿辉石在成分上具有更好的一致性,因此该方法更适用于该类型的初始组分再整合;在ⅲ类型实验中,方法(2)下获得的初始辉石在成分上具有更好的耦合性,更适用于该类型的初始组分再整合,不过这与前述实验结果相反,具体原因尚有待进一步探讨。上述两种再整合方法尽管具有一定的局限性,但因其便利性被广泛应用于变质岩石学研究。实际工作中,应该同时采用这两种方法进行对比分析,再�Pyroxene is an important rock-forming mineral in magmatic to metamorphic rocks,which often possesses certain breakdown texture with lamellae exsolution under temperature and pressure changes.In eclogite,the clinopyroxene,mostly omphacite may have different breakdown textures with quartz or amphibole-quartz,orthopyroxene and plagioclase lamellae exsolution during the retrogression process,and the latter is often further composed of clinopyroxene-plagioclase symplectite aggregate at lower grade metamorphism.To explore the temperature-pressure changes and the metamorphic evolution of the parent rock,it is necessary to restore the precursor pyroxene prior to breakdown from the residuary clinopyroxene and associated lamellae.Currently there are two major methods widely applied by different researchers in the pyroxene reintegration before its breakdown,an indirect mathematical fitting approach(method 1)and direct measurement(method 2),both of which are largely based on electron microprobe quantitative analysis.Method 1 largely relies on accurate microprobe analyses of host and exsolved components and appropriate estimates of their area percentages.Method 2 largely relies on the microprobe analytical conditions,involving a grid analysis option,raster mode for electron spot size,and accumulation calculation.Besides,correct standard material selection is also an essential factor.In this work,we exploited these two methods to reintegrate the precursor pyroxene prior to breakdown with(type-ii)orthopyroxene lamellae exsolution and(type-iii)clinopyroxene-plagioclase symplectite from eclogite and retrogressed eclogite samples.The results show that both types of clinopyroxene breakdown are a near-isochemical process that fits the requisite to apply the reintegration method.In type-ii,the reintegrated precursor pyroxene by method 1 tends to have analogous compositions with the unbroken relict omphacite in the sample,and thus this method is more suitable for such a scenario.In type-iii,in the opposite,the reintegrated precursor p

关 键 词：电子探针 斜方辉石出溶 后成合晶 辉石重组 变质作用 

分 类 号：P575.1[天文地球—矿物学]