金伯利岩中“熔离小球”的特征及其成因  

Characteristics and Their Genesis of Immiscible Small-balls in Kimberlites

作　　者：路凤香[1] 张宏福[2] 陈美华[3] 郑建平[1] 尹作为[3] 任迎新[1] 赵崇贺[4] 叶德隆[1] 

机构地区：[1]中国地质大学地球科学学院,湖北武汉430074 [2]中国科学院地质与地球物理研究所,北京100029 [3]中国地质大学珠宝学院,湖北武汉430074 [4]中国地质大学地球科学与资源学院,北京100083

出　　处：《现代地质》2007年第2期241-246,共6页Geoscience

基　　金：国家自然科学基金项目(40272021)

摘　　要：在金伯利岩人工重砂中发现的“熔离小球”，直径多数〈1mm，除个别出现微晶外，均为非晶质，属于熔体淬火冷却产物。提供了29个小球的主元素分析和3件微量元素分析结果。“熔离小球”按成分可分为3种类型：（1）高铁钛小球；（2）硫铁镍小球；（3）浅色硅铝质小球。其中高铁小球W（FeO）高达99．39％，高钛小球W（TiO2）达45．90％，它们含MnO也偏高，最高达23．75％。Fe、Mn、Ti都属于高负电性元素，在熔体中与氧结合的键强度大，容易发生熔离。硫铁镍小球的埘（SO3）变化于38．27％～51．95％，w（FeO）为0．31％～23．10％，加（NiO）为25．24％～61．05％。浅色小球W（SiO2）变化范围为24．01％～52．64％，Al：O3、CaO含量高但变化范围大，总体成分接近基性一超基性硅酸盐熔浆。主元素、微量元素特征以及硫铁镍小球中发现了高镁（Fo=0．95）橄榄石捕虏晶表明，小球形成于金伯利岩岩浆的介质环境。此外高铁及硅铝质两种成分呈交生结构的两相小球的发现，暗示二者为熔离作用成因。小球的熔离作用可以应用SiO2-FeS—FeO的液态不混溶相图做出解释。认为小球形成于岩浆结晶的晚期阶段，相对富含CO2、SO3、FeO、MnO、TiO2，在岩浆快速上升、快速降温、降压、熔体中出现了多种局部有序区的条件下发生的。Few＂ Immiscible small balls＂（ called ＂balls＂）were discovered in kimberlite heavy mineral concentrates. These balls are dominantly 〈 1 mm in diameter and have isotropic structure except few balls with microlite, which implies that they are quenching products from kimberlitic melts. Twenty-nine major element compositions and three trace element analyses are provided in this paper. According to the major element compositions, three types of balls can be divided ： （ 1 ） high Fe-Ti balls ; （2） S-Fe-Ni balls ; （ 3 ） light-colored sialic balls. The high Fe-Ti balls have the highest FeO（ up to 99. 39% ）and TiO2 （up to 45.90% ）contents. They also have relatively high MnO contents（up to 23.75% ）. Fe, Ti, and Mn elements have high electro negativity, which have penitential to form very strong bone with oxygen in melts. Thus they are easier to separate from the host melts through the immiscibility. The S-Fe-Ni balls have SO3, FeO and NiO contents ranging from 38.27% -51.95%, 0. 31% -23. 10% inant components in light-colored and 25.24% -61.05%, respectively. SiO2, Al2O3 and CaO oxides are domsialic balls with the SiO2 contents in a range of 24. 01% -52. 64%, cormsponding to the compositions of basic to uhra-basic magmas. Characteristics of major and trace elements and tile discover7 of olivine xenocryst（ Fo =0.95）in a S-Fe-Ni ball demonstrate that these balls were fbrmed from kimberlitic magma. The occurrence of two-phase（ high-Fe and sialic）inlergrown ball illustrates that high-Fe and sialic balls were produced from magma immiscibility. The formation of three type balls can be clearly explained with the SiO,-FeS-FeO phase diagram. We consider that immiscible small balls were formed at late stage of magma crystallization with decreasing temperature and pressure. The magma enriched in CO2, SO3, FeO, TiO2, MnO uplift and chilled very rapidly to form many local order areas in melts that produced these balls.

关 键 词：熔离小球 金伯利岩 高铁钛小球 硫铁镍小球 浅色硅铝质小球 岩浆不混熔 

分 类 号：P588.1[天文地球—岩石学]