LA-ICP-MS标准锆石原位微区U-Pb定年及微量元素的分析测定

LA-ICP-MS面扫描分析能直观细致地展示元素在矿物中的分布特征及相互关系，在揭示矿床成因、精细刻画成矿流体演化过程等方面具有显著优势。辽宁弓长岭铁矿床二矿区以产出沉积变质型磁铁矿富矿石而闻名，且富铁矿石的蚀变围岩中大量产出石榴石，其与富矿体成因关系密切。本文以二矿区富铁矿蚀变围岩中的石榴石为研究对象，为明确元素扩散对石榴石元素分布特征的影响，选择两颗大小不同的石榴石（1.5cm×1.5cm和0.6cm×0.7cm），应用LA-ICP-MS在10~20Hz、20~150μm正方形激光束斑、20~150μm/s扫描速度的条件下，在4h内完成其面扫描分析，并利用无内标法对数据进行半定量校正，详细研究石榴石主量、微量和稀土元素组合及分布特征，进而有效地分析热液流体演化过程和磁铁矿富矿体的成因。LA-ICP-MS面扫描结果揭示了弓长岭厘米级石榴石连续型环带和次厘米级石榴石突变型环带的特征，准确区分了突变环带的位置和界线。分析结果表明，弓长岭二矿区厘米级石榴石中Si、Al、Fe等主量元素成分较为均一，未显示环带特征；而Mg、Mn、Ca、重稀土及Y元素均保留了原始的生长环带，具有重要的成因指示意义。该石榴石从核部到边部，其Mg含量逐渐升高，Mn含量逐渐降低，指示石榴石形成温度从核部到边部逐渐升高；而Ca含量从核部至边部先升高后降低，指示压力先升高再降低，显示进变质成因石榴石的特点。同时，该石榴石δEu值变化规律指示变质热液流体的氧逸度先减小再增大；重稀土和Y元素与Ca元素一致的变化特征表明其分布主要受压力控制。因此，结合前人研究成果综合推测，弓长岭富铁矿蚀变围岩中的石榴石形成于早元古代晚期胶—辽—吉带大陆碰撞造山过程中的进变质作用阶段，在该阶段形成的变质热液流体沿断层运移，对断层两侧的贫铁矿和围岩进行改造，从而形成富铁矿石及蚀变围岩。

REE in skarn systems: A LA-ICP-MS study of garnets from the Crown Jewel gold deposit

With the advantage of high spatial resolution, low detection limit, and multi-element surface analysis, the mapping technique of laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) provides a new method for mineralogy research, which can display the element distribution characteristics in minerals, and constrain the evolution process of the ore-genesis fluid and ore genesis. The wall-rocks of magnetite-rich ore from the No.2 mining area of the Gongchangling iron deposit suffered obvious alteration, and the scale of magnetite-rich ore is roughly proportional to intensity of alteration. However, regarding the hydrothermal nature, it is argued for metamorphic or magmatic hydrothermal fluid. The garnet widely occurs in the altered wall-rock, which is closely related to the genesis of the magnetite-rich ore. Thus, by LA-ICP-MS mapping of garnet in the altered wall-rock of Gongchangling magnetite-rich ore, the element composition and distribution characteristics can be used to constrain the evolution process of hydrothermal fluid and the genesis of the magnetite-rich ore.To study the composition and distribution characteristics of major and trace elements in garnet by LA-ICP-MS mapping, and to constrain the evolution process of the ore-forming fluid and the genesis of magnetite-rich ore.The LA-ICP-MS mapping technique was applied to garnets from the Gongchangling No.2 mining area by simultaneously using Agilent 7700X inductively coupled plasma-mass spectrometry (ICP-MS) and Analyte Excite 193nm laser ablation system at the laboratory of mineralization and dynamics in Chang’an University, with laser frequencies of 10-20Hz, laser ablation spot sizes of 20-150μm square, laser ablation speeds of 20-150μm/s and laser ablation energy of 5.9J/cm², within 4 hours. Fifty-one elements (from ⁷Li to ²³⁸U) were chosen for ICP-MS analysis and the dwell time of each element was 6ms. This method adopted an external standard (NIST610) as the calibration standard without an internal standard. The result was semi-quantitative and the color brightness was used to represent the elemental content. The Iolite software can be used to generate multi-elemental pictures and elemental ratio mappings, to facilitate data analysis and interpretation for geologists. (1) LA-ICP-MS mapping indicates that the Si, Al, Fe, La, Ce, Pr and Nd compositions of the centimeter-scale garnet (Grt-1, particle size of 1.5cm×1.5cm) from the altered wall-rock are homogeneous, while the Mg, Mn, Ca, Li, Sc, V, heavy rare earth elements (HREEs) and Y retain the original compositional zonation. Most elements in the smaller garnet (Grt-2, particle size of 0.6cm×0.7cm) are mainly homogenized without zonation. (2) The two garnets from the altered wall-rock of the Gongchangling iron deposit show different elemental distribution. The centimeter-scale garnets (Grt-1) are more likely to retain the original compositional zonation when the metamorphic temperature is below 600℃. The results of LA-ICP-MS mapping of the centimeter-scale garnet (Grt-1) reveal the element correlations, to better understand the geochemical process in minerals. (3) The Mg content gradually increases and Mn content gradually decreases from the core to the rim of the garnet, indicating that the formation of the Gongchangling garnet is controlled by equilibrium growth and the formation temperature of the garnet gradually increases from the core to the rim. The Ca content of the garnet increases firstly and then decreases from the core to the rim, indicating that the pressure increases firstly and then decreases, which is consistent with the garnet formed during prograde metamorphism. The δEu anomalies of the garnet decreases firstly and then increases from the core to the rim, indicating that the oxygen fugacity of the metamorphic hydrothermal fluid decreases firstly and then increases. Since the characteristics of HREEs and Y in garnet are consistent with the characteristics of Ca, it is inferred that the distribution of the HREEs and Y is also mainly controlled by pressure.The centimeter-scale garnet from the Gongchangling altered wall-rock retains the original compositional zonation, and the LA-ICP-MS elemental mapping of the centimeter-scale garnet can be completed within 4 hours. The element distribution in the garnet indicates that the temperature gradually increases, the pressure increases firstly and then decreases, and the oxygen fugacity decreases firstly and then increases in the evolution process of the metamorphic hydrothermal fluid. Thus, it is inferred that the garnet in the altered wall-rock of the Gongchangling magnetite-rich ore was formed in the stage of prograde metamorphism associated with the Jiao—Liao—Ji Belt, and the magnetite-rich ore was derived from the reformation of BIF (low-grade iron ore) by metamorphic hydrothermal fluid.