单矿物中熔体包裹体研究进展及地质指示意义

熔体包裹体是矿物在生长过程中捕获的原始岩浆,能有效的保留大量主矿物结晶和周围岩浆介质的物理化学信息,是岩浆演化和成矿过程的良好指示刺.文章从熔体包裹体在主矿物中的赋存状态,均一化试验和测试技术等方面总结了目前的研究进展,并结合具体的矿床揭示熔体包裹体对岩浆和成矿方面的指示意义.同时笔者也建议,将熔体包裹体信息和岩石学、地球化学、找矿学联系起来,可以加深对岩浆演化过程的了解程度,提高勘查找矿的效果.熔体包裹体作为一种较新的微区分析对象具有十分广阔的应用前景.     

CCSD主孔榴辉岩中金红石微量元素特征：LA-ICPMS分析及其意义

对中国大陆科学钻探（CCSD）主孔200~1005m范围内8件榴辉岩样品的金红石进行了LA-ICPMS原位微区微量元素分析,结合前人已发表的全岩和金红石分析数据,研究结果发现：在不同类型榴辉岩中,金红石的微量元素与其全岩成分具有不同的相关关系。金红石中的Nb和Ta元素含量不同程度地受控于全岩Nb和Ta含量。在高钛和低镁钛榴辉岩中,金红石的Cr与全岩Cr/TiO₂正相关;在富镁榴辉岩中,金红石的Cr含量受全岩MgO含量的控制;在高钛和富镁榴辉岩中,全岩成分明显影响着金红石的Zr含量,金红石Zr温度计可能不适用。低镁钛榴辉岩的金红石的平衡温度普遍低于榴辉岩峰期变质温度,可能是变质流体参与下的扩散作用和退变质作用所致;多数情况下,单个样品中大部分金红石颗粒的Zr含量是均匀的,金红石Zr温度计所给出的温度可能代表着退变质再平衡的温度;CCSD榴辉岩的全岩Nb/Ta比值普遍低于其中金红石的Nb/Ta比值,不支持金红石榴辉岩可能是地球上超球粒陨石Nb/Ta比值储库的观点。     

Mineralizing fluids of the shallow epithermal Au–Ag deposits of the El Barqueño district, Jalisco, Mexico

La composición de elementos trazas y de isótopos de las secuencias magmáticas del terciario inferior en dos localidades de la Sierra Madre Occidental al norte de México, muestran una variación que manifiesta la composición y la edad del basamento por el cual fueron eyectadas. La corteza subyaciente en San Buenaventura corresponde a la parte asociada al basamento Norte Americano, en cambio en El Divisadero, la corteza subyaciente correspondería a los terranos alocthonos acrecionados durante el Mesozoico.En estas localidades las rocas volcánicas aparecen fuertemente diferenciadas, variando en su composición de basáltica a riolítica (SiO₂=50–76%). Las secuencias erupcionadas en los terranos de acreción presentan la más baja dispersión en su cociente isotópico, con un cociente mínimo inicial de Sr (>0.7044, corregido por la edad), y máximo de Nd (<0.5126) y de Pb (²⁰⁶Pb/²⁰⁴Pb 18.9).Los cocientes isotópicos correspondientes a la serie continental presentan una dispersión variable y una distribución fuera de la serie de los terranos hacia la composición más típica de la corteza antigua (⁸⁷Sr/⁸⁶Sr 0.710 y ¹⁴³Nd/¹⁴⁴Nd 0.5123). Las rocas volcánicas de la zona continental, muestran valores relativamente altos en elementos incompatibles en comparación con las rocas de los terranos acrecionados (Ce/Yb=25–45 vs. 13–33, respectivamente), y al mismo tiempo están empobrecidos en algunos elementos incompatibles como U y Rb (p.e. Th/U=3.8–7.5 vs. 2.5–4.0, respectivamente), lo cual indica una posición estratigráfica superior, con cocientes de ⁸⁷Sr/⁸⁶Sr, ²⁰⁸Pb/²⁰⁴Pb, y Th/U más altos, y ¹⁴³Nd/¹⁴⁴Nd más bajos.Los resultados tienen implicaciones sobre la composición original y la petrogénesis de las rocas volcánicas. Los cocientes isotópicos de los dos lugares revelan una composición que se sitúa entre las rocas de composición máfica de la Sierra Madre Occidental y los xenolithos intermediarios y félsicos característicos del norte de México o del sud-oeste de los Estados Unidos de América. La relación entre los cocientes isotópicos de las secuencias y la edad del basamento, así como el hecho de que la totalidad de los resultados obtenidos presenten una disposición bien definida, demuestran la fuerte contribución de la corteza en la química de los magmas silicatados. En la serie continental los cocientes de isótopos covarían con los cocientes de Th/Pb y U/Pb, aproximándose a la composición observada en los xenolithos intermediarios y en las rocas de composicion félsica. Esto indica claramente que no se trata de una fusión anatéxica de la corteza inferior, sino más bien de una interacción del magma basáltico del manto y la corteza. La contribución de la corteza es del orden de 20–70%. La estrecha variación del cociente isotópico en la zona de la secuencia de terranos acrecionados refleja la interacción del magma basáltico con una corteza relativamente joven, cuya composición isotópica es similar a los magmas derivados del manto. Los altos cocientes de Th/U y de Th/Rb indican que la contaminación de la corteza tiene lugar en su parte inferior. Además, los cocientes menos radiogénicos de ²⁰⁶Pb/²⁰⁴Pb y ²⁰⁷Pb/²⁰⁴Pb en la serie continental indican que el empobrecimiento de elementos fuertemente incompatibles en la corteza inferior corresponde a un rasgo antiguo. Los cambios seculares de los cocientes isotópicos de la sección estratigráfica indica que la contribución de la corteza superior aumenta con el tiempo, predominando al principio una corteza inferior máfica y, más tarde, una corteza intermediaria de rocas félsicas. Analizando rocas volcánicas externas a las dos secciones fuertemente muestreadas, las diferencias obtenidas en su composición isotópica de las rocas volcánicas, los terranos acrecionados y el basamento antiguo permiten deducir la localización del límite corteza continental-corteza oceánica.

Oxygen isotopic compositions of chondrules: Implications for evolution of oxygen isotopic reservoirs in the inner solar nebula

We review the oxygen isotopic compositions of minerals in chondrules and compound objects composed of a chondrule and a refractory inclusion, and bulk oxygen isotopic compositions of chondrules in unequilibrated ordinary, carbonaceous, enstatite, and Kakangari-like chondrites, focusing on data acquired using secondary ion mass-spectrometry and laser fluorination coupled with mass-spectrometry over the last decade. Most ferromagnesian chondrules from primitive (unmetamorphosed) chondrites are isotopically uniform (within 3–4‰ in Δ¹⁷O) and depleted in ¹⁶O (Δ¹⁷O>−7‰) relative to amoeboid olivine aggregates (AOAs) and most calcium–aluminum-rich inclusions (CAIs) (Δ¹⁷O<−20‰), suggesting that these classes of objects formed in isotopically distinct gaseous reservoirs, ¹⁶O-poor and ¹⁶O-rich, respectively. Chondrules uniformly enriched in ¹⁶O (Δ¹⁷O<−15‰) are exceptionally rare and have been reported only in CH chondrites. Oxygen isotopic heterogeneity in chondrules is mainly due to the presence of relict grains. These appear to consist of chondrules of earlier generations and rare refractory inclusions; with rare exceptions, the relict grains are ¹⁶O-enriched relative to chondrule phenocrysts and mesostasis. Within a chondrite group, the magnesium-rich (Type I) chondrules tend to be ¹⁶O-enriched relative to the ferrous (Type II) chondrules. Aluminum-rich chondrules in ordinary, enstatite, CR, and CV chondrites are generally ¹⁶O-enriched relative to ferromagnesian chondrules. No systematic differences in oxygen isotopic compositions have been found among these chondrule types in CB chondrites. Aluminum-rich chondrules in carbonaceous chondrites often contain relict refractory inclusions. Aluminum-rich chondrules with relict CAIs have heterogeneous oxygen isotopic compositions (Δ¹⁷O ranges from −20‰ to 0‰). Aluminum-rich chondrules without relict CAIs are isotopically uniform and have oxygen isotopic compositions similar to, or approaching, those of ferromagnesian chondrules. Phenocrysts and mesostases of the CAI-bearing chondrules show no clear evidence for ¹⁶O-enrichment compared to the CAI-free chondrules. Spinel, hibonite, and forsterite of the relict refractory inclusions largely retained their original oxygen isotopic compositions. In contrast, plagioclase and melilite of the relict CAIs experienced melting and ¹⁶O-depletion to various degrees, probably due to isotopic exchange with an ¹⁶O-poor nebular gas. Several igneous CAIs experienced isotopic exchange with an ¹⁶O-poor nebular gas during late-stage remelting in the chondrule-forming region. On a three-isotope diagram, bulk oxygen isotopic compositions of most chondrules in ordinary, enstatite, and carbonaceous chondrites plot above, along, and below the terrestrial fractionation line, respectively. Bulk oxygen isotopic compositions of chondrules in altered and/or metamorphosed chondrites show evidence for mass-dependent fractionation, reflecting either interaction with a gaseous/fluid reservoir on parent asteroids or open-system thermal metamorphism. Bulk oxygen isotopic compositions of chondrules and oxygen isotopic compositions of individual minerals in chondrules and refractory inclusions from primitive chondrites plot along a common line of slope of 1, suggesting that only two major reservoirs (gas and solids) are needed to explain the observed variations. However, there is no requirement that each had a permanently fixed isotopic composition. The absolute (²⁰⁷Pb–²⁰⁶Pb) and relative (²⁷Al–²⁶Mg) chronologies of CAIs and chondrules and the differences in oxygen isotopic compositions of most chondrules (¹⁶O-poor) and most refractory inclusions (¹⁶O-rich) can be interpreted in terms of isotopic self-shielding during UV photolysis of CO in the initially ¹⁶O-rich (Δ¹⁷O−25‰) parent molecular cloud or protoplanetary disk. According to these models, the UV photolysis preferentially dissociates C¹⁷O and C¹⁸O in the parent molecular cloud and in the peripheral zones of the protoplanetary disk. If this process occurs in the stability field of water ice, the released atomic ¹⁷O and ¹⁸O are incorporated into water ice, while the residual CO gas becomes enriched in ¹⁶O. During the earliest stages of evolution of the protoplanetary disk, the inner solar nebula had a solar H₂O/CO ratio and was ¹⁶O-rich. During this time, AOAs and the ¹⁶O-rich CAIs and chondrules formed. Subsequently, the inner solar nebula became H₂O- and ¹⁶O-depleted, because ice-rich dust particles, which were depleted in ¹⁶O, agglomerated outside the snowline (5 AU), drifted rapidly towards the Sun and evaporated. During this time, which may have lasted for 3 Myr, most chondrules and the ¹⁶O-depleted igneous CAIs formed. We infer that most chondrules formed from isotopically heterogeneous, but ¹⁶O-depleted precursors, and experienced isotopic exchange with an ¹⁶O-poor nebular gas during melting. Although the relative roles of the chondrule precursor materials and gas–melt isotopic exchange in establishing oxygen isotopic compositions of chondrules have not been quantified yet, mineralogical, chemical, and isotopic evidence indicate that Type I chondrules may have formed in chemical and isotopic equilibrium with nebular gas of variable isotopic composition. Whether these variations were spatial or temporal are not known yet.     

山西沁水盆地石炭-二叠系煤层生排烃史分析

通过对沁水盆地石炭-二叠系煤层夹矸及煤层顶板砂岩石样品中的包裹体类型、与矿物的共生关系以及包裹体均一化温度研究,结合山西南部上古生界煤层埋藏史及热史演化分析,讨论了太原组和山西组煤层的排烃历史。结果表明,太原组和山西组煤层排烃主要发生在晚三叠世末期煤层抬升释压以来,大体可以分为中侏罗世、早白垩世早期和晚白垩世一直到古新世三期。早期有过煤成油的阶段,并且有一定规模的排油过程;主要的排气阶段正好与热催化生油气阶段和热裂解生凝析气阶段相对应,那么可以认为热解气和热裂解气是煤层气的主要组成部分。煤储层中液态烃与煤层夹矸及顶板岩石中液态烃组成特征存在着明显差别,可能是由排烃过程吸附分馏作用和不同成熟度液态烃的混合作用不同所致。甾烷和藿烷总体组成特征相似,说明煤层、煤层夹矸及顶板砂岩中油气来源于煤层。     

胜利油气区奥陶系顶部风化壳有机包裹体初步研究及其意义

胜利油气区奥陶系顶部风化壳中的气态烃有机包裹体主要分布在方解石脉中,属晚期次生成因。均一温度测量结果表明其形成温度为140～160℃,明显低于奥陶系灰泥岩的成岩温度(170～208℃)。有机包裹体气态烃成份以CH4为主(占总量的44%～49%),同时含CO₂和H₂O等无机气体成份。有机包裹体气态烃δ¹³C₁为-29.2‰(PDB),δD为-128‰(SMOW),与本区已发现的来自石炭二叠系煤成天然气的δ¹³C₁和δD相近或相同,说明胜利油气区奥陶系顶部风化壳曾经有过来自石炭二叠系煤成天然气的运移与聚集作用。该区有与陕北大气田相似的成藏地质条件,奥陶系顶部风化壳具潜在的天然气勘探价值。     

Metal mobility during hydrothermal breakdown of Fe-Ti oxides: Insights from Sb-Au mineralizing event (Variscan Armorican Massif,France)

Hydrothermal alteration related to Sb-Au mineralization is widespread in the Variscan Armorican Massif, but mineral replacement reactions are not well characterized, in particular the hydrothermal breakdown of ilmenite-titanohematite. Based on petrography, electron probe micro-analyzer and laser ablation-inductively coupled plasma-mass spectrometer analyses, we document mineralogical change at rock- and mineral-scale and the redistribution of Sb and others trace elements during the recrystallization of ilmenite-titanohematite to hydrothermal rutile. Hydrothermal alteration is mainly potassic with associated carbonation. The replacement mechanism is interpreted to be an interface-coupled dissolution-reprecipitation process. Results show that Mn, Zn, Co, Ni, Sn, Mo and U are released during hydrothermal alteration, whereas Sb and W are incorporated in newly-formed hydrothermal rutile from the hydrothermal fluid. Furthermore, the concentration of Sb evolves through time suggesting a change in fluid composition likely related to an enrichment of fluid in Sb during rutile crystallization. Considering that Fe-Ti oxides breakdown during hydrothermal alteration is common within epithermal and mesothermal/orogenic Au-Sb mineralizing systems, results report in this study yield important constraints about metal mobility and exchanges in hydrothermal gold systems.     

简单体系水溶液包裹体pH和Eh的计算

流体包裹体pH和Eh参数的计算一直处于探索阶段。已经发表的计算公式,由于缺少高压(＞1bar)环境下化学组分反应平衡常数,常常利用常压(1大气压)下化学组分反应平衡常数代替而推导出的,对于大多数自然界捕获的包裹体,不可避免地产生较大计算误差。根据水溶液包裹体中离子反应热力学特征,结合前人推导的计算公式,我们分别建立简单体系水溶液包裹体pH和Eh计算公式。由于利用较高压(＞1bar)化学反应平衡常数,基本上解决不同条件下、特别在较高温度、压力下捕获的水溶液包裹体pH和Eh的计算难题。4种简单体系水溶液包裹体pH计算公式:① H₂O包裹体:pH=pK_w② CO₂-H₂O包裹体: ³－(m_CO2K_a,1+K_W)· －2mCO₂K_a,1K_a,2=0③ NaCl-H₂O包裹体: ²= ④ CO₂-H₂O-NaCl包裹体: ³+ ²－(m_CO2·K_a,1+K_w)· －2mCO₂·K_a,1·K_a,2－ =0计算数值精度分析表明:CO₂-H₂O和NaCl-H₂O包裹体的pH值按照公式计算,相同或接近于实际测定的天然酸雨、海水pH数值范围。CO₂-H₂O-NaCl包裹体与Crerar(1978)公式计算误差不超过10%。4种简单体系水溶液包裹体Eh的计算,引用Ryzhenko and Bryzgalin (1984)年推导的Eh公式。文中列举了3个实例,详细叙述不同类型包裹体捕获温度、压力下pH和Eh计算过程。自然界中水溶液包裹体成分十分复杂,本文涉及的只是自然界几种简单体系水溶液包裹体,给出的pH和Eh公式只对特定组分组合反应平衡条件有效,它不适合另外一种组分组合平衡条件下的关系,因此使用时务必注意平衡的组分组合条件。     

华北地块东南缘中生代侵入杂岩中所含榴辉岩类包体矿物微量元素地球化学特征及其意义

华北地块东南缘中生代侵入杂岩中所含榴辉岩类包体中矿物的分析结果表明, 组成榴辉岩类包体的各矿物之间元素的分配在榴辉岩相变质阶段已经达到了化学平衡;石榴石中Al、Fe、Mn, 单斜辉石中Na、Mg、Ca, 角闪石中Na、Mg、Fe等主量元素含量与其全岩之间均呈良好的正相关, 受全岩成分制约;石榴石明显富集HREE、贫LREE, 单斜辉石相对富集LREE、MREE、贫HREE, 两者轻、重稀土含量呈相互消长的互补关系, 石榴石和单斜辉石中分别富集Y、Sr, Ga和过渡族元素Sc、Ti、V、Cr、Co等含量较高, 其含量受全岩成分控制;金红石中稀土元素的含量很少, Fe、Al、Mg、Ca、Mn、Nb、Ta、Cr和Zr含量相对较高, Rb、Sr、Ba含量极低, 部分微量元素(如Nb、Cr、Fe、V、W等)含量与其原岩有继承和对应关系;部分原生或/和退变角闪石的REE配分模式分别与单斜辉石和石榴石相似, 过渡族金属元素等的含量与石榴石、单斜辉石和全岩具有一定的相关性, 角闪石容纳Na、K、Sr、Rb、Ba等元素的能力较强, 其成分除受全岩成分的制约外, 石榴石、单斜辉石对其形成也有影响。榴辉岩类包体与其寄主中生代侵入杂岩中部分高价态/高场强元素(HFSE)和重稀土元素(HREE)呈相互消长的关系。榴辉岩的形成与扬子地块和华北地块之间的俯冲-碰撞作用有关。     

阿尔泰南缘克兰盆地的脉状金-铜矿化及其流体演化

阿尔泰山南缘泥盆纪克兰火山-沉积盆地蕴藏有丰富的VMS锌铅铜多金属矿床。自晚泥盆世至早二叠世末, 阿尔泰山南缘为NE-SW向强烈挤压的构造环境, VMS矿石受到变形变质改造,脉状金铜矿化发育。金(铜)石英脉主要有2种产状:(1)白色-灰白色(硫化物)顺层石英脉(QI), 产于韧脆性剪切带发育地段,呈细脉状或透镜状产于绿泥片岩、黑云片岩中;(2)斜切黄铁矿化蚀变岩、层状铅锌矿和变质岩产状的黄铜矿-黄铁矿石英脉(QII),与晚期的脆性构造有关。金(铜)石英脉的流体包裹体发育,按室温下相态特征有3类。第I类为含子矿物的高盐度包裹体(L-V-S型),子晶为NaCl, 有时为KCl,包裹体呈孤立或无序分布,代表变质早期流体特征。一般NaCl子晶先消失(210~357℃),包裹体的最终均一温度369~512℃,其捕获温度与变质相的相平衡计算温度相当,反映了变质早期中高温热液活动的特征。第II类是富CO₂ 包裹体,包括单相的碳质流体包裹体(L _CO2、L _CO2-CH4或L _CO2-N2)和两相富CO₂包裹体(L _CO2-L _H2O)2个亚类。碳质流体包裹体是常见类型,有时与L _CO2-L_H2O型伴生,在较晚期的黄铜矿-黄铁矿石英脉中表现为原生特征,而在较早的石英脉中常表现为次生特征。萨热阔布的碳质流体可分为纯CO₂包裹体和CO₂-CH₄体系包裹体,纯CO₂包裹体的固体CO₂熔化温度(T_m,CO2)为 -60~-56.5℃,CO₂部分均一温度(T_h,CO2) 变化于-23~+31℃;密度一般为0.85~0.89g·m^-3。CO₂-CH₄包裹体的T_m,CO2＜-57℃,可低达-78.1℃,T_h,CO2低达-33.7~-17.7℃, 其密度高达1.01~1.07g·m^-3。VMS矿床中晚期叠加的黄铜矿石英脉中碳质流体包裹体可分为贫CH₄-N₂和富CH₄-N₂的CO₂-CH₄-N₂包裹体,贫CH₄-N₂的碳质包裹体T_m,CO2=-63.3~-57℃,T_h,CO2=-27.5~+29.7℃;富CH₄-N₂的CO₂-CH₄-N₂包裹体T_m,CO2=-83.4~-65.5℃,T_h,CO2=-56.0~+16.9℃。铜金石英脉中与碳质流体共生的L_CO2-L_H2O型包裹体均一温度T_h,total=205~370℃,略低于第I类高盐度包裹体的T_h,total=369~512℃。据CO₂流体高温高压相图估算包裹体的捕获压力至少为110~300MPa。金(铜)石英脉的主体在相当于445~566℃的高温条件下形成的,而金铜矿化则是在高于205~370℃、110~330MPa的中高温中深条件下发生的。流体包裹体的δ¹⁸O为7.54‰~11.84‰ (QI)和3.82‰~7.82‰ (QII), δD为-84.7‰~-98.2‰(QI)和-75.8‰~-108.8‰ (QII)。结合地质特征和流体研究,说明成矿热液来源与区域变质及相关的岩浆活动有关。