碳酸盐矿物氧同位素分馏的理论研究

应用增量方法系统地计算了碳酸盐矿物的同位素分馏系数，得到不同结构和成分的碳酸盐矿物的18O富集顺序为：菱铁矿〉铁白云石〉菱镁矿≥白云石≥方解石〉文石〉菱锶矿〉白铅矿〉碳钡矿。在0℃～1200℃范围内获得了一组内部一致的碳酸盐－水体系的理论分馏系数，这些计算结果与已知的实验和／或经验数据之间存在良好的一致性，因此本文对碳酸盐矿物氧同位素分馏系数的理论校准不仅可应用于共生矿物组合形成温度的确定，而且能够应用于其形成机理的示踪。 计算结果表明，白云石的氧同位素分馏行为与方解石相似，在25℃下白云石与方解石之间的平衡分馏为0.56‰ 。理论预测文石相对于方解石显著地亏损 δ18O，在25℃时方解石与文石之问的平衡分馏为4.47‰ 。文石向方解石的同质多相转变可能是通过一种没有同位素再造的惰性氧结构单元[CO3]2- 进行的，即只涉及Ca2+ 与[C03]2- 基团之间键的断裂和再组台而未出现[CO3]2- 基团内部C－O键的断裂和再组合。结果在自然界和实验室实验中，文石中氧同位索配分的温度关系能够传递副方解石中来。这种在同质多相转变形成方解石过程中的氧同位素继承性对于了解白云石－方解石－水体系分馏的难题至关重要。理论预测也能够用来解释对方解石分馏的经验估算与实验测定之间的分歧。     

文石-水体系氧同位素分馏系数的低温实验研究

采用缓慢分解法和“两步法”的附晶生长法,在低温(0℃～70℃)下实验合成纯文石型碳酸 钙矿物,以XRD和SEM技术对合成矿物的相组成和形貌进行了鉴定。将XRD与SEM及氧同位素分 析技术相结合,研究了文石的生成速率与氧同位素分馏之间关系。对0℃、25℃和50℃条件 下采用缓慢分解法合成的文石进行SEM观察发现,随着温度升高,矿物生成速率加快,氧同 位素分馏逐渐趋于不平衡,导致50℃条件下获得的文石-水体系氧同位素分馏是一种不平衡 分馏,而0℃和25℃条件下获得的低值代表平衡分馏。将0℃和25℃以下采用缓慢分解法获得 的文石-水体系分馏低值与采用“两步法”的附晶生长法在50℃和70℃条件下获得的文石- 水体系平衡分馏数据相结合,得到0℃～70℃范围内文石-水体系氧同位素平衡分馏方程为 ：103lnα=20.41×103T－41.42。这个实验结果不仅与增量方法理论计算结 果一致,而且与前人低温实验获得的文石或文石与方解石混合相碳酸钙-水体系,以及生物 成因文石-水体系的氧同位素分馏结果相近。这是首次根据实验确定的无机成因文石-水体 系热力学平衡氧同位素分馏系数,因此对于无机成因文石在古沉积环境和古气候研究中的应 用具有重要参考价值。     

云母族矿物的氧同位素分馏

利用增量方法对云母族矿物的氧同位素分馏进行了系统的理论计算。结果表明，不同化学成分和结构状态的云母之间存在一定的氧同位素分馏，其１８Ｏ富集顺序在热力学同位素平衡时为：多硅白云母＞钠云母＞锂云母＞白云母＝珍珠云母＞海绿石＞铁云母＞金云母＞黑云母。在４００℃以上的高温条件下，云母－水体系的氧同位素分馏与温度之间的相关性不明显，并且云母相对于水亏损１８Ｏ达１‰－２．５‰。石英－云母体系的氧同位素分馏与温度之间具有显著的负相关性，因此，能够作为灵敏的同位素地质温度计。不过，石英－黑云母对的氧同位素地质测温往往给出岩石冷却过程中的退化再平衡温度，而不是岩石形成温度。     

石英一水一盐体系氧同位素分馏作用

温度为180—550℃,盐度(wt.％)分别为0、5、25和40条件下,在高压釜内完成了由硅胶合成石英的氧同位素分馏作用实验研究,目的是了解:①盐同位素效应;②△t值对同位素分馏的影响;③温度与同位素分馏系数的关系。研究资料表明:低温条件下矿物和纯水之间同位素平衡作用不可能发生;影响含氧矿物(初)之间氧同位素平衡速率的因素包括盐度、△t值大小和温度等;我们的研究还表明,盐度对同位素分馏作用同系数无影响,即不存在所谓的“同位素盐效应”。在180—550℃温度范围内,不同盐度条件下获得的石英-水氧同位素分馏实验方程为:10001nα_(石英-水)=3.306×10~5T~(-2)—2.71。     

磷酸盐氧同位素组成的测定方法及分馏机理研究进展

磷酸盐氧同位素组成在古气候和磷的生物地球化学循环研究中都具有十分重要的意义.测定方法和同位素的分馏机理是该类研究的基础.国际上已开展了一系列磷酸盐氧同位素的测定方法和分馏机理研究.在测定方法上,由初期的间接法,经高温还原/裂解法到氟化法,再演化到改进后的高温还原法(包括TC/EA-IRMS法),甚至激光原位技术,样品由实验室纯化学试剂扩展到各种复杂地质样品,在测量精确度、测量速度、样品用量、安全性和技术要求方面都有巨大改进.在分馏机理上,①尽管Longinelli等建立的关系式已获得了天然样品的验证,并认为是平衡分馏,但实验室模拟结果与其还存在较大差异(即没有达到平衡分馏).②在地表温度和pH条件下,无机过程均不会造成水体中溶解态磷酸盐和水之间的氧同位素交换.在高温(>70℃)及不同pH条件下,即使没有生物作用也会造成溶解磷酸盐和水分子之间进行氧的同位素交换,但不同实验室之间结果不一致.③在生物作用存在下,溶解无机磷酸盐和水之间在地表环境会发生强烈氧同位素交换,但除了PPase外,其余均没有达到平衡值.④磷灰石的氧同位素组成要比形成它的溶解态磷酸盐的值高1‰～1.4‰,因此在把Longinelli等关系式用于溶解态磷酸盐和水体系时,需要考虑该因素.同位素平衡分馏和条件有关,认为无机条件下的高温(>70℃)实验结果不一致,以及有生物参与的培养实验结果偏离平衡值,都是实验条件不同所致,包括pH、磷酸盐浓度、生物种类、生物量等.     

碳酸钙-水体系氧同位素分馏系数的低温实验研究

碳酸钙是古气候和沉积岩稳定同位素地球化学研究中最常用的矿物 ,因此对碳酸钙水体系氧同位素分馏系数的实验校准已成为稳定同位素地球化学诞生以来的热点和前沿课题。但由于碳酸钙在自然界存在 3种同质多象变体 (方解石、文石和六方方解石 ) ,使人们对碳酸钙矿物与水之间氧同位素分馏系数的实验测定结果存在较大差别 ,当应用到同位素地质测温时 ,会给出显著不同的温度值。正确选用合理的方解石水或文石水体系分馏曲线 ,对低温和环境地球化学研究和应用具有重要价值。文章系统总结和评述了碳酸钙水体系氧同位素分馏系数实验校准的历史、方法和结果 ,对前人在表达方式上的不一致进行了统一 ,对氧同位素分馏的盐效应、动力氧同位素分馏效应和同质多象转变过程中的氧同位素继承性进行了讨论。通过对前人大量实验数据的系统处理并与理论计算相比较 ,推荐了热力学上平衡的方解石水体系氧同位素分馏方程 ,而对于文石水体系 ,理论计算结果尚有待于实验证实。     

新疆吐-哈地区硝酸盐矿床的氧同位素非质量效应

随着分析测试技术的提高以及同位素分馏理论的进步,氧同位素非质量分馏的应用范围已经从太空拓展到地球,逐渐发展成为研究行星和地球早期演化、地球表面各圈层相互作用的重要途径和手段.硝酸盐和硫酸盐是地球上少数几个具有明显氧同位素非质量分馏效应的矿物.本文测定了新疆吐-哈地区硝酸盐矿床中硝酸盐和硫酸盐矿物的氧同位素组成,结果显示,硝酸盐的氧同位素存在明显的非质量分馏效应,△17O高达14‰,δ18O高达40‰.为该超大型硝酸盐矿床的大气沉积成因提供了可靠依据.     

白云母、高岭石矿物中羟基的o同位素分析方法研究

羟基矿物内部存在两种位于不同结构位置上的氧原子硅氧四面体氧和羟基氧,二者之间的Ｏ同位素分馏可能比任何共生矿物对都大,是一种潜在的单矿物同位素地质温度计。单矿物同位素地质温度计较矿物对同位素地质温度计有很多优点。准确测量矿物中羟基的Ｏ同位素组成是建立单矿物同位素地质温度计的关键。本文介绍了一种精确测量白云母、高岭石矿物中羟基的Ｏ同位素组成的新方法火焰加热真空脱水氟化法。δ１８ＯＯＨ的分析精度达到０３‰,羟基氧的提取率达到９９％～１００％。实验证明羟基矿物在高温真空脱水过程中不存在Ｏ同位素动力学分馏,羟基水     

湖北大冶铜绿山铜铁矿床夕卡岩矿物学及碳氧硫同位素特征

湖北大冶铜绿山铜铁矿床是长江中下游西段鄂东南矿集区一个大型夕卡岩矿床。围岩为三叠系大理岩及白云质大理岩,决定了其发育丰富的钙镁质复合夕卡岩矿物组合,包括石榴子石、辉石、角闪石、绿帘石、金云母、绿泥石等。本文详细描述了夕卡岩不同阶段矿物的特征,并对矿物进行了电子探针分析(EPMA)及碳、氧、硫稳定同位素研究。结果表明石榴子石形成于三期,成分上属于钙铝—钙铁系列,且从早到晚具有从钙铝向钙铁榴石演化趋势,反映出成矿溶液由酸性向碱性演化。环带结构的石榴子石和绿帘石从核部到边部Fe含量增高,说明磁铁矿是在Fe浓度升高的碱性溶液中沉淀。辉石为透辉石。角闪石属于单斜角闪石中的钙质角闪石,包括透闪石,韭闪石和少量阳起石。矿物成分分析表明辉石和石榴子石的Mn/Fe值与矿化金属元素存在一定的联系。相对于钙质夕卡岩,镁质或含镁质夕卡岩是铜铁矿体交代的更有利岩石。矿床硫化物的δ34SV-CDT均为正值且变化范围较窄,介于0.6‰～3.8‰。成矿阶段方解石δ13CV-PDB变化于-2.9‰～6.3‰,δ18OV-SMOW变化于9.6‰～12.6‰,成矿后方解石的同位素值明显增大,δ13CV-PDB为-0.9‰～1.3‰,δ18OV-SMOW为15.2‰～17.3‰,趋向于围岩的同位素值。研究结果说明成矿阶段的硫和碳来自于深源或地幔,而成矿后期碳与地层发生明显的同位素交换反应。     

Skarn Mineral and Stable Isotopic Characteristics of Tonglushan Cu-Fe Deposit in Hubei Province

湖北大冶铜绿山铜铁矿床是长江中下游西段鄂东南矿集区一个大型夕卡岩矿床.围岩为三叠系大理岩及白云质大理岩,决定了其发育丰富的钙镁质复合夕卡岩矿物组合,包括石榴子石、辉石、角闪石、绿帘石、金云母、绿泥石等.本文详细描述了夕卡岩不同阶段矿物的特征,并对矿物进行了电子探针分析(EPMA)及碳、氧、硫稳定同位素研究.结果表明石榴子石形成于三期,成分上属于钙铝—钙铁系列,且从早到晚具有从钙铝向钙铁榴石演化趋势,反映出成矿溶液由酸性向碱性演化.环带结构的石榴子石和绿帘石从核部到边部Fe含量增高,说明磁铁矿是在Fe浓度升高的碱性溶液中沉淀.辉石为透辉石.角闪石属于单斜角闪石中的钙质角闪石,包括透闪石,韭闪石和少量阳起石.矿物成分分析表明辉石和石榴子石的Mn/Fe值与矿化金属元素存在一定的联系.相对于钙质夕卡岩,镁质或含镁质夕卡岩是铜铁矿体交代的更有利岩石.矿床硫化物的δ34 SV-CDT均为正值且变化范围较窄,介于0.6‰～3.8‰.成矿阶段方解石δ13CV-PDB变化于-2.9‰～6.3‰,δ18OV-SMOW变化于9.6‰ ～ 12.6‰,成矿后方解石的同位素值明显增大,δ13CV-PDB为-0.9‰ ～ 1.3‰,δ18OV-SMOW为15.2‰ ～ 17.3‰,趋向于围岩的同位素值.研究结果说明成矿阶段的硫和碳来自于深源或地幔,而成矿后期碳与地层发生明显的同位素交换反应.     

On the oxygen isotope distribution among mineral triplets in igneous and metamorphic rocks

A compilation of ¹⁸O analyses of minerals separated from about 400 igneous and metamorphic rocks from published investigations reveals regularity in the fractionation of ¹⁸O among associated minerals, suggesting that an approach to isotopic equilibrium may be common. However, for only a minority of terrestrial rocks are these regularities sufficiently systematic to be compatible with the actual attainment and preservation of isotopic equilibrium among three minerals. Fractionations among triplets of quartz, calcite, feldspar, muscovite, and magnetite show some correspondence to those expected on the basis of experimental calibrations; however, there are also considerable deviations. The variability of natural data is such that less than half of the rocks analyzed to date would yield concordant ¹⁸O-derived temperatures. Of the additional 52 mineral triplets studied, plagioclase-pyrox-ene-ilmenite, plagioclase-pyroxene-magnetite, plagioclase-pyroxene-olivine, quartz-amphibole-garnet, pyroxene-ilmenite-magnetite, muscovite-biotite-magnetite, and quartz-muscovite-amphibole show the most systematic oxygen isotope fractionations. For 12 other mineral triplets a defined isotope fractionation relationship may be postulated to underlie the data; however for these a close approach to isotopic equilibrium is not commonly observed. For 33 of the mineral triplets an approach to isotopic equilibrium can be noted; however, the scatter of the available data is such that a systematic influence of a factor, such as temperature, on the size of the ¹⁸O fractionation could not be detected. In the past, regularities of oxygen isotope fractionations among three minerals have been used to establish secondary isotope geothermometers. Before this can be done with any reliability, however, the effects of possible retrograde isotope exchange and spurious correlation must be accounted for.     

Oxygen isotopic compositions of IVA iron meteorites: implications for the thermal evolution derived from in situ ultraviolet laser microprobe analyses

Oxygen isotopic compositions of silicate inclusions in IVA iron meteorites have been measured with an in situ UV laser microprobe technique. The homogeneity of oxygen isotopic compositions within and among individual mineral grains has also been examined. Oxygen isotope fractionations between coexisting mineral pairs were utilized in oxygen isotope thermometry. Our measured Δ¹⁷O values, ranging from 0.97 to 1.25‰, are characteristic of a single reservoir and fully confirm the oxygen isotopic similarity between IVA irons and L/LL chondrites. Steinbach and São João Nepomuceno, containing inclusions of two silicate minerals in mutual contact, exhibit a mass-dependent fractionation of ¹⁸O/¹⁶O between tridymite and bronzite with apparent oxygen isotopic heterogeneity. The SiO₂-bearing member, Gibeon, gives homogeneous oxygen isotopic compositions without detectable fractionation of ¹⁸O/¹⁶O between tridymite and quartz. Oxygen isotope equilibrium temperatures are estimated for coexisting tridymite and bronzite in the same sample slabs or clusters in Steinbach and São João Nepomuceno. The fractionations of ¹⁸O/¹⁶O between bronzite and tridymite range from 1.6 to 2.3‰ in different sample slabs or clusters. On the basis of the closure temperature concept, cooling rates are estimated at approximately 20 to 1000°C/Myr between 800 and 1000°C, a range of temperatures not accessible to other cooling rate methods. Using the Fast Grain Boundary diffusion model, we have demonstrated that significant oxygen heterogeneity both in tridymite and bronzite is probably due to isotope exchange during cooling between minerals with various grain sizes and mineral abundances in different regions of the samples. The new estimates of cooling rate by oxygen isotope thermometry refine previous cooling curves of IVA irons and support the breakup-reassembly model for the IVA parent body.     

Oxygen isotope fractionation between calcite and tremolite: an experimental study

Oxygen isotope partitioning between calcite and tremolite was experimentally calibrated in the presence of small amounts of a supercritical CO₂–H₂O fluid at temperatures from 520 to 680° C and pressures from 3 to 10 kbar. The experiments were carried out within the stability field of the calcite-tremolite assemblage based on phase equilibrium relationships in the system CaO–MgO–SiO₂–CO₂–H₂O, so that decomposition of calcite and tremolite was avoided under the experimental conditions. Appropriate proportions of carbon dioxide to water were used to meet this requirement. Large weight ratios of mineral to fluid were employed in order to make the isotopic exchange between calcite and tremolite in the presence of a fluid close to that without fluid. The data processing method for isotopic exchange in a three-phase system has been applied to extrapolate partial equilibrium data to equilibrium values. The determined fractionation factors between calcite (Cc) and tremolite (Tr) are expressed as:10³1n _Cc-Tr=3.80 × 10⁶/T ²-1.67By combining the present data with the experimental calibrations of Clayton et al. (1989) on the calcite-quartz system, we obtain the fractionation for the quartztremolite system: 10³1n _Qz-Tr=4.18 × 10⁶/T ²-1.67Our experimental calibrations are in good agreement with the theoretical calculations of Hoffbauer et al. (1994) and the empirical estimates of Bottinga and Javoy (1975) based on isotopic data from naturall assemblages. At 700 C good agreement also exists between our experimental data and theoretical values calculated by Zheng (1993b). With decreasing temperature, however, an increasing difference between these data appears.Retrograde isotopic reequilibration by oxygen diffusion may be common for amphibole relative to diopside in metamorphic rocks. However, isotopic equilibrium in amphibole can be preserved in cases of rapid cooling.     

Oxygen isotope exchange and disequilibrium between calcite and tremolite in the absence and presence of an experimental C–O–H fluid

Oxygen isotope exchange between minerals during metamorphism can occur in either the presence or the absence of aqueous fluids. Oxygen isotope partitioning among minerals and fluid is governed by both chemical and isotopic equilibria during these processes, which progress by intragranular and intergranular diffusion as well as by surface reactions. We have carried out isotope exchange experiments in two- and three-phase systems, respectively, between calcite and tremolite at high temperatures and pressures. The two-phase system experiments were conducted without fluid either at 1 GPa and 680 °C for 7 days or at 500 MPa and 560 °C for 20 days. Extrapolated equilibrium fractionations between calcite and tremolite are significantly lower than existing empirical estimates and experimental determinations in the presence of small amounts of fluid, but closely match calculated fractionations by means of the increment method for framework oxygen in tremolite. The small fractionations measured in the direct calcite–tremolite exchange experiments are interpreted by different rates of oxygen isotope exchange between hydroxyl oxygen, framework oxygen and calcite during the solid–solid reactions where significant recrystallization occurs. The three-phase system experiments were accomplished in the presence of a large amount of fluid (CO₂+H₂O) at 500 MPa and 560 °C under conditions of phase equilibrium for 5, 10, 20, 40, 80, 120, 160, and 200 days. The results show that oxygen isotope exchange between minerals and fluid proceeds in two stages: first, through a mechanism of dissolution-recrystallization and very rapidly; second, through a mechanism of diffusion and very slowly. Synthetic calcite shows a greater rate of isotopic exchange with fluid than natural calcite in the first stage. The rate of oxygen diffusion in calcite is approximately equal to or slightly greater than that in tremolite in the second stage. A calculation using available diffusion coefficients for calcite suggests that grain boundary diffusion, rather than volume diffusion, has been the dominant mechanism of oxygen transport between the fluid and the mineral grains in the later stage.Editorial responsibility: T.L. Grove     

Oxygen isotope salt effects at high pressure and high temperature and the calibration of oxygen isotope geothermometers

The influence of NaCl, CaCl₂, and dissolved minerals on the oxygen isotope fractionation in mineral-water systems at high pressure and high temperature was studied experimentally. The salt effects of NaCl (up to 37 molal) and 5-molal CaCl₂ on the oxygen isotope fractionation between quartz and water and between calcite and water were measured at 5 and 15 kbar at temperatures from 300 to 750°C. CaCl₂ has a larger influence than NaCl on the isotopic fractionation between quartz and water. Although NaCl systematically changes the isotopic fractionation between quartz and water, it has no influence on the isotopic fractionation between calcite and water. This difference in the apparent oxygen isotope salt effects of NaCl must relate to the use of different minerals as reference phases. The term oxygen isotope salt effect is expanded here to encompass the effects of dissolved minerals on the fractionations between minerals and aqueous fluids. The oxygen isotope salt effects of dissolved quartz, calcite, and phlogopite at 15 kbar and 750°C were measured in the three-phase systems quartz-calcite-water and phlogopite-calcite-water. Under these conditions, the oxygen isotope salt effects of the three dissolved minerals range from ∼0.7 to 2.1‰. In both three-phase hydrothermal systems, the equilibrium fractionation factors between the pairs of minerals are the same as those obtained by anhydrous direct exchange between each pair of minerals, proving that the use of carbonate as exchange medium provides correct isotopic fractionations for a mineral pair.When the oxygen isotope salt effects of two minerals are different, the use of water as an indirect exchange medium will give erroneous fractionations between the two minerals. The isotope salt effect of a dissolved mineral is also the main reason for the observation that the experimentally calibrated oxygen isotope fractionations between a mineral and water are systematically 1.5 to 2‰ more positive than the results of theoretical calculations. Dissolved minerals greatly affect the isotopic fractionation in mineral-water systems at high pressure and high temperature. If the presence of a solute changes the solubility of a mineral, the real oxygen isotope salt effect of the solute at high pressure and high temperature cannot be correctly derived by using the mineral as reference phase.     

The non-mass-dependent oxygen isotope effect in the electrodissociation of carbon dioxide: A step toward understanding NoMaD chemistry

A non-mass dependent (NoMaD) oxygen isotope effect is demonstrated in the dissociation of CO₂ similar to that observed in the electrosynthesis of ozone. The molecular oxygen produced carries the signature of two separate isotopic fractionation processes; a mass-dependent fractionation probably due to CO₂ + O isotopic exchange, and a secondary NoMaD fractionation (δ¹⁷O = 0.97 ± 0.09δ¹⁸O, with the O₂ depleted in ¹⁷O and ¹⁸O). It is suggested that the effect is due to either the formation or relaxation of ozone in an excited electronic state. This represents the latest advance in the understanding of chemical NoMaD effects which may be essential to the explanation of non-mass-dependent fractionations observed in meteorites.     

Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate ‘clumped isotope’ analysis. The CO₂ recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H₂CO₃ reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of ¹³C-¹⁸O-containing isotopologues (‘clumped’ isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25 °C are 10.72‰, 0.220‰, 0.137‰, 0.593‰ for, respectively, ¹⁸O/¹⁶O ratios (1000 lnα^∗) and three indices that measure proportions of multiply-substituted isotopologues . We also predict that oxygen isotope fractionations follow the mass dependence exponent, λ of 0.5281 (where ). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of ¹³C-¹⁸O bonds (the measured change in Δ₄₇ = 0.23‰) during phosphoric acid digestion of calcite at 25 °C.We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H₂CO₃ interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important factor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of ¹³C-¹⁸O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of for different carbonate minerals. Combined with previous theoretical evaluations of ¹³C-¹⁸O clumping effects in carbonate minerals, this enables us to predict the temperature calibration relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H₂CO₃.     

Kinetic mechanism of oxygen isotope disequilibrium in precipitated witherite and aragonite at low temperatures: an experimental study

To study what dictates oxygen isotope equilibrium fractionation between inorganic carbonate and water during carbonate precipitation from aqueous solutions, a direct precipitation approach was used to synthesize witherite, and an overgrowth technique was used to synthesize aragonite. The experiments were conducted at 50 and 70°C by one- and two-step approaches, respectively, with a difference in the time of oxygen isotope exchange between dissolved carbonate and water before carbonate precipitation. The two-step approach involved sufficient time to achieve oxygen isotope equilibrium between dissolved carbonate and water, whereas the one-step approach did not. The measured witherite-water fractionations are systematically lower than the aragonite-water fractionations regardless of exchange time between dissolved carbonate and water, pointing to cation effect on oxygen isotope partitioning between the barium and calcium carbonates when precipitating them from the solutions. The two-step approach experiments provide the equilibrium fractionations between the precipitated carbonates and water, whereas the one-step experiments do not. The present experiments show that approaching equilibrium oxygen isotope fractionation between precipitated carbonate and water proceeds via the following two processes:

1.

Oxygen isotope exchange between [CO₃]²⁻ and H₂O:

(1)     

Fe isotope systematics of coexisting amphibole and pyroxene in the alkaline igneous rock suite of the Ilímaussaq Complex,South Greenland

The Ilímaussaq intrusion, South Greenland, provides an exceptional test case for investigating the changes of stable Fe isotope fractionation of solidus phases with changes in the Fe³⁺/∑Fe ratio of an evolving melt. The intrusion comprises a sequence of four melt batches that were fed from the same parental alkali basaltic magma. Differentiation produced cumulate rocks that range from augite syenite (phase I) over peralkaline granite (phase II) to agpaitic syenites (phases IIIa and IIIb). Fe³⁺/∑Fe ratios in amphiboles increase substantially from phase I to phase II and III rocks and mark a major change in the parental magma composition from augite syenites to peralkaline granites and agpaitic syenites. Before this transition, olivine, clinopyroxene, and amphibole in augite syenite, the most primitive rock type in the Ilímaussaq Complex, have a uniform Fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts (δ^56/54Fe_IRMM-014 = 0.072 ± 0.046‰). After the transition, amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier Fe isotope compositions with δ^56/54Fe_IRMM-014 values ranging from 0.123 to 0.237‰. Contamination of the Ilímaussaq magma by ongoing crustal assimilation as cause for this increase can be excluded on the grounds of Nd isotope data. Large-scale metasomatic overprint with an external fluid can also be dismissed based on amphibole O and Li isotope systematics. Rather, the increase towards heavy Fe isotope compositions most likely reflects the change in chemical compositions of amphiboles (calcic in augite syenite to sodic in the agpaitic syenites) and their Fe³⁺/ΣFe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity. A sensitive adjustment of equilibrium Fe isotope fractionation factors to amphibole ferric/ferrous ratios is also supported by beta-factors calculated from Mössbauer spetroscopy data. Comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from Mössbauer data reveal Fe isotope systematics close to equilibrium in augite syenites but Fe isotopic disequilibrium between these two phases in phase IIIa agpaitic syenites. These results are in agreement with O and Li isotope systematics. While amphiboles in all Ilímaussaq lithologies crystallized at temperatures between 650 and 850 °C, textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300–400 °C. Therefore, isotopic equilibrium at crystallization conditions between these two phases can not be expected, but importantly, subsolidus reequilibration can also be dismissed.     

Effects of cation substitutions in garnet and pyroxene on equilibrium oxygen isotope fractionations

High-grade metamorphic rocks were used to explore oxygen isotope fractionations between pyroxene and garnet, and to investigate the effects on fractionation factors of the cation substitutions Fe³⁺Al_?1 and Ca(Fe,Mg)_?1. Recrystallized, granulite facies (725 °C) wollastonite ores from the northern Adirondack highlands contain essentially only the minerals clinopyroxene (a Di–Hd solid solution)+garnet (a Grs–Adr solid solution)±wollastonite, and exhibit a systematic dependence of measured fractionations on the Fe³⁺ content of calcic garnet: Δ(Cpx–CaGrt)=(0.14±0.12)+(0.78±0.20)X_Adr and Δ(Wo–CaGrt)=(0.15±0.22)+(0.57±0.33)X_Adr. In eclogites formed at T ≤650 °C, measured compositions of Ca-poor garnet and omphacite combined with experimental data indicate that Ca-poor, Fe-rich garnet is enriched in ¹⁸O compared to both diopside and grossular: extrapolating to 1000 K, Δ(Alm–Di)≈c. 0.2 and Δ(Alm–Grs)≈c. 0.5. Orthopyroxene and clinopyroxene from Gore Mountain, New York, show a constant fractionation that is independent of rock type, as expected if they have the same closure temperature. These data imply Δ(Opx-Cpx)≈c. 0.7 at 1000 K. Measured fractionations among Ca-poor garnet, orthopyroxene, clinopyroxene and hornblende in the Gore Mountain rocks further indicate an ¹⁸O enrichment in Ca-poor garnet over Grs (≈c. 0.5 at 1000 K). The new measurements are indistinguishable from expected equilibrium values based on experiments for the minerals enstatite, diopside, grossular, wollastonite and feldspar, but consistently indicate a significant isotope effect for the simple octahedral cation substitutions Fe³⁺Al_?1 (Grs vs. Adr) and Ca(Fe,Mg)_?1 (Ca-poor garnet vs. Grs; Opx vs. Cpx). Neither cation substitution has been directly investigated for its effect on ¹⁸O/¹⁶O fractionation with experiments in silicates. Chemical characterization of minerals is required prior to petrological interpretation of oxygen isotope trends.