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。     

黑铜矿氧同位素分馏的理论计算

黑铜矿为铜的氧化矿物，对其结构、力场、光谱前人做过较详细的工作。本文在黑铜矿内部力场计算的同位素位移基础上，通过光谱简约配分函数比的计算，得到黑铜矿－水氧同位素分馏方程：１０＾３ｌｈα＝２．５１Ｘ＾２－１４．８７Ｘ＋６．３１（Ｘ＝１０＾３／Ｔ）同时，利用Ｚｈｅｎｇ的加和增量模式法对黑铜矿的氧同位素分馏进行了计算，得到如下黑铜矿－水体系的分馏方程：１０＾３ｌｎα＝２．８９Ｘ＾２－１３．１０Ｘ＋３．９     

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。     

氢氧化物族矿物的氧同位素分馏

应用增量方法计算了氢氧化物族矿物的氧同位素分馏,得到常见氢氧化物的18O富集顺序为：褐铁矿＞三水铝石＞针铁矿＞水镁石＞硬水铝石。氢氧化物与其对应的氧化物相比显著地富集18O。三价阳离子的氢氧化物和氧化物的18O富集顺序为：M（OH）3＞MO（OH）＞M2O3。Al（OH）3同质多象变体之间也存在一定的分馏。对于石英-氢氧化物、方解石-氢氧化物和氢氧化物-水体系,本文计算提供了在0-1200℃温度范围内三组内部一致的分馏系数方程。这些理论校准与合成实验结果和／或地表温度下的天然样品相吻合,特别针铁矿、勃姆石和硬水铝石与水之间的氧同位素分馏关系能够满足地质测温的要求。因此,对氢氧化物-水体系的氧同位素分析可望提供表生环境下可靠的地质温度计。     

同位素分馏理论计算新进展

稳定同位素分馏的理论计算，既是同位素理论研究的重要方面，也是矿物物理研究的重要课题，它涉及矿物的宏观性质和微观性质两个方面。自从Ｕｒｅｙ（１９４７）以来，人们提出了许多计算模式，但绝大多数模式都是以还原配分函数比作为出发点，以自由能变化为基础。从方法上表现出两种趋势，一是简化的趋势，以尽可能少的参数得到尽可能合理的结果；二是大型化和精确化趋势，以复杂的晶格动力学模式和计算机的大量应用为特征。随着这方面研究的深入，理论计算将成为人们了解矿物同位素分馏性质的重要途径。     

水镁石—水体系氧同位素分馏系数的低温实验研究

在15—120℃的低温范围内分别应用氮化镁法、氯化镁法和氧化镁法3种化学合成方法,对水镁石-水体系氧同位素分馏系数进行了实验测定。所有合成样品的晶体结构均由XRD测定,其形貌特征则由SEM确定。应用3种不同合成方法得到了一致的水镁石—水体系氧同位素分馏系数,证明同位素平衡分馏已经达到。在实验温度范围内,水镁石—水体系氧同位素分馏系数主要决定于温度,而溶液的酸碱度、化学组分和陈化时间的影响不明显。由实验数据得到的氧同位素分馏曲线方程为:10~3Inα=1.59×10~6/T~2-14.10(r=0.9921)。结合前人对三水铝石—水体系和针铁矿—水体系氧同位素分馏系数的低温实验测定,可以得到氢氧化物中金属M—OH键的~(18)O富集顺序:Al~(3+)-OH>Fe~(3+)-OH>Mg~(2+)-OH。应用化学合成方法实验测定低温条件下水镁石—水体系氧同位素分馏系数,不仅克服了同位素交换反应实验的一些缺陷(如交换速率缓慢、仪器设备复杂昂贵等),而且可以应用不同的化学合成反应机理来检验同位素平衡是否达到,这为研究低温地球化学过程作用提供了有价值的基本参数。     

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。     

二氧化钛—水体系氧同位素分馏系数的实验研究

对ＴｉＯ２的两种常见同质多像变体金红石和锐钛矿与水之间的氧同位素分馏系数的实验研究进行了系统的总结和评述。水热晶化法和水热氧化法以及低温水解法获得的实验金红石－水体系分馏系数与增量方法理论计算结果相一致。低温水解法获得的够太矿－水体系分馏可能是一种不平衡分馏,其分馏系数的大小决定于锐钛矿的形成机制。同时,低温水解实验还揭示,在低温矿物形成和同质多像转变过程中可能存在氧同位素继承性,这对解释低温环境     

文石—水体系氧同位素分馏机理的实验研究

采用“附晶生长法”分别在５０和７０℃下合成文石下矿物,获得了两种不同的文石与水之间的氧同位素分馏关系。结果证明,文石与水之间氧同位素分馏的化学动力学机 为两步：（１）碳酸根与水之间进行氧同位素交换和平衡,即：「Ｃ＾１６Ｏ３」＾＾３－＋２Ｈ２＾１８Ｏ＝「Ｃ＾１８Ｏ３＾１６Ｏ」＾２－＋２Ｈ２Ｏ１６Ｏ;（２）与水平衡以后的「ＣＯ２」＾２－离子与Ｃａ＾２＋结合生成文石,即：Ｃａ＾２＋＋＿「Ｃ＾１８Ｏ２＾１     

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。     

Calculation of oxygen isotope fractionation in magmatic rocks

The increment method is applied to calculation of oxygen isotope fractionation factors for common magmatic rocks. The ¹⁸O-enrichment degree of the different compositions of magmatic rocks is evaluated by the oxygen isotope indices of both CIPW normative minerals and normalized chemical composition. The consistent results are obtained from the two approaches, pointing to negligible oxygen isotope fractionation between rock and melt of the same compositions. The present calculations verify the following sequence of ¹⁸O-enrichment in the magmatic rocks: felsic rocks>intermediate rocks>mafic rocks>ultramafic rocks. Two sets of internally consistent fractionation factors are acquired for phenocryst–lava systems at the temperatures above 1000 K and rock–water systems in the temperatures range of 0–1200 °C, respectively. The present calculations are consistent with existing data from experiments and/or empirical calibrations. The obtained results can be used to quantitatively determine the history of water–rock interaction and to serve geological thermometry for various types of magmatic rocks (especially extrusive rocks).     

Theoretical calculation of oxygen isotope fractionation factors in carbonate systems

Using established methods of statistical mechanical calculation and a recent compilation of vibrational frequency data, we have computed oxygen isotope reduced partition function ratios (β values) for a large number of carbonate minerals. The oxygen isotope β values of carbonates are inversely correlated to both the mass and radius of the cation bonded to the carbonate anion but neither correlation is good enough to be used as a precise and accurate predictor of β values. There is an approximately 0.6% relative increase in the β values of aragonite per 10 kbar increase in pressure. These estimates of the pressure effect on β values are broadly similar to those deduced previously for calcite using the methods of mineral physics. In comparing the β values of our study with those derived recently from first-principles lattice dynamics calculations, we find near-perfect agreement for calcite and witherite (<0.3% deviation), reasonable agreement for dolomite (<0.9% deviation) and somewhat poorer agreement for aragonite and magnesite (1.5-2% deviation). In the system for which we have the most robust constraints, CO₂-calcite, there is excellent agreement between our calculations and experimental data over a broad range of temperatures (0-900 °C). Similarly, there is good to excellent correspondence between calculation and experiment for most other low to moderate atomic mass carbonate minerals (aragonite to strontianite). The agreement is not as good for high atomic mass carbonates (witherite, cerussite, otavite). In the case of witherite and cerussite, the discrepancy may be due, in part, to our calculation methodology, which does not account for the effect of cation mass on the magnitude of vibrational frequency shifts associated with heavy isotope substitution. However, the calculations also reveal an incompatibility between the high- and low-temperature experimental datasets for witherite and cerussite. Specifically, the shapes of fractionation factor versus 1/T² curves in the calcite-witherite and calcite-cerussite systems do not conform to the robust constraints on the basic shape of these curves provided by theory. This suggests that either the high- or low-temperature datasets for both minerals is in error. Dolomite-calcite fractionation factors derived from our calculations fall within the wide range of fractionations for this system given by previous experimental and natural sample studies. However, our compilation of available low-temperature (25-80 °C) experimental data reveal an unusual temperature dependence of fractionations in this system; namely, the data indicate an increase in the magnitude of fractionations between dolomite (or proto-dolomite) and calcite with increasing temperature. Such a trend is incompatible with theory, which stipulates that fractionations between carbonate minerals must decrease monotonically with increasing temperature. We propose that the anomalous temperature dependence seen in the low-temperature experimental data reflect changes in the crystallinity and degree of cation ordering of the dolomite phase over this temperature interval and the effect these changes have on the vibrational frequencies of dolomite. Similar effects may be present in natural systems at low-temperature and must be considered in applying experimental or theoretical fractionation data to these systems. In nearly all cases, carbonate mineral-calcite fractionation factors given by the present calculations are in as good or better agreement with experimental data than are fractionations derived from semi-empirical bond strength methods.     

Theoretical Calculation of Oxygen Isotopic Fractionation of Tenorite

Tenorite is an oxide of cupper which has been well documented on its structure,force constants and spectrum.Based on the isotopic shifts of its spectrum calculated from the force constants of tenorite,its reduced partition function ratios were calculated,from which an oxygen isotopic fractionation curve between tenorite and water was obtained:10^3lnα=2.51X^2-14.87X 6.31(X=10^3/T)Oxygen isotopic fractionation of tenorite was also calculated with the modified increment method of Zheng(1991),and another equation was obtained:10^3lnα=2.89X^2-13.10X 3.92(X=10^3/T) Calculated results of the two different models were fitted and possible deviation involved in the calcuation was also discussed.     

Silicon isotope fractionation during magmatic differentiation

The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ³⁰Si = −0.29 ± 0.08‰, 2 s.d.) and not greatly affected by partial melting and recycling. Previous analyses of evolved igneous material indicate that such rocks are isotopically heavy relative to the mantle. To understand this variation, it is necessary to investigate the degree of Si isotopic fractionation that takes place during magmatic differentiation. Here we report Si isotopic compositions of lavas from Hekla volcano, Iceland, which has formed in a region devoid of old, geochemically diverse crust. We show that Si isotopic composition varies linearly as a function of silica content, with more differentiated rocks possessing heavier isotopic compositions. Data for samples from the Afar Rift Zone, as well as various igneous USGS standards are collinear with the Hekla trend, providing evidence of a fundamental relationship between magmatic differentiation and Si isotopes. The effect of fractionation has been tested by studying cumulates from the Skaergaard Complex, which show that olivine and pyroxene are isotopically light, and plagioclase heavy, relative to the Si isotopic composition of the Earth’s mantle. Therefore, Si isotopes can be utilised to model the competing effects of mafic and felsic mineral fractionation in evolving silicate liquids and cumulates.At an average SiO₂ content of ∼60 wt.%, the predicted δ³⁰Si value of the continental crust that should result from magmatic fractionation alone is −0.23 ± 0.05‰ (2 s.e.), barely heavier than the mantle. This is, at most, a maximum estimate, as this does not take into account weathered material whose formation drives the products toward lighter δ³⁰Si values. Mass balance calculations suggest that removal of continental crust of this composition from the upper mantle will not affect the Si isotopic composition of the mantle.     

Theoretical calculation of equilibrium Mg isotope fractionation between silicate melt and its vapor

Isotope fractionation during the evaporation of silicate melt and condensation of vapor has been widely used to explain various isotope signals observed in lunar soils, cosmic spherules, calcium–aluminum-rich inclusions, and bulk compositions of planetary materials. During evaporation and condensation, the equilibrium isotope fractionation factor (α) between high-temperature silicate melt and vapor is a fundamental parameter that can constrain the melt’s isotopic compositions. However, equilibrium α is difficult to calibrate experimentally. Here we used Mg as an example and calculated equilibrium Mg isotope fractionation in MgSiO₃ and Mg₂SiO₄ melt–vapor systems based on first-principles molecular dynamics and the high-temperature approximation of the Bigeleisen–Mayer equation. We found that, at 2500 K, δ²⁵Mg values in the MgSiO₃ and Mg₂SiO₄ melts were 0.141?±?0.004 and 0.143?±?0.003‰ more positive than in their respective vapors. The corresponding δ²⁶Mg values were 0.270?±?0.008 and 0.274?±?0.006‰ more positive than in vapors, respectively. The general \(\alpha - T\) equations describing the equilibrium Mg α in MgSiO₃ and Mg₂SiO₄ melt–vapor systems were: \(\alpha_{{{\text{Mg}}\left( {\text{l}} \right) - {\text{Mg}}\left( {\text{g}} \right)}} = 1 + \frac{{5.264 \times 10^{5} }}{{T^{2} }}\left( {\frac{1}{m} - \frac{1}{{m^{\prime}}}} \right)\) and \(\alpha_{{{\text{Mg}}\left( {\text{l}} \right) - {\text{Mg}}\left( {\text{g}} \right)}} = 1 + \frac{{5.340 \times 10^{5} }}{{T^{2} }}\left( {\frac{1}{m} - \frac{1}{{m^{\prime}}}} \right)\), respectively, where m is the mass of light isotope ²⁴Mg and m′ is the mass of the heavier isotope, ²⁵Mg or ²⁶Mg. These results offer a necessary parameter for mechanistic understanding of Mg isotope fractionation during evaporation and condensation that commonly occurs during the early stages of planetary formation and evolution.     

Coupled lithium- and iron isotope fractionation during magmatic differentiation

In this study, we investigated Fe and Li isotope fractionation between mineral separates of olivine pheno- and xenocrysts (including one clinopyroxyene phenocryst) and their basaltic hosts. Samples were collected from the Canary Islands (Teneriffa, La Palma) and some German volcanic regions (Vogelsberg, Westerwald and Hegau). All investigated bulk samples fall in a tight range of Li and Fe isotope compositions (δ⁵⁶Fe_wr = 0.06–0.17‰ and δ⁷Li_ma = 2.5–5.2‰, assuming δ⁷Li of the olivine-free matrix is virtually identical to that of the bulk sample for mass balance reasons). In contrast, olivine phenocrysts display highly variable, but generally light Fe and mostly light Li isotope compositions compared to their respective olivine-free basaltic matrix, which was considered to represent the melt (with δ⁵⁶Fe_ol = ? 0.24 to 0.14‰ and δ⁷Li_ol = ? 10.5 to + 6.5‰, respectively). Single olivine crystals from one sample display even a larger range of δ⁵⁶Fe_ol between ? 0.7 and + 0.1‰. One single clinopyroxene phenocryst displays the lightest Li isotope composition (δ⁷Li_cpx = ? 17.7‰), but no Fe isotope fractionation relative to melt. The olivine phenocrysts show variable Mg# and Ni (correlated in most cases) that range between 0.89 and 0.74 and between 300 and 3000 μg/g, respectively. These olivines likely grew by fractional crystallization in an evolving magma. One sample from the Vogelsberg volcano contained olivine xenocrysts (Mg# > 0.89 and Ni > 3000 μg/g), in addition to olivine phenocrysts. This sample displays the highest Li- and the second highest Fe-isotope fractionation between olivine and melt (Δ⁷Li_ol-melt = ? 13; Δ⁵⁶Fe_ol-melt = ? 0.29).Our data, i.e. the variable olivine- at constant whole rock and matrix isotope compositions, strongly indicate disequilibrium, i.e. kinetic Fe and Li isotope fractionation between olivine and melt (for Li also between cpx and melt) during fractional crystallization. Δ⁷Li_ol-melt is correlated with the Li partitioning between olivine and melt (i.e. with Li_ol/Li_melt), indicating Li isotope fractionation due to preferential (faster) diffusion of ⁶Li into olivine during fractional crystallization. Olivine with low Δ⁷Li_ol-melt, also have low Δ⁵⁶Fe_ol-melt, indicating that Fe isotope fractionation is also driven by diffusion of isotopically light Fe into olivine, potentially, as Fe–Mg inter-diffusion. The lowest Δ⁵⁶Fe_ol-melt (? 0.40) was observed in a sample from Westerwald (Germany) with abundant magnetite, indicating relatively oxidizing conditions during magma differentiation. This may have enhanced equilibrium Fe isotope fractionation between olivine and melt or fine dispersed magnetite in the basalt matrix may have shifted its Fe isotope composition towards higher δ⁵⁶Fe. The decoupling of Li- and Fe isotope fractionation in cpx is likely due to faster diffusion of Li relative to Fe in cpx, implying that the large investigated cpx phenocryst resided in the magma for only a short period of time which was sufficient for Li- but not for Fe diffusion. The absence of any equilibrium Fe isotope fractionation between the investigated cpx phenocryst and its basaltic host may be related to the similar Fe^3 +/Fe^2 + of cpx and melt. In contrast to cpx, the generally light Fe isotope composition of all investigated olivine separates implies the existence of equilibrium- (in addition to diffusion-driven) isotope fractionation between olivine and melt, on the order of 0.1‰.     

Nitrogen in peridotite xenoliths: Lithophile behavior and magmatic isotope fractionation

In order to document the origin and speciation of nitrogen in mantle-derived rocks and minerals, the N and Ar contents and isotopic compositions were investigated for hydrous and anhydrous peridotite xenoliths from Ataq, Yemen, from Eifel, Germany, and from Massif Central, France. Nitrogen and Ar were extracted by stepwise combustion with a fine temperature resolution, followed by fusion in a platinum crucible. A large isotopic disequilibrium of up to 25.4‰ is observed within single peridotite xenoliths, with δ¹⁵N values as low as −17.3‰ in phlogopite whereas clinopyroxene and olivine show positive δ¹⁵N values. Identical Sr isotopic ratios of phlogopite, clinopyroxene and whole rock in this wehrlite sample are consistent with crystallization from a common reservoir, suggesting that the light N signature of phlogopite might be the result of isotopic fractionation during N uptake from the host magma. The nitrogen concentration is systematically high in phlogopite, (7.6-25.7 ppm), whereas that of bulk peridotite xenoliths is between 0.1 and 0.8 ppm. The high N content of phlogopite is at least partly due to host magma-mineral interaction, and may also suggest the occurrence of N as that substituted for K⁺ during mineral growth in mafic magmas. Such speciation is consistent with the fact that N and Rb contents correlate well for a set of samples from mantle regions that were affected by subduction-related metasomatism and magmatism. The N/Rb ratios of these samples are comparable with values estimated for subduction zone magmas, but are one order of magnitude lower than the N/Rb ratios characterizing subducting slabs. This difference suggests preferential release of N relative to alkalis in the forearc region. N/⁴⁰Ar^∗ ratios of minerals from analyzed mantle xenoliths are much higher than those of vesicles in MORBs and OIBs, requiring either the occurrence of nitrogen speciation in the mantle more compatible than Ar, significant loss of fluid phase during entrainment, or long residence time of volatile elements in the mantle source(s) of fluids to increase drastically the ⁴⁰Ar^∗ budget of the latter.     

Empirical calibration of oxygen isotope fractionation in zircon

New empirical calibrations for the fractionation of oxygen isotopes among zircon, almandine-rich garnet, titanite, and quartz are combined with experimental values for quartz-grossular. The resulting A-coefficients (‰K²) are:

	Zrc	Alm	Grs	Ttn
Qtz	2.64	2.71	3.03	3.66
Zrc		0.07	0.39	1.02
Alm			0.32	0.95
Grs				0.63

Full-size table

     

The oxygen isotope geochemistry of igneous rocks

Oxygen isotope analyses have been obtained for 443 igneous rock and mineral samples from various localities throughout the world. Detailed studies were made on the Medicine Lake, Newberry, Lassen, Clear Lake, S. E. Guatemala, Hawaii and Easter I. volcanic complexes and on the Bushveld, Muskox, Kiglapait, Guadalupe, Duluth, Nain, Egersund, Lac St. Jean, Laramie, Skaergaard, Mull, Skye, Ardnamurchan and Alta, Utah plutonic complexes, as well as upon several of the zoned ultramafic intrusions of S. E. Alaska. Basalts, gabbros, syenites and andesites are very uniform in O¹⁸/O¹⁶, commonly with δ-values of 5.5 to 7.0 per mil. Many rhyolite obsidians, particularly those from oceanic areas and the Pacific Coast of the United States, also lie in this range; this indicates that such obsidians are differentiates of basaltic or andesitic magma at high temperatures (about 1,000° C). They cannot represent melted sialic crust. The only plutonic granites with such low δ-values are some of the hypersolvus variety, suggesting that these also might form by fractional crystallization. Obsidians from the continental interior, east of the quartz-diorite line, have higher δ-values. This is compatible with their having assimilated O¹⁸-rich sialic crust. A correlation generally exists between the O¹⁸/O¹⁶ ratios of SiO₂-rich differentiates and the chemical trends in volcanic complexes. High O¹⁸/O¹⁶ ratios accompany those trends having the lower Fe/Mg ratios, while ferrogabbro trends are associated with depletion in O¹⁸. Variations in oxygen fugacity may be responsible for these effects, as abundant early precipitation of magnetite should lead to both O¹⁸-enrichment and Fe-depletion in later differentiates. Plutonic granites have higher O¹⁸/O¹⁶ ratios than their volcanic equivalents, because (a) their differentiation occurred at much lower temperatures, or (b) they are in large part derived from O¹⁸-rich sialic crust by partial melting or assimilation. Also, the oxygen isotope fractionations among coexisting minerals are distinctly larger in plutonic rocks than in volcanic rocks. This is in keeping with their lower crystallization temperatures and their longer cooling history, which promotes post-crystallization oxygen isotope exchange. Hydrated obsidians and perlites have δO¹⁸-values that are much different from their primary, magmatic values. A correlation exists between D/H and O¹⁸/O¹⁶ ratios in hydrated volcanic glass from the western U.S.A., proving that the isotopic compositions are a result of exchange with meteoric waters. The O¹⁸ contents of the glasses appear to be about 25 per mil higher than their associated waters; hence, these hydrated glasses have not simply absorbed H₂O, but they have exchanged with large quantities of it. The igneous rocks from Mull, Skye, Ardnamurchan and the Skaergaard intrusion are all abnormally depleted in O¹⁸ relative to “normal” igneous rocks. This is a result of their having exchanged at high temperatures with meteoric water that was apparently abundant in the highly jointed plateau lavas into which these igneous rocks were intruded. In part, this exchange occurred with liquid magma and in part with the crystalline rock; in the latter case the feldspar was more easily exchanged and has become much more depleted in O¹⁸ than has coexisting quartz or pyroxene. The later differentiates of the Muskox intrusion are markedly O¹⁸-rich, but this is not a result of fractional crystallization. It is in large part a result of deuteric exchange between feldspars and an oxygen-bearing fluid (H₂O ?) that was either O¹⁸-rich or had a relatively low temperature. This phenomenon was also observed in a number of granophyres from other localities, particularly those containing brick-red alkali feldspar. The exchanged feldspars in all these examples are turbid or cloudy, and may be filled with hematite dust. It is concluded that most such feldspar in nature is the result of deuteric exchange and is probably drastically out of oxygen isotopic equilibrium with its coexisting quartz.     

Oxygen isotope variations in magmatic differentiation processes of the volcanic rocks in Japan

The ¹⁸O/¹⁶O ratios were analyzed for 35 volcanic rocks from the Izu-Hakone region, Oki-Dogo Island, Asama Volcano and Kiso-Ontake Volcano in the Japanese Islands. The ¹⁸O-enrichment in magma during crystallization differentiation is recognized in every rock series. The magnitude of solid-liquid isotope fractionation in the magma of tholeiite series is similar to that of alkali rock series, while the apparent isotope fractionation of calc-alkali rock series is larger than that of the above two series. The ¹⁸O value of the tholeiite magma in the Izu- Hakone region is +5.7 relative to SMOW, which suggests its origin from the fresh upper mantle material. The primary magma of the calc-alkali rock series in Asama Volcano is assumed to have the ¹⁸O value similar to or slightly higher than that of the Izu-Hakone primary magma. The alkali rock series of Oki-Dogo Island in the continental side of the island arc is by 1 heavier than the Izu-Hakone rock suite. This may imply that Oki-Dogo rock series might have exchanged their oxygen with ¹⁸O-rich crustal materials, or they might have originated from somewhat ¹⁸O-rich upper mantle material.