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1.
查向平  郑永飞  龚冰 《地质论评》2010,56(4):595-603
在激光BrF5法分析硅酸盐和氧化物矿物的真空提取流程中,通常采用氧气作为工作气体来测定样品的氧同位素组成。这需要使用分子筛吸附并转移氧气到质谱进样系统,而分子筛的活化对于有效吸附和解吸氧气至关重要,否则会引起分析过程中的氧同位素分馏。通过采用13X分子筛进行多组实验分析,笔者发现用不同活化程度或中毒失活分子筛沸石转移时可能存在氧同位素分馏。在低温下分子筛是不能够活化的(或活化不完全),吸附能力很低,所测定的δ18O值是波动的。在200℃马弗炉中预加热24h,然后迅速转移到温度为100℃、真空度为10-3Pa系统中加热12h的分子筛,对氧气吸附能力很强,氧同位素分析的绝对误差为±0.05‰(1σ),能够满足地球化学氧同位素分析的要求。如果分子筛受BrF5污染,其吸附能力大为降低,测定的δ18O值呈下降趋势,氧同位素分馏高达0.7‰。如果分子筛受水汽污染,质谱接受电压不断地降低,存在解吸温度下的再吸附过程,测定δ18O值呈上升趋势。因此,分子筛在使用前要在适当条件下的活化,在发现污染或测量值发生波动变化时要及时更换,否则难以保证氧同位素分析数据的可靠性。  相似文献   

2.
张峰  杨小平 《第四纪研究》2006,26(5):879-880
由于石英的δ~(18)O值能够提供有力的地球化学证据[1,2],这一方法近来被应用于沙漠物源研究[3～5]。在测定沙漠石英δ~(18)O值的实验中,样品前处理多采用焦硫酸钠熔融-氟硅酸浸泡法[6]提纯石英和五氟化溴法[7]提取石英中的氧。而这一流程在处理具体的沙漠沙样品时,部分环节还有  相似文献   

3.
硅酸盐和金属氧化物矿物氧同位素组成的CO2激光氟化分析   总被引：9，自引：0，他引：9  
我室采用MIR-10型CO2激光器,在一种富BrF5的氛围中使激光对硅酸盐和氧化物矿物样品加热形成O2,经多次纯化后用5A的分子筛吸收,再直接送至气体质谱仪进行氧同位素比值测定.这个实验流程与传统方法相比的改进不仅在使用激光加热技术及样品的放置上,而且在直接采用O2而不是CO2进行质谱测定.采用O2进行直接分析的优点不仅避免了向CO2转化过程中的潜在同位素分馏,而且能够得到样品的δ17O值,因此为宇宙样品分析提供了可能.CO2激光氟化技术的优点是所需样品量小(可低达1～2 mg),因此能够分析微小岩石区域或单矿物晶体内的氧同位素分布.同时,激光可以达到非常高的温度(＞4000K),因此能够对某些难熔矿物(如锆石、蓝晶石、橄榄石等)进行氧同位素分析.  相似文献   

4.
我室采用MIR 10型CO2 激光器 ,在一种富BrF5的氛围中使激光对硅酸盐和氧化物矿物样品加热形成O2 ,经多次纯化后用 5 的分子筛吸收 ,再直接送至气体质谱仪进行氧同位素比值测定。这个实验流程与传统方法相比的改进不仅在使用激光加热技术及样品的放置上 ,而且在直接采用O2 而不是CO2 进行质谱测定。采用O2 进行直接分析的优点不仅避免了向CO2 转化过程中的潜在同位素分馏 ,而且能够得到样品的δ1 7O值 ,因此为宇宙样品分析提供了可能。CO2 激光氟化技术的优点是所需样品量小 (可低达 1～ 2mg) ,因此能够分析微小岩石区域或单矿物晶体内的氧同位素分布。同时 ,激光可以达到非常高的温度 (>40 0 0K) ,因此能够对某些难熔矿物 (如锆石、蓝晶石、橄榄石等 )进行氧同位素分析。  相似文献   

5.
目前的发现指出，硫酸盐的δ17O和δ18O值，特别是Δ17O值（＝δ17O-0.52δ18O）能够提供关于大气圈中和地表环境中硫酸盐的起源、混合和迁移的独特的信息，仅仅使用δ18O的测量结果不能圆满地解决相关间题。现有的分析硫酸盐的δ17O和δ18O的方法极其辛苦，而且要求高纯的BrF5。文中报道了从重晶石（BaSO4）中直接产生O2供同位素比值质谱计（IRMS）同时分析δ17O和δ18O的新方法。该方法使用了CO2-激光氟化系统，该系统也可用于从硅酸盐和氧化物中定量的产生O2。对于样品大于4 mg来说，可以获得部分的但是一致的来自BaSO4的氧的产量。每个工作日用该系统可以处理12个以上的样品。Δ17O的分析误差为±0.05‰, δ18O的分析误差为±0.8‰。该方法可有效和准确地研究硫酸盐的Δ17O值。  相似文献   

6.
五氟化溴法在分析氧化物和硅酸盐矿物的氧同位素组成时,反应温度与反应时间是关键要素.在保证反应时间的前提下,研究较高反应温度条件（550~800 ℃）对五氟化溴法氧同位素组成分析的影响.在高反应温度条件下对国家标准物质GBW04409进行氧同位素样品制备与同位素组成分析表明:反应温度在550~675 ℃,获得了较足量的O2产率,δ18O集中在10.4‰~11.8‰范围,准确度较高;反应温度高于700 ℃后,O2产率降低,δ18O分布在10.8‰~26.8‰范围,δ18O产生明显的正偏差;通过分次氟化反应、合并收集气体的方式获得了与标准物质推荐值相吻合的δ18O分析结果.在高于700 ℃的反应温度条件下,BrF5与镍反应器发生反应,增加了试剂消耗.由于BrF5试剂量不足,导致O2产率偏低从而引起氧同位素分馏.   相似文献   

7.
氢氧同位素体系成矿流体示踪若干问题   总被引：4，自引：2，他引：2  
综合分析国内外氢氧同位素研究进展结合部分热液矿床成矿流体的氢氧同位素示踪体系的可靠性,认为该方法存在以下问题:(1)通过石英矿物δ18 O计算流体的δ18O,同位素平衡温度厘定具有主观性,并且石英与流体之间的同位素平衡状态是否达到很难确定。同样的石英矿物δ18 O通过不同的平衡温度计算,得到不同的流体δ18 O,得出晚期大气降水混合的模糊认识;(2)石英中除流体包裹体外,由于晶格结构缺陷而含有部分结构水。在热爆裂法提取流体包裹体时石英中结构水的释放严重影响石英中流体包裹体δD测试结果。大量的已发表数据没有明确指定热爆裂温度,因此无法评价石英结构水对测试结果的影响程度;(3)石英作为流体包裹体寄主矿物对流体包裹体封闭性不佳,流体包裹体捕获后容易与外界流体发生物质交换,使流体包裹体δD不能代表成矿流体同位素组成,这直接挑战了石英作为寄主矿物进行流体包裹体δD测定的可靠性;(4)通过含差劲基热液蚀变矿物δD计算成矿流体δD,由于高温下压力对D/H分馏的影响难以定量评价,已有平衡方程大多忽略压力对D/H同位素分馏的影响,使得计算结果意义不明确。因此硫化物中流体包裹体的δD-δ18 O系统测试,需要辅以微量气体比值、H...  相似文献   

8.
氧同位素组成是一个非常有效的示踪硫酸盐的起源和形成环境的工具.过去主要针对硫酸盐中的氧同位素δ(18O)的研究,近年来,随着硫酸盐中氧同位素质量不相关分馏现象的陆续发现,其δ(17O)的研究成为热点与前沿.在综述了硫酸盐三氧同位素测试技术的研究现状及进展情况后,重点介绍了从硫酸盐中直接提取O2供质谱计同时测定δ(17O)与δ(18O)值的CO2-激光氟化法;同时介绍了硫酸盐中氧同位素异常[Δ(17O)]的研究历程,探讨了硫酸盐中氧同位素质量不相关分馏的机理及三氧同位素的应用前景,展望了今后的研究趋势.  相似文献   

9.
杜广鹏  王旭  张福松 《岩矿测试》2010,29(6):631-638
较为系统地研究了在特定排空时间下GasBenchⅡ顶空瓶内背景CO2的量和同位素组成变化,并检查了该空白CO2组分对碳酸盐C、O同位素在线测定的影响。结果发现,由于排空时间较短所导致的顶空瓶内剩余的空气CO2或由于排空方法不当所导致的外部空气少量回流都会对小样品量(100μg)碳酸盐C、O同位素测定结果产生显著影响。其影响程度取决于空白CO2的量和空白CO2与碳酸盐样品之间C、O同位素的差别大小。由此,对小样品量碳酸盐C、O同位素的测定结果进行了空白校正。当碳酸盐样品质量在20~100μg,校正后δ13C和δ18O值的标准偏差分别小于0.12‰和0.13‰,这证明了该空白校正方法的可行性。由于顶空瓶中空白CO2的量很小,所以对痕量CO2的量、δ13C和δ18O测定值的测定会存在一定波动,这对碳酸盐δ13C和δ18O校正值产生小于0.2‰的不确定度。采用准确测定的实验室空气CO2的δ13C和δ18O值则会大大减少该不确定度的影响。  相似文献   

10.
应用五氟化溴法(BrF5)测定硅酸盐及氧化物矿物的氧同位素组成,试剂纯化、样品预处理、环境湿度、反应器洁净程度以及实验质量控制是影响该法准确度的主要因素。为确定BrF5法最佳实验条件,提高分析精度,本文以国家标准物质为参考,对硅酸盐及氧化物矿物在不同焙烧温度下的氧同位素组成进行分析,并对实验环境湿度和反应器洁净程度对分析数据质量的影响进行了系统研究。实验确定了焙烧温度设置为650～750℃,可去除硅酸盐全岩样品中的炭质成分,避免了温度因素引起的同位素分馏;将实验环境相对湿度降低至50%RH,并在0. 5min内完成样品装填,可解决由于水汽造成δ18O值明显偏低的突出问题;对于新使用的反应器,或反应器在使用中有黄色物质脱落、内壁受到污染,及时进行氟化处理和清洗,可避免因氧气产率降低所引起的氧同位素分馏;反应器交替进行样品和标准物质CO2试样的制备,可对样品同位素数据的可靠性进行监测。对国家标准GBW04409的氧同位素组成进行测试,δ18O测试结果与推荐值一致,分析精度优于0. 1‰。以上途径有效控制了氧同位素分馏,提高了BrF5氧同位素分析方法的适用性。  相似文献   

11.
《Chemical Geology》2002,182(2-4):179-194
A technique using isotope-ratio-monitoring gas chromatography-mass spectrometry (irmGCMS) and excimer laser fluorination for in situ oxygen isotope analysis of silicates is described. The irmGCMS and oxygen extraction line is connected by a newly developed interface, reducing the time for a single analysis to less than 10 min. The precision obtained for δ18O is similar to what has been reported for excimer laser fluorination using dual inlet systems. δ18O values of two olivine standards had 1σ precision of ±0.14‰ (n=19 and n=10) and that of Dörentrup quartz had ±0.17‰. Eleven analyses of a large zircon crystal had a precision of ±0.12‰. However, between 300 and 600 nmol oxygen was liberated for a single analysis, equivalent to cylindrical laser holes 250 to 350 μm in diameter and depth. In the future it will be feasible to measure the isotope ratio of cylindrical volumes 150 μm in diameter simply by reducing the volume of the extraction line. While this is still significantly larger than what is possible with ion probes, the ratios obtained by excimer laser-irmGCMS are highly accurate and precise without correction. The value of this technique for in situ oxygen isotope measurements is demonstrated with two rock slabs from metamorphic rocks of the Dabie–Sulu ultra-high-pressure belt, China.  相似文献   

12.
GasBench Ⅱ-IRMS水平衡氢氧同位素分析方法研究   总被引：10，自引：5，他引：5  
考察了参考气信号强度、样品水体积和水平衡时间对水平衡氢、氧同位素组成分析的影响,建立了GasBench Ⅱ-IRMS连续流稳定同位素质谱仪、CO2-H2O平衡在线连续分析水样中氧同位素组成、疏水铂催化H2-H2O平衡在线连续分析水样中氢同位素组成的分析方法。分析实验室和标准水样,测量值在误差允许范围内与推荐值一致;实验室水标准氢同位素组成分析精度(1σ)为1‰,氧同位素组成分析精度(1σ)为0.1‰。方法能减少样品的损失,提高分析速度和稳定性。  相似文献   

13.
Two quartz samples of igneous origin, UNIL‐Q1 (Torres del Paine Intrusion, Chile) and BGI‐Q1 (Shandong province, China), were calibrated for their oxygen isotope composition for SIMS measurements. UNIL‐Q1 and BGI‐Q1 were evaluated for homogeneity using SIMS. Their reference δ18O values were determined by CO2 laser fluorination. The average δ18O value found for UNIL‐Q1 is 9.8 ± 0.06‰ and that for BGI‐Q1 is 7.7 ± 0.11‰ (1s). The intermediate measurement precision of SIMS oxygen isotope measurements was 0.32–0.41‰ (2s; UNIL‐Q1) and 0.40–0.48‰ (2s; BGI‐Q1), respectively. While less homogeneous in its oxygen isotope composition, BGI‐Q1 is also suitable for SIMS trace element measurements.  相似文献   

14.
A number of distinct methodologies are available for determining the oxygen isotope composition of minerals and rocks, these include laser-assisted fluorination, secondary ion mass spectrometry (SIMS) and UV laser ablation. In this review we focus on laser-assisted fluorination, which currently achieves the highest levels of precision available for oxygen isotope analysis. In particular, we examine how results using this method have furthered our understanding of early-formed differentiated meteorites. Due to its rapid reaction times and low blank levels, laser-assisted fluorination has now largely superseded the conventional externally-heated Ni “bomb” technique for bulk analysis. Unlike UV laser ablation and SIMS analysis, laser-assisted fluorination is not capable of focused spot analysis. While laser fluorination is now a mature technology, further analytical improvements are possible via refinements to the construction of sample chambers, clean-up lines and the use of ultra-high resolution mass spectrometers.High-precision oxygen isotope analysis has proved to be a particularly powerful technique for investigating the formation and evolution of early-formed differentiated asteroids and has provided unique insights into the interrelationships between various groups of achondrites. A clear example of this is seen in samples that lie close to the terrestrial fractionation line (TFL). Based on the data from conventional oxygen isotope analysis, it was suggested that the main-group pallasites, the howardite eucrite diogenite suite (HEDs) and mesosiderites could all be derived from a single common parent body. However, high precision analysis demonstrates that main-group pallasites have a Δ17O composition that is fully resolvable from that of the HEDs and mesosiderites, indicating the involvement of at least two parent bodies. The range of Δ17O values exhibited by an achondrite group provides a useful means of assessing the extent to which their parent body underwent melting and isotopic homogenization. Oxygen isotope analysis can also highlight relationships between ungrouped achondrites and the more well-populated groups. A clear example of this is the proposed link between the evolved GRA 06128/9 meteorites and the brachinites.The evidence from oxygen isotopes, in conjunction with that from other techniques, indicates that we have samples from approximately 110 asteroidal parent bodies (∼60 irons, ∼35 achondrites and stony-iron, and ∼15 chondrites) in our global meteorite collection. However, compared to the likely size of the original protoplanetary asteroid population, this is an extremely low value. In addition, almost all of the differentiated samples (achondrites, stony-iron and irons) are derived from parent bodies that were highly disrupted early in their evolution.High-precision oxygen isotope analysis of achondrites provides some important insights into the origin of mass-independent variation in the early Solar System. In particular, the evidence from various primitive achondrite groups indicates that both the slope 1 (Y&R) and CCAM lines are of primordial significance. Δ17O differences between water ice and silicate-rich solids were probably the initial source of the slope 1 anomaly. These phases most likely acquired their isotopic composition as a result of UV photo-dissociation of CO that took place either in the early solar nebula or precursor giant molecular cloud. Such small-scale isotopic heterogeneities were propagated into larger-sized bodies, such as asteroids and planets, as a result of early Solar System processes, including dehydration, aqueous alteration, melting and collisional interactions.There is increasing evidence that chondritic parent bodies accreted relatively late compared to achondritic asteroids. This may account for the fact that apart from a few notable exceptions’ such as the aubrite-enstatite chondrite association, known chondrite groups could not have been the parents to the main achondrite groups.  相似文献   

15.
A detailed oxygen isotope study of detrital quartz and authigenic quartz overgrowths from shallowly buried (<1 km) quartz arenites of the St. Peter Sandstone (in SW Wisconsin) constrains temperature and fluid sources during diagenesis. Quartz overgrowths are syntaxial (optically continuous) and show complex luminescent zonation by cathodoluminescence. Detrital quartz grains were separated from 53 rocks and analyzed for oxygen isotope ratio by laser fluorination, resulting in an average δ18O of 10.0 ± 0.2‰ (1SD, n = 109). Twelve thin sections were analyzed by CAMECA-1280 ion microprobe (6-10 μm spot size, analytical precision better than ±0.2‰, 1SD). Detrital quartz grains have an average δ18O of 10.0 ± 1.4‰ (1SD, n = 91) identical to the data obtained by laser fluorination. The ion microprobe data reveal true variability that is otherwise lost by homogenization of powdered samples necessary for laser fluorination. Laser fluorination uses samples that are one million times larger than the ion microprobe. Whole rock (WR) samples from the 53 rocks were analyzed by laser fluorination, giving δ18O between 9.8‰ and 16.7‰ (n = 110). Quartz overgrowths in thin sections from 10 rocks were analyzed by ion microprobe and average δ18O = 29.3 ± 1.0‰ (1SD, n = 161).Given the similarity, on average, of δ18O for all detrital quartz grains and for all quartz overgrowths, samples with higher δ18O(WR) values can be shown to have more cement. The quartz cement in the 53 rocks, calculated by mass balance, varies from <1 to 21 vol.% cement, with one outlier at 33 vol.% cement. Eolian samples have an average of 11% cement compared to marine samples, which average 4% cement.Two models for quartz cementation have been investigated: high temperature (50-110 °C) formation from ore-forming brines related to Mississippi Valley Type (MVT) mineralization and formation as silcretes at low temperature (10-30 °C). The homogeneity of δ18O for quartz overgrowths determined by ion microprobe rules out a systematic regional variation of temperature as predicted for MVT brines and there are no other known heating events in these sediments that were never buried to depths >1 km. The data in this study suggest that quartz overgrowths formed as silcretes in the St. Peter Sandstone from meteoric water with δ18O values of −10‰ to −5‰ at 10-30 °C. This interpretation runs counter to conventional wisdom based on fibrous or opaline silica cements suggesting that the formation of syntaxial quartz overgrowths requires higher temperatures. While metastable silica cements commonly form at high degrees of silica oversaturation following rapid break-down reactions of materials such as of feldspars or glass, the weathering of a clean quartz arenite is slower facilitating chemical equilibrium and precipitation of crystallographically oriented overgrowths of α-quartz.  相似文献   

16.
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.  相似文献   

17.
Oxygen isotope ratios of quartz inclusions (QI) within garnet from granulite and amphibolite facies gneisses in the Adirondack Mountains, NY were analysed and used to determine metamorphic temperatures. Primary QI for eight of 12 samples have δ18O values significantly lower than matrix quartz (MQ). The primary QI retain δ18O values representative of thermal conditions during garnet crystallization, whereas the δ18O values of MQ were raised by diffusive exchange with other matrix minerals (e.g. mica and feldspar) during cooling. The δ18O differences between QI and MQ show that garnet (a mineral with slow diffusion of oxygen) can armour QI from isotopic exchange with surrounding matrix, even during slow cooling. These differences between δ18O in MQ and QI can further be used to test cooling rates by Fast Grain Boundary diffusion modelling. Criteria for identifying QI that preserve primary compositions and are suitable for thermometry were developed based on comparative tests. Relations between δ18O and inclusion size, distance of inclusion to host–garnet rim, core–rim zonation of individual inclusions, and presence or absence of petrological features (healed cracks in QI, inclusions in contact with garnet cracks lined by secondary minerals, and secondary minerals along the inclusion grain boundary) were investigated. In this study, 61% of QI preserve primary δ18O and 39% were associated with features that were linked to reset δ18O values. If δ18O in garnet is homogeneous and inclusions are removed, laser‐fluorination δ18O values of bulk garnet are more precise, more accurate, and best for thermometry. Intragrain δ18O(Grt) profiles measured in situ by ion microprobe show no δ18O zonation. Almandine–rich garnet (Alm60–75) from each sample was measured by laser‐fluorination mass‐spectrometry (LF‐MS) for δ18O and compared with ion microprobe measurements of δ18O in QI for thermometry. The Δ18O(Qz–Grt) values for Adirondack samples range from 2.66 to 3.24‰, corresponding to temperatures of 640–740 °C (A[Qz–Alm] = 2.71). Out of 12 samples that were used for thermometry, nine are consistent with previous estimates of peak temperature (625–800 °C) based on petrological and carbon–isotope thermometry for regional granulite and upper amphibolite facies metamorphism. The three samples that disagree with independent thermometry for peak metamorphism are from the anorthosite–mangerite–charnockite–granite suite in the central Adirondacks and yield temperatures of 640–665 °C, ~100 °C lower than previous estimates. These low temperatures could be interpreted as thermal conditions during late (post‐peak) crystallization of garnet on the retrograde path.  相似文献   

18.
硫酸盐的氧同位素测量方法   总被引：7，自引：0，他引：7  
硫酸盐不仅是常见矿物,还是自然界少数几个具有氧同位素非质量分馏效应的矿物之一。硫酸盐矿物的氧同位素组成,特别是硫酸盐的氧同位素非质量分馏效应,可以为研究其形成过程和条件提供大碹有用信息,揭示一些元素浓度甚至单个同位素比值测量无法获得的特殊作用过程。但由于硫酸盐的氧同位素分析技术难度大,这一方法在国内尚未建立起来。论文介绍了BrF5法测量硫酸盐氧同位素组成的实验装置、分析流程和测量结果。该方法是目前唯一可以同时测量硫酸盐^17O／^16O和^18O／^16O比值的方法。对BaSO4国际氧同位素标准样品NBS-127和一种BaSO4化学试剂进行了多次重复测量。测量的NBS-127国际标样的δ^18Ov-SMOW=（9．20±0．11）‰,与标准值完全一致;BaSO4化学试剂的δ^18Ov-SMOW=（14．64±0．13）‰。分析精度（标准偏差）达到0．13％。（1σ）,优于国外（0．15～0．29）‰（1σ）的水平  相似文献   

19.
We report a study of the oxygen isotope ratios of chondrules and their constituent mineral grains from the Mokoia, oxidized CV3 chondrite. Bulk oxygen isotope ratios of 23 individual chondrules were determined by laser ablation fluorination, and oxygen isotope ratios of individual grains, mostly olivine, were obtained in situ on polished mounts using secondary ion mass spectrometry (SIMS). Our results can be compared with data obtained previously for the oxidized CV3 chondrite, Allende. Bulk oxygen isotope ratios of Mokoia chondrules form an array on an oxygen three-isotope plot that is subparallel to, and slightly displaced from, the CCAM (carbonaceous chondrite anhydrous minerals) line. The best-fit line for all CV3 chondrite chondrules has a slope of 0.99, and is displaced significantly (by δ17O ∼ −2.5‰) from the Young and Russell slope-one line for unaltered calcium-aluminum-rich inclusion (CAI) minerals. Oxygen isotope ratios of many bulk CAIs also lie on the CV-chondrule line, which is the most relevant oxygen isotope array for most CV chondrite components. Bulk oxygen isotope ratios of most chondrules in Mokoia have δ18O values around 0‰, and olivine grains in these chondrules have similar oxygen isotope ratios to their bulk values. In general, it appears that chondrule mesostases have higher δ18O values than olivines in the same chondrules. Our bulk chondrule data spread to lower δ18O values than any ferromagnesian chondrules that have been measured previously. Two chondrules with the lowest bulk δ18O values (−7.5‰ and −11.7‰) contain olivine grains that display an extremely wide range of oxygen isotope ratios, down to δ17O, δ18O around -50‰ in one chondrule. In these chondrules, there are no apparent relict grains, and essentially no relationships between olivine compositions, which are homogeneous, and oxygen isotopic compositions of individual grains. Heterogeneity of oxygen isotope ratios within these chondrules may be the result of incorporation of relict grains from objects such as amoeboid olivine aggregates, followed by solid-state chemical diffusion without concomitant oxygen equilibration. Alternatively, oxygen isotope exchange between an 16O-rich precursor and an 16O-poor gas may have taken place during chondrule formation, and these chondrules may represent partially equilibrated systems in which isotopic heterogeneities became frozen into the crystallizing olivine grains. If this is the case, we can infer that the earliest nebular solids from which chondrules formed had δ17O and δ18O values around -50‰, similar to those observed in refractory inclusions.  相似文献   

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