RUI: Oxygen Isotope Thermometry and Hygrometry in Quartzites

RUI：石英岩中的氧同位素测温和测湿

• 批准号: 0106890
• 负责人: William Peck
• 金额: $ 6.44万
• 依托单位: Colgate University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-06-15 至 2004-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0106890&HistoricalAwards=false
• 关键词: RUI; Oxygen; Isotope; Thermometry; Hygrometry

PeckEAR-0106890This proposal details approaches for using the oxygen isotope systematics of metaquartzites to: (1) retrieve peak (800 deg C) garnet-quartz fractionations for thermometry and (2) use water-controlled retrograde diffusion in rutile and other trace phases as a paleohygrometer. Pure (90% Qtz) quartzite is the ideal lithology for this study because it satisfies the 'infinite reservoir' assumption with respect to modeling oxygen diffusion in accessory minerals. Laser fluorination will allow high-precision analysis of equilibrium mineral pairs. Samples will be selected from well-studied localities in the southern Grenville Province with published constraints on metamorphic conditions (T, P, cooling rates, and fluids).The thermometry portion of this project will use the slow oxygen diffusion rate in garnet (as determined by experimental studies) as a way to 'see-past' multiple thermal events in polymetamorphic rocks. Garnet commonly forms below its closure temperature, so it will preserve oxygen isotope ratios from garnet formation even through subsequent granulite-facies metamorphism. Quartzite is essentially momomineralic, so closed-system retrogression will not affect quartz-garnet fractionations because quartz will have no other minerals with which to exchange during cooling. Samples will be rigorously characterized with respect to oxygen isotope ratio, and also with BSE, CL, and conventional microscopy to determine if they have behaved as closed systems. This thermometer will be extremely useful in high-grade polymetamorphic terrains where more conventional cation thermometers are not applicable.Experiments have shown that oxygen diffusion rates in minerals are a strong function of water fugacity. The proposed research will use published experiments and measured oxygen isotope ratios to constrain water fugacities in slowly cooled metamorphic rocks. Intermineral fractionations and isotopic zoning will be used, in conjunction with diffusion models, to develop an oxygen isotope hygrometer. Rutile will be a focus of this study because it behaves in the opposite manner to other rock-forming minerals with respect to water effects on oxygen diffusion rates (in rutile water causes slow oxygen diffusion). Thus, when used with other minerals, rutile provides good leverage for determining the interplay between water fugacity and oxygen diffusion rate. This oxygen isotope hygrometer will allow the detection of even chemically unreactive fluids which leave no chemical signature but can still flux melting and weaken rocks.The last decade has seen an explosion of experimental work on oxygen diffusion rates and equilibrium oxygen isotope fractionation factors, and also the modeling of oxygen exchange between minerals during cooling. Unfortunately, few studies have combined these resources with the high precision, small sample sizes, and high spatial resolution available from laser fluorination to allow genuinely new information to be extracted from rocks. High-temperature thermometry and hygrometry are important avenues of timely research which will provide new tools for understanding deep-crustal processes.

PeckEAR-0106890该提案详细介绍了使用变石英岩的氧同位素系统学的方法：(1) 检索峰值 (800 摄氏度) 石榴石石英分馏以进行测温；(2) 使用金红石和其他痕量相中的水控制逆行扩散作为古湿度计。 纯（90% Qtz）石英岩是本研究的理想岩性，因为它满足模拟副矿物中氧扩散的“无限储层”假设。 激光氟化将允许对平衡矿物对进行高精度分析。 样本将从格伦维尔省南部经过充分研究的地点选取，并公布了对变质条件（T、P、冷却速率和流体）的限制。该项目的测温部分将利用石榴石中缓慢的氧扩散速率（由实验研究确定）作为“了解”多变质岩中多次热事件的方法。 石榴石通常在其闭合温度以下形成，因此即使在随后的麻粒岩相变质作用中，它也会保留石榴石形成过程中的氧同位素比率。 石英岩本质上是单矿物，因此封闭系统回归不会影响石英-石榴石分馏，因为石英在冷却过程中没有其他矿物可以与之交换。 样品将根据氧同位素比进行严格表征，并使用 BSE、CL 和传统显微镜来确定它们是否表现为封闭系统。 该温度计在传统阳离子温度计不适用的高级多变质地形中非常有用。实验表明，矿物中的氧扩散速率是水逸度的重要函数。 拟议的研究将使用已发表的实验和测量的氧同位素比率来限制缓慢冷却的变质岩中的水逸度。 矿间分馏和同位素分区将与扩散模型结合使用，开发氧同位素湿度计。 金红石将成为本研究的重点，因为在水对氧扩散速率的影响方面，金红石的行为与其他造岩矿物相反（在金红石水中会导致氧扩散缓慢）。 因此，当与其他矿物一起使用时，金红石为确定水逸度和氧扩散速率之间的相互作用提供了良好的杠杆作用。 这种氧同位素湿度计甚至可以检测化学不反应的流体，这些流体不会留下化学特征，但仍然可以助熔熔化并削弱岩石。过去十年，关于氧扩散速率和平衡氧同位素分馏因子的实验工作激增，并且还对冷却过程中矿物之间的氧交换进行了建模。 不幸的是，很少有研究将这些资源与激光氟化所提供的高精度、小样本量和高空间分辨率相结合，以便从岩石中提取真正的新信息。 高温测温和测湿是及时研究的重要途径，将为理解深部地壳过程提供新工具。