RUI: Oxygen Isotope Thermometry and Hygrometry in Quartzites

本提案详细介绍了使用变石英岩氧同位素系统学的方法：（1）检索峰值（800摄氏度）石榴石-石英分馏温度计和（2）使用水控制的金红石和其他痕量相的逆行扩散作为古湿度计。 纯石英岩（90%Qtz）是这项研究的理想岩性，因为它满足了“无限水库”的假设，在模拟辅助矿物中的氧扩散。 激光扫描仪将允许对平衡矿物对进行高精度分析。 样品将从格伦维尔省南部经过充分研究的地区选择，这些地区已公布了变质条件（T、P、冷却速率和流体）的限制条件。该项目的测温部分将使用石榴石中缓慢的氧扩散速率（由实验研究确定）作为“看穿”多变质岩中多个热事件的一种方式。 石榴石通常在其闭合温度以下形成，因此即使经过随后的麻粒岩相变质作用，它也会保持石榴石形成时的氧同位素比值。 石英岩基本上是单矿物的，因此封闭系统的退变质作用不会影响石英-石榴石分馏，因为石英在冷却过程中没有其他矿物可与之交换。 将严格表征样品的氧同位素比，以及BSE、CL和常规显微镜，以确定它们是否表现为封闭系统。 这种温度计在高级多变质地区将是非常有用的，在那里更传统的阳离子温度计是不适用的。实验表明，氧在矿物中的扩散速率是水逸度的强函数。 拟议中的研究将使用已发表的实验和测得的氧同位素比来限制缓慢冷却的变质岩中的水逸度。 将使用矿物间分馏和同位素分区，结合扩散模型，开发氧同位素湿度计。 金红石将是这项研究的重点，因为它的行为方式与其他造岩矿物相反，就水对氧扩散速率的影响而言（在金红石水中导致氧扩散缓慢）。 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将允许检测甚至化学不反应的流体，这些流体没有留下化学特征，但仍然可以熔化和削弱岩石。过去十年来，关于氧扩散速率和平衡氧同位素分馏因子的实验工作以及冷却过程中矿物之间氧交换的建模工作激增。 不幸的是，很少有研究将这些资源与激光扫描的高精度、小样本量和高空间分辨率相结合，从而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，将为了解地壳深部过程提供新的工具。

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.