GOALI: Geochronology of Ultra-Fine-Grained Clay, an Industry-University Collaboration

GOALI：超细粒粘土的地质年代学，产学合作

• 批准号: 9725576
• 负责人: Tullis Onstott
• 金额: $ 13.66万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1998
• 资助国家: 美国
• 起止时间: 1998-03-01 至 2001-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9725576&HistoricalAwards=false
• 关键词: GOALI; Geochronology; Ultra; Fine; Grained

9725576 Onstott This proposal requests funds for further refinement of methodologies for 40Ar/39Ar dating of fine-grained clay using the laser microprobe. We have successfully developed a vacuum micro-encapsulation technique for 40Ar/39Ar dating of microgram amounts of clay. This break through will enable 40Ar/39Ar dating of pore filling illite in sandstone and syndeformational illite in fault gouge when sample yields are too small for K/Ar. Furthermore, we have shown that laser incremental heating can distinguish between authigenic illite/smectite and detrital muscovite in clay mixtures and can even detect trace quantities of detrital muscovite in a shale sample. This result provides a partial solution to a vexatious problem for clay geochronologists. As with every application of a new technology to an old problem, however, controversies have arisen regarding the interpretation of the 40Ar/39Ar spectra for illite/smectite. When the 39Ar lost from clay during irradiation, but captured by the encapsulating tube, is added to the Ar retained in the clay, then the 40Ar/39Ar age (total gas age) agrees with the K/Ar age. The 40Ar/39Ar age calculated from just the Ar retained in the clay (ignoring the Ar lost during irradiation) yields an age, referred to as "retention age" that is older than the K/Ar age. In some cases, the "retention age" appears concordant with Rb/Sr and Sm/Nd ages; whereas the total gas age and K/Ar age is significantly younger. If this phenomenon is generally valid, then the "retention age" is a more accurate estimate of the timing of clay crystallization than the K/Ar age. This proposition critically impacts the underpinning of clay geochronology and hence is extremely debatable. Fortunately, the hypothesis is easily tested, because a requisite corollary is that a portion of the K in the clay resides in sites that are non-retentive to Ar and that the amount of K in these sites equals the amount 39Ar lost from the clay during irradiation. By performing 40Ar/39Ar analyses on clay samples before and after they are subjected to cation exchange experiments, the amount of K in Ar non-retentive sites can be determined. With respect to detrital contamination of clay, the 40Ar/39Ar step-heating results would be far more beneficial if they could be used to simultaneously estimate the ages of the authigenic illite/smectite and the detrital micaceous end members. End member ages in clay samples are currently ascertained by plotting the K/Ar dates of separates of varying grain size versus their proportions of diagenetic and detrital components as determined by XRD, and then extrapolating to 100% of each component. This approach referred to as Illite Age Analysis (IAA) is quite successful, but the accuracy's of the resulting ages are limited by the imprecision's of the XRD estimates. The combination of 40Ar/39Ar, K/Ar and XRD analyses could potentially reduce these errors significantly. Inferring the end member ages from the 40Ar/39Ar step-heating requires an independent measure of proportion of Ar contributed by each end member for each heating step. Because Ar release in clays occurs by dehydroxylation, we propose determining the H2O/K of the end members by TGA and AEM/TEM, the proportion of Ar from each end member can be calculated for each heating step, and their ages extrapolated in a manner directly analogous to IAA, correcting for 39Ar recoil by modeling. In summary, the scope of this two year proposal is to perform experiments that will fully elucidate; 1) the proportion of K in Ar non-retentive sites, thereby determining which age (retention vs. K/Ar) represents the clay crystallization age; 2) the Ar mixing ratio in 40Ar/39Ar incremental analyses for two component clay mixtures in order to deduce end member ages. This will be accomplished by performing 40Ar/39Ar, quadrupole gas, high resolution TEM, and thermal stage XRD analyses on well-characterized samples of varying grain size. These samples are authigenic illite/smectite, detrital musco vite, and mixtures thereof, each of known age, crystallinity and composition. The research will be undertaken with the collaboration of Drs. Pevear and Vrolijk of Exxon Production Research Company following the guidelines of GOALI. Drs. Pevear and Vrolijk are acknowledged experts in the field of clay mineralogy and clay geochronology. The proposed duration of the research program is two years.

小行星9725576 该提案要求提供资金，以进一步完善使用激光微探针对细粒粘土进行40 Ar/39 Ar测年的方法。 我们成功地开发了一种真空微胶囊技术，用于微克量粘土的40 Ar/39 Ar定年。 这一突破将使40 Ar/39 Ar定年的孔隙填充伊利石在砂岩和同变形伊利石断层泥时，样品产量太小的K/Ar。 此外，我们已经表明，激光增量加热可以区分自生伊利石/蒙皂石和碎屑白云母粘土混合物，甚至可以检测到微量的碎屑白云母在页岩样品。 这一结果为粘土年代学家们的一个棘手问题提供了部分解决方案。 然而，正如每一个新技术应用于一个老问题一样，关于伊利石/蒙皂石的40 Ar/39 Ar光谱的解释也出现了争议。 当辐照过程中从粘土中损失但被包封管捕获的~（39）Ar加入到粘土中保留的Ar中时，~（40）Ar/~（39）Ar年龄（总气体年龄）与K/Ar年龄一致。 仅从粘土中保留的Ar计算的40 Ar/39 Ar年龄（忽略照射过程中损失的Ar）产生的年龄，称为“保留年龄”，比K/Ar年龄更老。 在某些情况下，“保留年龄”似乎与Rb/Sr和Sm/Nd年龄一致，而总气体年龄和K/Ar年龄显着年轻。 如果这种现象是普遍有效的，那么“保留年龄”是一个更准确的估计粘土结晶的时间比K/Ar年龄。 这一命题严重影响了粘土地质年代学的基础，因此极具争议。 幸运的是，这一假设很容易检验，因为一个必要的推论是，粘土中的一部分K存在于对Ar不具保持性的位点中，并且这些位点中的K量等于在辐照期间从粘土中损失的39 Ar量。 通过对粘土样品进行阳离子交换实验前后的~（40）Ar/~（39）Ar分析，可以确定Ar非滞留位中的K含量。 对于粘土的碎屑污染，40 Ar/39 Ar逐步加热的结果将是更有益的，如果它们可以用来同时估计自生伊/蒙皂石和碎屑云母端员的年龄。 粘土样品中的端元年龄目前是通过绘制不同粒度的分离物的K/Ar年龄与其由XRD确定的成岩和碎屑组分的比例，然后外推到100%的每种组分来确定的。 这种被称为伊利石年龄分析（IAA）的方法是相当成功的，但所得到的年龄的准确性受到XRD估计的不精确性的限制。 40 Ar/39 Ar、K/Ar和XRD分析的组合可以潜在地显著减少这些误差。 从40 Ar/39 Ar分步加热推断端元年龄需要独立测量每个加热步骤中每个端元贡献的Ar比例。 由于Ar在粘土中的释放发生脱羟基，我们建议确定的H2O/K的端员TGA和AEM/TEM，从每个端员的Ar的比例可以计算每个加热步骤，和他们的年龄外推的方式直接类似于IAA，校正39 Ar反冲建模。 总之，这一为期两年的提案的范围是进行实验，以充分阐明：1）Ar非保留位点中K的比例，从而确定哪个年龄（保留与K/Ar）代表粘土结晶年龄; 2）双组分粘土混合物的40 Ar/39 Ar增量分析中的Ar混合比，以推断端元年龄。 这将通过对不同粒度的表征良好的样品进行40 Ar/39 Ar、四极气体、高分辨率TEM和热台XRD分析来实现。 这些样品为自生伊利石/蒙皂石、碎屑白云母及其混合物，每种样品的年龄、结晶度和组成均已知。 这项研究将与埃克森生产研究公司的Pevear和Vrolijk博士合作，按照GOALI的指导方针进行。 Pevear和Vrolijk博士是粘土矿物学和粘土地质年代学领域公认的专家。 研究计划的拟议期限为两年。