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

该提案要求资金用于进一步改进使用激光微探针对细粒粘土进行40Ar/39Ar测年的方法。我们成功地开发了一种真空微包封技术，用于测定微量粘土的40Ar/39Ar年代。这一突破将有助于在样品产率小于K/Ar值的情况下，对砂岩孔隙充填伊利石和断层泥同变形伊利石进行40Ar/39Ar定年。此外，我们已经证明，激光增量加热可以区分粘土混合物中的自生伊利石/蒙脱石和碎屑白云母，甚至可以检测页岩样品中微量的碎屑白云母。这一结果部分地解决了黏土地质年代学家的一个棘手问题。然而，就像每次将新技术应用于老问题一样，关于伊利石/蒙脱石的40Ar/39Ar光谱的解释也出现了争议。当辐照过程中粘土中流失的39Ar被包封管捕获后，加入到粘土中保留的Ar中，则40Ar/39Ar年龄（总气龄）与K/Ar年龄一致。仅根据粘土中保留的Ar（忽略辐照过程中损失的Ar）计算出的40Ar/39Ar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”与Rb/Sr和Sm/Nd年龄一致；而总气体年龄和K/Ar年龄明显年轻。如果这种现象是普遍有效的，那么“保留年龄”比K/Ar年龄更准确地估计粘土结晶的时间。这一命题严重影响了粘土地质年代学的基础，因此极具争议性。幸运的是，这个假设很容易被验证，因为一个必要的推论是，粘土中的一部分K存在于对Ar不保留的位置，这些位置的K的数量等于在辐照期间从粘土中损失的39Ar的数量。通过对粘土样品进行阳离子交换实验前后进行40Ar/39Ar分析，可以确定Ar非保留位中K的含量。对于粘土的碎屑污染，如果40Ar/39Ar的阶梯加热结果可以同时用于估计自生伊利石/蒙脱石和碎屑云母端元的年龄，则将更加有利。目前，粘土样品中端元年龄的确定是通过绘制不同粒度分离物的K/Ar日期与XRD测定的成岩和碎屑组分的比例，然后外推到每种组分的100%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。结合40Ar/39Ar、K/Ar和XRD分析可以显著降低这些误差。从40Ar/39Ar阶梯加热推断端元年龄需要对每个加热步骤中每个端元贡献的Ar比例进行独立测量。由于Ar在粘土中的释放是通过去羟基化发生的，我们建议通过TGA和AEM/TEM来确定末端成员的H2O/K，每个加热步骤中每个末端成员的Ar比例可以计算出来，并且它们的年龄可以用直接类似于IAA的方式推断出来，通过建模来纠正39Ar反冲。总之，这个为期两年的提案的范围是进行实验，将充分阐明；1) K在Ar非保留位中的比例，从而确定哪个年龄（保留vs. K/Ar）代表粘土结晶年龄；2)通过对双组分粘土混合物进行40Ar/39Ar增量分析，确定其掺氩比例，从而推断端元年龄。这将通过对不同晶粒尺寸的样品进行40Ar/39Ar、四极气相、高分辨率TEM和热阶段XRD分析来完成。这些样品是自生的伊利石/蒙脱石、碎屑麝香云母及其混合物，每种样品的年龄、结晶度和组成都是已知的。这项研究将与博士合作进行。埃克森生产研究公司的Pevear和Vrolijk遵循了GOALI的指导方针。Drs。Pevear和Vrolijk是粘土矿物学和粘土地质年代学领域公认的专家。研究计划的建议期限为两年。

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.