Isotope Investigation of Mass Transfer in a Sedimentary Basin

我们建议调查流体-岩石相互作用和质量转移率在圣华金盆地。这项工作将测试沉积盆地中的流体通量模型，包括：压实驱动流，热驱动对流和流星补给。我们的方法是使用87 Sr/86 Sr和143 Nd/144 Nd比值作为同位素示踪剂，结合我们目前对成岩反应的理解，以确定盆地中的钙的流动性。我们将比较钙汇（碳酸盐和浊沸石水泥）与潜在的钙源，包括：海洋生物成因碳酸盐，碎屑钙蒙脱石，钙斜长石。初步工作表明，钙斜长石是最有可能的来源，钙的方解石水泥。我们相信，这项研究将提供一个独特的洞察盆地成岩过程，由于在这个地区的良好约束的地质，我们相信这是理想的这种类型的研究。我们的成功将取决于能够识别每个潜在来源的同位素“指纹”，能够可靠地测量水泥中的同位素比率，并能够预测反应的时间。迄今为止，我们的工作强烈表明，Sierran斜长石是唯一的锶同位素来源的比率低到足以占那些观察到的水泥在北科尔斯大堤，中新世水库，我们已经详细研究。此外，斜长石的溶解将唯一地解释在盆地的逐渐改性的现代孔隙沃茨中观察到的比率下降。我们建议将这些研究在北科尔斯大堤扩大到流域范围。为了检验斜长石来源的Ca的假设，我们建议检查四个不同长石蚀变的区域：1）具有未蚀变斜长石的区域，2）具有丰富的斜长石溶解的区域，3）具有丰富的斜长石钠长石化的区域，以及4）具有钾长石溶解但没有斜长石蚀变的区域。在每个区域，我们将测量水泥和潜在钙源（包括斜长石）的同位素值。还将测量孔隙沃茨，以监测流体-岩石相互作用对Sr同位素比值的影响。如可能，将计算每个区域的传质速率。最后，通过与其他小组的合作研究，我们计划将我们对传质的估计与各种水文模型的预测进行比较。我们相信这个受控的、多学科的项目对于理解流域范围内的质量传递具有巨大的潜力。

We propose to investigate fluid-rock interaction and mass transfer rates in the San Joaquin basin. The work will test models of fluid flux in sedimentary basins including: compaction driven flow, thermal driven convection, and meteoric recharge. Our approach is to use 87Sr/86Sr and 143Nd/144Nd ratios as isotopic tracers, in combination with our present understanding of diagenetic reactions, to determine calcium mobility in the basin. We will compare calcium sinks (carbonate and laumontite cements) with potential calcium sources including: marine biogenetic carbonate, detrital Ca-smectite, and Ca-plagioclase. Preliminary work suggests that Ca-plagioclase is the most likely source for calcium in calcite cements. We believe that this study will offer a unique insight into basin diagenetic processes, and owing to the well-constrained geology in this area, we believe it is ideal for a study of this type. Our success will depend on being able to recognize isotopic "fingerprints" for each potential source, to be able to reliably measure isotopic ratios in cements, and to be able to predict timing of reactions. Our work to date strongly indicates that Sierran plagioclase is the only Sr isotopic source with ratios low enough to account for those observed in cements at North Coles Levee, a Miocene reservoir we have examined in detail. In addition, dissolution of plagioclase would uniquely account for the decreasing ratios observed in progressively modified modern pore waters of the basin. We propose to extend these studies at North Coles Levee into a basinwide scale. To test the hypothesis of plagioclase sourced Ca, we propose to examine four areas of varying feldspar alteration: 1) areas with unaltered plagioclase, 2) with abundant plagioclase dissolution, 3) with abundant plagioclase albitization, and 4) with K-feldspar dissolution but without plagioclase alteration. In each area we will measure isotopic values for cements and potential Ca sources including plagioclase. Pore waters will also be measured to monitor the effects of fluid-rock interaction on Sr isotopic ratios. Mass transfer rates will be calculated for each area where possible. Finally by cooperative research with other groups we plan to compare our estimates of mass transfer with those predicted by various hydrologic models. We believe this controlled, multi- disciplinary project has great potential for understanding mass transfer on a basin-wide scale.