Isotope Investigation of Mass Transfer in a Sedimentary Basin

我们建议研究圣华金盆地的流体-岩石相互作用和质量传输率。这项工作将测试沉积盆地中流体流动的模型，包括：压实驱动流、热驱动对流和大气补给。结合我们目前对成岩反应的认识，我们的方法是用87Sr/86Sr和143Nd/144Nd作为同位素示踪剂来确定盆地中的钙活动性。我们将比较钙汇(碳酸盐和浊沸石胶结物)和潜在的钙源，包括：海洋生物成因碳酸盐、碎屑钙蒙脱石和钙斜长石。初步研究表明，钙斜长石是方解石水泥中最有可能的钙来源。我们相信，这项研究将为盆地成岩过程提供一个独特的见解，由于该地区地质条件良好，我们认为它是进行这类研究的理想选择。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。到目前为止，我们的工作有力地表明，Sierran斜长石是唯一一个锶同位素源，其比值低到足以解释我们详细研究过的北科尔斯利维的胶结物中观察到的那些比值。此外，斜长石的溶解作用将是盆地现代孔隙水中观察到的比例递减的唯一原因。我们建议将北科尔勒维的这些研究扩展到盆地范围内。为了验证斜长石来源钙的假说，我们建议考察四个不同的长石蚀变区域：1)斜长石未改变的区域，2)斜长石溶解丰富的区域，3)斜长石钠长石丰富的区域，4)钾长石溶解但没有斜长石蚀变的区域。在每个地区，我们将测量水泥和包括斜长石在内的潜在钙源的同位素值。还将测量孔隙水，以监测流体-岩石相互作用对锶同位素比率的影响。在可能的情况下，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预测的结果进行比较。我们相信，这个受控的、多学科的项目在了解盆地范围内的质量传递方面具有巨大的潜力。

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.