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.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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John A. Maccini
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University of California Santa Barbara
Santa Barbara
United States
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