GRANTEE: UNIVERSITY OF CALIFORNIA, SANTA BARBARA

Department of Geological Sciences

Office of Research and Development Administration

Santa Barbara, California 93106

GRANT: DE-FG03096ER 14620

TITLE: The Hydrodynamics of Geochemical Mass Transport and Clastic Diagenesis: San Joaquin Basin, California

PERSONS IN CHARGE: James Boles (805-893-3719; Fax 805-893-2314; E-mail boles@magic.ucsb.edu) and Grant Garven (The John Hopkins University)

Objectives: The purpose of this study is to reconstruct the different hydrologic regimes in the San Joaquin basin and to model the importance of these regimes to porosity reducing mineral reactions in the sediments.

Project Description: The project is to reconstruct the pore fluid history of the Southern San Joaquin basin from the Miocene to the present and to compare the documented clastic diagenesis in the basin to the flow model. My part of the joint project is to provide geologic background data on the San Joaquin basin and identify diagenetic reactions to be incorporated into the Johns Hopkins hydrogeologic models.

Results: In FY97 data sets were assembled from oil exploration and production wells along a east-west cross section through the San Joaquin basin. The data includes lithology, age, temperature, pressure and pore fluid chemistry and has been incorporated into the 2D time slice modeling. In addition, I have began to summarize diagenetic data from the basin for comparison to the hydrologic flow modeling. The goal is to constrain the relative importance of compaction driven flow, convection, and meteoric influx to important diagenetic reactions in the basin.

Currently I am compiling existing diagenetic data for the basin including evidence for carbonate cementation, feldspar and clay alteration. The isotopic data for the carbonates cements has shown unexpected trends with respect to carbon isotopes. Cements forming during burial are expected to become progressively enriched in carbon 12 from thermogenic processes. Although the data from cements formed at intermediate burial depths have thermogenic carbon values, anomously, cements formed during deep burial have relatively heavy carbon isotope values. This trend has been noted in at least four hydrocarbon reservoirs. Sr isotopic data suggest that the plagioclase is the dominant souce of Ca and Sr in the cements. The importance of these observation is that it indicates carbonate cements are not formed simply from mobilization and/or recrystallization of preexisting carbonate, but that the cements reflect distinct independent processes which are producing calcium and carbon. The carbon incorporated in late calcite cements in the basin may be related to the presence of organic acids associated with oil in the reservoirs. These acids have been shown by Franks et al (1996) to have highly variable and relatively heavy carbon isotopic compositions. Results were presented at the 1997 AAPG Pacific Section, Bakersfield, California.

GRANTEE: UNIVERSITY OF CALIFORNIA, SANTA BARBARA

Institute for Crustal Studies

Santa Barbara, California 93106-1100

GRANT: DE-FG03-91ER14211

TITLE: Physical and Experimental Studies of Magma Rheology, Sedimentary Basins and Molecular Dynamics of Silicates

PERSON IN CHARGE: F. J. Spera (805-893-4880; Fax 805-893-8649; E-mail spera@magma.geol.ucsb.edu)

Objectives: (1) Construction of high viscosity rheometer and laboratory measurements on magma; (2) Determination of the structure and properties of multicomponent silicate melts and glasses at elevated temperature and pressure; (3) Numerical modeling of magmatic underplating and dynamics of granitic magma ascent; (4) Geochemical material balance modeling of assimilation and fractional crystallization subject to energy constraints.

Project Description: This collaborative project with D. A. Yuen at the University of Minnesota will improve our understanding of the thermal, chemical, dynamical and mechanical state of the continental crust and subcrustal lithosphere with particular focus on the interactions between the various subsystems. The work-plan includes: (1) Construction of new rheological apparatus and laboratory measurements on melts and magmatic suspensions, (2) Determination of the thermodynamical and transport properties of molten silicates by MD simulations, (3) Mixing processes of rheological fluids in convection and visualization of complex processes, (4) Numerical modeling of magmatic underplating and the formation of granitic diapirs, (5) Coupling between mantle convection with temperature-dependent and non-Newtonian rheology and mantle diapirs on the thermal regime and subsidence curves of rift-related basins, (6) The dynamical influences of lithospheric phase transitions on the thermal-mechanical evolution of sedimentary basins, (7) The development of stress fields and criteria for faulting in the crust and finally, (8) Numerical modeling of heat and mass transport driven by thermal and compositional heterogeneities in geothermal systems.

Results: Results cited below are for the UCSB part of this project. Additional results are given in the summary of activities by the University of Minnesota team lead by D. A. Yuen. Molecular Dynamics (MD) simulations have been carried out on a number of silicate melts and glasses including NaAlO₂-SiO₂, CaMgSi₂O₆ and CaAl₂Si₂O₈. The use of simple effective pair potentials enables one to estimate thermodynamic and transport properties and their temperature- and pressure-derivatives quite well. We have studied changes in melt structure as a function of pressure to better comprehend the known pressure-dependent properties of network and partially networked silicate melts. One result is that the activation volume for diffusion of Oxygen in melts across the join NaAlO₂-SiO₂ varies smoothly with composition such that Va = -10cm³/mol for silica and +4cm³/mol for soda aluminate (NaAlO₂) at low pressures. We ascribe this systematic behavior to the weakening of the network due to substitution of Al for Si in the tetrahedral subunits and possibly also to the destruction of planar rings associated with the Al for Si substitution. In other MD work, Bryce and Spera (1997) compared extant laboratory measurements of the pressure and temperature-dependence of the self diffusivity of oxygen and silicon to MD computed values. Activation energies and activation volumes for oxygen agreed quite well (Ea=275 kJ/mol (lab) vs 294kJ/mol (MD) and Va=-6.2 cm³/mol (lab) vs -5.9 cm³/mol(MD)). At the microscopic scale it was noted that in the pressure range zero to 15GPa, the proportion of four-fold Si and Al drops as the fraction of 5-fold (trigonal bipyramids) climbs from zero up to about 0.5. Six-fold Si and Al climb monotonically throughout the interval where 5-fold Si, Al peaks. Work on molten CaAl₂Si₂O₈ shows that the melt structure at 6 GPa is consistent with edge-sharing Si and Al octahedra and not n-membered tetrahedral rings as at low pressure. These rings define `holes' and make the low pressure melts possess a relatively high anionic porosity responsible for the anomalous properties of network (fully polymerized) silicate melts such as the well-known effect of decreasing viscosity with increasing pressure. This effect disappears at higher pressures because the dominant subunit is no longer the tetrahedral one. Instead, trigonal bipyramids (5-fold Al and Si) and octahedra (6-fold Al and Si) become the dominant short-range units, and hence rings of tetrahedra do not form.

A study of the mechanics of crustal anatexis has been completed. The critical factors governing the dynamics of anatexis include the intensity of the heat input, the rheological properties of the source rocks and the bulk composition and compositional structure of the region undergoing partial fusion. Simulations have been performed to evaluate these factors using phase equilibria and thermochemical and transport property data for the binary eutectic system CaAl₂Si₂O₈- CaMgSi₂O₆ at 0.1 Mpa. The role of enthalpy power is tested by varying the enthalpy (or temperature) along the base of the crustal block while maintaining a fixed temperature at the top. As ratio T_bot/T_top increases modestly from 1.05 to 1.15, the average fraction of melt (1-f_s) at steady state increases from 50% to 75%. Time to attain steady state scales inversely with T_bot/T_top with an increase by a factor of two for a 10% decrease in T_bot/T_top. Typical anatexic timescales are in the range 10³ to 10⁵ yr for length scales in range 10² to 10⁴ m. The consequences of different rheological models, especially the importance of Darcy percolative flow relative to en masse flow within the partial melt region was also investigated. At high values of fs_CRIT(e.g., > 0.5) relatively large volumes of nearly homogeneous melt are rapidly generated. There is a factor of two difference in volume of melt generated when the bulk composition of the source is 10 modal percent less refractory. The effects of anatextic events are to fundamentally reorganize the pattern of compositional structure within the crust. Intracrustal differentiation, is an inevitable process associated with magma underplating.

In additional work, the design, fabrication and assembly of a new high-precision concentric cylinder rheometer with capability in the range 10^-3 to 3 Nm of torque and shear rates in the range 10^-4 to 1 s^-1 at 0.1 MPa and temperatures to 1600ºC has been completed. Measurements of melts and magmatic suspensions are currently underway.