Geology topics

P. Smart

Publications and source records attributed to P. Smart.

3 recordsLinked to original sources

USGS2001

Spatial patterns of diagenesis during geothermal circulation in carbonate platforms

Geothermal convection of seawater deep in carbonate platforms could provide the necessary supply of magnesium for dolomitization at temperatures high enough to overcome kinetic limitations. We used reactive-transport simulations to predict the rates and spatial patterns of dolomitization during geothermal convection in a platform that was 40 km across and 2 km thick. In the simulations, porosity and permeability decrease with depth to account for sediment compaction. Dolomitization of a platform consisting of medium grained (∼0.05 mm) sediments occurred in a broad band ranging from ∼2.5 km depth near the margin to ∼1.5 km depth near the platform center. The area of dolomitization is deep enough that temperatures exceed ∼50°C but not so deep that low permeabilities restrict mass transport. Complete dolomitization in the center of this zone is estimated to require at least 60 my. Incorporation of permeability contrasts, permeable beds, and reactive beds focused dolomitization strongly and reduced the estimated time required for dolomitization by as much as 50 percent. Dolomitization created magnesium-depleted, calcium-rich fluids in less than 10 ky, and results support a link between dolomitization and anhydrite precipitation where adequate sulfate is available.

American Journal of Science↗

USGS2000

Numerical modelling of geothermal and reflux circulation in Enewetak Atoll: Implications for dolomitization

Two types of regional-scale seawater circulation have been proposed to explain the formation of Enewetak Atoll dolomites: geothermal and reflux circulation. We have used a finite element groundwater flow model to examine the pattern, magnitude and dynamic interaction of these two different circulation mechanisms in Enewetak Atoll. Geothermal circulation is concentrated around the atoll-margin whereas refluxing mesosaline brines flow from the atoll interior towards the margin to restrict and eventually shut off geothermal circulation. Refluxing brines of 36-80??? can account for the salinity signature recorded in dolomite fluid inclusions. Distributions of fluid flux and Mg mass-balance calculations suggest that both geothermal and reflux circulation mechanisms could account for the observed distribution of dolomite in Enewetak Atoll. Furthermore, the atoll interior may be extensively dolomitized as observed in other atolls. (C) 2000 Elsevier Science B.V. All rights reserved.Two types of regional-scale seawater circulation have been proposed to explain the formation of Enewetak Atoll dolomites: geothermal and reflux circulation. We have used a finite element groundwater flow model to examine the pattern, magnitude and dynamic interaction of these two different circulation mechanisms in Enewetak Atoll. Geothermal circulation is concentrated around the atoll-margin whereas refluxing mesosaline brines flow from the atoll interior towards the margin to restrict and eventually shut off geothermal circulation. Refluxing brines of 36-80 per mil can account for the salinity signature recorded in dolomite fluid inclusions. Distributions of fluid flux and Mg mass-balance calculations suggest that both geothermal and reflux circulation mechanisms could account for the observed distribution of dolomite in Enewetak Atoll. Furthermore, the atoll interior may be extensively dolomitized as observed in other atolls.

Conference Paper↗

USGS2000

Geothermal convection: a mechanism for dolomitization at Enewetak Atoll?

Geothermal convection in carbonate platforms could drive massive dolomitization by supplying mass transport of magnesium over long periods and at temperatures high enough to overcome kinetic limitations. Reactive-transport simulations based on Enewetak Atoll show dolomitization in a thin band at a permeability contrast near the base of the platform, which is consistent with field observations of dolomitized Eocene deposits. Dolomitization is predicted at approximately 6% per My at temperatures of 45–60°C, and complete dolomitization could be accomplished in ∼16 My. Calcium enrichment of pore fluids and upward transport of these fluids is established early, prior to 30 ky.

Book↗