Uptake of Ammonium By Potassium-Bearing Silicates in
Guaymas Basin Hydrothermal
System, Gulf of California
Ann E. Sturdivant
Potassium (as K2O) and fixed (lattice) and exchangeable (surface)
ammonium (NH4+) content of sediment samples from the
Guaymas basin were measured to determine the relationship between poor fluid
composition and the chemical compositions of potassium-bearing minerals from an
active hydrothermal
system.
In the Guaymas basin, NH4+ is generated and concentrated in the pore fluid during thermal maturation and biological consumption of organic matter (up to 3.3% total organic carbon in the sediment). The NH4+ is then available for ion substitution into lattice sites otherwise occupied by the potassium ion (K+) in clays.
The pore fluid NH4+ content was found to correspond with either the fixed or exchangeable NH4+ content in the sediment. Other factors such as total organic carbon content, clay mineralogy, and variable permeability may affect this relationship. Total NH4+ content is as high as 1,779 ppm in the Guaymas basin sediment. No apparent relationship exists between pore fluid K* and sediment K2O content. No evidence has been found for a relationship between NH4+ in the sediment and K+ in the pore fluid. This implies that a complex series of chemical reactions is occurring, and the pore fluid and the sediment may not be in equilibrium at this time.
Fluid-mineral relationships such as those in the Guaymas basin may be used to
infer similar relationships between paleofluids and sediments from fossil
hydrothermal
systems. This information could be of economic importance because
metallic mineral concentrations are often formed by
hydrothermal
activity, and
NH4+ anomalies could be used as an indicator of proximity
to ore deposits.
AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.