TY - JOUR
T1 - On the secular variations in the composition of Phanerozoic marine potash evaporites
AU - Holland, Heinrich D.
AU - Horita, Juske
AU - Seyfried, William E.
PY - 1996/11
Y1 - 1996/11
N2 - In a recent paper, L. A. Hardie proposed that the secular changes in the mineralogy of marine nonskeletal limestones and in the mineralogy of marine potash evaporites during the Phanerozoic are the result of changes in the composition of seawater caused primarily by fluctuations in the flux of seawater through mid-ocean ridges. He suggested that even quite small variations (25%) in this flux have a profound effect on the composition of seawater. We show that the effects of changes in the hydrothermal flux are much smaller than he proposed. Nevertheless, a doubling of the hydrothermal flux could probably alter the composition of seawater sufficiently to affect the mineralogy of marine potash evaporites. An alternative explanation for the variations in the mineralogy of these deposits invokes differences in the degree of dolomitization during the evaporation of seawater. The apparent near constancy of the K+ content of seawater during the Phanerozoic supports this hypothesis. It seems likely that during periods of rapid seafloor spreading the rate of seawater cycling is greater than today, that sea level is higher, that very large carbonate platforms are more abundant, and that changes in seawater chemistry together with increases in dolomitization during the evaporation of seawater lead to MgSO4-free potash deposits. The relative importance of these several effects can probably be determined with certainty only when we have analyses of relatively unaltered seawater from the several Phanerozoic periods.
AB - In a recent paper, L. A. Hardie proposed that the secular changes in the mineralogy of marine nonskeletal limestones and in the mineralogy of marine potash evaporites during the Phanerozoic are the result of changes in the composition of seawater caused primarily by fluctuations in the flux of seawater through mid-ocean ridges. He suggested that even quite small variations (25%) in this flux have a profound effect on the composition of seawater. We show that the effects of changes in the hydrothermal flux are much smaller than he proposed. Nevertheless, a doubling of the hydrothermal flux could probably alter the composition of seawater sufficiently to affect the mineralogy of marine potash evaporites. An alternative explanation for the variations in the mineralogy of these deposits invokes differences in the degree of dolomitization during the evaporation of seawater. The apparent near constancy of the K+ content of seawater during the Phanerozoic supports this hypothesis. It seems likely that during periods of rapid seafloor spreading the rate of seawater cycling is greater than today, that sea level is higher, that very large carbonate platforms are more abundant, and that changes in seawater chemistry together with increases in dolomitization during the evaporation of seawater lead to MgSO4-free potash deposits. The relative importance of these several effects can probably be determined with certainty only when we have analyses of relatively unaltered seawater from the several Phanerozoic periods.
UR - http://www.scopus.com/inward/record.url?scp=0030433487&partnerID=8YFLogxK
U2 - 10.1130/0091-7613(1996)024<0993:OTSVIT>2.3.CO;2
DO - 10.1130/0091-7613(1996)024<0993:OTSVIT>2.3.CO;2
M3 - Article
AN - SCOPUS:0030433487
SN - 0091-7613
VL - 24
SP - 993
EP - 996
JO - Geology
JF - Geology
IS - 11
ER -