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Oxygen and hydrogen isotopes of waters in sedimentary basins with emphasis on the Illinois Basin waters
134
Abstracts
hydrocarbon deposits. The explanation ofa caprock holding down both types of gas is inadequate, because there are many regions devoid of substantial hydrocarbon deposits where equally good caprocks would have to be expected to hold down high He concentrations. This is not found. The transport of gases through the solid crust requires sufficiently abundant fluids to fracture the rock and hold open the porosity created. The abundance of He is nowhere sufficiently high to achieve this. Therefore, its transport is always dependent on another more abundant fluid, and the indications we have are that CH4 and N 2 are the most common transporting agents. Where high 3He/4He ratios indicate a primordial source, this then implies that the flushing process in the rocks concerned is of recent origin. Where flushing has been going on for a long time, the primordial He has been removed and only radiogenic He is available. On this basis, the 3He/4He ratio is mostly an indicator of the age of the pathway. Even mantle He may show a low ratio if it comes from an old pathway. Study of uranium series disequilibrium in cores of crystalline rocks from URL, Pinawa, Canada. M. IVANOVICH,Nuclear Physics Division, Building 7, Atomic Energy Research Establishment, Harwell, Oxon O X l l 0RA, U.K.
SOLUTIONS and fluids migrating through fractures in the Lac du Bonnet granite, Manitoba, Canada, have interacted with the rock for over 2.5 Ga and the penetration of alteration effects into the rock matrix varies from about 10 cm around small single fractures to several metres around large or composite fractures. U p to 30% depletion of 234U relative to parent 23sU has been observed in the most altered parts of the three cores under investigation by Gascoyne and Cramer (this conference), indicating leaching of "~34 "- U within the last one million years although the 231~Fh/234U and 231pa/235U activity ratios are close to secular equilibrium value of unity throughout the profiles. To explain the radiometric data obtained, more than one alteration process is required. The most recent low temperature alteration is probably superimposed on an older hydrothermal alteration. Evidence for Ra isotope mobility in this rock-water system is apparent from the measurement of 22SRa/228Th and 226Ra/23°Th activity ratios in the core material. Current work consists of the U-series disequilibrium measurement in different mineralogical phases of both altered and unaltered rock in order to establish which mineralogical phases are the most affected by these alteration processes and to what degree. Such an approach may yield information on the nature and detailed chronology of the responsible geochemical processes. Isotope geochemistry of groundwater in the Four Corners area, Utah, U.S.A. BRYANT A. K~MBALL, U.S. Geological Survey, Denver, CO, U.S.A.
STABLE isotope ratios can be used to study mixing and chemical reactions among groundwaters of different sources. To help determine sources of saline water in the aquifer of the Mesozoic Navajo Sandstone, 12 groundwater samples were obtained from Mesozoic aquifers, along a flow path of nearly 80 km (50 miles) in the Four Corners area of Utah. Possible sources of the saline water are: (1) upward leakage of brines that originated from the underlying middle Paleozoic aquifer; (2) downward leakage of saline water from the overlying Mesozoic Carmel Formation; or (3) reinjection of oilfield brines from the Paleozoic aquifers. Hydraulic-head data indicate that water moves downward from Mesozoic aquifers to Paleozoic aquifers, rather than upward from Paleozoic aquifers to Mesozoic aquifers. Thus, reinjection would be the only means of introducing water from Paleozoic aquifers into the Navajo Sandstone.
These 12 groundwater samples represent 3 different water types: (1) dilute recharge water; (2) diagenetic water that has evolved from the recharge water through chemical reactions; and (3) saline waters. Spatial changes in chemistry and isotope ratios of the water help identify the source of the saline water as the Middle Paleozoic aquifer. 34S values in the aquifer vary from - 1 0 to +10%o, progressively approaching the value of +12.4%o present in salts of the Paradox Member of the Paleozoic Hermosa Formation. Deuterium and lSo also indicate mixing of local recharge water, having the isotopic composition of modern precipitation, with deeper basin waters. Geochemical reaction models, constructed to account for changes in water chemistry along the flow path, were verified by independent calculations using stable isotope ratios. The calculations indicate that the largest chemical change along the flow path is caused by mixing of water from the Mesozoic and the Paleozoic aquifers and not by reactions with aquifer minerals. Corrected 14C ages of groundwater moving along the flow paths indicate that velocities of the groundwater are about 0.3 cm/d (0.01 ft/d), which indicates a hydraulic conductivity of about 3 m/d (1 ft/d). This value of hydraulic conductivity is similar to that determined from computer simulations of groundwater flow in the same area. Groundwater flow around a mine site as reflected by isotopic and geochemical data. I. MACDONALD, S. K. FRAPE and P. FRITZ, Department of Earth Science, University of Waterloo, Waterloo, Ontario, Canada.
AN INVESTmATIONof groundwaters occurring in felsic-dominated rock environments was undertaken to determine if geochemical processes were different in felsic and marie environments. Fresh and brackish waters dominated at most sites with ~so and 2H data close to the Meteoric Water Line (MWL). Deviation from the MWL occurred only where saline waters or brines were encountered. All brines had a Ca-Na-C1 composition. The data indicate that geochemical processes were dependent on rock type, as long as groundwater salinities remained low, hut that both felsic and marie systems evolved towards the same Ca-Na-CI end-member brine. Sampling at the Con Mine, Yellowknife, NWT, over a five-year period has allowed for the observation of how mining practices have affected previously undisturbed water regimes. Total dissolved solids (TDS) contents have decreased in some boreholes with each succeeding sampling, but TDS increases and no change in TDS have also been observed. Although these TDS results have a direct correlation to the isotopic changes, the overall geochemical composition of the brines has not changed. Stable isotopes and major element geochemistry indicated that a great deal of groundwater mixing has occurred between present-day surface waters, present-day shallow groundwaters, present-day deep brines, and older, colder climate, groundwaters. Initial data suggested that the mine was acting as a sink, funnelling fresh, tritiated waters down to mix with deep brine. More recent data indicate that shallow, non-tritiated groundwaters may also be mixing with the brines. Mixing of two isotopically different brines and isolated pockets of water that have undergone no mixing are also possible. Oxygen and hydrogen isotopes of waters in sedimentary basins with emphasis on the Illinois Basin waters. H. W. NESBtTr, Department of Geology, University of Western Ontario, London, Ontario N6A 5B7, Canada
THE bD AND blSo compositions of subsurface waters of sedimentary basins often are strongly and positively correlated with salinites, as shown for the Illinois Basin. The subsurface waters react, and attain equilibrium, with common minerals of the basins, including calcite and clay miner-
Abstracts als of the kandite, illite and smectite groups. Reaction with these minerals controls the compositions of the subsurface waters of the Illinois Basin and probably the Michigan and Alberta Basins. The isotopic compositions of the waters are also influenced by these reactions. Although Clayton and his co-workers conclude little about the H isotopes, they argue that the shift to heavy O isotopic values results from reaction with calcite, the O shift reflecting the temperature at which the water and carbonate reacted. There is, however, significant reaction with the clay minerals and 6D and b180 values undoubtedly have been affected by the reactions. The postulate is supported by laboratory studies, where NaCl-bearing waters have been passed through clay membranes. At low temperatures, the residual solutions were enriched in the heavy isotopes and total dissolved solids relative to the starting solutions, and relative to the solutions that passed through the clay membrane (filtrates). At higher temperatures (140-180°C), however, the residual solutions are isotopically lighter than the filtrate. Coplen and Hanshaw emphasize that exchange of isotopes between the waters and clay minerals of the clay filter is the principal reason for the isotopic shifts. The complexities inherent in the interpretation of O isotopes in "open systems" during diagenesis and low-grade metamorphism are apparent from these data. The results are important to the interpretation of subsurface waters, d JSO values of solutions generally change during reaction with silicate minerals (regardless of the type of mineral), and hence may not be particularly diagnostic of geological setting. Of the common minerals, only the clay minerals and micas can substantially influence H isotopic ratios. The clay minerals are a large reservoir for H isotopes and they are very reactive, hence, even at low rock-water ratios, the clay minerals may influence D values of the waters. Furthermore, clay minerals are abundant in rocks of sedimentary basins, whereas they are scarce in igneous and metamorphic rocks. These aspects make D/H ratios a likely tool for assessing the genesis of subsurface waters, and in this regard probably a more useful tool than O isotopes. Frape and Fritz argue that the stable isotope characteristics of Precambrian-hosted brines result from mixing of two water types, dilute meteoric water and CaCl-rich brines of uncertain origin. They present convincing evidence that the CaC1 brines have interacted extensively, at relatively low temperatures, with the host rocks. Kelly and co-workers argue, but do not prove, that the waters were once sedimentary basin brines. The suggestion (previously considered) does not provide an obvious explanation for the compositions of the Precambrian-hosted brines and must remain a possibility rather than a likely explanation. Additional study and experiments of isotopic exchange with hydrated minerals, at different temperatures, are required if a generally accepted explanation is to be offered for the isotopic compositional trends of the Precambrian-hosted waters.
135
used (e.g. Rb-Sr, K-At, Ar-Ar, U-Th-Pb, U-series) can be applied to this problem providing that suitable parentenriched minerals formed in the alteration zones, or that recrystallization of existing minerals and/or diffusion has resulted in loss of previously accumulated daughter isotopes. Some secondary minerals may be highly enriched in the daughter isotope of a particular radioisotope system (e.g. Sr in calcite) and depleted in, or devoid of, the parent isotope. The amount of the unsupported daughter isotope may yield age information if certain assumptions regarding extent and degree of isotopic equilibration with the enclosing host rock can be made with confidence. Correlating an isotopic age with a specific alteration event can be fraught with uncertainty. The isotope systems of low-temperature minerals in alteration zones can be easily perturbed by subsequent cryptic thermal or tectonic overprinting. Ductile shear zones are likely to be reactivated at higher levels resulting in a complex protracted history of movement, and the isotope systems of early-formed minerals may be partially or totally reset during continued movement in the brittle zone. Any age attributed to a particular event should be verified by independent methods. Concordancy would suggest a discrete event, whereas discordant results are generally ambiguous, and at best provide only time limits. Rarely is this sort of independent verification possible and more often it is necessary to argue an interpretation on the basis of internal consistency within data sets and external reasonableness among data sets. Some recent technological advances such as the ion-probe and the laserprobe mass spectrometers offer considerable promise in improving our ability to credibly date alteration events in crystalline rocks.
Isotopic composition of uranium and thorium in crystalline rocks. JOHN N. ROSHOLT, U.S. Geological Survey, Denver Federal Center, Denver, CO 80225, U.S.A. THE 238U-234U-23°TH-232TH SYSTEMhas been investigated in 84 silicic crystalline rocks obtained from drill cores, surface, and near-surface samples in California, Wyoming, Colorado, and Illinois. Results of these analyses displaye~d on ternary diagrams with apexes for 23su, 234U, and 23°Th indicate five predominant geochemical processes that affected U in the rock: (1) bulk U leaching where 238U and 234U were removed with little or no fractionation; (2) preferential 234U leaching by alpha recoil displacement (234U recoil loss) with lesser 238Uloss; (3) 234Urecoil loss with little or no 23su loss; (4) U assimilation where both 23su and 234U were added with present-day234/238U activity ratios varying from 0.8 to 1.2; and (5) addition of 234U and 23°Th by daughter emplacement processes (234Uq- 23°Th recoil gain). Evidence for the existence of 234U and 23~I'h recoil gain in rocks is the most important finding of this investigation. Radioactive disequilibrium occurs in the majority of rocks analysed where 234U recoil loss is the predominant process associated with incipient weathering; U assimilation and Isotopic dating of alteration events in crystalline rocks. ZELL 234U + 23°Th recoil gain occur under conditions of substanE. PETERMANand JOHN N. ROSHOLT, U.S. Geological Sur- tial water penetration along fractures and into weathered vey, MS 963, Box 25046 DPC, Denver, CO 80226, U.S.A. zones in the rocks. Relatively unfractured and petrographiMost mNEOUS and metamorphic crystalline rocks residing cally fresh rocks from the UPH-3 drill hole in northern in the upper part of the Earth's crust have been imprinted by Illinois are closest to being in radioactive equilibrium for numerous secondary events that are manifested in ductile any suite of rocks included in this study, and they demonshear zones, brittle fractures, joints, and mineralogic altera- strate that equilibrium during the last 0.5 Ma can be maintion associated with cooling and with ingress of fluids into tained over a substantial vertical distance where there has fractures and joints at high crustal levels. Relative dating of been little or no movement of water in the basement rock. alteration events can be done quite successfully through The 238U-234U-23°Th system can be a sensitive indicator of detailed field and petrographic studies. Calibration of this geologically recent U mobility and rock-water interaction relative alteration sequence by isotopic dating is a worthy both in petrographically fresh core samples and in "sealed" goal, but one that can be burdened with failed assumptions, fracture zones. [Abstract taken from J. Geophys. Res. 88, analytical difficulties, and interpretative ambiguities. 7315-7330 (1983).] Ideally, many of the isotope geochronometers currently
Abstracts
hydrocarbon deposits. The explanation ofa caprock holding down both types of gas is inadequate, because there are many regions devoid of substantial hydrocarbon deposits where equally good caprocks would have to be expected to hold down high He concentrations. This is not found. The transport of gases through the solid crust requires sufficiently abundant fluids to fracture the rock and hold open the porosity created. The abundance of He is nowhere sufficiently high to achieve this. Therefore, its transport is always dependent on another more abundant fluid, and the indications we have are that CH4 and N 2 are the most common transporting agents. Where high 3He/4He ratios indicate a primordial source, this then implies that the flushing process in the rocks concerned is of recent origin. Where flushing has been going on for a long time, the primordial He has been removed and only radiogenic He is available. On this basis, the 3He/4He ratio is mostly an indicator of the age of the pathway. Even mantle He may show a low ratio if it comes from an old pathway. Study of uranium series disequilibrium in cores of crystalline rocks from URL, Pinawa, Canada. M. IVANOVICH,Nuclear Physics Division, Building 7, Atomic Energy Research Establishment, Harwell, Oxon O X l l 0RA, U.K.
SOLUTIONS and fluids migrating through fractures in the Lac du Bonnet granite, Manitoba, Canada, have interacted with the rock for over 2.5 Ga and the penetration of alteration effects into the rock matrix varies from about 10 cm around small single fractures to several metres around large or composite fractures. U p to 30% depletion of 234U relative to parent 23sU has been observed in the most altered parts of the three cores under investigation by Gascoyne and Cramer (this conference), indicating leaching of "~34 "- U within the last one million years although the 231~Fh/234U and 231pa/235U activity ratios are close to secular equilibrium value of unity throughout the profiles. To explain the radiometric data obtained, more than one alteration process is required. The most recent low temperature alteration is probably superimposed on an older hydrothermal alteration. Evidence for Ra isotope mobility in this rock-water system is apparent from the measurement of 22SRa/228Th and 226Ra/23°Th activity ratios in the core material. Current work consists of the U-series disequilibrium measurement in different mineralogical phases of both altered and unaltered rock in order to establish which mineralogical phases are the most affected by these alteration processes and to what degree. Such an approach may yield information on the nature and detailed chronology of the responsible geochemical processes. Isotope geochemistry of groundwater in the Four Corners area, Utah, U.S.A. BRYANT A. K~MBALL, U.S. Geological Survey, Denver, CO, U.S.A.
STABLE isotope ratios can be used to study mixing and chemical reactions among groundwaters of different sources. To help determine sources of saline water in the aquifer of the Mesozoic Navajo Sandstone, 12 groundwater samples were obtained from Mesozoic aquifers, along a flow path of nearly 80 km (50 miles) in the Four Corners area of Utah. Possible sources of the saline water are: (1) upward leakage of brines that originated from the underlying middle Paleozoic aquifer; (2) downward leakage of saline water from the overlying Mesozoic Carmel Formation; or (3) reinjection of oilfield brines from the Paleozoic aquifers. Hydraulic-head data indicate that water moves downward from Mesozoic aquifers to Paleozoic aquifers, rather than upward from Paleozoic aquifers to Mesozoic aquifers. Thus, reinjection would be the only means of introducing water from Paleozoic aquifers into the Navajo Sandstone.
These 12 groundwater samples represent 3 different water types: (1) dilute recharge water; (2) diagenetic water that has evolved from the recharge water through chemical reactions; and (3) saline waters. Spatial changes in chemistry and isotope ratios of the water help identify the source of the saline water as the Middle Paleozoic aquifer. 34S values in the aquifer vary from - 1 0 to +10%o, progressively approaching the value of +12.4%o present in salts of the Paradox Member of the Paleozoic Hermosa Formation. Deuterium and lSo also indicate mixing of local recharge water, having the isotopic composition of modern precipitation, with deeper basin waters. Geochemical reaction models, constructed to account for changes in water chemistry along the flow path, were verified by independent calculations using stable isotope ratios. The calculations indicate that the largest chemical change along the flow path is caused by mixing of water from the Mesozoic and the Paleozoic aquifers and not by reactions with aquifer minerals. Corrected 14C ages of groundwater moving along the flow paths indicate that velocities of the groundwater are about 0.3 cm/d (0.01 ft/d), which indicates a hydraulic conductivity of about 3 m/d (1 ft/d). This value of hydraulic conductivity is similar to that determined from computer simulations of groundwater flow in the same area. Groundwater flow around a mine site as reflected by isotopic and geochemical data. I. MACDONALD, S. K. FRAPE and P. FRITZ, Department of Earth Science, University of Waterloo, Waterloo, Ontario, Canada.
AN INVESTmATIONof groundwaters occurring in felsic-dominated rock environments was undertaken to determine if geochemical processes were different in felsic and marie environments. Fresh and brackish waters dominated at most sites with ~so and 2H data close to the Meteoric Water Line (MWL). Deviation from the MWL occurred only where saline waters or brines were encountered. All brines had a Ca-Na-C1 composition. The data indicate that geochemical processes were dependent on rock type, as long as groundwater salinities remained low, hut that both felsic and marie systems evolved towards the same Ca-Na-CI end-member brine. Sampling at the Con Mine, Yellowknife, NWT, over a five-year period has allowed for the observation of how mining practices have affected previously undisturbed water regimes. Total dissolved solids (TDS) contents have decreased in some boreholes with each succeeding sampling, but TDS increases and no change in TDS have also been observed. Although these TDS results have a direct correlation to the isotopic changes, the overall geochemical composition of the brines has not changed. Stable isotopes and major element geochemistry indicated that a great deal of groundwater mixing has occurred between present-day surface waters, present-day shallow groundwaters, present-day deep brines, and older, colder climate, groundwaters. Initial data suggested that the mine was acting as a sink, funnelling fresh, tritiated waters down to mix with deep brine. More recent data indicate that shallow, non-tritiated groundwaters may also be mixing with the brines. Mixing of two isotopically different brines and isolated pockets of water that have undergone no mixing are also possible. Oxygen and hydrogen isotopes of waters in sedimentary basins with emphasis on the Illinois Basin waters. H. W. NESBtTr, Department of Geology, University of Western Ontario, London, Ontario N6A 5B7, Canada
THE bD AND blSo compositions of subsurface waters of sedimentary basins often are strongly and positively correlated with salinites, as shown for the Illinois Basin. The subsurface waters react, and attain equilibrium, with common minerals of the basins, including calcite and clay miner-
Abstracts als of the kandite, illite and smectite groups. Reaction with these minerals controls the compositions of the subsurface waters of the Illinois Basin and probably the Michigan and Alberta Basins. The isotopic compositions of the waters are also influenced by these reactions. Although Clayton and his co-workers conclude little about the H isotopes, they argue that the shift to heavy O isotopic values results from reaction with calcite, the O shift reflecting the temperature at which the water and carbonate reacted. There is, however, significant reaction with the clay minerals and 6D and b180 values undoubtedly have been affected by the reactions. The postulate is supported by laboratory studies, where NaCl-bearing waters have been passed through clay membranes. At low temperatures, the residual solutions were enriched in the heavy isotopes and total dissolved solids relative to the starting solutions, and relative to the solutions that passed through the clay membrane (filtrates). At higher temperatures (140-180°C), however, the residual solutions are isotopically lighter than the filtrate. Coplen and Hanshaw emphasize that exchange of isotopes between the waters and clay minerals of the clay filter is the principal reason for the isotopic shifts. The complexities inherent in the interpretation of O isotopes in "open systems" during diagenesis and low-grade metamorphism are apparent from these data. The results are important to the interpretation of subsurface waters, d JSO values of solutions generally change during reaction with silicate minerals (regardless of the type of mineral), and hence may not be particularly diagnostic of geological setting. Of the common minerals, only the clay minerals and micas can substantially influence H isotopic ratios. The clay minerals are a large reservoir for H isotopes and they are very reactive, hence, even at low rock-water ratios, the clay minerals may influence D values of the waters. Furthermore, clay minerals are abundant in rocks of sedimentary basins, whereas they are scarce in igneous and metamorphic rocks. These aspects make D/H ratios a likely tool for assessing the genesis of subsurface waters, and in this regard probably a more useful tool than O isotopes. Frape and Fritz argue that the stable isotope characteristics of Precambrian-hosted brines result from mixing of two water types, dilute meteoric water and CaCl-rich brines of uncertain origin. They present convincing evidence that the CaC1 brines have interacted extensively, at relatively low temperatures, with the host rocks. Kelly and co-workers argue, but do not prove, that the waters were once sedimentary basin brines. The suggestion (previously considered) does not provide an obvious explanation for the compositions of the Precambrian-hosted brines and must remain a possibility rather than a likely explanation. Additional study and experiments of isotopic exchange with hydrated minerals, at different temperatures, are required if a generally accepted explanation is to be offered for the isotopic compositional trends of the Precambrian-hosted waters.
135
used (e.g. Rb-Sr, K-At, Ar-Ar, U-Th-Pb, U-series) can be applied to this problem providing that suitable parentenriched minerals formed in the alteration zones, or that recrystallization of existing minerals and/or diffusion has resulted in loss of previously accumulated daughter isotopes. Some secondary minerals may be highly enriched in the daughter isotope of a particular radioisotope system (e.g. Sr in calcite) and depleted in, or devoid of, the parent isotope. The amount of the unsupported daughter isotope may yield age information if certain assumptions regarding extent and degree of isotopic equilibration with the enclosing host rock can be made with confidence. Correlating an isotopic age with a specific alteration event can be fraught with uncertainty. The isotope systems of low-temperature minerals in alteration zones can be easily perturbed by subsequent cryptic thermal or tectonic overprinting. Ductile shear zones are likely to be reactivated at higher levels resulting in a complex protracted history of movement, and the isotope systems of early-formed minerals may be partially or totally reset during continued movement in the brittle zone. Any age attributed to a particular event should be verified by independent methods. Concordancy would suggest a discrete event, whereas discordant results are generally ambiguous, and at best provide only time limits. Rarely is this sort of independent verification possible and more often it is necessary to argue an interpretation on the basis of internal consistency within data sets and external reasonableness among data sets. Some recent technological advances such as the ion-probe and the laserprobe mass spectrometers offer considerable promise in improving our ability to credibly date alteration events in crystalline rocks.
Isotopic composition of uranium and thorium in crystalline rocks. JOHN N. ROSHOLT, U.S. Geological Survey, Denver Federal Center, Denver, CO 80225, U.S.A. THE 238U-234U-23°TH-232TH SYSTEMhas been investigated in 84 silicic crystalline rocks obtained from drill cores, surface, and near-surface samples in California, Wyoming, Colorado, and Illinois. Results of these analyses displaye~d on ternary diagrams with apexes for 23su, 234U, and 23°Th indicate five predominant geochemical processes that affected U in the rock: (1) bulk U leaching where 238U and 234U were removed with little or no fractionation; (2) preferential 234U leaching by alpha recoil displacement (234U recoil loss) with lesser 238Uloss; (3) 234Urecoil loss with little or no 23su loss; (4) U assimilation where both 23su and 234U were added with present-day234/238U activity ratios varying from 0.8 to 1.2; and (5) addition of 234U and 23°Th by daughter emplacement processes (234Uq- 23°Th recoil gain). Evidence for the existence of 234U and 23~I'h recoil gain in rocks is the most important finding of this investigation. Radioactive disequilibrium occurs in the majority of rocks analysed where 234U recoil loss is the predominant process associated with incipient weathering; U assimilation and Isotopic dating of alteration events in crystalline rocks. ZELL 234U + 23°Th recoil gain occur under conditions of substanE. PETERMANand JOHN N. ROSHOLT, U.S. Geological Sur- tial water penetration along fractures and into weathered vey, MS 963, Box 25046 DPC, Denver, CO 80226, U.S.A. zones in the rocks. Relatively unfractured and petrographiMost mNEOUS and metamorphic crystalline rocks residing cally fresh rocks from the UPH-3 drill hole in northern in the upper part of the Earth's crust have been imprinted by Illinois are closest to being in radioactive equilibrium for numerous secondary events that are manifested in ductile any suite of rocks included in this study, and they demonshear zones, brittle fractures, joints, and mineralogic altera- strate that equilibrium during the last 0.5 Ma can be maintion associated with cooling and with ingress of fluids into tained over a substantial vertical distance where there has fractures and joints at high crustal levels. Relative dating of been little or no movement of water in the basement rock. alteration events can be done quite successfully through The 238U-234U-23°Th system can be a sensitive indicator of detailed field and petrographic studies. Calibration of this geologically recent U mobility and rock-water interaction relative alteration sequence by isotopic dating is a worthy both in petrographically fresh core samples and in "sealed" goal, but one that can be burdened with failed assumptions, fracture zones. [Abstract taken from J. Geophys. Res. 88, analytical difficulties, and interpretative ambiguities. 7315-7330 (1983).] Ideally, many of the isotope geochronometers currently