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Loss of uranium from crystallized silicic volcanic rocks
EARTH AND PLANETARY SCIENCE LETTERS 6 (1969) 268-270. NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM
LOSS OF URANIUM FROM CRYSTALLIZED
SILICIC VOLCANIC
ROCKS
*
J.N.ROSHOLT U.S. GeologicalSurvey, Denver, Colorado, USA and
D.C.NOBLE Department
of Geological Sciences, Harvard University, Cam bridge, Massachusetts, USA
Received 20 May 1969
Dense primarily crystallized silicic groundmass material from two welded ash-flow units and one lava flow of Tertiary age from the Western United States contain only 20 to 60 percent of the uranium present in nonhydrated glass from the rock units. These differences reflect loss of uranium from the crystallized specimens, probably as a result of groundwater leaching. It appears likely that most crystallized silicic volcanic rocks have lost comparable amounts of uranium.
1. INTRODUCTION It is well known that uranium is extremely mobile in near-surface environments. We have investigated the process of postdepositional mobilization of uranium in silicic volcanic rocks by directly comparing the umaium contents of nonhydrated glass and dense primarily crystallized groundmass material from two welded ash-flow tuff units and one lava unit, all of Tertiary age, from the Western United States.
2. SPECIMENTS STUDIED 2.1. Nonhydrated glass The glass is from the quickly cooled basal vitrophyres of the ash-flow units and from the marginal glassy zone of the lava. The nonhydrated glass occurs as relict “obsidian cores” surrounded by secondarily hydrated glass [ 1,2,3] . The compositions of the non* Publication authorized by the Director, U.S. Geological Survey.
hydrated glass have not changed since deposition: inasmuch as water has not migrated into the glass, it would appear most unlikely that uranium, thorium, and other elements have migrated into or out of the glass. This conclusion is corroborated by the fact that nonhydrated glasses are in equilibrium with respect to 238U-234U-230Th decay and 235U-231Pa decay [4,5,6] , by oxygen and hydrogen isotope data [7,8], and by the identical alkali and alkaline earth, contents of nonhydrated glass from various parts of the same rock unit [9, lo] . 2.2. Crystallized specimens The crystallized specimens are all dense (nonporous), although specimens WPN-109J and WPN-21B contain lenticular gas cavities [ 1 l] . AU the specimens are primarily crystallized, and they crystallized at elevated, but subsolidus, temperatures during the initial cooling of the rock mass [ 121. Some of the specimens are devitrified, containing cristobalite as the silica polymorph; others are granophyrically crystallized, with primary quartz in the groundmass [ 131 . The specimens are all extremely fresh appearing and
LOSS OF URANIUM
269
Table 1 Sample descriptions and analytical data. Sample number
Rock type
U (ppm) *
Th (ppm) *
234U/238U (activity ratio)
Fe+3/Fe+2 + Fe+3 (atomic ratio)
Spearhead Member of the Thirsty Canyon Tuff (Pliocene), southern Nevada (comendite ash-flow tuff) [ 14] Ttsu-035
Nonhydrated glass separate Densely welded devitrified tuff Calculated groundm ass composition **
5.31 + 0.09 3.18 +--0.05 3.38
23.5 + 0.5 19.8 + 0.4 21.1
0.998 + 0.005 1.005 + 0.005
0.44 0.87
Grouse Canyon Member (lower parO of the Belted Range Tuff (Miocene) southern Nevada (aphyric comendite ash-flow tuff) [ 15] WPN-23 WPN-109J BG-UN WPN-21B
Densely welded nonhyd~rated glassy tuff Densely welded granophyrically crystallized tuff Densely welded devitrigied tuff Densely welded devitrified tuff
5.47 + 0.09 1.35 +0.02
21.8 + 0.5 19.5 +0.4
1.005 + 0.01 0.85 + 0.01
0.46 0.61
2.03 -+0.03 1.01 -+0.02
22.9 + 0.5 21.7 + 0.5
0.94 +-0.01 0.98 + 0.01
1.0
Nathrop Volcanics (Oligocene), Colorado (aphyric rhyolite lava) [ 16, 17] RM-I RM-2
Dense nonhydrated glassy lava Dense devitrified lava
15.3 + 0.2 6.66 + 0.11
37.7 +0.8 35.0 +-0.8
1.001 + 0.005 0.892 + 0.005
0.57 0.77
* The estimated analytical errors are two sigma values. ** Contains 6 percent phenocrysts, mainly sanidine but contains small amounts of quartz and sodie plagioclase and mafic minerais. The calculated groundmass composition assumes that all the U and Th is present in the groundmass.
show no megascopically or microscopically discernible evidence of alteration. Field relations, nearly identical thorium contents, and a wide variety o f other major- and minor-element data show that, originally, the crystallized and glassy specimens were virtually identical in chemical composition.
3. A N A L Y T I C A L DATA AND DISCUSSION Selected analytical data are given in table 1. Uranium and thorium concentrations were determined by mass spectrometry using isotope dilution techniques [18] ; the material used for 230Th and 235U spikes was furnished on loan from the U.S. A t o m i c Energy Commission. The 234U/238Uratios were determined by mass spectrometry measurements of 234U/235 U and 238U/235U. The uranium and thorium contents o f the nonhydrated glasses from the Grouse Canyon Member o f
the Belted Range Tuff and the Spearhead Member o f the Thirsty Canyon Tuff are similar to those o f other obsidians from the Western United States [6, 19]. The Ruby Mountain glass is unusually rich in uranium. The thorium content o f the nonhydrated glass and o f the crystallized specimens is identical or nearly identical within the limits o f analytical error. The uranium content o f the crystallized specimens, however, is much less than that o f the comparable nonhydrated glasses. Since all the evidence indicates that the uranium content o f the nonhydrated glass represents the original uranium content o f the rock units, we conclude that uranium has been lost by the crystallized groundmass material. The present data do not show whether most o f the uranium was lost during the period o f crystallization and cooling or whether it was lost at some later time. We suspect, however, that most o f the uranium lost from the rocks was leached by groundwater from accessible intergranular positions soon after the rocks cooled, but that some leaching continued over an
270
J.N.ROSHOULT and D.C.NOBLE
extended period o f time. Except for specimen Ttsu-D, there is little evidence o f more recent uranium leaching based on 230Th/234U and 231pa/235U ratios [20] in the samples [6]. In almost all cases, ferrous iron is partly or completely oxidized during the primary cr)~stallization o f silicic volcanic rocks [ 10, 21], apparently as the result o f dissociation o f magmatic water with attendant loss o f gaseous hydrogen from the rock system. Three o f the specimens o f crystallized rock have a significant 234U deficiency with respect to 238U (table 1); Fe+3/(Fe +2 + Fe +3) ratios indicate that those three are the least oxidized o f the crystallized specimens. Additional isotopic analyses and other data are required to fully evaluate the significance o f oxidation during primary crystallization on the postcooling behavior o f uranium and its daughter products.
4. CONCLUSIONS Direct comparison o f nonhydrated glassy and dense very fresh-appearing primarily crystallized groundmass material from three silicic rock units of Tertiary age shows that the crystalhzed specimens have lost from 40 to 80 percent o f the uranium originally present in the rocks. This loss probably was caused by groundwater leaching which was facilitated by oxidation o f part o f all o f the uranium during crystallization. A reasonable conclusion is that many, and probably most, crystallized silicic and intermediate volcanic rocks o f Cenozoic age have been affected in a similar manner.
REFERENCES [1] C.S.Ross and R.L.Smith, Water and other volatiles in volcanic glasses, Am. Mineral. 40 (1955) 1071. [2] 1.Friedman, R.L.Smith and W.D.Long, Hydration of natural glass and formation of perlite, Geol. Soc. Am. Bull. 77 (1966) 323.
[3 ] D.C.Noble, Stress-corrosion failure and the hydration of glassy silicic rocks, Am. Mineral. 53 (1968) 1756. [4] J.N.Rosholt and B.J.Szabo, Determination of protactinium by neutron activation and alpha spectrometry, Prec. Modern Trends in Activation Analysis, 3d Internat. Conf., in press. [5] J.N.Rosholt and B.J.Szabo, unpublished data. [6] J.N.Rosholt and D.C.Noble, unpublished data. [7] I.Friedman and R.L.Smith, The deuterium content of water in some volcanic glasses, Geochim. Cosmochim. Acta 15 (1959) 218. [8] H.P.Taylor, The oxygen isotope geochemistry of igneous rocks, Contr. Mineral. Petrol. 19 (1968) 1. [9] D.C.Noble, J.Haffty and C.E.Hedge, Strontium and magnesium contents of some natural peralkaline silicic glasses and their petrogenetic significance, Am. J. Sci. 267 (1969) in press. [10] D.C.Noble, unpublished data. [ 11 ] D.C.Noble, Laminar viscous towage structures in ashflow tuff from Gran Canaria, Canary Islands: A discussion, J. Geol. 76 (1968) 721. [12] P.W.Lipman, Chemical comparison of glassy and crystalline volcanic rocks, U.S. Geol. Survey Bull. 1201-D (1965) D1. [13] R.L.Smith, Zones and zonal variations in welded ash flows, U.S. Geol. Survey Prof. Paper 354-F (1960) 149. [14] D.C.Noble, G.D.Bath, R.L.Christiansen and P.P.Orkild, Zonal relations and paleomagnetism of the Spearhead and Rocket Wash Members of the Thirsty Canyon Tuff, southern Nevada, in: Geological survey research 1968, U.S. Geol. Survey Prof. Paper 600-C (1968) C61. [15] D.C.Noble, K.A.Sargent, H.H.Mehnert, E.B.Ekren and F.M.Byers Jr., Silent Canyon volcanic center, Nye County, Nevada, Geol. Soc. Am. Mem. 110 (1968) 65. [16] C.W.Cross, On the occurrence of topaz and garnet in lithophyses of rhyolite, Am. J. Sci. 31 (1886) 432. [17] C.T.Wrucke and D.C.Noble, unpublished data. [18] J.N.Rosholt, B.R.Doe and M.Tatsumoto, Evolution of the isotopic composition of uranium and thorium in soil profiles, Bull. Geol. Soc. Am. 77 (1966) 987. [19] B.R.Doe, The bearing of lead isotopes on the source of g~anitic magma, J. Petrology (1967) 51. [ 20] J.R.Dooley Jr., H.C.Granger and J.N.Rosholt, Uranium234 fractionation in the sandstone-type uranium deposits of the Ambrosia Lake district, New Mexico, Econ. Geol. 61 (1966) 1362. [ 21 ] H.S.Washington, The rhyolites of Lipari, Am. J. Sci., 4th set., 200 (1920) 446.
LOSS OF URANIUM FROM CRYSTALLIZED
SILICIC VOLCANIC
ROCKS
*
J.N.ROSHOLT U.S. GeologicalSurvey, Denver, Colorado, USA and
D.C.NOBLE Department
of Geological Sciences, Harvard University, Cam bridge, Massachusetts, USA
Received 20 May 1969
Dense primarily crystallized silicic groundmass material from two welded ash-flow units and one lava flow of Tertiary age from the Western United States contain only 20 to 60 percent of the uranium present in nonhydrated glass from the rock units. These differences reflect loss of uranium from the crystallized specimens, probably as a result of groundwater leaching. It appears likely that most crystallized silicic volcanic rocks have lost comparable amounts of uranium.
1. INTRODUCTION It is well known that uranium is extremely mobile in near-surface environments. We have investigated the process of postdepositional mobilization of uranium in silicic volcanic rocks by directly comparing the umaium contents of nonhydrated glass and dense primarily crystallized groundmass material from two welded ash-flow tuff units and one lava unit, all of Tertiary age, from the Western United States.
2. SPECIMENTS STUDIED 2.1. Nonhydrated glass The glass is from the quickly cooled basal vitrophyres of the ash-flow units and from the marginal glassy zone of the lava. The nonhydrated glass occurs as relict “obsidian cores” surrounded by secondarily hydrated glass [ 1,2,3] . The compositions of the non* Publication authorized by the Director, U.S. Geological Survey.
hydrated glass have not changed since deposition: inasmuch as water has not migrated into the glass, it would appear most unlikely that uranium, thorium, and other elements have migrated into or out of the glass. This conclusion is corroborated by the fact that nonhydrated glasses are in equilibrium with respect to 238U-234U-230Th decay and 235U-231Pa decay [4,5,6] , by oxygen and hydrogen isotope data [7,8], and by the identical alkali and alkaline earth, contents of nonhydrated glass from various parts of the same rock unit [9, lo] . 2.2. Crystallized specimens The crystallized specimens are all dense (nonporous), although specimens WPN-109J and WPN-21B contain lenticular gas cavities [ 1 l] . AU the specimens are primarily crystallized, and they crystallized at elevated, but subsolidus, temperatures during the initial cooling of the rock mass [ 121. Some of the specimens are devitrified, containing cristobalite as the silica polymorph; others are granophyrically crystallized, with primary quartz in the groundmass [ 131 . The specimens are all extremely fresh appearing and
LOSS OF URANIUM
269
Table 1 Sample descriptions and analytical data. Sample number
Rock type
U (ppm) *
Th (ppm) *
234U/238U (activity ratio)
Fe+3/Fe+2 + Fe+3 (atomic ratio)
Spearhead Member of the Thirsty Canyon Tuff (Pliocene), southern Nevada (comendite ash-flow tuff) [ 14] Ttsu-035
Nonhydrated glass separate Densely welded devitrified tuff Calculated groundm ass composition **
5.31 + 0.09 3.18 +--0.05 3.38
23.5 + 0.5 19.8 + 0.4 21.1
0.998 + 0.005 1.005 + 0.005
0.44 0.87
Grouse Canyon Member (lower parO of the Belted Range Tuff (Miocene) southern Nevada (aphyric comendite ash-flow tuff) [ 15] WPN-23 WPN-109J BG-UN WPN-21B
Densely welded nonhyd~rated glassy tuff Densely welded granophyrically crystallized tuff Densely welded devitrigied tuff Densely welded devitrified tuff
5.47 + 0.09 1.35 +0.02
21.8 + 0.5 19.5 +0.4
1.005 + 0.01 0.85 + 0.01
0.46 0.61
2.03 -+0.03 1.01 -+0.02
22.9 + 0.5 21.7 + 0.5
0.94 +-0.01 0.98 + 0.01
1.0
Nathrop Volcanics (Oligocene), Colorado (aphyric rhyolite lava) [ 16, 17] RM-I RM-2
Dense nonhydrated glassy lava Dense devitrified lava
15.3 + 0.2 6.66 + 0.11
37.7 +0.8 35.0 +-0.8
1.001 + 0.005 0.892 + 0.005
0.57 0.77
* The estimated analytical errors are two sigma values. ** Contains 6 percent phenocrysts, mainly sanidine but contains small amounts of quartz and sodie plagioclase and mafic minerais. The calculated groundmass composition assumes that all the U and Th is present in the groundmass.
show no megascopically or microscopically discernible evidence of alteration. Field relations, nearly identical thorium contents, and a wide variety o f other major- and minor-element data show that, originally, the crystallized and glassy specimens were virtually identical in chemical composition.
3. A N A L Y T I C A L DATA AND DISCUSSION Selected analytical data are given in table 1. Uranium and thorium concentrations were determined by mass spectrometry using isotope dilution techniques [18] ; the material used for 230Th and 235U spikes was furnished on loan from the U.S. A t o m i c Energy Commission. The 234U/238Uratios were determined by mass spectrometry measurements of 234U/235 U and 238U/235U. The uranium and thorium contents o f the nonhydrated glasses from the Grouse Canyon Member o f
the Belted Range Tuff and the Spearhead Member o f the Thirsty Canyon Tuff are similar to those o f other obsidians from the Western United States [6, 19]. The Ruby Mountain glass is unusually rich in uranium. The thorium content o f the nonhydrated glass and o f the crystallized specimens is identical or nearly identical within the limits o f analytical error. The uranium content o f the crystallized specimens, however, is much less than that o f the comparable nonhydrated glasses. Since all the evidence indicates that the uranium content o f the nonhydrated glass represents the original uranium content o f the rock units, we conclude that uranium has been lost by the crystallized groundmass material. The present data do not show whether most o f the uranium was lost during the period o f crystallization and cooling or whether it was lost at some later time. We suspect, however, that most o f the uranium lost from the rocks was leached by groundwater from accessible intergranular positions soon after the rocks cooled, but that some leaching continued over an
270
J.N.ROSHOULT and D.C.NOBLE
extended period o f time. Except for specimen Ttsu-D, there is little evidence o f more recent uranium leaching based on 230Th/234U and 231pa/235U ratios [20] in the samples [6]. In almost all cases, ferrous iron is partly or completely oxidized during the primary cr)~stallization o f silicic volcanic rocks [ 10, 21], apparently as the result o f dissociation o f magmatic water with attendant loss o f gaseous hydrogen from the rock system. Three o f the specimens o f crystallized rock have a significant 234U deficiency with respect to 238U (table 1); Fe+3/(Fe +2 + Fe +3) ratios indicate that those three are the least oxidized o f the crystallized specimens. Additional isotopic analyses and other data are required to fully evaluate the significance o f oxidation during primary crystallization on the postcooling behavior o f uranium and its daughter products.
4. CONCLUSIONS Direct comparison o f nonhydrated glassy and dense very fresh-appearing primarily crystallized groundmass material from three silicic rock units of Tertiary age shows that the crystalhzed specimens have lost from 40 to 80 percent o f the uranium originally present in the rocks. This loss probably was caused by groundwater leaching which was facilitated by oxidation o f part o f all o f the uranium during crystallization. A reasonable conclusion is that many, and probably most, crystallized silicic and intermediate volcanic rocks o f Cenozoic age have been affected in a similar manner.
REFERENCES [1] C.S.Ross and R.L.Smith, Water and other volatiles in volcanic glasses, Am. Mineral. 40 (1955) 1071. [2] 1.Friedman, R.L.Smith and W.D.Long, Hydration of natural glass and formation of perlite, Geol. Soc. Am. Bull. 77 (1966) 323.
[3 ] D.C.Noble, Stress-corrosion failure and the hydration of glassy silicic rocks, Am. Mineral. 53 (1968) 1756. [4] J.N.Rosholt and B.J.Szabo, Determination of protactinium by neutron activation and alpha spectrometry, Prec. Modern Trends in Activation Analysis, 3d Internat. Conf., in press. [5] J.N.Rosholt and B.J.Szabo, unpublished data. [6] J.N.Rosholt and D.C.Noble, unpublished data. [7] I.Friedman and R.L.Smith, The deuterium content of water in some volcanic glasses, Geochim. Cosmochim. Acta 15 (1959) 218. [8] H.P.Taylor, The oxygen isotope geochemistry of igneous rocks, Contr. Mineral. Petrol. 19 (1968) 1. [9] D.C.Noble, J.Haffty and C.E.Hedge, Strontium and magnesium contents of some natural peralkaline silicic glasses and their petrogenetic significance, Am. J. Sci. 267 (1969) in press. [10] D.C.Noble, unpublished data. [ 11 ] D.C.Noble, Laminar viscous towage structures in ashflow tuff from Gran Canaria, Canary Islands: A discussion, J. Geol. 76 (1968) 721. [12] P.W.Lipman, Chemical comparison of glassy and crystalline volcanic rocks, U.S. Geol. Survey Bull. 1201-D (1965) D1. [13] R.L.Smith, Zones and zonal variations in welded ash flows, U.S. Geol. Survey Prof. Paper 354-F (1960) 149. [14] D.C.Noble, G.D.Bath, R.L.Christiansen and P.P.Orkild, Zonal relations and paleomagnetism of the Spearhead and Rocket Wash Members of the Thirsty Canyon Tuff, southern Nevada, in: Geological survey research 1968, U.S. Geol. Survey Prof. Paper 600-C (1968) C61. [15] D.C.Noble, K.A.Sargent, H.H.Mehnert, E.B.Ekren and F.M.Byers Jr., Silent Canyon volcanic center, Nye County, Nevada, Geol. Soc. Am. Mem. 110 (1968) 65. [16] C.W.Cross, On the occurrence of topaz and garnet in lithophyses of rhyolite, Am. J. Sci. 31 (1886) 432. [17] C.T.Wrucke and D.C.Noble, unpublished data. [18] J.N.Rosholt, B.R.Doe and M.Tatsumoto, Evolution of the isotopic composition of uranium and thorium in soil profiles, Bull. Geol. Soc. Am. 77 (1966) 987. [19] B.R.Doe, The bearing of lead isotopes on the source of g~anitic magma, J. Petrology (1967) 51. [ 20] J.R.Dooley Jr., H.C.Granger and J.N.Rosholt, Uranium234 fractionation in the sandstone-type uranium deposits of the Ambrosia Lake district, New Mexico, Econ. Geol. 61 (1966) 1362. [ 21 ] H.S.Washington, The rhyolites of Lipari, Am. J. Sci., 4th set., 200 (1920) 446.