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Roberta L Rudnick - One of the best experts on this subject based on the ideXlab platform.
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potassium isotope fractionation during continental weathering and implications for global k isotopic balance
Geochimica et Cosmochimica Acta, 2020Co-Authors: Fangzhen Teng, Yan Hu, Roberta L RudnickAbstract:Abstract Potassium isotopic compositions of profiles through Saprolites developed on a diabase in South Carolina, U.S.A., and on a granite in Guangdong, China, allow characterization of the behavior of K isotopes during continental weathering. Saprolites from the diabase profile are heavily weathered with chemical index of alteration (CIA) values up to 95; however, their K isotopic variation is limited, with δ41K ranging from −0.475 ± 0.028‰ in the unweathered diabase to −0.407 ± 0.021‰ for the Saprolites. The lack of significant K isotope fractionation mainly reflects the conservative behavior of K in the diabase weathering profile, with >50% of the original K remaining in the Saprolites. By contrast, K isotopes are fractionated during granite weathering and correlate with sample depth, CIA, and kaolinite abundance, with δ41K decreasing from −0.493 ± 0.030‰ in the unweathered granites at the bottom to −0.628 ± 0.021‰ in the most weathered Saprolite close to the surface. These observations suggest the preference of light K isotopes in Saprolites relative to fluids, which is further supported by the overall isotopically heavy nearby stream water samples (δ41K = −0.709 ± 0.017 to −0.339 ± 0.018‰). These results demonstrate that continental weathering plays an important role in the global K isotopic budget through the formation of isotopically heterogeneous rivers and weathered regolith. Recycling of K-rich crustal materials with distinct K isotopic signatures may produce distinct mantle K isotopic end members.
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contrasting lithium and magnesium isotope fractionation during continental weathering
Earth and Planetary Science Letters, 2010Co-Authors: Fangzhen Teng, Roberta L Rudnick, Robert L GardnerAbstract:Abstract Magnesium isotopic compositions of a profile through Saprolites developed on a diabase dike from South Carolina have been measured in order to study the behavior of Mg isotopes during continental weathering. As weathering progresses, Mg isotopes are greatly fractionated and are correlated with Mg concentration, clay mineral proportions and density of the Saprolites. δ 26 Mg values increase from −0.22 in the unweathered diabase to + 0.65 in the most weathered Saprolite. These observations are consistent with the release of light Mg to the hydrosphere and formation of isotopically heavy Mg in the weathered products. The loss of Mg during weathering can be modeled by Rayleigh distillation with an apparent fractionation factor between the Saprolite and fluid (α) of 1.00005 to 1.0004, i.e., up to 0.4‰ fractionation in the 26 Mg/ 24 Mg ratio between the Saprolite and fluid. The large variation in α value reflects a mineralogical control on Mg isotope fractionation during primary dissolution of Mg-rich minerals and formation of secondary minerals during continental weathering. Like Mg isotopes, Li isotopes in the Saprolite profile are also greatly fractionated, with δ 7 Li values ranging from −6.7 down to −20. The large Li isotope fractionation and variation in Li concentration, as well as irregularities in the δ 7 Li profile with depth, however, cannot be explained by Li loss during weathering alone. Instead, Li can be modeled by a two-step process: (1) equilibrium isotope fractionation during continental weathering, which lowered δ 7 Li and Li concentrations and produced a Li concentration gradient in the Saprolites like that seen in Mg, and (2) subsequent kinetic isotope fractionation produced by diffusion of Li in the Saprolites, possibly across a paleo-water table. The results presented here suggest that continental weathering will shift the Mg isotopic composition of the continental crust to values higher than the mantle value, whereas crustal recycling over the history of the Earth will have no discernible effect on the Mg isotopic composition of the mantle.
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extreme lithium isotopic fractionation during continental weathering revealed in Saprolites from south carolina
Chemical Geology, 2004Co-Authors: Roberta L Rudnick, Paul B Tomascak, Robert L GardnerAbstract:The lithium concentration and isotopic composition of two Saprolites developed on a granite and diabase dike from South Carolina have been measured in order to document the behavior of lithium isotopes during continental weathering. Both Saprolites show a general trend of decreasing d 7 Li with increasing weathering intensity, as measured by both bulk density and the chemical index of alteration (CIA). The Saprolite developed on the granite is isotopically lighter than the fresh igneous rock (d 7 Li=6.8x to +1.4x vs. +2.3x, respectively), and is generally depleted in lithium. These observations are consistent with leaching of lithium via Rayleigh distillation during progressive weathering; most Saprolites fall on a Rayleigh distillation curve corresponding to an apparent fractionation factor (a) of 0.997. However, two samples have higher lithium contents than the fresh granite and thus point to additional processes affecting lithium in the Saprolite (e.g., sorption of lithium on to clay minerals). The Saprolite profile developed on the diabase dike shows highly variable d 7 Li values, ranging down to extremely light compositions (20x). Previous work has identified a chemical and mineralogical discontinuity at a depth of 2 m, but our lithium data show a marked discontinuity at 6 m depth. Saprolite samples at or above 6 m depth are highly weathered (CIA=88– 95), depleted in lithium (having b50% of the original diabase lithium) and isotopically light (10x to 20x vs. 4.3x for the unweathered diabase). Most of the data are consistent with leaching of lithium via Rayleigh distillation during intense weathering, with apparent a values of 0.995 to 0.980. Samples experiencing lower apparent a values tend to have higher kaolinite/smectite ratios, suggesting a mineralogical control on isotopic fractionation. However, the lightest sample (at 20x) is only slightly depleted in lithium and would require extremely low a values to explain via Rayleigh distilliation. This extreme composition remains enigmatic. Saprolite samples below 6 m have highly variable d 7 Li (5x to 14x) and, importantly, lithium concentrations that are higher than that of the unweathered diabase (up to 2.4 times the concentration of the fresh diabase). These deeper Saprolites thus cannot be explained by Rayleigh distillation. A positive correlation between d 7 Li and Li
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Assessing molybdenum isotope fractionation during continental weathering as recorded by weathering profiles in Saprolites and bauxites
Chemical Geology, 2024Co-Authors: Allison T. Greaney, Roberta L Rudnick, Stephen J. Romaniello, Aleisha C. Johnson, Ariel D. Anbar, Michael L. CummingsAbstract:Abstract Molybdenum isotopes in three deep and well-characterized weathering profiles – a Saprolite formed on meta-diabase from South Carolina, USA, and two ferruginous bauxites formed on Columbia River Basalts in Oregon and Washington, USA – elucidate Mo isotope behavior during continental weathering. The Saprolite records an overall loss of Mo relative to the fresh bedrock, as indicated by negative τMoTi, which defines the loss of Mo relative to the relatively immobile element Ti. The Saprolites are also isotopically light: δ98Mo values range from −0.89‰ (relative to NIST 3134) to −0.05‰, mean δ98Mo = −0.40‰, compared to +0.55‰ NIST3134 for the underlying unweathered bedrock. By contrast, the ferruginous bauxites generally record addition of Mo relative to the fresh bedrock (zero to positive τMoTi) and generally have higher δ98Mo values than the parental basalts: δ98Mo of the bauxites range from −0.14‰ to +0.38‰ compared to −0.33‰ and +0.02‰ for the unweathered parental basalt. Low δ98Mo values in the Saprolites likely reflect preferential retention of isotopically light Mo adsorbed onto accessory Fe-oxy-hydroxides and clays during weathering, whereas the high δ98Mo values in the bauxites reflect the addition of isotopically heavy Mo from groundwater. When the three profiles are combined, there is a positive correlation between τMoTi and δ98Mo, suggesting that when Mo is lost during continental weathering, the resulting regolith is isotopically light, whereas groundwater addition can shift the regolith to heavier values. Because Saprolites are a more common weathering product than bauxites, we conclude that, in general, continental weathering fractionates Mo isotopes such that the weathered upper crust retains isotopically light Mo. In contrast, the groundwater that leaches Mo from the weathered crust is isotopically heavy. Thus, chemical weathering of continents generates the isotopically heavy riverine signature observed globally, and partially contributes to the isotopically heavy seawater signature. Finally, these data, in conjunction with previously published data for glacial diamictites, can be used to assess changes in the crustal Mo isotope signature over the last 2.9 Ga.
Robert L Gardner - One of the best experts on this subject based on the ideXlab platform.
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contrasting lithium and magnesium isotope fractionation during continental weathering
Earth and Planetary Science Letters, 2010Co-Authors: Fangzhen Teng, Roberta L Rudnick, Robert L GardnerAbstract:Abstract Magnesium isotopic compositions of a profile through Saprolites developed on a diabase dike from South Carolina have been measured in order to study the behavior of Mg isotopes during continental weathering. As weathering progresses, Mg isotopes are greatly fractionated and are correlated with Mg concentration, clay mineral proportions and density of the Saprolites. δ 26 Mg values increase from −0.22 in the unweathered diabase to + 0.65 in the most weathered Saprolite. These observations are consistent with the release of light Mg to the hydrosphere and formation of isotopically heavy Mg in the weathered products. The loss of Mg during weathering can be modeled by Rayleigh distillation with an apparent fractionation factor between the Saprolite and fluid (α) of 1.00005 to 1.0004, i.e., up to 0.4‰ fractionation in the 26 Mg/ 24 Mg ratio between the Saprolite and fluid. The large variation in α value reflects a mineralogical control on Mg isotope fractionation during primary dissolution of Mg-rich minerals and formation of secondary minerals during continental weathering. Like Mg isotopes, Li isotopes in the Saprolite profile are also greatly fractionated, with δ 7 Li values ranging from −6.7 down to −20. The large Li isotope fractionation and variation in Li concentration, as well as irregularities in the δ 7 Li profile with depth, however, cannot be explained by Li loss during weathering alone. Instead, Li can be modeled by a two-step process: (1) equilibrium isotope fractionation during continental weathering, which lowered δ 7 Li and Li concentrations and produced a Li concentration gradient in the Saprolites like that seen in Mg, and (2) subsequent kinetic isotope fractionation produced by diffusion of Li in the Saprolites, possibly across a paleo-water table. The results presented here suggest that continental weathering will shift the Mg isotopic composition of the continental crust to values higher than the mantle value, whereas crustal recycling over the history of the Earth will have no discernible effect on the Mg isotopic composition of the mantle.
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extreme lithium isotopic fractionation during continental weathering revealed in Saprolites from south carolina
Chemical Geology, 2004Co-Authors: Roberta L Rudnick, Paul B Tomascak, Robert L GardnerAbstract:The lithium concentration and isotopic composition of two Saprolites developed on a granite and diabase dike from South Carolina have been measured in order to document the behavior of lithium isotopes during continental weathering. Both Saprolites show a general trend of decreasing d 7 Li with increasing weathering intensity, as measured by both bulk density and the chemical index of alteration (CIA). The Saprolite developed on the granite is isotopically lighter than the fresh igneous rock (d 7 Li=6.8x to +1.4x vs. +2.3x, respectively), and is generally depleted in lithium. These observations are consistent with leaching of lithium via Rayleigh distillation during progressive weathering; most Saprolites fall on a Rayleigh distillation curve corresponding to an apparent fractionation factor (a) of 0.997. However, two samples have higher lithium contents than the fresh granite and thus point to additional processes affecting lithium in the Saprolite (e.g., sorption of lithium on to clay minerals). The Saprolite profile developed on the diabase dike shows highly variable d 7 Li values, ranging down to extremely light compositions (20x). Previous work has identified a chemical and mineralogical discontinuity at a depth of 2 m, but our lithium data show a marked discontinuity at 6 m depth. Saprolite samples at or above 6 m depth are highly weathered (CIA=88– 95), depleted in lithium (having b50% of the original diabase lithium) and isotopically light (10x to 20x vs. 4.3x for the unweathered diabase). Most of the data are consistent with leaching of lithium via Rayleigh distillation during intense weathering, with apparent a values of 0.995 to 0.980. Samples experiencing lower apparent a values tend to have higher kaolinite/smectite ratios, suggesting a mineralogical control on isotopic fractionation. However, the lightest sample (at 20x) is only slightly depleted in lithium and would require extremely low a values to explain via Rayleigh distilliation. This extreme composition remains enigmatic. Saprolite samples below 6 m have highly variable d 7 Li (5x to 14x) and, importantly, lithium concentrations that are higher than that of the unweathered diabase (up to 2.4 times the concentration of the fresh diabase). These deeper Saprolites thus cannot be explained by Rayleigh distillation. A positive correlation between d 7 Li and Li
Susan L. Brantley - One of the best experts on this subject based on the ideXlab platform.
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links between physical and chemical weathering inferred from a 65 m deep borehole through earth s critical zone
Scientific Reports, 2019Co-Authors: Susan L. Brantley, Steven W Holbrook, B A Flinchum, Virginia Marcon, Allan R Bacon, Bradley J Carr, Daniel Richter, Clifford S RiebeAbstract:As bedrock weathers to regolith – defined here as weathered rock, Saprolite, and soil – porosity grows, guides fluid flow, and liberates nutrients from minerals. Though vital to terrestrial life, the processes that transform bedrock into soil are poorly understood, especially in deep regolith, where direct observations are difficult. A 65-m-deep borehole in the Calhoun Critical Zone Observatory, South Carolina, provides unusual access to a complete weathering profile in an Appalachian granitoid. Co-located geophysical and geochemical datasets in the borehole show a remarkably consistent picture of linked chemical and physical weathering processes, acting over a 38-m-thick regolith divided into three layers: soil; porous, highly weathered Saprolite; and weathered, fractured bedrock. The data document that major minerals (plagioclase and biotite) commence to weather at 38 m depth, 20 m below the base of Saprolite, in deep, weathered rock where physical, chemical and optical properties abruptly change. The transition from Saprolite to weathered bedrock is more gradational, over a depth range of 11–18 m. Chemical weathering increases steadily upward in the weathered bedrock, with intervals of more intense weathering along fractures, documenting the combined influence of time, reactive fluid transport, and the opening of fractures as rock is exhumed and transformed near Earth’s surface.
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links between physical and chemical weathering inferred from a 65 m deep borehole through earth s critical zone
Scientific Reports, 2019Co-Authors: Susan L. Brantley, Steven W Holbrook, B A Flinchum, Virginia Marcon, Allan R Bacon, Bradley J Carr, Daniel Richter, Clifford S RiebeAbstract:As bedrock weathers to regolith – defined here as weathered rock, Saprolite, and soil – porosity grows, guides fluid flow, and liberates nutrients from minerals. Though vital to terrestrial life, the processes that transform bedrock into soil are poorly understood, especially in deep regolith, where direct observations are difficult. A 65-m-deep borehole in the Calhoun Critical Zone Observatory, South Carolina, provides unusual access to a complete weathering profile in an Appalachian granitoid. Co-located geophysical and geochemical datasets in the borehole show a remarkably consistent picture of linked chemical and physical weathering processes, acting over a 38-m-thick regolith divided into three layers: soil; porous, highly weathered Saprolite; and weathered, fractured bedrock. The data document that major minerals (plagioclase and biotite) commence to weather at 38 m depth, 20 m below the base of Saprolite, in deep, weathered rock where physical, chemical and optical properties abruptly change. The transition from Saprolite to weathered bedrock is more gradational, over a depth range of 11–18 m. Chemical weathering increases steadily upward in the weathered bedrock, with intervals of more intense weathering along fractures, documenting the combined influence of time, reactive fluid transport, and the opening of fractures as rock is exhumed and transformed near Earth’s surface.
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Links between physical and chemical weathering inferred from a 65-m-deep borehole through Earth’s critical zone
Nature Publishing Group, 2019Co-Authors: Steven W Holbrook, Susan L. Brantley, B A Flinchum, Virginia Marcon, Allan R Bacon, Bradley J Carr, Daniel D. Richter, Clifford S RiebeAbstract:Abstract As bedrock weathers to regolith – defined here as weathered rock, Saprolite, and soil – porosity grows, guides fluid flow, and liberates nutrients from minerals. Though vital to terrestrial life, the processes that transform bedrock into soil are poorly understood, especially in deep regolith, where direct observations are difficult. A 65-m-deep borehole in the Calhoun Critical Zone Observatory, South Carolina, provides unusual access to a complete weathering profile in an Appalachian granitoid. Co-located geophysical and geochemical datasets in the borehole show a remarkably consistent picture of linked chemical and physical weathering processes, acting over a 38-m-thick regolith divided into three layers: soil; porous, highly weathered Saprolite; and weathered, fractured bedrock. The data document that major minerals (plagioclase and biotite) commence to weather at 38 m depth, 20 m below the base of Saprolite, in deep, weathered rock where physical, chemical and optical properties abruptly change. The transition from Saprolite to weathered bedrock is more gradational, over a depth range of 11–18 m. Chemical weathering increases steadily upward in the weathered bedrock, with intervals of more intense weathering along fractures, documenting the combined influence of time, reactive fluid transport, and the opening of fractures as rock is exhumed and transformed near Earth’s surface
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regolith formation rate from u series nuclides implications from the study of a spheroidal weathering profile in the rio icacos watershed puerto rico
Geochimica et Cosmochimica Acta, 2013Co-Authors: François Chabaux, Heather L. Buss, Eric Pelt, R Di Chiara Roupert, Anthony Dosseto, E Blaes, Peter Stille, Susan L. BrantleyAbstract:Abstract A 2 m-thick spheroidal weathering profile, developed on a quartz diorite in the Rio Icacos watershed (Luquillo Mountains, eastern Puerto Rico), was analyzed for major and trace element concentrations, Sr and Nd isotopic ratios and U-series nuclides ( 238 U– 234 U– 230 Th– 226 Ra). In this profile a 40 cm thick soil horizon is overlying a 150 cm thick Saprolite which is separated from the basal corestone by a ∼40 cm thick rindlet zone. The Sr and Nd isotopic variations along the whole profile imply that, in addition to geochemical fractionations associated to water–rock interactions, the geochemical budget of the profile is influenced by a significant accretion of atmospheric dusts. The mineralogical and geochemical variations along the profile also confirm that the weathering front does not progress continuously from the top to the base of the profile. The upper part of the profile is probably associated with a different weathering system (lateral weathering of upper corestones) than the lower part, which consists of the basal corestone, the associated rindlet system and the Saprolite in contact with these rindlets. Consequently, the determination of weathering rates from 238 U– 234 U– 230 Th– 226 Ra disequilibrium in a series of samples collected along a vertical depth profile can only be attempted for samples collected in the lower part of the profile, i.e. the rindlet zone and the lower Saprolite. Similar propagation rates were derived for the rindlet system and the Saprolite by using classical models involving loss and gain processes for all nuclides to interpret the variation of U-series nuclides in the rindlet-Saprolite subsystem. The consistency of these weathering rates with average weathering and erosion rates derived via other methods for the whole watershed provides a new and independent argument that, in the Rio Icacos watershed, the weathering system has reached a geomorphologic steady-state. Our study also indicates that even in environments with differential weathering, such as observed for the Puerto Rico site, the radioactive disequilibrium between the nuclides of a single radioactive series (here 238 U– 234 U– 230 Th– 226 Ra) can still be interpreted in terms of a simplified scenario of congruent weathering. Incidentally, the U–Th–Ra disequilibrium in the corestone samples confirms that the outermost part of the corestone is already weathered.
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weathering of the rio blanco quartz diorite luquillo mountains puerto rico coupling oxidation dissolution and fracturing
Geochimica et Cosmochimica Acta, 2008Co-Authors: Heather L. Buss, Samuel M Webb, Susan L. BrantleyAbstract:In the mountainous Rio Icacos watershed in northeastern Puerto Rico, quartz diorite bedrock weathers spheroidally, producing a 0.2-2 m thick zone of partially weathered rock layers ({approx}2.5 cm thickness each) called rindlets, which form concentric layers around corestones. Spheroidal fracturing has been modeled to occur when a weathering reaction with a positive {Delta}V of reaction builds up elastic strain energy. The rates of spheroidal fracturing and Saprolite formation are therefore controlled by the rate of the weathering reaction. Chemical, petrographic, and spectroscopic evidence demonstrates that biotite oxidation is the most likely fracture-inducing reaction. This reaction occurs with an expansion in d (0 0 1) from 10.0 to 10.5 {angstrom}, forming 'altered biotite'. Progressive biotite oxidation across the rindlet zone was inferred from thin sections and gradients in K and Fe(II). Using the gradient in Fe(II) and constraints based on cosmogenic age dates, we calculated a biotite oxidation reaction rate of 8.2 x 10{sup -14} mol biotite m{sup -2} s{sup -1}. Biotite oxidation was documented within the bedrock corestone by synchrotron X-ray microprobe fluorescence imaging and XANES. X-ray microprobe images of Fe(II) and Fe(III) at 2 {micro}m resolution revealed that oxidized zones within individual biotite crystals are the first evidence ofmore » alteration of the otherwise unaltered corestone. Fluids entering along fractures lead to the dissolution of plagioclase within the rindlet zone. Within 7 cm surrounding the rindlet-Saprolite interface, hornblende dissolves to completion at a rate of 6.3 x 10{sup -13} mol hornblende m{sup -2} s{sup -1}: the fastest reported rate of hornblende weathering in the field. This rate is consistent with laboratory-derived hornblende dissolution rates. By revealing the coupling of these mineral weathering reactions to fracturing and porosity formation we are able to describe the process by which the quartz diorite bedrock disaggregates and forms Saprolite. In the corestone, biotite oxidation induces spheroidal fracturing, facilitating the influx of fluids that react with other minerals, dissolving plagioclase and chlorite, creating additional porosity, and eventually dissolving hornblende and precipitating secondary minerals. The thickness of the resultant Saprolite is maintained at steady state by a positive feedback between the denudation rate and the weathering advance rate driven by the concentration of pore water O{sub 2} at the bedrock-Saprolite interface.« less
Heather L. Buss - One of the best experts on this subject based on the ideXlab platform.
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regolith formation rate from u series nuclides implications from the study of a spheroidal weathering profile in the rio icacos watershed puerto rico
Geochimica et Cosmochimica Acta, 2013Co-Authors: François Chabaux, Heather L. Buss, Eric Pelt, R Di Chiara Roupert, Anthony Dosseto, E Blaes, Peter Stille, Susan L. BrantleyAbstract:Abstract A 2 m-thick spheroidal weathering profile, developed on a quartz diorite in the Rio Icacos watershed (Luquillo Mountains, eastern Puerto Rico), was analyzed for major and trace element concentrations, Sr and Nd isotopic ratios and U-series nuclides ( 238 U– 234 U– 230 Th– 226 Ra). In this profile a 40 cm thick soil horizon is overlying a 150 cm thick Saprolite which is separated from the basal corestone by a ∼40 cm thick rindlet zone. The Sr and Nd isotopic variations along the whole profile imply that, in addition to geochemical fractionations associated to water–rock interactions, the geochemical budget of the profile is influenced by a significant accretion of atmospheric dusts. The mineralogical and geochemical variations along the profile also confirm that the weathering front does not progress continuously from the top to the base of the profile. The upper part of the profile is probably associated with a different weathering system (lateral weathering of upper corestones) than the lower part, which consists of the basal corestone, the associated rindlet system and the Saprolite in contact with these rindlets. Consequently, the determination of weathering rates from 238 U– 234 U– 230 Th– 226 Ra disequilibrium in a series of samples collected along a vertical depth profile can only be attempted for samples collected in the lower part of the profile, i.e. the rindlet zone and the lower Saprolite. Similar propagation rates were derived for the rindlet system and the Saprolite by using classical models involving loss and gain processes for all nuclides to interpret the variation of U-series nuclides in the rindlet-Saprolite subsystem. The consistency of these weathering rates with average weathering and erosion rates derived via other methods for the whole watershed provides a new and independent argument that, in the Rio Icacos watershed, the weathering system has reached a geomorphologic steady-state. Our study also indicates that even in environments with differential weathering, such as observed for the Puerto Rico site, the radioactive disequilibrium between the nuclides of a single radioactive series (here 238 U– 234 U– 230 Th– 226 Ra) can still be interpreted in terms of a simplified scenario of congruent weathering. Incidentally, the U–Th–Ra disequilibrium in the corestone samples confirms that the outermost part of the corestone is already weathered.
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weathering of the rio blanco quartz diorite luquillo mountains puerto rico coupling oxidation dissolution and fracturing
Geochimica et Cosmochimica Acta, 2008Co-Authors: Heather L. Buss, Samuel M Webb, Susan L. BrantleyAbstract:In the mountainous Rio Icacos watershed in northeastern Puerto Rico, quartz diorite bedrock weathers spheroidally, producing a 0.2-2 m thick zone of partially weathered rock layers ({approx}2.5 cm thickness each) called rindlets, which form concentric layers around corestones. Spheroidal fracturing has been modeled to occur when a weathering reaction with a positive {Delta}V of reaction builds up elastic strain energy. The rates of spheroidal fracturing and Saprolite formation are therefore controlled by the rate of the weathering reaction. Chemical, petrographic, and spectroscopic evidence demonstrates that biotite oxidation is the most likely fracture-inducing reaction. This reaction occurs with an expansion in d (0 0 1) from 10.0 to 10.5 {angstrom}, forming 'altered biotite'. Progressive biotite oxidation across the rindlet zone was inferred from thin sections and gradients in K and Fe(II). Using the gradient in Fe(II) and constraints based on cosmogenic age dates, we calculated a biotite oxidation reaction rate of 8.2 x 10{sup -14} mol biotite m{sup -2} s{sup -1}. Biotite oxidation was documented within the bedrock corestone by synchrotron X-ray microprobe fluorescence imaging and XANES. X-ray microprobe images of Fe(II) and Fe(III) at 2 {micro}m resolution revealed that oxidized zones within individual biotite crystals are the first evidence ofmore » alteration of the otherwise unaltered corestone. Fluids entering along fractures lead to the dissolution of plagioclase within the rindlet zone. Within 7 cm surrounding the rindlet-Saprolite interface, hornblende dissolves to completion at a rate of 6.3 x 10{sup -13} mol hornblende m{sup -2} s{sup -1}: the fastest reported rate of hornblende weathering in the field. This rate is consistent with laboratory-derived hornblende dissolution rates. By revealing the coupling of these mineral weathering reactions to fracturing and porosity formation we are able to describe the process by which the quartz diorite bedrock disaggregates and forms Saprolite. In the corestone, biotite oxidation induces spheroidal fracturing, facilitating the influx of fluids that react with other minerals, dissolving plagioclase and chlorite, creating additional porosity, and eventually dissolving hornblende and precipitating secondary minerals. The thickness of the resultant Saprolite is maintained at steady state by a positive feedback between the denudation rate and the weathering advance rate driven by the concentration of pore water O{sub 2} at the bedrock-Saprolite interface.« less
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a spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady state denudation
Earth and Planetary Science Letters, 2006Co-Authors: Raymond C Fletcher, Heather L. Buss, Susan L. BrantleyAbstract:Abstract Spheroidal weathering, a common mechanism that initiates the transformation of bedrock to Saprolite, creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets. In the spheroidally weathering Rio Blanco quartz diorite (Puerto Rico), diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides. A positive ΔV of reaction results in the build-up of elastic strain energy in the rock. Formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock. Substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the Rio Icacos bedrock (≈ 2–3 cm) and a time interval between fractures (≈ 200–300 a) based on an assumption of steady-state denudation at the measured rate of 0.01 cm/a. Averaged over times longer than this interval, the rate of advance of the bedrock–Saprolite interface during spheroidal weathering (the weathering advance rate) is constant with time. Assuming that the oxygen concentration at the bedrock–Saprolite interface varies with the thickness of soil/Saprolite yields predictive equations for how weathering advance rate and steady-state Saprolite/soil thickness depend upon atmospheric oxygen levels and upon denudation rate. The denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the Saprolite. In our model for spheroidal weathering of the Rio Blanco quartz diorite, fractures occur every ∼ 250 yr, ferric oxide is fully depleted over a four rindlet set in ∼ 1000 yr, and saprolitization is completed in ∼ 5000 yr in the zone containing ∼ 20 rindlets. Spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing. Coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures, but for the same kinetics and transport parameters, the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the Rio Blanco system. The spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time.
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the coupling of biological iron cycling and mineral weathering during Saprolite formation luquillo mountains puerto rico
Geobiology, 2005Co-Authors: Heather L. Buss, Mary Ann Bruns, M J Schultz, Joel Moore, C F Mathur, Susan L. BrantleyAbstract:Corestones of quartz diorite bedrock in the Rio Icacos watershed in Puerto Rico weather spheroidally to form concentric sets of partially weathered rock layers (referred to here as rindlets) that slowly transform to Saprolite. The rindlet zone (0.2–2 m thick) is overlain by Saprolite (2–8 m) topped by soil (0.5–1 m). With the objective of understanding interactions between weathering, substrate availability, and resident micro-organisms, we made geochemical and microbiological measurements as a function of depth in 5 m of regolith (soil + Saprolite). We employed direct microscopic counting of total cell densities; enumeration of culturable aerobic heterotrophs; extraction of microbial DNA for yield calculations; and biochemical tests for iron-oxidizing bacteria. Total cell densities, which ranged from 2.5 × 106 to 1.6 × 1010 g−1 regolith, were higher than 108 g−1 at three depths: in the upper 1 m, at 2.1 m, and between 3.7 and 4.9 m, just above the rindlet zone. High proportions of inactive or unculturable cells were indicated throughout the profile by very low percentages of culturable heterotrophs (0.0004% to 0.02% of total cell densities). The observed increases in total and culturable cells and DNA yields at lower depths were not correlated with organic carbon or total iron but were correlated with moisture and HCl-extractable iron. Biochemical tests for aerobic iron-oxidizers were also positive at 0.15–0.6 m, at 2.1–2.4 m, and at 4.9 m depths. To interpret microbial populations within the context of weathering reactions, we developed a model for estimating growth rates of lithoautotrophs and heterotrophs based on measured substrate fluxes. The calculations and observations are consistent with a model wherein electron donor flux driving bacterial growth at the Saprolite–bedrock interface is dominated by Fe(II) and where autotrophic iron-oxidizing bacteria support the heterotrophic population and contribute to bedrock disaggregation and Saprolite formation.
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Assessing molybdenum isotope fractionation during continental weathering as recorded by weathering profiles in Saprolites and bauxites
Chemical Geology, 2024Co-Authors: Allison T. Greaney, Roberta L Rudnick, Stephen J. Romaniello, Aleisha C. Johnson, Ariel D. Anbar, Michael L. CummingsAbstract:Abstract Molybdenum isotopes in three deep and well-characterized weathering profiles – a Saprolite formed on meta-diabase from South Carolina, USA, and two ferruginous bauxites formed on Columbia River Basalts in Oregon and Washington, USA – elucidate Mo isotope behavior during continental weathering. The Saprolite records an overall loss of Mo relative to the fresh bedrock, as indicated by negative τMoTi, which defines the loss of Mo relative to the relatively immobile element Ti. The Saprolites are also isotopically light: δ98Mo values range from −0.89‰ (relative to NIST 3134) to −0.05‰, mean δ98Mo = −0.40‰, compared to +0.55‰ NIST3134 for the underlying unweathered bedrock. By contrast, the ferruginous bauxites generally record addition of Mo relative to the fresh bedrock (zero to positive τMoTi) and generally have higher δ98Mo values than the parental basalts: δ98Mo of the bauxites range from −0.14‰ to +0.38‰ compared to −0.33‰ and +0.02‰ for the unweathered parental basalt. Low δ98Mo values in the Saprolites likely reflect preferential retention of isotopically light Mo adsorbed onto accessory Fe-oxy-hydroxides and clays during weathering, whereas the high δ98Mo values in the bauxites reflect the addition of isotopically heavy Mo from groundwater. When the three profiles are combined, there is a positive correlation between τMoTi and δ98Mo, suggesting that when Mo is lost during continental weathering, the resulting regolith is isotopically light, whereas groundwater addition can shift the regolith to heavier values. Because Saprolites are a more common weathering product than bauxites, we conclude that, in general, continental weathering fractionates Mo isotopes such that the weathered upper crust retains isotopically light Mo. In contrast, the groundwater that leaches Mo from the weathered crust is isotopically heavy. Thus, chemical weathering of continents generates the isotopically heavy riverine signature observed globally, and partially contributes to the isotopically heavy seawater signature. Finally, these data, in conjunction with previously published data for glacial diamictites, can be used to assess changes in the crustal Mo isotope signature over the last 2.9 Ga.
