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Papers by Brian Marshall

Geological, mineralogical, chemical, and isotopic evidence from coatings of calcite and silica on... more Geological, mineralogical, chemical, and isotopic evidence from coatings of calcite and silica on open fractures and lithophysal cavities within welded tuffs at Yucca Mountain indicate an origin from meteoric water percolating through a thick (500 to 700 m) unsaturated zone (UZ) rather than from pulses of ascending ground water. Geologic evidence for a UZ setting

Initial uranium-234/uranium-238 activity ratios and conventional thorium-230/uranium ages for sub... more Initial uranium-234/uranium-238 activity ratios and conventional thorium-230/uranium ages for subsamples of calcite and opal from the Exploratory Studies Facility tunnel, Yucca Mountain, Nevada......... 18. Carbon and oxygen isotope compositions of subsamples of calcite relative to distance from the north portal of the Exploratory Studies Facility tunnel, Yucca Mountain, Nevada.

Applied Geochemistry, 2008

Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities recor... more Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry, temperatures, and timing of past fluid movement in the unsaturated zone at Yucca Mountain, Nevada, the proposed site of a high-level radioactive waste repository. The distribution and geochemistry of these deposits are consistent with low-temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone. However, the discovery of fluid inclusions in calcite with homogenization temperatures (T h) up to 80°Cwasconstruedbysomescientistsasstrongevidenceforhydrothermaldeposition.Thispaperreportstheresultsofinvestigationstotestthehypothesisofhydrothermaldepositionandtodeterminethetemperatureandtimingofsecondarymineraldeposition.Mineralprecipitationtemperaturesintheunsaturatedzoneareestimatedfromcalcite−andfluorite−hostedfluidinclusionsandcalcited18Ovalues,anddepositionaltimingisconstrainedbythe207Pb/235Uagesofchalcedonyoropalinthedeposits.FluidinclusionThfrom50samplesofcalciteandfoursamplesoffluoriterangefrom80°C was construed by some scientists as strong evidence for hydrothermal deposition. This paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition. Mineral precipitation temperatures in the unsaturated zone are estimated from calcite-and fluorite-hosted fluid inclusions and calcite d 18 O values, and depositional timing is constrained by the 207 Pb/ 235 U ages of chalcedony or opal in the deposits. Fluid inclusion T h from 50 samples of calcite and four samples of fluorite range from 80°Cwasconstruedbysomescientistsasstrongevidenceforhydrothermaldeposition.Thispaperreportstheresultsofinvestigationstotestthehypothesisofhydrothermaldepositionandtodeterminethetemperatureandtimingofsecondarymineraldeposition.Mineralprecipitationtemperaturesintheunsaturatedzoneareestimatedfromcalcite−andfluorite−hostedfluidinclusionsandcalcited18Ovalues,anddepositionaltimingisconstrainedbythe207Pb/235Uagesofchalcedonyoropalinthedeposits.FluidinclusionThfrom50samplesofcalciteandfoursamplesoffluoriterangefrom35 to 90°C.Calcited18Ovaluesrangefrom90°C. Calcite d 18 O values range from 90°C.Calcited18Ovaluesrangefrom0 to 22‰(SMOW)butmostfallbetween12and20‰.ThehighestThandthelowestd18Ovaluesarefoundintheoldercalcite.CalciteThandd18Ovaluesindicatethatmostcalciteprecipitatedfromwaterwithd18OvaluesbetweenAˋ13andAˋ7‰,similartomodernmeteoricwaters.Twenty−two207Pb/235Uagesofchalcedonyoropalthatgenerallypostdateelevateddepositionaltemperaturesrangefrom22‰ (SMOW) but most fall between 12 and 20‰. The highest T h and the lowest d 18 O values are found in the older calcite. Calcite T h and d 18 O values indicate that most calcite precipitated from water with d 18 O values between À13 and À7‰, similar to modern meteoric waters. Twenty-two 207 Pb/ 235 U ages of chalcedony or opal that generally postdate elevated depositional temperatures range from 22‰(SMOW)butmostfallbetween12and20‰.ThehighestThandthelowestd18Ovaluesarefoundintheoldercalcite.CalciteThandd18Ovaluesindicatethatmostcalciteprecipitatedfromwaterwithd18OvaluesbetweenAˋ13andAˋ7‰,similartomodernmeteoricwaters.Twenty−two207Pb/235Uagesofchalcedonyoropalthatgenerallypostdateelevateddepositionaltemperaturesrangefrom9.5 to 1.9 Ma. New and published 207 Pb/ 235 U and 230 Th/Uages coupled with the T h values and estimates of temperature from calcite d 18 O values indicate that maximum unsaturated zone temperatures probably predate 10Maandthattheunsaturatedzonehadcooledtonear−present−daytemperatures(24−26°Catadepthof250m)by2−4Ma.Theevidenceofelevatedtemperaturespersistinginashflowtuffsadjacenttoparentcalderasforasmuchas10 Ma and that the unsaturated zone had cooled to near-present-day temperatures (24-26°C at a depth of 250 m) by 2-4 Ma. The evidence of elevated temperatures persisting in ash flow tuffs adjacent to parent calderas for as much as 10Maandthattheunsaturatedzonehadcooledtonear−present−daytemperatures(24−26°Catadepthof250m)by2−4Ma.Theevidenceofelevatedtemperaturespersistinginashflowtuffsadjacenttoparentcalderasforasmuchas8 Ma is a new finding, but consistent with thermal modeling. Simulations using the HEAT code demonstrate that prolonged cooling of the unsaturated zone is consistent with magmatic heat inputs and deep-seated (sub-water table) hydrothermal activity generated by the large magma body $8 km to the north that produced the 15-11 Ma ash flows and ash falls that make up Yucca Mountain. The evidence discussed in this and preceding papers strongly supports unsaturated zone deposition of the secondary minerals from descending meteoric waters. Although depositional temperatures reflect conductive

DOE Scientific and Technical Information. DOE Scientific and Technical Information. ...

Geochim Cosmochim Acta, 1982

Geological Society of America Memoirs, 2012

Fluid geochemistry of Yucca Mountain and vicinity Email alerting services cite this article to re... more Fluid geochemistry of Yucca Mountain and vicinity Email alerting services cite this article to receive free e-mail alerts when new articles www.gsapubs.org/cgi/alerts click Subscribe America Memoirs to subscribe to Geological Society of www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click viewpoint. Opinions presented in this publication do not reflect official positions of the Society. positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political article's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to the science. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to

Strontium isotope studies of carbonates from soils, veins, eolian dust and Paleozoic basement sam... more Strontium isotope studies of carbonates from soils, veins, eolian dust and Paleozoic basement samples near Crater Flat, southwest of Yucca Mountain, provide evidence for the origins of these materials. Vein and soil carbonates have nearly identical ranges of ⁸⁷Sr/⁸⁶Sr ratios at the lower end of the pedogenic range. The average ⁸⁷Sr/⁸⁶Sr of Paleozoic basement from Black Marble Hill is similar

Yucca Mountain, a ridge of shallowly dipping, Miocene-age volcanic rocks in southwest Nevada, is ... more Yucca Mountain, a ridge of shallowly dipping, Miocene-age volcanic rocks in southwest Nevada, is the proposed site for a nuclear waste repository to be constructed in the 500- to 700-m-thick unsaturated zone (UZ). At the proposed repository, the 300-m-thick Topopah Spring Tuff welded unit (TSw) is overlain by approximately 30 m of nonwelded tuffs (PTn); the Tiva Canyon Tuff welded unit (TCw) overlies the PTn with a range in thickness from 0 to approximately 130 m at the site. The amount of water percolation through the UZ is low and difficult to measure directly, but local seepage into mined tunnels has been observed in the TCw. Past water seepage in the welded tuffs is recorded by widespread, thin (0.3 cm) coatings of calcite and silica on fracture surfaces and within cavities. Abundances of calcite and silica in the coatings were determined by X-ray microfluorescence mapping and subsequent multispectral image analysis of over 200 samples. The images were classified into constituen...

Tunneling of the Exploratory Studies Facility has offered the opportunity to sample and examine o... more Tunneling of the Exploratory Studies Facility has offered the opportunity to sample and examine occurrences of secondary mineralization found in the unsaturated-zone tuffs of Yucca Mountain, Nevada. Petrographic and paragenetic analyses, calcite and silica-phase stable isotopic analyses, and preliminary strontium tracer isotope and radiocarbon age analyses of these samples indicate that (1) an early stage of secondary mineralization consisting largely of chalcedony and quartz, but possibly with or slightly preceded by calcite, probably formed at warmer than ambient temperatures; (2) later secondary mineralization consisting of calcite and opal appears completely consistent with formation from percolation of surface infiltration whose solute load and carbon isotopic compositions reflect passage through the overlying soils; (3) based on textural studies, all unsaturated-zone secondary mineral occurrences exposed within the Exploratory Studies Facility tunnel, with the exception of the vapor-phase assemblages that formed at high temperatures during cooling of the tuffs, probably formed in unsaturated settings; and (4) calcite radiocarbon ages, based on preliminary results, have not been compromised by post-depositional exchange with carbon-bearing water and gases in the unsaturated zone.

Strontium isotopic analyses of barren and mineralized Paleozoic carbonate rocks show that hydroth... more Strontium isotopic analyses of barren and mineralized Paleozoic carbonate rocks show that hydrothermal fluids added radiogenic strontium (⁸⁷Sr) to the mineralized zones. At Bare Mountain, samples collected from mineralized areas have δ⁸⁷Sr{sub t} values ranging from +3.0 to +23.0, whereas unmineralized carbonate rocks have δ⁸⁷Sr, values of -0.6 to +2.9. In other ranges, δ⁸⁷Sr, values of the unmineralized carbonate rocks

Geological, mineralogical, chemical, and isotopic evidence from coatings of calcite and silica on... more Geological, mineralogical, chemical, and isotopic evidence from coatings of calcite and silica on open fractures and lithophysal cavities within welded tuffs at Yucca Mountain indicate an origin from meteoric water percolating through a thick (500 to 700 m) unsaturated zone (UZ) rather than from pulses of ascending ground water. Geologic evidence for a UZ setting

Initial uranium-234/uranium-238 activity ratios and conventional thorium-230/uranium ages for sub... more Initial uranium-234/uranium-238 activity ratios and conventional thorium-230/uranium ages for subsamples of calcite and opal from the Exploratory Studies Facility tunnel, Yucca Mountain, Nevada......... 18. Carbon and oxygen isotope compositions of subsamples of calcite relative to distance from the north portal of the Exploratory Studies Facility tunnel, Yucca Mountain, Nevada.

Applied Geochemistry, 2008

Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities recor... more Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry, temperatures, and timing of past fluid movement in the unsaturated zone at Yucca Mountain, Nevada, the proposed site of a high-level radioactive waste repository. The distribution and geochemistry of these deposits are consistent with low-temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone. However, the discovery of fluid inclusions in calcite with homogenization temperatures (T h) up to 80°Cwasconstruedbysomescientistsasstrongevidenceforhydrothermaldeposition.Thispaperreportstheresultsofinvestigationstotestthehypothesisofhydrothermaldepositionandtodeterminethetemperatureandtimingofsecondarymineraldeposition.Mineralprecipitationtemperaturesintheunsaturatedzoneareestimatedfromcalcite−andfluorite−hostedfluidinclusionsandcalcited18Ovalues,anddepositionaltimingisconstrainedbythe207Pb/235Uagesofchalcedonyoropalinthedeposits.FluidinclusionThfrom50samplesofcalciteandfoursamplesoffluoriterangefrom80°C was construed by some scientists as strong evidence for hydrothermal deposition. This paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition. Mineral precipitation temperatures in the unsaturated zone are estimated from calcite-and fluorite-hosted fluid inclusions and calcite d 18 O values, and depositional timing is constrained by the 207 Pb/ 235 U ages of chalcedony or opal in the deposits. Fluid inclusion T h from 50 samples of calcite and four samples of fluorite range from 80°Cwasconstruedbysomescientistsasstrongevidenceforhydrothermaldeposition.Thispaperreportstheresultsofinvestigationstotestthehypothesisofhydrothermaldepositionandtodeterminethetemperatureandtimingofsecondarymineraldeposition.Mineralprecipitationtemperaturesintheunsaturatedzoneareestimatedfromcalcite−andfluorite−hostedfluidinclusionsandcalcited18Ovalues,anddepositionaltimingisconstrainedbythe207Pb/235Uagesofchalcedonyoropalinthedeposits.FluidinclusionThfrom50samplesofcalciteandfoursamplesoffluoriterangefrom35 to 90°C.Calcited18Ovaluesrangefrom90°C. Calcite d 18 O values range from 90°C.Calcited18Ovaluesrangefrom0 to 22‰(SMOW)butmostfallbetween12and20‰.ThehighestThandthelowestd18Ovaluesarefoundintheoldercalcite.CalciteThandd18Ovaluesindicatethatmostcalciteprecipitatedfromwaterwithd18OvaluesbetweenAˋ13andAˋ7‰,similartomodernmeteoricwaters.Twenty−two207Pb/235Uagesofchalcedonyoropalthatgenerallypostdateelevateddepositionaltemperaturesrangefrom22‰ (SMOW) but most fall between 12 and 20‰. The highest T h and the lowest d 18 O values are found in the older calcite. Calcite T h and d 18 O values indicate that most calcite precipitated from water with d 18 O values between À13 and À7‰, similar to modern meteoric waters. Twenty-two 207 Pb/ 235 U ages of chalcedony or opal that generally postdate elevated depositional temperatures range from 22‰(SMOW)butmostfallbetween12and20‰.ThehighestThandthelowestd18Ovaluesarefoundintheoldercalcite.CalciteThandd18Ovaluesindicatethatmostcalciteprecipitatedfromwaterwithd18OvaluesbetweenAˋ13andAˋ7‰,similartomodernmeteoricwaters.Twenty−two207Pb/235Uagesofchalcedonyoropalthatgenerallypostdateelevateddepositionaltemperaturesrangefrom9.5 to 1.9 Ma. New and published 207 Pb/ 235 U and 230 Th/Uages coupled with the T h values and estimates of temperature from calcite d 18 O values indicate that maximum unsaturated zone temperatures probably predate 10Maandthattheunsaturatedzonehadcooledtonear−present−daytemperatures(24−26°Catadepthof250m)by2−4Ma.Theevidenceofelevatedtemperaturespersistinginashflowtuffsadjacenttoparentcalderasforasmuchas10 Ma and that the unsaturated zone had cooled to near-present-day temperatures (24-26°C at a depth of 250 m) by 2-4 Ma. The evidence of elevated temperatures persisting in ash flow tuffs adjacent to parent calderas for as much as 10Maandthattheunsaturatedzonehadcooledtonear−present−daytemperatures(24−26°Catadepthof250m)by2−4Ma.Theevidenceofelevatedtemperaturespersistinginashflowtuffsadjacenttoparentcalderasforasmuchas8 Ma is a new finding, but consistent with thermal modeling. Simulations using the HEAT code demonstrate that prolonged cooling of the unsaturated zone is consistent with magmatic heat inputs and deep-seated (sub-water table) hydrothermal activity generated by the large magma body $8 km to the north that produced the 15-11 Ma ash flows and ash falls that make up Yucca Mountain. The evidence discussed in this and preceding papers strongly supports unsaturated zone deposition of the secondary minerals from descending meteoric waters. Although depositional temperatures reflect conductive

DOE Scientific and Technical Information. DOE Scientific and Technical Information. ...

Geochim Cosmochim Acta, 1982

Geological Society of America Memoirs, 2012

Fluid geochemistry of Yucca Mountain and vicinity Email alerting services cite this article to re... more Fluid geochemistry of Yucca Mountain and vicinity Email alerting services cite this article to receive free e-mail alerts when new articles www.gsapubs.org/cgi/alerts click Subscribe America Memoirs to subscribe to Geological Society of www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click viewpoint. Opinions presented in this publication do not reflect official positions of the Society. positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political article's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to the science. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to

Strontium isotope studies of carbonates from soils, veins, eolian dust and Paleozoic basement sam... more Strontium isotope studies of carbonates from soils, veins, eolian dust and Paleozoic basement samples near Crater Flat, southwest of Yucca Mountain, provide evidence for the origins of these materials. Vein and soil carbonates have nearly identical ranges of ⁸⁷Sr/⁸⁶Sr ratios at the lower end of the pedogenic range. The average ⁸⁷Sr/⁸⁶Sr of Paleozoic basement from Black Marble Hill is similar

Yucca Mountain, a ridge of shallowly dipping, Miocene-age volcanic rocks in southwest Nevada, is ... more Yucca Mountain, a ridge of shallowly dipping, Miocene-age volcanic rocks in southwest Nevada, is the proposed site for a nuclear waste repository to be constructed in the 500- to 700-m-thick unsaturated zone (UZ). At the proposed repository, the 300-m-thick Topopah Spring Tuff welded unit (TSw) is overlain by approximately 30 m of nonwelded tuffs (PTn); the Tiva Canyon Tuff welded unit (TCw) overlies the PTn with a range in thickness from 0 to approximately 130 m at the site. The amount of water percolation through the UZ is low and difficult to measure directly, but local seepage into mined tunnels has been observed in the TCw. Past water seepage in the welded tuffs is recorded by widespread, thin (0.3 cm) coatings of calcite and silica on fracture surfaces and within cavities. Abundances of calcite and silica in the coatings were determined by X-ray microfluorescence mapping and subsequent multispectral image analysis of over 200 samples. The images were classified into constituen...

Tunneling of the Exploratory Studies Facility has offered the opportunity to sample and examine o... more Tunneling of the Exploratory Studies Facility has offered the opportunity to sample and examine occurrences of secondary mineralization found in the unsaturated-zone tuffs of Yucca Mountain, Nevada. Petrographic and paragenetic analyses, calcite and silica-phase stable isotopic analyses, and preliminary strontium tracer isotope and radiocarbon age analyses of these samples indicate that (1) an early stage of secondary mineralization consisting largely of chalcedony and quartz, but possibly with or slightly preceded by calcite, probably formed at warmer than ambient temperatures; (2) later secondary mineralization consisting of calcite and opal appears completely consistent with formation from percolation of surface infiltration whose solute load and carbon isotopic compositions reflect passage through the overlying soils; (3) based on textural studies, all unsaturated-zone secondary mineral occurrences exposed within the Exploratory Studies Facility tunnel, with the exception of the vapor-phase assemblages that formed at high temperatures during cooling of the tuffs, probably formed in unsaturated settings; and (4) calcite radiocarbon ages, based on preliminary results, have not been compromised by post-depositional exchange with carbon-bearing water and gases in the unsaturated zone.

Strontium isotopic analyses of barren and mineralized Paleozoic carbonate rocks show that hydroth... more Strontium isotopic analyses of barren and mineralized Paleozoic carbonate rocks show that hydrothermal fluids added radiogenic strontium (⁸⁷Sr) to the mineralized zones. At Bare Mountain, samples collected from mineralized areas have δ⁸⁷Sr{sub t} values ranging from +3.0 to +23.0, whereas unmineralized carbonate rocks have δ⁸⁷Sr, values of -0.6 to +2.9. In other ranges, δ⁸⁷Sr, values of the unmineralized carbonate rocks