Karlis Muehlenbachs | University of Alberta (original) (raw)
Papers by Karlis Muehlenbachs
Journal of Geophysical Research: Biogeosciences, 2019
The biogeochemistry of two alkaline permeable reactive barriers (PRBs) installed for remediation ... more The biogeochemistry of two alkaline permeable reactive barriers (PRBs) installed for remediation in a mining‐affected wetland was investigated in order to assess the importance of colloidal particles on metal removal processes in such systems. At the time of investigation, both PRBs were effective in removing U, Cu, and Zn (>95%) from groundwater but were slightly less efficient for Ni and Co (<90%). Previously installed groundwater wells allowed an in‐depth analysis of groundwater passing through the first PRB. Here, in an alkaline environment (pH 6.0–9.7), 11–14% of Ni, 36–37% of Co, 77–81% of Cu, 14–17% of U, and 10–19% of Fe were associated with organic matter and inorganic colloids, while upgradient in the more acidic environments (pH <6.0), ionic species and complexes (e.g., Co2+, Ni2+, Cu2+, and UO2H3SiO4+) dominated. Copper and U preferentially bound to larger colloidal fractions (>1 kDa), which might have promoted their sequestration. Uranium removal was likely ...
Earth and Planetary Science Letters, 2017
Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes ... more Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their coexistence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr-Nd-Hf-Pb-Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ε Hf coupled with >+5h δ 7 Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05-3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (−3.6 to −2.4h δ 13 C for carbonatites versus −5.7 to −3.6h δ 13 C for kimberlites) require open-system fractionation at magmatic temperatures. Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of 'pure' carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16-33 wt.% SiO 2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly differentiated 'cratonic carbonatites' have only little in common with those of metasomatic agents that act on the deeper lithosphere. Consequently, carbonatite trace element systematics should only be used with caution when constraining carbon mobility and metasomatism at mantle depths. Regardless of the exact nature of carbonate-bearing melts within the mantle lithosphere, they play an important role in enrichment processes, thereby decreasing the stability of buoyant cratons and promoting rift initiationas exemplified by the Mesozoic-Cenozoic breakup of the North Atlantic craton.
Soil Science Society of America Journal, 1995
The formation of dolomite is poorly understood and highly controversial, since well‐ordered dolom... more The formation of dolomite is poorly understood and highly controversial, since well‐ordered dolomite has never been synthesized under earth surface conditions and natural occurrences of modern dolomite have been reported only rarely from restricted sedimentary environments. It is generally accepted that dolomite does not form in soils. This study investigates the characteristics and origin of dolomite in a sulfatic saline soil in east‐central Alberta, Canada, in which dolomite makes up more than half of the <2‐µm fraction. X‐ray diffraction analysis shows that the dolomite is stoichiometric and ordered, with only slight deviations in unit cell parameters relative to ideal dolomite. The clay‐sized dolomite is enriched in 18O and depleted in 13C relative to detrital dolomite and dolomite present in larger size fractions, and has a radiocarbon age ranging from 1270 to 5270 yr before present, clearly indicating an authigenic origin. The very small dolomite particle size and differenc...
Soil Science Society of America Journal, 1985
... centrifugation to remove the excess electro-lyte. Oriented specimens for x-ray diffraction an... more ... centrifugation to remove the excess electro-lyte. Oriented specimens for x-ray diffraction analysis were prepared on glass slides using the paste method (Thiesen and Harward, 1962). The remaining portions of the clay fractions ...
Journal of Materials Science, 1978
American Journal of Science, 1973
De Gruyter eBooks, Dec 31, 1986
Canada is the third largest oil and gas producer, with over 500,000 active and inactive wells, mo... more Canada is the third largest oil and gas producer, with over 500,000 active and inactive wells, mostly located in the Western Canada Sedimentary Basin (WCSB). A large, undetermined fraction of Canada’s GHG emissions emanate from oil and gas infrastructure. Governments and industry are all committed to immediately reducing methane leaks to the atmosphere from surface casing vent flows (SCVF) and ground migration (GM) of both new and old wells. Methane carbon isotopic composition offers insight into the source of unwanted gas emissions. A geospatial tool would help to attribute and reduce GHG emissions from contour maps of ẟ13C of methane and other hydrocarbons of production, SCVF, and GM gases across the WCSB. These “Isoscapes” of production gases vary systematically, reflecting the local geology. SCVF and GM isoscapes are offset from the production ones because the SCVF most often are shallower than the target formations, and the GM gas may be oxidized in soils. The difference between the production and SCVF isoscapes can be used to attribute methane emissions from tanks and production infrastructure, compared to leaks from the wells themselves. The isoscapes directly facilitate the plugging of problem wells. The maps are based on over 3,000 locations where we used isotope fingerprinting (i.e., Rowe & Muehlenbachs, 1999) to identify the source depth of a leak. Regulatory measurements mandate that the leaks are sealed at their source depth, greatly adding to the cost of plugging any well. The SCVF isoscapes suggest the likely source depth of an unsampled leaking well, thus greatly simplifying its remediation. Applying such information beyond a local case study may contribute to accounting for the GH contribution from regional oil and gas activities in Canada and elsewhere. ReferenceRowe, D., & Muehlenbachs, A. (1999). Low-temperature thermal generation of hydrocarbon gases in shallow shales. Nature, 398(6722), 61-63.
This database provides the raw data used for calculation of (i) carbon input flux carried by sedi... more This database provides the raw data used for calculation of (i) carbon input flux carried by sediments and altered oceanic crust along Central-Northern Lesser Antilles trench; (ii) carbon output flux through the dominant volcanoes at Central-Northern Lesser Antilles arc. Carbon input flux is calculated based on the measured carbon concentration of sediments and altered oceanic crust samples recovered directly from Deep Sea Drilling Programs Site 543. Carbon output flux is calculated based on the previous volcanic gas data. Through the comparison between carbon input and output flux at Central-Northern Lesser Antilles arc, carbon recycling efficiency of arc of Central-Northern Lesser Antilles is obtained as ~100%. This indicate that subducted slab carbon is efficiently recycled at the sub-arc depth of Central-Northern Lesser Antilles subduction zone with minor carbon loss within the forearc region. Furthermore, our study highlights that slab carbon is recycled back to the surface with a different degree at different subduction zones, and a highly variable carbon recycling efficiency of arc volcanoe is observed on a global scale. More works are needed to constrain the carbon recycling efficiency of arc at individual subduction zones.
Leg 83 of the Deep Sea Drilling Project has deepened Hole 504B to over 1 km into basement, 1350 m... more Leg 83 of the Deep Sea Drilling Project has deepened Hole 504B to over 1 km into basement, 1350 m below the seafloor (BSF). The hole previously extended through 274.5 m of sediment and 561.5 m of pillow basalts altered at low temperature (< 100°C), to 836 m BSF. Leg 83 drilling penetrated an additional 10 m of pillows, a 209-m transition zone, and 295 m into a sheeted dike complex. Leg 83 basalts (836-1350 m BSF) generally contain superimposed greenschist and zeolite-facies mineral parageneses. Alteration of pillows and dikes from 836 to 898 m BSF occurred under reducing conditions at low water/rock ratios, and at temperatures probably greater than 100°C. Evolution of fluid composition resulted in the formation of (1) clay minerals, followed by (2) zeolites, anhydrite, and calcite. Alteration of basalts in the transition zone and dike sections (898-1350 m BSF) occurred in three basic stages, defined by the opening of fractures and the formation of characteristic secondary minerals. (1) Chlorite, actinolite, pyrite, albite, sphene, and minor quartz formed in veins and host basalts from partially reacted seawater (Mg-bearing, locally metal-and Si-enriched) at temperatures of at least 200-250°C. (2) Quartz, epidote, and sulfides formed in veins at temperatures of up to 380°C, from more evolved (Mg-depleted, metal-, Si-, and 18 O-enriched) fluids. (3) The last stage is characterized by zeolite formation: (a) analcite and stilbite formed locally, possibly at temperatures less than 200°C followed by (b) formation of laumontite, heulàndite, scolecite, calcite, and prehnite from solutions depleted in Mg and enriched in Ca and 18 O, at temperatures of up to 250°C. The presence of small amounts of anhydrite locally may be due to ingress of relatively unaltered seawater into the system during Stage 3. Alteration was controlled by the permeability of the crust and is characterized by generally incomplete recrystallization and replacement reactions among secondary minerals. Secondary mineralogy in the host basalts is strongly controlled by primary mineralogy. The alteration of Leg 83 basalts can be interpreted in terms of an evolving hydrothermal system, with (a) changes in solution composition because of reaction of seawater fluids with basalts at high temperatures; (b) variations in permeability caused by several stages of sealing and reopening of cracks; and (c) a general cooling of the system, caused either by the cooling of a magma chamber beneath the spreading center and/or the movement of the crust away from the heat source. The relationship of the high-temperature alteration in the transition zone and dike sections to the low-temperature alteration in the overlying pillow section remains uncertain.
Delta180 and 87Sr/86Sr isotopic data from smectites, calcites, and whole rocks, together with pub... more Delta180 and 87Sr/86Sr isotopic data from smectites, calcites, and whole rocks, together with published isotopic age determinations, alkali element concentration data and petrographic observations suggest a sequential model of ocean floor alteration. The early stage lasts about 3 m.y. and is characterized by palagonite and smectite formation, and solutions with a large basaltic component, increasing with temperature which varies from 15° to 80° C at DSDP site 418A. Most carbonates are deposited after this stage from solutions with a negligible basaltic Sr component and temperatures of 15° to 40° C. Water of seawater Sr and O isotopic composition is shown to percolate to at least 500 m into the basaltic basement. No evidence was found for continuing exchange of strontium or oxygen after 3 m.y.
A novel strip-sampling technique has been applied to the 500-m gabbroic section drilled at site 7... more A novel strip-sampling technique has been applied to the 500-m gabbroic section drilled at site 735 during Leg 118. Twenty-two continuous strips of 1.1- to 4.5-m length were cut longitudinally from the core, allowing for a more representative sampling of this section of the deep ocean crust. A full suite of trace element and isotopic (Sr, Nd, Pb, Os, d18O) analyses were conducted on these strip samples; for comparison, analyses were conducted on a small suite of protolith samples, selected for their fresh and unaltered appearance. Amphibole, diopside, and plagioclase from 18 vein samples were also analyzed for Sr and Nd isotopes. Although the evidence for a seawater component in these gabbros is clear (87/86 Sr up to 0.70316; 206/204 Pb up to 19.3; d18O down to 2.0‰; 187/188 Os up to 0.44), the trace element signatures are dominated by magmatic effects (infiltration and impregnation by late-stage melts derived locally or from deeper levels of the crust). The average upper 500 m 735B gabbro section is somewhat lower than average N-MORB in trace elements such as Ba (30%), Nb (50%), U (40%), and heavy REE (Yb and Lu, 30%), but somewhat enriched in others such as La (23%), Ce (24%), Pb (23%), and Sr (40%). Although the section is largely comprised of cumulate gabbros (Natland et al., 1991), and many of the strip samples show marked Sr and Eu anomalies (plagioclase cumulation), the average composition of the total 500 m section shows no Sr or Eu anomalies (<1%). This implies that there has been local separation of melt and solids, but no large scale removal of melts from this 500-m gabbro section.
Textural and DNA studies of pillow lavas in DSDP/ODP cores from the Atlantic Ocean, the Lau Basin... more Textural and DNA studies of pillow lavas in DSDP/ODP cores from the Atlantic Ocean, the Lau Basin and the Costa Rica Rift indicate that microbes had a significant role in the alteration of basaltic glasses. Carbon isotopes (d13C) in carbonates from glassy and crystalline basalts from these locations also show differences that may relate to microbial activity during alteration. The generally low d13C values (< -7 per mil) in the basaltic glass of a high proportion of samples from the Atlantic, and most of the Costa Rica Rift material were attributed to alteration influenced by Bacteria and oxidation of organic matter. Positive d13C values of some samples from the Atlantic suggest lithotrophic utilization of CO2, in which methanogenic Archaea produced CH4 from H2 and CO2. This may result from higher abiotic production of H2 in the slow-spreading, fault-dominated Atlantic crust, due to more extensive serpentinization than at the intermediate-spreading Costa Rica Rift.
Journal of Geophysical Research: Biogeosciences, 2019
The biogeochemistry of two alkaline permeable reactive barriers (PRBs) installed for remediation ... more The biogeochemistry of two alkaline permeable reactive barriers (PRBs) installed for remediation in a mining‐affected wetland was investigated in order to assess the importance of colloidal particles on metal removal processes in such systems. At the time of investigation, both PRBs were effective in removing U, Cu, and Zn (>95%) from groundwater but were slightly less efficient for Ni and Co (<90%). Previously installed groundwater wells allowed an in‐depth analysis of groundwater passing through the first PRB. Here, in an alkaline environment (pH 6.0–9.7), 11–14% of Ni, 36–37% of Co, 77–81% of Cu, 14–17% of U, and 10–19% of Fe were associated with organic matter and inorganic colloids, while upgradient in the more acidic environments (pH <6.0), ionic species and complexes (e.g., Co2+, Ni2+, Cu2+, and UO2H3SiO4+) dominated. Copper and U preferentially bound to larger colloidal fractions (>1 kDa), which might have promoted their sequestration. Uranium removal was likely ...
Earth and Planetary Science Letters, 2017
Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes ... more Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their coexistence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr-Nd-Hf-Pb-Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ε Hf coupled with >+5h δ 7 Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05-3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (−3.6 to −2.4h δ 13 C for carbonatites versus −5.7 to −3.6h δ 13 C for kimberlites) require open-system fractionation at magmatic temperatures. Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of 'pure' carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16-33 wt.% SiO 2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly differentiated 'cratonic carbonatites' have only little in common with those of metasomatic agents that act on the deeper lithosphere. Consequently, carbonatite trace element systematics should only be used with caution when constraining carbon mobility and metasomatism at mantle depths. Regardless of the exact nature of carbonate-bearing melts within the mantle lithosphere, they play an important role in enrichment processes, thereby decreasing the stability of buoyant cratons and promoting rift initiationas exemplified by the Mesozoic-Cenozoic breakup of the North Atlantic craton.
Soil Science Society of America Journal, 1995
The formation of dolomite is poorly understood and highly controversial, since well‐ordered dolom... more The formation of dolomite is poorly understood and highly controversial, since well‐ordered dolomite has never been synthesized under earth surface conditions and natural occurrences of modern dolomite have been reported only rarely from restricted sedimentary environments. It is generally accepted that dolomite does not form in soils. This study investigates the characteristics and origin of dolomite in a sulfatic saline soil in east‐central Alberta, Canada, in which dolomite makes up more than half of the <2‐µm fraction. X‐ray diffraction analysis shows that the dolomite is stoichiometric and ordered, with only slight deviations in unit cell parameters relative to ideal dolomite. The clay‐sized dolomite is enriched in 18O and depleted in 13C relative to detrital dolomite and dolomite present in larger size fractions, and has a radiocarbon age ranging from 1270 to 5270 yr before present, clearly indicating an authigenic origin. The very small dolomite particle size and differenc...
Soil Science Society of America Journal, 1985
... centrifugation to remove the excess electro-lyte. Oriented specimens for x-ray diffraction an... more ... centrifugation to remove the excess electro-lyte. Oriented specimens for x-ray diffraction analysis were prepared on glass slides using the paste method (Thiesen and Harward, 1962). The remaining portions of the clay fractions ...
Journal of Materials Science, 1978
American Journal of Science, 1973
De Gruyter eBooks, Dec 31, 1986
Canada is the third largest oil and gas producer, with over 500,000 active and inactive wells, mo... more Canada is the third largest oil and gas producer, with over 500,000 active and inactive wells, mostly located in the Western Canada Sedimentary Basin (WCSB). A large, undetermined fraction of Canada’s GHG emissions emanate from oil and gas infrastructure. Governments and industry are all committed to immediately reducing methane leaks to the atmosphere from surface casing vent flows (SCVF) and ground migration (GM) of both new and old wells. Methane carbon isotopic composition offers insight into the source of unwanted gas emissions. A geospatial tool would help to attribute and reduce GHG emissions from contour maps of ẟ13C of methane and other hydrocarbons of production, SCVF, and GM gases across the WCSB. These “Isoscapes” of production gases vary systematically, reflecting the local geology. SCVF and GM isoscapes are offset from the production ones because the SCVF most often are shallower than the target formations, and the GM gas may be oxidized in soils. The difference between the production and SCVF isoscapes can be used to attribute methane emissions from tanks and production infrastructure, compared to leaks from the wells themselves. The isoscapes directly facilitate the plugging of problem wells. The maps are based on over 3,000 locations where we used isotope fingerprinting (i.e., Rowe & Muehlenbachs, 1999) to identify the source depth of a leak. Regulatory measurements mandate that the leaks are sealed at their source depth, greatly adding to the cost of plugging any well. The SCVF isoscapes suggest the likely source depth of an unsampled leaking well, thus greatly simplifying its remediation. Applying such information beyond a local case study may contribute to accounting for the GH contribution from regional oil and gas activities in Canada and elsewhere. ReferenceRowe, D., & Muehlenbachs, A. (1999). Low-temperature thermal generation of hydrocarbon gases in shallow shales. Nature, 398(6722), 61-63.
This database provides the raw data used for calculation of (i) carbon input flux carried by sedi... more This database provides the raw data used for calculation of (i) carbon input flux carried by sediments and altered oceanic crust along Central-Northern Lesser Antilles trench; (ii) carbon output flux through the dominant volcanoes at Central-Northern Lesser Antilles arc. Carbon input flux is calculated based on the measured carbon concentration of sediments and altered oceanic crust samples recovered directly from Deep Sea Drilling Programs Site 543. Carbon output flux is calculated based on the previous volcanic gas data. Through the comparison between carbon input and output flux at Central-Northern Lesser Antilles arc, carbon recycling efficiency of arc of Central-Northern Lesser Antilles is obtained as ~100%. This indicate that subducted slab carbon is efficiently recycled at the sub-arc depth of Central-Northern Lesser Antilles subduction zone with minor carbon loss within the forearc region. Furthermore, our study highlights that slab carbon is recycled back to the surface with a different degree at different subduction zones, and a highly variable carbon recycling efficiency of arc volcanoe is observed on a global scale. More works are needed to constrain the carbon recycling efficiency of arc at individual subduction zones.
Leg 83 of the Deep Sea Drilling Project has deepened Hole 504B to over 1 km into basement, 1350 m... more Leg 83 of the Deep Sea Drilling Project has deepened Hole 504B to over 1 km into basement, 1350 m below the seafloor (BSF). The hole previously extended through 274.5 m of sediment and 561.5 m of pillow basalts altered at low temperature (< 100°C), to 836 m BSF. Leg 83 drilling penetrated an additional 10 m of pillows, a 209-m transition zone, and 295 m into a sheeted dike complex. Leg 83 basalts (836-1350 m BSF) generally contain superimposed greenschist and zeolite-facies mineral parageneses. Alteration of pillows and dikes from 836 to 898 m BSF occurred under reducing conditions at low water/rock ratios, and at temperatures probably greater than 100°C. Evolution of fluid composition resulted in the formation of (1) clay minerals, followed by (2) zeolites, anhydrite, and calcite. Alteration of basalts in the transition zone and dike sections (898-1350 m BSF) occurred in three basic stages, defined by the opening of fractures and the formation of characteristic secondary minerals. (1) Chlorite, actinolite, pyrite, albite, sphene, and minor quartz formed in veins and host basalts from partially reacted seawater (Mg-bearing, locally metal-and Si-enriched) at temperatures of at least 200-250°C. (2) Quartz, epidote, and sulfides formed in veins at temperatures of up to 380°C, from more evolved (Mg-depleted, metal-, Si-, and 18 O-enriched) fluids. (3) The last stage is characterized by zeolite formation: (a) analcite and stilbite formed locally, possibly at temperatures less than 200°C followed by (b) formation of laumontite, heulàndite, scolecite, calcite, and prehnite from solutions depleted in Mg and enriched in Ca and 18 O, at temperatures of up to 250°C. The presence of small amounts of anhydrite locally may be due to ingress of relatively unaltered seawater into the system during Stage 3. Alteration was controlled by the permeability of the crust and is characterized by generally incomplete recrystallization and replacement reactions among secondary minerals. Secondary mineralogy in the host basalts is strongly controlled by primary mineralogy. The alteration of Leg 83 basalts can be interpreted in terms of an evolving hydrothermal system, with (a) changes in solution composition because of reaction of seawater fluids with basalts at high temperatures; (b) variations in permeability caused by several stages of sealing and reopening of cracks; and (c) a general cooling of the system, caused either by the cooling of a magma chamber beneath the spreading center and/or the movement of the crust away from the heat source. The relationship of the high-temperature alteration in the transition zone and dike sections to the low-temperature alteration in the overlying pillow section remains uncertain.
Delta180 and 87Sr/86Sr isotopic data from smectites, calcites, and whole rocks, together with pub... more Delta180 and 87Sr/86Sr isotopic data from smectites, calcites, and whole rocks, together with published isotopic age determinations, alkali element concentration data and petrographic observations suggest a sequential model of ocean floor alteration. The early stage lasts about 3 m.y. and is characterized by palagonite and smectite formation, and solutions with a large basaltic component, increasing with temperature which varies from 15° to 80° C at DSDP site 418A. Most carbonates are deposited after this stage from solutions with a negligible basaltic Sr component and temperatures of 15° to 40° C. Water of seawater Sr and O isotopic composition is shown to percolate to at least 500 m into the basaltic basement. No evidence was found for continuing exchange of strontium or oxygen after 3 m.y.
A novel strip-sampling technique has been applied to the 500-m gabbroic section drilled at site 7... more A novel strip-sampling technique has been applied to the 500-m gabbroic section drilled at site 735 during Leg 118. Twenty-two continuous strips of 1.1- to 4.5-m length were cut longitudinally from the core, allowing for a more representative sampling of this section of the deep ocean crust. A full suite of trace element and isotopic (Sr, Nd, Pb, Os, d18O) analyses were conducted on these strip samples; for comparison, analyses were conducted on a small suite of protolith samples, selected for their fresh and unaltered appearance. Amphibole, diopside, and plagioclase from 18 vein samples were also analyzed for Sr and Nd isotopes. Although the evidence for a seawater component in these gabbros is clear (87/86 Sr up to 0.70316; 206/204 Pb up to 19.3; d18O down to 2.0‰; 187/188 Os up to 0.44), the trace element signatures are dominated by magmatic effects (infiltration and impregnation by late-stage melts derived locally or from deeper levels of the crust). The average upper 500 m 735B gabbro section is somewhat lower than average N-MORB in trace elements such as Ba (30%), Nb (50%), U (40%), and heavy REE (Yb and Lu, 30%), but somewhat enriched in others such as La (23%), Ce (24%), Pb (23%), and Sr (40%). Although the section is largely comprised of cumulate gabbros (Natland et al., 1991), and many of the strip samples show marked Sr and Eu anomalies (plagioclase cumulation), the average composition of the total 500 m section shows no Sr or Eu anomalies (<1%). This implies that there has been local separation of melt and solids, but no large scale removal of melts from this 500-m gabbro section.
Textural and DNA studies of pillow lavas in DSDP/ODP cores from the Atlantic Ocean, the Lau Basin... more Textural and DNA studies of pillow lavas in DSDP/ODP cores from the Atlantic Ocean, the Lau Basin and the Costa Rica Rift indicate that microbes had a significant role in the alteration of basaltic glasses. Carbon isotopes (d13C) in carbonates from glassy and crystalline basalts from these locations also show differences that may relate to microbial activity during alteration. The generally low d13C values (< -7 per mil) in the basaltic glass of a high proportion of samples from the Atlantic, and most of the Costa Rica Rift material were attributed to alteration influenced by Bacteria and oxidation of organic matter. Positive d13C values of some samples from the Atlantic suggest lithotrophic utilization of CO2, in which methanogenic Archaea produced CH4 from H2 and CO2. This may result from higher abiotic production of H2 in the slow-spreading, fault-dominated Atlantic crust, due to more extensive serpentinization than at the intermediate-spreading Costa Rica Rift.