The subsurface evolution of shallow-sea hydrothermal fluids is a function of many factors includi... more The subsurface evolution of shallow-sea hydrothermal fluids is a function of many factors including fluid?mineral equilibria, phase separation, magmatic inputs, and mineral precipitation, all of which influence discharging fluid chemistry and consequently associated seafloor microbial communities. Shallow-sea vent systems, however, are understudied in this regard. In order to investigate subsurface processes in a shallow-sea hydrothermal vent, and determine how these physical and chemical parameters influence the metabolic potential of the microbial communities, three shallow-sea hydrothermal vents associated with Panarea Island (Italy) were characterized. Vent fluids, pore fluids and gases at the three sites were sampled and analyzed for major and minor elements, redox-sensitive compounds, free gas compositions, and strontium isotopes. The corresponding data were used to 1) describe the subsurface geochemical evolution of the fluids and 2) to evaluate the catabolic potential of 61 inorganic redox reactions for in situ microbial communities. Generally, the vent fluids can be hot (up to 135 °C), acidic (pH 1.9-5.7), and sulfidic (up to 2.5 mM H2S). Three distinct types of hydrothermal fluids were identified, each with higher temperatures and lower pH, Mg and SO4, relative to seawater. Type 1 was consistently more saline than Type 2, and both were more saline than seawater. Type 3 fluids were similar to or slightly depleted in most major ions relative to seawater. End-member calculations of conservative elements indicate that Type 1 and Type 2 fluids are derived from two different sources, most likely 1) a deeper, higher salinity reservoir and 2) a shallower, lower salinity reservoir, respectively, in a layered hydrothermal system. The deeper reservoir records some of the highest end-member Cl concentrations to date, and developed as a result of recirculation of brine fluids with long term loss of steam and volatiles due to past phase separation. No strong evidence for ongoing phase separation is observed. Type 3 f [...]
ABSTRACT The northern Cascadia margin of North America represents the classic example of a &q... more ABSTRACT The northern Cascadia margin of North America represents the classic example of a "hot" subduction system. The downgoing Juan de Fuca plate ranges in age from about 10 m.y. along the coast of central Washington to less than 5 m.y. off central Vancouver Island. The Garibaldi volcanic belt (GVB) is a trench parallel, NNW-trending chain of small composite volcanoes, subglacial volcanic domes, and lava fields that lies 250 km east of the convergen margin. Beneath the GVB, the oceanic lithosphere attains an age that decreases northward from about 22 m.y to13 m.y. Primitive mafic lavas, which occur trenchward of the GVB volcanic front, range northward from high-alumina olivine tholeiites, magnesian andesites and LILE- and LREE-enriched calc-alkaline basalts at Glacier Peak, northern Washington, through transitional basalts in southern British Columbia to alkali olivine basalts and trachybasalts at Meager Mountain and Bridge River at the northern end of the volcanic belt. Concentrations of fluid-mobile-elements (FMEs) in the basalts suggest a significant interrelationship between forearc metamorphic dehydration/devolatization processes in the subducted plate and melting conditions within the mantle wedge beneath the GVB volcanic front. The forearc GVB mafic lavas generally possess an exceptionally low and more restricted range of boron abundances (0.9-5.7 ppm) compared to lavas of other subduction systems, including those of the High Cascades. The basalts also are characterized by low abundances of other FMEs (e.g., 1.5-7.9 ppm Pb, 5-11 ppm Rb, 0.10-0.36 ppm Cs, 0.7-1.7 ppm As, and 0.1-0.4 ppm Sb). Concentrations of B and FMEs in the lavas, as well as FME/LREE (e.g. Cs/La, Rb/La, and Pb/Ce) and 87Sr/86Sr ratios, correlate positively with the inferred age of the subducted slab immediately beneath the volcanic front. Such variations are consistent with increased metamorphic devolatilization of the Juan de Fuca lithosphere within the Cascadia seismogenic zone as the downgoing plate becomes younger and hotter northward along the convergent margin. Correlations between FME and major-element, REE and HFSE abundances suggest that more extensive dehydration of the slab beneath the northernmost basaltic centers resulted in lower degrees of wedge melting (5-10%) at higher P-T than conditions responsible for their more southerly counterparts (10-22%). The GVB mafic lavas therefore record a close relationship between the forearc metamorphic conditions and the degree of mantle melting beneath the volcanic front (i.e., the nature of primary arc magmas).
We explore the consequences of "hot" subduction zone thermal structure for the recyclin... more We explore the consequences of "hot" subduction zone thermal structure for the recycling of fluid mobile elements across convergent margins. We selected the Late Cenozoic Garibaldi Volcanic Belt (GVB) of the Northern Cascadia Subduction System because it is associated with the subduction of extremely young (< 23 Ma) and "hot" oceanic lithosphere. The volcanic belt is ~ 15 km wide
ABSTRACT We have studied glasses from the Southernmost Mariana Trough (13.4-14.3° N and 144-144.6... more ABSTRACT We have studied glasses from the Southernmost Mariana Trough (13.4-14.3° N and 144-144.6° E) recovered during the Cook 7 expedition (2001). Due to their differing proximities to the volcanic front of the Mariana island arc (33 to 106 km), these samples are critical for evaluating across arc elemental and isotope transport and possible linkages between back-arc spreading ridges and the adjacent volcanic arc chains. Petrographic examination shows all of the samples to be unaltered dark colored volcanic glasses with rare crystals of olivine, pyroxene and plagioclase. Their Mg #s range between 33 and 60, with most Mg# >50. Although samples were recovered from water depths of 3.2 to 4.2 km, there is lack of major element and Cl/K concentrations that indicate any seawater alterations (e.g. LOI and Na contents). The lack of seawater signature is also shown by low (0.70275 to 0.70302) 87Sr/86Sr, which is depleted MORB range. The glasses also have low H2O (~1 wt%). This lack of secondary glass alteration allows study of the behavior of volatile and low- temperature alteration sensitive tracers (e.g. B, Cl and B and Sr isotopes). There is no correlation between depth of recovery and B, Cl or d11B systematics, indicating that the degree of vesiculation did not control the d11B. The B content of the glasses is low (generally MORB- like) and the Cl contents show progressive increase with distance from the volcanic front and range between 150 and 1300 ppm. The d11B ratios vary widely and are between +5.3 to -5.6 per mil. As expected, the d11B values become progressively heavier closer to the volcanic front, where they are indistinguishable from the Mariana arc lavas. Interestingly, the heaviest d11B samples are also associated with elevated ratios of Ba/La and U/Nb, and to some extent also with elevated Th/Yb. In arcs, U and Ba are fluid mobile, while Th is a sediment melt sensitive element. From the preliminary dataset it appears that the samples from the S. Mariana Trough are transitional between typical island arc and back-arc magmas. Our new data confirms the proposed highly variable mantle sources along the strike of the Izu-Bonin-Mariana back-arc basalts (Pearce et al., 2005) and stress the fact that distance from the volcanic front may be an important constraint to consider when evaluating high resolution tomographic imaging and modeling mantle flow in the southern Marianas.
The early Izu-Bonin-Mariana (IBM) arc formed during the Eocene along a margin of the West Philipp... more The early Izu-Bonin-Mariana (IBM) arc formed during the Eocene along a margin of the West Philippine Basin (WPB). At that time, the WPB had at least one active spreading center, the Central Basin Ridge (CBR) and it was rimmed by fragments of older, Early Cretaceous to Paleocene island arc and ocean floor lithosphere. Published models of IBM arc initiation suggest
Serpentinized harzburgites recently drilled during ODP Leg 195 at South Chamorro Seamount in the ... more Serpentinized harzburgites recently drilled during ODP Leg 195 at South Chamorro Seamount in the Mariana forearc region have been studied for their platinum group element (PGE) concentrations and Os isotopic compositions. The samples allow a look at the slab fluid-modified subarc mantle immediately overlaying the actively subducting Pacific slab at depths of ~ 30 km. The average PGE (Os 2.3
The subsurface evolution of shallow-sea hydrothermal fluids is a function of many factors includi... more The subsurface evolution of shallow-sea hydrothermal fluids is a function of many factors including fluid?mineral equilibria, phase separation, magmatic inputs, and mineral precipitation, all of which influence discharging fluid chemistry and consequently associated seafloor microbial communities. Shallow-sea vent systems, however, are understudied in this regard. In order to investigate subsurface processes in a shallow-sea hydrothermal vent, and determine how these physical and chemical parameters influence the metabolic potential of the microbial communities, three shallow-sea hydrothermal vents associated with Panarea Island (Italy) were characterized. Vent fluids, pore fluids and gases at the three sites were sampled and analyzed for major and minor elements, redox-sensitive compounds, free gas compositions, and strontium isotopes. The corresponding data were used to 1) describe the subsurface geochemical evolution of the fluids and 2) to evaluate the catabolic potential of 61 inorganic redox reactions for in situ microbial communities. Generally, the vent fluids can be hot (up to 135 °C), acidic (pH 1.9-5.7), and sulfidic (up to 2.5 mM H2S). Three distinct types of hydrothermal fluids were identified, each with higher temperatures and lower pH, Mg and SO4, relative to seawater. Type 1 was consistently more saline than Type 2, and both were more saline than seawater. Type 3 fluids were similar to or slightly depleted in most major ions relative to seawater. End-member calculations of conservative elements indicate that Type 1 and Type 2 fluids are derived from two different sources, most likely 1) a deeper, higher salinity reservoir and 2) a shallower, lower salinity reservoir, respectively, in a layered hydrothermal system. The deeper reservoir records some of the highest end-member Cl concentrations to date, and developed as a result of recirculation of brine fluids with long term loss of steam and volatiles due to past phase separation. No strong evidence for ongoing phase separation is observed. Type 3 f [...]
ABSTRACT The northern Cascadia margin of North America represents the classic example of a &q... more ABSTRACT The northern Cascadia margin of North America represents the classic example of a "hot" subduction system. The downgoing Juan de Fuca plate ranges in age from about 10 m.y. along the coast of central Washington to less than 5 m.y. off central Vancouver Island. The Garibaldi volcanic belt (GVB) is a trench parallel, NNW-trending chain of small composite volcanoes, subglacial volcanic domes, and lava fields that lies 250 km east of the convergen margin. Beneath the GVB, the oceanic lithosphere attains an age that decreases northward from about 22 m.y to13 m.y. Primitive mafic lavas, which occur trenchward of the GVB volcanic front, range northward from high-alumina olivine tholeiites, magnesian andesites and LILE- and LREE-enriched calc-alkaline basalts at Glacier Peak, northern Washington, through transitional basalts in southern British Columbia to alkali olivine basalts and trachybasalts at Meager Mountain and Bridge River at the northern end of the volcanic belt. Concentrations of fluid-mobile-elements (FMEs) in the basalts suggest a significant interrelationship between forearc metamorphic dehydration/devolatization processes in the subducted plate and melting conditions within the mantle wedge beneath the GVB volcanic front. The forearc GVB mafic lavas generally possess an exceptionally low and more restricted range of boron abundances (0.9-5.7 ppm) compared to lavas of other subduction systems, including those of the High Cascades. The basalts also are characterized by low abundances of other FMEs (e.g., 1.5-7.9 ppm Pb, 5-11 ppm Rb, 0.10-0.36 ppm Cs, 0.7-1.7 ppm As, and 0.1-0.4 ppm Sb). Concentrations of B and FMEs in the lavas, as well as FME/LREE (e.g. Cs/La, Rb/La, and Pb/Ce) and 87Sr/86Sr ratios, correlate positively with the inferred age of the subducted slab immediately beneath the volcanic front. Such variations are consistent with increased metamorphic devolatilization of the Juan de Fuca lithosphere within the Cascadia seismogenic zone as the downgoing plate becomes younger and hotter northward along the convergent margin. Correlations between FME and major-element, REE and HFSE abundances suggest that more extensive dehydration of the slab beneath the northernmost basaltic centers resulted in lower degrees of wedge melting (5-10%) at higher P-T than conditions responsible for their more southerly counterparts (10-22%). The GVB mafic lavas therefore record a close relationship between the forearc metamorphic conditions and the degree of mantle melting beneath the volcanic front (i.e., the nature of primary arc magmas).
We explore the consequences of "hot" subduction zone thermal structure for the recyclin... more We explore the consequences of "hot" subduction zone thermal structure for the recycling of fluid mobile elements across convergent margins. We selected the Late Cenozoic Garibaldi Volcanic Belt (GVB) of the Northern Cascadia Subduction System because it is associated with the subduction of extremely young (< 23 Ma) and "hot" oceanic lithosphere. The volcanic belt is ~ 15 km wide
ABSTRACT We have studied glasses from the Southernmost Mariana Trough (13.4-14.3° N and 144-144.6... more ABSTRACT We have studied glasses from the Southernmost Mariana Trough (13.4-14.3° N and 144-144.6° E) recovered during the Cook 7 expedition (2001). Due to their differing proximities to the volcanic front of the Mariana island arc (33 to 106 km), these samples are critical for evaluating across arc elemental and isotope transport and possible linkages between back-arc spreading ridges and the adjacent volcanic arc chains. Petrographic examination shows all of the samples to be unaltered dark colored volcanic glasses with rare crystals of olivine, pyroxene and plagioclase. Their Mg #s range between 33 and 60, with most Mg# >50. Although samples were recovered from water depths of 3.2 to 4.2 km, there is lack of major element and Cl/K concentrations that indicate any seawater alterations (e.g. LOI and Na contents). The lack of seawater signature is also shown by low (0.70275 to 0.70302) 87Sr/86Sr, which is depleted MORB range. The glasses also have low H2O (~1 wt%). This lack of secondary glass alteration allows study of the behavior of volatile and low- temperature alteration sensitive tracers (e.g. B, Cl and B and Sr isotopes). There is no correlation between depth of recovery and B, Cl or d11B systematics, indicating that the degree of vesiculation did not control the d11B. The B content of the glasses is low (generally MORB- like) and the Cl contents show progressive increase with distance from the volcanic front and range between 150 and 1300 ppm. The d11B ratios vary widely and are between +5.3 to -5.6 per mil. As expected, the d11B values become progressively heavier closer to the volcanic front, where they are indistinguishable from the Mariana arc lavas. Interestingly, the heaviest d11B samples are also associated with elevated ratios of Ba/La and U/Nb, and to some extent also with elevated Th/Yb. In arcs, U and Ba are fluid mobile, while Th is a sediment melt sensitive element. From the preliminary dataset it appears that the samples from the S. Mariana Trough are transitional between typical island arc and back-arc magmas. Our new data confirms the proposed highly variable mantle sources along the strike of the Izu-Bonin-Mariana back-arc basalts (Pearce et al., 2005) and stress the fact that distance from the volcanic front may be an important constraint to consider when evaluating high resolution tomographic imaging and modeling mantle flow in the southern Marianas.
The early Izu-Bonin-Mariana (IBM) arc formed during the Eocene along a margin of the West Philipp... more The early Izu-Bonin-Mariana (IBM) arc formed during the Eocene along a margin of the West Philippine Basin (WPB). At that time, the WPB had at least one active spreading center, the Central Basin Ridge (CBR) and it was rimmed by fragments of older, Early Cretaceous to Paleocene island arc and ocean floor lithosphere. Published models of IBM arc initiation suggest
Serpentinized harzburgites recently drilled during ODP Leg 195 at South Chamorro Seamount in the ... more Serpentinized harzburgites recently drilled during ODP Leg 195 at South Chamorro Seamount in the Mariana forearc region have been studied for their platinum group element (PGE) concentrations and Os isotopic compositions. The samples allow a look at the slab fluid-modified subarc mantle immediately overlaying the actively subducting Pacific slab at depths of ~ 30 km. The average PGE (Os 2.3
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