Dépouillements

Metallogenesis and mineralization of intraoceanic arcs I: kermadec arc / Cornel E. J. de Ronde in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1521-1525 Titre : Metallogenesis and mineralization of intraoceanic arcs I: kermadec arc : introduction Type de document : texte imprimé Auteurs : Cornel E. J. de Ronde, Auteur ; D. A. Butterfield, Auteur ; M. I. Leybourne, Auteur Année de publication : 2013 Article en page(s) : pp. 1521-1525 Note générale : economic geology Langues : Anglais (eng) Mots-clés : intraoceanic  arcs;  kermadec  arc Résumé : The Kermadec intraoceanic arc marks the southernmost section of the Pacific Ring of Fire in the western Pacific region, a chain of mostly underwater volcanoes that stretches from north of Japan southward to New Zealand. The Kermadec arc, striking for 1,200 km from Monowai volcano to the North Island of New Zealand, represents the southern portion of the contiguous ~2,500-km Kermadec-Tonga arc (Fig. 1; de Ronde et al., 2003). The Kermadec arc is populated by almost 30 volcanic centers comprising single, large caldera or cone volcanoes, or less commonly, groups of relatively smaller cones (de Ronde et al., 2001, 2007; Graham et al., 2008). All of these volcanic centers lie to the west of the Kermadec Ridge in the southern and mid-parts of the arc—by up to 70 km in the case of Rumble II West—then merge with the Kermadec Ridge near the Kermadec islands around 30° S. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/8/1521.extract [article] Metallogenesis and mineralization of intraoceanic arcs I: kermadec arc : introduction [texte imprimé] / Cornel E. J. de Ronde, Auteur ; D. A. Butterfield, Auteur ; M. I. Leybourne, Auteur . - 2013 . - pp. 1521-1525. economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1521-1525 Mots-clés : intraoceanic  arcs;  kermadec  arc Résumé : The Kermadec intraoceanic arc marks the southernmost section of the Pacific Ring of Fire in the western Pacific region, a chain of mostly underwater volcanoes that stretches from north of Japan southward to New Zealand. The Kermadec arc, striking for 1,200 km from Monowai volcano to the North Island of New Zealand, represents the southern portion of the contiguous ~2,500-km Kermadec-Tonga arc (Fig. 1; de Ronde et al., 2003). The Kermadec arc is populated by almost 30 volcanic centers comprising single, large caldera or cone volcanoes, or less commonly, groups of relatively smaller cones (de Ronde et al., 2001, 2007; Graham et al., 2008). All of these volcanic centers lie to the west of the Kermadec Ridge in the southern and mid-parts of the arc—by up to 70 km in the case of Rumble II West—then merge with the Kermadec Ridge near the Kermadec islands around 30° S. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/8/1521.extract

Sources of chalcophile and siderophile elements in kermadec arc lavas / Christian Timm in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1527-1538 Titre : Sources of chalcophile and siderophile elements in kermadec arc lavas Type de document : texte imprimé Auteurs : Christian Timm, Auteur ; Cornel E. J. de Ronde, Auteur Année de publication : 2013 Article en page(s) : pp. 1527-1538 Note générale : economic geology Langues : Anglais (eng) Mots-clés : kermadec  arc;  chalcophile  elements;  siderophile  elements Résumé : Volcanogenic massive sulfide (VMS) deposits typically contain significant proportions of magma-derived chalcophile (Cu affinity) and siderophile (Fe affinity) elements such as Au, Cu, V, Zn, Mo, Bi, Sb, and As that relate to the composition of associated (host) magmatic rocks. Here, we combine new and published trace element data for lavas recovered from 15 volcanic centers along the Kermadec arc. The data show that mafic back-arc and arc-front lavas are enriched in most of the chalcophile and siderophile elements when compared with mid-ocean ridge basalts (MORB). Elevated (Cu, Zn, V, Mo, Pb)/Yb, Ba/La, As/Ce, and Sb/Pr ratios indicate that the chalcophile and siderophile elements are either transported into the mantle wedge via hydrous fluids derived from the subducting slab, or are liberated from residual mantle wedge sulfides that are oxidized by hydrous fluids. Lower ratios of (Cu, Zn, Mo, Sb, and Pb)/(MREE, HREE) in basalts from the Kermadec back arc (Havre Trough) when compared to the arc front suggests decreasing slab-related input into the mantle source away from the arc front. Unusually high contents of LILE, Ag, Sn, Mo, Th, LREE, MREE, Nb, Zr, Hf, and positive trends in (Ag, Sn)/Yb with Th/Yb, Hf/Y, (La/Sm)N, but low Sr/Y, in dacites from the Brothers volcanic center, southern Kermadec arc, indicate the additional transport of Ag and Sn via a solute-rich supercritical fluid, or via a sediment-derived melt. Magmas generated through partial melting of a sub-arc mantle metasomatized by hydrous melts thus appear to play an important role in the formation of Cu-Au-Ag−rich arc-type VMS deposits. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/8/1527.short [article] Sources of chalcophile and siderophile elements in kermadec arc lavas [texte imprimé] / Christian Timm, Auteur ; Cornel E. J. de Ronde, Auteur . - 2013 . - pp. 1527-1538. economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1527-1538 Mots-clés : kermadec  arc;  chalcophile  elements;  siderophile  elements Résumé : Volcanogenic massive sulfide (VMS) deposits typically contain significant proportions of magma-derived chalcophile (Cu affinity) and siderophile (Fe affinity) elements such as Au, Cu, V, Zn, Mo, Bi, Sb, and As that relate to the composition of associated (host) magmatic rocks. Here, we combine new and published trace element data for lavas recovered from 15 volcanic centers along the Kermadec arc. The data show that mafic back-arc and arc-front lavas are enriched in most of the chalcophile and siderophile elements when compared with mid-ocean ridge basalts (MORB). Elevated (Cu, Zn, V, Mo, Pb)/Yb, Ba/La, As/Ce, and Sb/Pr ratios indicate that the chalcophile and siderophile elements are either transported into the mantle wedge via hydrous fluids derived from the subducting slab, or are liberated from residual mantle wedge sulfides that are oxidized by hydrous fluids. Lower ratios of (Cu, Zn, Mo, Sb, and Pb)/(MREE, HREE) in basalts from the Kermadec back arc (Havre Trough) when compared to the arc front suggests decreasing slab-related input into the mantle source away from the arc front. Unusually high contents of LILE, Ag, Sn, Mo, Th, LREE, MREE, Nb, Zr, Hf, and positive trends in (Ag, Sn)/Yb with Th/Yb, Hf/Y, (La/Sm)N, but low Sr/Y, in dacites from the Brothers volcanic center, southern Kermadec arc, indicate the additional transport of Ag and Sn via a solute-rich supercritical fluid, or via a sediment-derived melt. Magmas generated through partial melting of a sub-arc mantle metasomatized by hydrous melts thus appear to play an important role in the formation of Cu-Au-Ag−rich arc-type VMS deposits. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/8/1527.short

The tectonomagmatic source of ore metals and volatile elements in the southern kermadec arc / R. J. Wysoczanski in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1539-1556 Titre : The tectonomagmatic source of ore metals and volatile elements in the southern kermadec arc Type de document : texte imprimé Auteurs : R. J. Wysoczanski, Auteur ; M. R. Handler, Auteur ; C. I. Schipper, Auteur Année de publication : 2013 Article en page(s) : pp. 1539-1556 Note générale : economic geology Langues : Anglais (eng) Mots-clés : chalcophile  metals;  volatile  elements Résumé : Deciphering the ultimate source of chalcophile metals (e.g., Cu, Zn, Pb) in volcanogenic massive sulfide (VMS) deposits and the volatiles that help drive their formation is critical for understanding where, how, and why VMS deposits form. The southern Kermadec arc volcanic front is known to be highly hydrothermally active and host to at least three VMS deposits, whereas the associated back -arc system is apparently hydrothermally inactive, although this may simply be due to a lack of exploration in this region. We have analyzed major, trace, and volatile element concentrations in a suite of basaltic glasses and olivine-hosted melt inclusions from volcanoes, ridges, and rifts of the southern Kermadec arc volcanic front and Havre Trough back-arc basin. These data indicate clear compositional differences in the mantle beneath the arc front and the back arc, with the arc front having higher extents of prior melt extraction and enrichment in volatile and metal elements from slab-derived aqueous fluids. The magmatic budget of Pb is supplied to the mantle source by these slab-derived fluids, whereas magmatic concentrations of Zn and Cu are primarily controlled by the degree of partial melting, with a Cu-bearing residual phase required in the mantle. Consequently, the relative enrichment of chalcophile metals in the mantle-derived melts is Pb >> Cu > Zn. The magma volatile history recorded by the glasses and melt inclusions indicates that degassing during crystal fractionation is significant and, notably, leads to the reduction of sulfur in the evolving magma as oxidized sulfur degasses preferentially. The apparent absence (or retention) of volatiles (notably CO2 and SO3) in magma chambers, lack of magmatic activity, and absence of hot, mafic dikes beneath volcanoes may be important factors in inhibiting the formation of active hydrothermal venting, and by extension the potential for VMS deposition. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/8/1539.short [article] The tectonomagmatic source of ore metals and volatile elements in the southern kermadec arc [texte imprimé] / R. J. Wysoczanski, Auteur ; M. R. Handler, Auteur ; C. I. Schipper, Auteur . - 2013 . - pp. 1539-1556. economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1539-1556 Mots-clés : chalcophile  metals;  volatile  elements Résumé : Deciphering the ultimate source of chalcophile metals (e.g., Cu, Zn, Pb) in volcanogenic massive sulfide (VMS) deposits and the volatiles that help drive their formation is critical for understanding where, how, and why VMS deposits form. The southern Kermadec arc volcanic front is known to be highly hydrothermally active and host to at least three VMS deposits, whereas the associated back -arc system is apparently hydrothermally inactive, although this may simply be due to a lack of exploration in this region. We have analyzed major, trace, and volatile element concentrations in a suite of basaltic glasses and olivine-hosted melt inclusions from volcanoes, ridges, and rifts of the southern Kermadec arc volcanic front and Havre Trough back-arc basin. These data indicate clear compositional differences in the mantle beneath the arc front and the back arc, with the arc front having higher extents of prior melt extraction and enrichment in volatile and metal elements from slab-derived aqueous fluids. The magmatic budget of Pb is supplied to the mantle source by these slab-derived fluids, whereas magmatic concentrations of Zn and Cu are primarily controlled by the degree of partial melting, with a Cu-bearing residual phase required in the mantle. Consequently, the relative enrichment of chalcophile metals in the mantle-derived melts is Pb >> Cu > Zn. The magma volatile history recorded by the glasses and melt inclusions indicates that degassing during crystal fractionation is significant and, notably, leads to the reduction of sulfur in the evolving magma as oxidized sulfur degasses preferentially. The apparent absence (or retention) of volatiles (notably CO2 and SO3) in magma chambers, lack of magmatic activity, and absence of hot, mafic dikes beneath volcanoes may be important factors in inhibiting the formation of active hydrothermal venting, and by extension the potential for VMS deposition. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/8/1539.short

Detailed morphology and structure of an active submarine arc caldera / R. W. Embley in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1557-1570 Titre : Detailed morphology and structure of an active submarine arc caldera : Brothers volcano, Kermadec arc Type de document : texte imprimé Auteurs : R. W. Embley, Auteur ; Cornel E. J. de Ronde, Auteur ; S. G. Merle, Auteur Année de publication : 2013 Article en page(s) : pp. 1557-1570 Note générale : economic geology Langues : Anglais (eng) Mots-clés : Brothers  caldera  volcano;  submarine  frontal  arc.  morphology Résumé : A survey of the Brothers caldera volcano (Kermadec arc) with the autonomous underwater vehicle ABE has revealed new details of the morphology and structure of this submarine frontal arc caldera and the geologic setting of its hydrothermal activity. Brothers volcano has formed between major SW-NE–trending faults within the extensional field of the Havre Trough. Brothers may be unique among known submarine calderas in that it has four active hydrothermal systems, two high-temperature sulfide-depositing sites associated with faulting on the northwestern and western walls (i.e., the NW caldera and W caldera hydrothermal sites, respectively), and gas-rich sites on the summits of the constructional cones that fill most of the southern part of the caldera (i.e., the Upper and Lower cone sites). The 3.0- × 3.4-km caldera is well defined by a topographic rim encompassing ∼320° of its circumference and which lies between the bounds of two outer half-graben–shaped faults in the northwest and southeast sectors. There is not a morphologically well defined continuous ring fault (at the map resolution), although near-vertical scarps are present discontinuously at the base of sections of the wall. The width of the wall varies from <200 m at its southwest portion to ∼750 m on its northern section. The widest part of the wall is its northwest sector, which also has the largest documented area of hydrothermal alteration and where sea-floor magnetization is lowest. In addition to primary northwest-southeast elongation and southwest-northeast structures caused by faulting within the regional back-arc strain field, there are also less well developed west-southwest–north-northeast regional structures intersecting the volcano that is apparent on the ABE bathymetry and at outcrop scale from submersible observations. Asymmetrical trap-door–style caldera collapse is considered a possible mechanism for the formation of the Brothers caldera. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/8/1557.short [article] Detailed morphology and structure of an active submarine arc caldera : Brothers volcano, Kermadec arc [texte imprimé] / R. W. Embley, Auteur ; Cornel E. J. de Ronde, Auteur ; S. G. Merle, Auteur . - 2013 . - pp. 1557-1570. economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1557-1570 Mots-clés : Brothers  caldera  volcano;  submarine  frontal  arc.  morphology Résumé : A survey of the Brothers caldera volcano (Kermadec arc) with the autonomous underwater vehicle ABE has revealed new details of the morphology and structure of this submarine frontal arc caldera and the geologic setting of its hydrothermal activity. Brothers volcano has formed between major SW-NE–trending faults within the extensional field of the Havre Trough. Brothers may be unique among known submarine calderas in that it has four active hydrothermal systems, two high-temperature sulfide-depositing sites associated with faulting on the northwestern and western walls (i.e., the NW caldera and W caldera hydrothermal sites, respectively), and gas-rich sites on the summits of the constructional cones that fill most of the southern part of the caldera (i.e., the Upper and Lower cone sites). The 3.0- × 3.4-km caldera is well defined by a topographic rim encompassing ∼320° of its circumference and which lies between the bounds of two outer half-graben–shaped faults in the northwest and southeast sectors. There is not a morphologically well defined continuous ring fault (at the map resolution), although near-vertical scarps are present discontinuously at the base of sections of the wall. The width of the wall varies from <200 m at its southwest portion to ∼750 m on its northern section. The widest part of the wall is its northwest sector, which also has the largest documented area of hydrothermal alteration and where sea-floor magnetization is lowest. In addition to primary northwest-southeast elongation and southwest-northeast structures caused by faulting within the regional back-arc strain field, there are also less well developed west-southwest–north-northeast regional structures intersecting the volcano that is apparent on the ABE bathymetry and at outcrop scale from submersible observations. Asymmetrical trap-door–style caldera collapse is considered a possible mechanism for the formation of the Brothers caldera. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/8/1557.short

Crustal magnetization of Brothers volcano, New Zealand, measured by autonomous underwater vehicles: / F. Caratori Tontini in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1571-1581 Titre : Crustal magnetization of Brothers volcano, New Zealand, measured by autonomous underwater vehicles: : geophysical expression of a submarine hydrothermal system Type de document : texte imprimé Auteurs : F. Caratori Tontini, Auteur ; B. Davy, Auteur ; Cornel E. J. de Ronde, Auteur Année de publication : 2013 Article en page(s) : pp. 1571-1581 Note générale : economic geology Langues : Anglais (eng) Mots-clés : Near-bottom  magnetic  anomaly;  Brothers  volcano;  crustal  magnetization Résumé : Near-bottom magnetic anomaly data have been acquired using autonomous underwater vehicles at Brothers volcano, southern Kermadec arc, New Zealand. Crustal magnetization for the study area was obtained by inverting the magnetic data and shows a strong correlation between areas of low magnetization and four hydrothermal fields, one of which was unknown prior to our surveys. The magnetization pattern is consistent with a model of discrete, individual zones of fluid upflow focused along caldera ring faults that provide preferred pathways for the ascent of the hydrothermal fluids. Differences in the amplitude of the magnetization over the vent fields appear to correlate with age and temperature variations of the hydrothermal fields. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/8/1571.short [article] Crustal magnetization of Brothers volcano, New Zealand, measured by autonomous underwater vehicles: : geophysical expression of a submarine hydrothermal system [texte imprimé] / F. Caratori Tontini, Auteur ; B. Davy, Auteur ; Cornel E. J. de Ronde, Auteur . - 2013 . - pp. 1571-1581. economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1571-1581 Mots-clés : Near-bottom  magnetic  anomaly;  Brothers  volcano;  crustal  magnetization Résumé : Near-bottom magnetic anomaly data have been acquired using autonomous underwater vehicles at Brothers volcano, southern Kermadec arc, New Zealand. Crustal magnetization for the study area was obtained by inverting the magnetic data and shows a strong correlation between areas of low magnetization and four hydrothermal fields, one of which was unknown prior to our surveys. The magnetization pattern is consistent with a model of discrete, individual zones of fluid upflow focused along caldera ring faults that provide preferred pathways for the ascent of the hydrothermal fluids. Differences in the amplitude of the magnetization over the vent fields appear to correlate with age and temperature variations of the hydrothermal fields. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/8/1571.short

High-resolution hydrothermal mapping of Brothers Caldera, Kermadec arc / Edward T. Baker in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1583-1593 Titre : High-resolution hydrothermal mapping of Brothers Caldera, Kermadec arc Type de document : texte imprimé Auteurs : Edward T. Baker, Auteur ; Sharon L. Walker, Auteur ; Robert W. Embley, Auteur Année de publication : 2013 Article en page(s) : pp. 1583-1593 Note générale : economic gzology Langues : Anglais (eng) Mots-clés : autonomous  underwater  vehicle;  hydrothermal  tracers;  Brothers  volcano Résumé : Submarine edifices with caldera summits are common along volcanic arcs and much more likely than simple cones to host hydrothermal venting. Compared with cones, however, locating all vent field locations on a caldera's complex bathymetry is a daunting logistical challenge. Here we describe the first use of an autonomous underwater vehicle, ABE, to fully map the distribution of near-bottom hydrothermal tracers over the caldera walls and cone complex of Brothers Volcano, the most active hydrothermal source on the southern Kermadec arc. Sensors on ABE simultaneously measured hydrothermal plume anomalies in temperature, light backscattering (particle concentration), and oxidation-reduction potential (dissolved reduced species) every 2 to 3 m along track. Local maxima in these tracers confirmed known sites on the northwestern wall and the cone summits, and more precisely mapped their extents. We discovered evidence for new sites throughout the entire northwestern half of the caldera wall, at the base of the southeastern wall, and on the flanks of both cones. Systematic variations in the backscattering/temperature ratio identified different types of fluid discharge. Plumes from high-temperature, metal-rich sources dominated the caldera wall, while on top of the caldera wall and on the cones we found only plumes from low-temperature diffuse flow, including occasional S-rich plumes. Source distribution on the walls appears fault controlled, but we detected no sources along the deepest fault defining the rim of the downdropped caldera floor. Advanced deep-sea survey techniques using autonomous vehicles is an indispensible and cost-effective tool for acquiring a comprehensive understanding of the relationship between hydrothermal discharge and geology on individual volcanoes. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1583.abstract [article] High-resolution hydrothermal mapping of Brothers Caldera, Kermadec arc [texte imprimé] / Edward T. Baker, Auteur ; Sharon L. Walker, Auteur ; Robert W. Embley, Auteur . - 2013 . - pp. 1583-1593. economic gzology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1583-1593 Mots-clés : autonomous  underwater  vehicle;  hydrothermal  tracers;  Brothers  volcano Résumé : Submarine edifices with caldera summits are common along volcanic arcs and much more likely than simple cones to host hydrothermal venting. Compared with cones, however, locating all vent field locations on a caldera's complex bathymetry is a daunting logistical challenge. Here we describe the first use of an autonomous underwater vehicle, ABE, to fully map the distribution of near-bottom hydrothermal tracers over the caldera walls and cone complex of Brothers Volcano, the most active hydrothermal source on the southern Kermadec arc. Sensors on ABE simultaneously measured hydrothermal plume anomalies in temperature, light backscattering (particle concentration), and oxidation-reduction potential (dissolved reduced species) every 2 to 3 m along track. Local maxima in these tracers confirmed known sites on the northwestern wall and the cone summits, and more precisely mapped their extents. We discovered evidence for new sites throughout the entire northwestern half of the caldera wall, at the base of the southeastern wall, and on the flanks of both cones. Systematic variations in the backscattering/temperature ratio identified different types of fluid discharge. Plumes from high-temperature, metal-rich sources dominated the caldera wall, while on top of the caldera wall and on the cones we found only plumes from low-temperature diffuse flow, including occasional S-rich plumes. Source distribution on the walls appears fault controlled, but we detected no sources along the deepest fault defining the rim of the downdropped caldera floor. Advanced deep-sea survey techniques using autonomous vehicles is an indispensible and cost-effective tool for acquiring a comprehensive understanding of the relationship between hydrothermal discharge and geology on individual volcanoes. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1583.abstract

Fluid-flow patterns at Brothers volcano, southern Kermadec arc / Gillian Gruen in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1595-1611 Titre : Fluid-flow patterns at Brothers volcano, southern Kermadec arc : insights from geologically constrained numerical simulations Type de document : texte imprimé Auteurs : Gillian Gruen, Auteur ; Philipp Weis, Auteur ; Thomas Driesner, Auteur Année de publication : 2013 Article en page(s) : pp. 1595-1611 Note générale : economic geology Langues : Anglais (eng) Mots-clés : hydrothermal  venting;  Brothers  volcano Résumé : Numerical, multiphase pure-water simulations were performed to study the first-order geologic and physical parameters controlling the style and distribution of hydrothermal venting at Brothers volcano, southern Kermadec arc, New Zealand. By comparing the results for different permeability scenarios, we can show that the location of venting on the inner slopes of the caldera (e.g., at the NW caldera site) requires the presence of higher permeability faults. Venting at the top of the Upper cone develops naturally by hydrothermal flow in porous rocks above an underlying magma body. However, this magmatic reservoir cannot alone account for present-day hydrothermal venting at the NW caldera site, which implies that a larger magma chamber, which was responsible for caldera collapse, is still active. Modeled venting temperatures for scenarios with homogeneous host-rock permeability correspond well with formation temperatures determined by sulfate-sulfide mineral pairs from different vent sites at Brothers volcano. Direct measurements of vent fluids at the NW caldera site today, however, show higher temperatures than modeled. This may be due to rapid ascent of hot fluids in individual fractures that are not resolved in the simulations. At the cone sites, measured temperatures are lower than modeled, likely the result of mixing with ambient seawater in near-surface permeable rocks. The inferred presence of a constant magmatic fluid source underneath the volcanic edifice leads to a more rapid development of the hydrothermal circulation and stabilizes the system at higher temperatures. We suggest that the hydrothermal evolution and fluid-flow patterns at Brothers volcano are controlled by episodes of varying magmatic fluid input into a seawater-dominated convection system. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1595.short [article] Fluid-flow patterns at Brothers volcano, southern Kermadec arc : insights from geologically constrained numerical simulations [texte imprimé] / Gillian Gruen, Auteur ; Philipp Weis, Auteur ; Thomas Driesner, Auteur . - 2013 . - pp. 1595-1611. economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1595-1611 Mots-clés : hydrothermal  venting;  Brothers  volcano Résumé : Numerical, multiphase pure-water simulations were performed to study the first-order geologic and physical parameters controlling the style and distribution of hydrothermal venting at Brothers volcano, southern Kermadec arc, New Zealand. By comparing the results for different permeability scenarios, we can show that the location of venting on the inner slopes of the caldera (e.g., at the NW caldera site) requires the presence of higher permeability faults. Venting at the top of the Upper cone develops naturally by hydrothermal flow in porous rocks above an underlying magma body. However, this magmatic reservoir cannot alone account for present-day hydrothermal venting at the NW caldera site, which implies that a larger magma chamber, which was responsible for caldera collapse, is still active. Modeled venting temperatures for scenarios with homogeneous host-rock permeability correspond well with formation temperatures determined by sulfate-sulfide mineral pairs from different vent sites at Brothers volcano. Direct measurements of vent fluids at the NW caldera site today, however, show higher temperatures than modeled. This may be due to rapid ascent of hot fluids in individual fractures that are not resolved in the simulations. At the cone sites, measured temperatures are lower than modeled, likely the result of mixing with ambient seawater in near-surface permeable rocks. The inferred presence of a constant magmatic fluid source underneath the volcanic edifice leads to a more rapid development of the hydrothermal circulation and stabilizes the system at higher temperatures. We suggest that the hydrothermal evolution and fluid-flow patterns at Brothers volcano are controlled by episodes of varying magmatic fluid input into a seawater-dominated convection system. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1595.short

Mineralogy and formation of black smoker chimneys from Brothers submarine volcano, Kermadec arc / H. A. Berkenbosch in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1613-1633 Titre : Mineralogy and formation of black smoker chimneys from Brothers submarine volcano, Kermadec arc Type de document : texte imprimé Auteurs : H. A. Berkenbosch, Auteur ; Cornel E. J. de Ronde, Auteur ; J. B. Gemmell, Auteur Année de publication : 2013 Article en page(s) : pp. 1613-1633 Note générale : economic geology Langues : Anglais (eng) Mots-clés : submarine  volcano;  black  smoker  chimneys;  Brothers  volcano Résumé : Brothers caldera volcano is a submarine volcano of dacitic composition, located on the Kermadec arc, New Zealand. It hosts the NW caldera vent field perched on the steep slope of the caldera walls and includes numerous, active, high-temperature (max 302°C) chimneys and a greater amount of dead, sulfide-rich spires. Petrographic studies of these chimneys show that three main zones can occur within the chimneys: a chalcopyrite-rich core, surrounded by a sulfate-dominated zone, which is in turn mantled by an external rind of Fe oxides, calcite, and silicates. Four chimney types are identified based on the relative proportions of the chalcopyrite and sulfate layers and the presence or absence of anhydrite. Two are Cu rich, i.e., chalcopyrite-sulfate and chalcopyrite-bornite chimneys, and two are Zn rich, i.e., sphalerite-barite and sphalerite-chalcopyrite. Chimney growth begins with the formation of a sulfate wall upon which sulfides precipitate. Later, zone refining results in a chalcopyrite-rich core with pyrite/marcasite and sphalerite occurring predominantly near the outer margins. In chalcopyrite-bornite chimneys, the chalcopyrite core rapidly loses permeability and limits the thickness of the surrounding sulfate layer. In these chimneys, bornite, chalcocite, and covellite form along the outer margin of the chalcopyrite zone as a result of oxidation by seawater. Zinc-rich chimneys display a more vertical zonation and their growth involves an upward-advancing barite cap followed by chalcopyrite deposition (if present) nearer the base. The vertical zonation and lack of anhydrite in these chimneys also implies that larger chalcopyrite and anhydrite deposits may exist subsea floor. The different chimney types are related to subsea-floor permeability, the amount of fluid mixing that occurs prior to venting, and heterogeneous fluid compositions. The occurrence of specular hematite and Bi or Au tellurides associated with chalcopyrite are consistent with magmatic contributions to the NW caldera vent site. These tellurides are the first gold-bearing phase to be identified in these chimneys, and the Bi-Au association suggests that gold enrichment up to 91 ppm is due to scavenging by liquid bismuth. The presence of tellurides in Brothers chimneys have implications for other telluride-bearing deposits, such those in the Urals. Likewise, other aspects of the mineralogy (i.e., textures) and zonation, including the implied subsea-floor deposition, presented here from an active, undeformed environment can aid in understanding ancient volcanogenic massive sulfide (VMS) deposits that have undergone various degrees of metamorphism. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1613.short [article] Mineralogy and formation of black smoker chimneys from Brothers submarine volcano, Kermadec arc [texte imprimé] / H. A. Berkenbosch, Auteur ; Cornel E. J. de Ronde, Auteur ; J. B. Gemmell, Auteur . - 2013 . - pp. 1613-1633. economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1613-1633 Mots-clés : submarine  volcano;  black  smoker  chimneys;  Brothers  volcano Résumé : Brothers caldera volcano is a submarine volcano of dacitic composition, located on the Kermadec arc, New Zealand. It hosts the NW caldera vent field perched on the steep slope of the caldera walls and includes numerous, active, high-temperature (max 302°C) chimneys and a greater amount of dead, sulfide-rich spires. Petrographic studies of these chimneys show that three main zones can occur within the chimneys: a chalcopyrite-rich core, surrounded by a sulfate-dominated zone, which is in turn mantled by an external rind of Fe oxides, calcite, and silicates. Four chimney types are identified based on the relative proportions of the chalcopyrite and sulfate layers and the presence or absence of anhydrite. Two are Cu rich, i.e., chalcopyrite-sulfate and chalcopyrite-bornite chimneys, and two are Zn rich, i.e., sphalerite-barite and sphalerite-chalcopyrite. Chimney growth begins with the formation of a sulfate wall upon which sulfides precipitate. Later, zone refining results in a chalcopyrite-rich core with pyrite/marcasite and sphalerite occurring predominantly near the outer margins. In chalcopyrite-bornite chimneys, the chalcopyrite core rapidly loses permeability and limits the thickness of the surrounding sulfate layer. In these chimneys, bornite, chalcocite, and covellite form along the outer margin of the chalcopyrite zone as a result of oxidation by seawater. Zinc-rich chimneys display a more vertical zonation and their growth involves an upward-advancing barite cap followed by chalcopyrite deposition (if present) nearer the base. The vertical zonation and lack of anhydrite in these chimneys also implies that larger chalcopyrite and anhydrite deposits may exist subsea floor. The different chimney types are related to subsea-floor permeability, the amount of fluid mixing that occurs prior to venting, and heterogeneous fluid compositions. The occurrence of specular hematite and Bi or Au tellurides associated with chalcopyrite are consistent with magmatic contributions to the NW caldera vent site. These tellurides are the first gold-bearing phase to be identified in these chimneys, and the Bi-Au association suggests that gold enrichment up to 91 ppm is due to scavenging by liquid bismuth. The presence of tellurides in Brothers chimneys have implications for other telluride-bearing deposits, such those in the Urals. Likewise, other aspects of the mineralogy (i.e., textures) and zonation, including the implied subsea-floor deposition, presented here from an active, undeformed environment can aid in understanding ancient volcanogenic massive sulfide (VMS) deposits that have undergone various degrees of metamorphism. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1613.short

Radiometric dating of volcanogenic massive sulfides and associated iron oxide crusts with an emphasis on 226Ra/Ba and 228Ra/226Ra in volcanic and hydrothermal processes at intraoceanic arcs / Robert G. Ditchburn in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1635-1648 Titre : Radiometric dating of volcanogenic massive sulfides and associated iron oxide crusts with an emphasis on 226Ra/Ba and 228Ra/226Ra in volcanic and hydrothermal processes at intraoceanic arcs Type de document : texte imprimé Auteurs : Robert G. Ditchburn, Auteur ; Cornel E. J. de Ronde, Auteur ; Bernard J. Barry, Auteur Année de publication : 2013 Article en page(s) : pp. 1635-1648 Note générale : economic geology Langues : Anglais (eng) Mots-clés : dating  volcanogenic  massive  sulfide  mineralization;  submarine  arc  volcano;  hydrothermal  systems;  radiometric  dating  methods Résumé : Radiometric dating methods using 238U and 232Th decay chain isotopes have been developed and applied to volcanogenic massive sulfide (VMS) samples, most of which were collected from hydrothermal sites at volcanoes along the Kermadec and Mariana intraoceanic arcs. The activity ratios, 228Th/228Ra, 228Ra/226Ra (Bq.Bq−1), and 226Ra/Ba values (Bq.g−1), are used for dating VMS in the age ranges 0.3 to 12, 3 to 35, and 500 to 15,000 years, respectively. Black smoker chimneys 35 to 100 years old are dated using the activity ratio, 210Pb/226Ra, derived from 210Pb/[Pb] versus 226Ra/[Pb] isochron plots. Iron oxide crusts are dated using 210Pb in combination with either As or 10Be. The 228Ra/226Ra values for recent VMS mineralization from Brothers and East Diamante volcanoes are reasonably constant but greater than the theoretical ratios calculated from the Th and U contents of volcanic rock from the same area. The high initial 228Ra/226Ra values for VMS are most likely due to excess 228Ra (the daughter of 232Th) that is contained within sediment on the subducted plate. This implies that the residence time for Ba and Ra in the volcano would be less than ∼35 years, considering the half-life of 228Ra (5.75 years). These Ra isotopes are used as a proxy for better understanding the transfer of Ba from magmas to the sea floor, via circulating fluids of the hydrothermal system. Radiometric dating shows VMS chimneys have been forming at Brothers volcano for at least 1,000 years. The supply of Ba and Ra to the chimneys and associated VMS deposits comes from magmas rich in these elements continuously emplaced via dike intrusion into a zone of hydrothermally altered rock. There, the dikes interact with evolved seawater to produce the metal-rich hydrothermal fluid discharging on the sea floor, forming the chimneys. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1635.short [article] Radiometric dating of volcanogenic massive sulfides and associated iron oxide crusts with an emphasis on 226Ra/Ba and 228Ra/226Ra in volcanic and hydrothermal processes at intraoceanic arcs [texte imprimé] / Robert G. Ditchburn, Auteur ; Cornel E. J. de Ronde, Auteur ; Bernard J. Barry, Auteur . - 2013 . - pp. 1635-1648. economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1635-1648 Mots-clés : dating  volcanogenic  massive  sulfide  mineralization;  submarine  arc  volcano;  hydrothermal  systems;  radiometric  dating  methods Résumé : Radiometric dating methods using 238U and 232Th decay chain isotopes have been developed and applied to volcanogenic massive sulfide (VMS) samples, most of which were collected from hydrothermal sites at volcanoes along the Kermadec and Mariana intraoceanic arcs. The activity ratios, 228Th/228Ra, 228Ra/226Ra (Bq.Bq−1), and 226Ra/Ba values (Bq.g−1), are used for dating VMS in the age ranges 0.3 to 12, 3 to 35, and 500 to 15,000 years, respectively. Black smoker chimneys 35 to 100 years old are dated using the activity ratio, 210Pb/226Ra, derived from 210Pb/[Pb] versus 226Ra/[Pb] isochron plots. Iron oxide crusts are dated using 210Pb in combination with either As or 10Be. The 228Ra/226Ra values for recent VMS mineralization from Brothers and East Diamante volcanoes are reasonably constant but greater than the theoretical ratios calculated from the Th and U contents of volcanic rock from the same area. The high initial 228Ra/226Ra values for VMS are most likely due to excess 228Ra (the daughter of 232Th) that is contained within sediment on the subducted plate. This implies that the residence time for Ba and Ra in the volcano would be less than ∼35 years, considering the half-life of 228Ra (5.75 years). These Ra isotopes are used as a proxy for better understanding the transfer of Ba from magmas to the sea floor, via circulating fluids of the hydrothermal system. Radiometric dating shows VMS chimneys have been forming at Brothers volcano for at least 1,000 years. The supply of Ba and Ra to the chimneys and associated VMS deposits comes from magmas rich in these elements continuously emplaced via dike intrusion into a zone of hydrothermally altered rock. There, the dikes interact with evolved seawater to produce the metal-rich hydrothermal fluid discharging on the sea floor, forming the chimneys. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1635.short

Geology, hydrothermal activity, and sea-floor massive sulfide mineralization at the rumble II west mafic Caldera / Matthew I. Leybourne in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1649-1668 Titre : Geology, hydrothermal activity, and sea-floor massive sulfide mineralization at the rumble II west mafic Caldera Type de document : texte imprimé Auteurs : Matthew I. Leybourne, Auteur ; Cornel E. J. de Ronde, Auteur ; Richard J. Wysoczanski, Auteur Année de publication : 2013 Article en page(s) : pp. 1649-1668 Note générale : economic geology Langues : Anglais (eng) Mots-clés : sea-floor;  massive  sulfide  mineralization;  Caldera Résumé : Sea-floor imagery, volcanic rock, massive sulfide, and hydrothermal plume samples (δ3He, pH, dissolved Fe and Mn, and particulate chemistry) have been collected from the Rumble II West volcano, southern Kermadec arc, New Zealand. Rumble II West is a caldera volcano with an ∼3-km-diameter summit depression bounded by ring faults with a resurgent central cone. Rocks recovered to date are predominantly mafic in composition (i.e., basalt to basaltic andesite) with volumetrically lesser intermediate rocks (i.e., andesite). On the basis of its size, geometry, volcanic products, and composition, Rumble II West can be classified as a mafic caldera volcano. Rumble II West has a weak hydrothermal plume signature characterized by a small but detectable δ3He anomaly (25%). Time-series light scattering data though, obtained from vertical casts and tow-yos, do show that hydrothermal activity has increased in intensity between 1999 and 2011. Massive sulfides recovered from the eastern caldera wall and eastern flank of the central cone are primarily comprised of barite and chalcopyrite, with lesser sphalerite, pyrite, and traces of galena. The weak hydrothermal plume signal indicates that the volcano is in a volcanic-hydrothermal quiescent stage compared to other volcanoes along the southern Kermadec arc, although the preponderance of barite with massive sulfide mineralization indicates higher temperature venting in the past. Of the volcanoes along the Kermadec-Tonga arc known to host massive sulfides (i.e., Clark, Rumble II West, Brothers, Monowai, Volcano 19, and Volcano 1), the majority (five out of six) are dominantly mafic in composition and all but one of these mafic volcanoes form moderate-size to large calderas. To date, mafic calderas have been largely ignored as hosts to sea-floor massive sulfide deposits. That 75% of the presently known massive sulfide-bearing calderas along the arc are mafic in composition (the dacitic Brothers volcano is the exception) has important implications for sea-floor massive sulfide mineral exploration in the modern oceans and ancient rock record on land. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1649.short [article] Geology, hydrothermal activity, and sea-floor massive sulfide mineralization at the rumble II west mafic Caldera [texte imprimé] / Matthew I. Leybourne, Auteur ; Cornel E. J. de Ronde, Auteur ; Richard J. Wysoczanski, Auteur . - 2013 . - pp. 1649-1668. economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1649-1668 Mots-clés : sea-floor;  massive  sulfide  mineralization;  Caldera Résumé : Sea-floor imagery, volcanic rock, massive sulfide, and hydrothermal plume samples (δ3He, pH, dissolved Fe and Mn, and particulate chemistry) have been collected from the Rumble II West volcano, southern Kermadec arc, New Zealand. Rumble II West is a caldera volcano with an ∼3-km-diameter summit depression bounded by ring faults with a resurgent central cone. Rocks recovered to date are predominantly mafic in composition (i.e., basalt to basaltic andesite) with volumetrically lesser intermediate rocks (i.e., andesite). On the basis of its size, geometry, volcanic products, and composition, Rumble II West can be classified as a mafic caldera volcano. Rumble II West has a weak hydrothermal plume signature characterized by a small but detectable δ3He anomaly (25%). Time-series light scattering data though, obtained from vertical casts and tow-yos, do show that hydrothermal activity has increased in intensity between 1999 and 2011. Massive sulfides recovered from the eastern caldera wall and eastern flank of the central cone are primarily comprised of barite and chalcopyrite, with lesser sphalerite, pyrite, and traces of galena. The weak hydrothermal plume signal indicates that the volcano is in a volcanic-hydrothermal quiescent stage compared to other volcanoes along the southern Kermadec arc, although the preponderance of barite with massive sulfide mineralization indicates higher temperature venting in the past. Of the volcanoes along the Kermadec-Tonga arc known to host massive sulfides (i.e., Clark, Rumble II West, Brothers, Monowai, Volcano 19, and Volcano 1), the majority (five out of six) are dominantly mafic in composition and all but one of these mafic volcanoes form moderate-size to large calderas. To date, mafic calderas have been largely ignored as hosts to sea-floor massive sulfide deposits. That 75% of the presently known massive sulfide-bearing calderas along the arc are mafic in composition (the dacitic Brothers volcano is the exception) has important implications for sea-floor massive sulfide mineral exploration in the modern oceans and ancient rock record on land. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1649.short

Submarine magmatic-hydrothermal systems at the Monowai volcanic center, Kermadec arc / Matthew I. Leybourne in Economic geology, Vol. 107 N° 8 (Décembre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1669-1694 Titre : Submarine magmatic-hydrothermal systems at the Monowai volcanic center, Kermadec arc Type de document : texte imprimé Auteurs : Matthew I. Leybourne, Auteur ; Ulrich Schwarz-Schampera, Auteur ; Cornel E. J. de Ronde, Auteur Année de publication : 2013 Article en page(s) : pp. 1669-1694 Note générale : economic geology Langues : Anglais (eng) Mots-clés : Monowai  volcanic  center;  magmatic  hydrothermal  systems Résumé : The Monowai volcanic center is located at the midpoint along the ∼2,530-km-long Tonga-Kermadec arc system. The Monowai volcanic center is comprised of a large elongate caldera (Monowai caldera area ∼35 km2; depth to caldera floor 1,590 m), which has formed within an older caldera some 84 km2 in area. To the south of this nested caldera system is a large composite volcano, Monowai cone, which rises to within ∼100 m of the sea surface and which has been volcanically active for the past several decades. Mafic volcanic rocks dominate the Monowai volcanic center; basalts are the most common rock type recovered from the cone, whereas basaltic andesites are common within the caldera. Hydrothermal plume mapping has shown at least three major hydrothermal systems associated with the caldera and cone: (1) the summit of the cone, (2) low-temperature venting (<60°C; Mussel Ridge) on the southwestern wall of the caldera, and (3) a deeper caldera source with higher temperature venting that has yet to be observed. The cone summit plume shows large anomalies in pH (a shift of −2.00 pH units) and δ3He (≤358%), and noticeable H2S (up to 32 μm), and CH4 (up to 900 nm). The summit plume is also metal rich, with elevated total dissolvable Fe (TDFe up to 4,200 nm), TDMn (up to 412 nm), and TDFe/TDMn (up to 20.4). Particulate samples have elevated Fe, Si, Al, and Ti consistent with addition to the hydrothermal fluid from acidic water-rock reaction. Plumes extending from ∼1,000- to 1,400-m depth provide evidence for a major hydrothermal vent system in the caldera. The caldera plume has lower values for TDFe and TDMn, although some samples show higher TDMn concentrations than the cone summit plume; caldera plume samples are also relatively gas poor (i.e., no H2S detected, pH shift of −0.06 pH units, CH4 concentrations up to 26 nm). The composition of the hydrothermal plumes in the caldera have higher metal contents than the sampled vent fluids along Mussel Ridge, requiring that the source of the caldera plumes is at greater depth and likely of higher temperature. Minor plumes detected as light scattering anomalies but with no 3He anomalies down the northern flank of the Monowai caldera most likely represent remobilization of volcanic debris from the volcano flanks. We believe the Monowai volcanic center is host to a robust magmatic-hydrothermal system, with significant differences in the style and composition of venting at the cone and caldera sites. At the cone, the large shifts in pH, very high δ3He% values, elevated TDFe and TDFe/TDMn, and the H2S- and CH4-rich nature of the plume fluids, together with elevated Ti, P, V, S, and Al in hydrothermal particulates, indicates significant magmatic volatile ± metal contributions in the hydrothermal system coupled with aggressive acidic water-rock interaction. By contrast, the caldera has low TDFe/TDMn in hydrothermal plumes; however, elevated Al and Ti contents in caldera particulate samples, combined with the presence of alunite, pyrophyllite, sulfide minerals, and native sulfur in samples from Mussel Ridge suggest past, and perhaps recent, acid volatile-rich venting and active Fe sulfide formation in the subsurface. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1669.short [article] Submarine magmatic-hydrothermal systems at the Monowai volcanic center, Kermadec arc [texte imprimé] / Matthew I. Leybourne, Auteur ; Ulrich Schwarz-Schampera, Auteur ; Cornel E. J. de Ronde, Auteur . - 2013 . - pp. 1669-1694. economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 8 (Décembre 2012) . - pp. 1669-1694 Mots-clés : Monowai  volcanic  center;  magmatic  hydrothermal  systems Résumé : The Monowai volcanic center is located at the midpoint along the ∼2,530-km-long Tonga-Kermadec arc system. The Monowai volcanic center is comprised of a large elongate caldera (Monowai caldera area ∼35 km2; depth to caldera floor 1,590 m), which has formed within an older caldera some 84 km2 in area. To the south of this nested caldera system is a large composite volcano, Monowai cone, which rises to within ∼100 m of the sea surface and which has been volcanically active for the past several decades. Mafic volcanic rocks dominate the Monowai volcanic center; basalts are the most common rock type recovered from the cone, whereas basaltic andesites are common within the caldera. Hydrothermal plume mapping has shown at least three major hydrothermal systems associated with the caldera and cone: (1) the summit of the cone, (2) low-temperature venting (<60°C; Mussel Ridge) on the southwestern wall of the caldera, and (3) a deeper caldera source with higher temperature venting that has yet to be observed. The cone summit plume shows large anomalies in pH (a shift of −2.00 pH units) and δ3He (≤358%), and noticeable H2S (up to 32 μm), and CH4 (up to 900 nm). The summit plume is also metal rich, with elevated total dissolvable Fe (TDFe up to 4,200 nm), TDMn (up to 412 nm), and TDFe/TDMn (up to 20.4). Particulate samples have elevated Fe, Si, Al, and Ti consistent with addition to the hydrothermal fluid from acidic water-rock reaction. Plumes extending from ∼1,000- to 1,400-m depth provide evidence for a major hydrothermal vent system in the caldera. The caldera plume has lower values for TDFe and TDMn, although some samples show higher TDMn concentrations than the cone summit plume; caldera plume samples are also relatively gas poor (i.e., no H2S detected, pH shift of −0.06 pH units, CH4 concentrations up to 26 nm). The composition of the hydrothermal plumes in the caldera have higher metal contents than the sampled vent fluids along Mussel Ridge, requiring that the source of the caldera plumes is at greater depth and likely of higher temperature. Minor plumes detected as light scattering anomalies but with no 3He anomalies down the northern flank of the Monowai caldera most likely represent remobilization of volcanic debris from the volcano flanks. We believe the Monowai volcanic center is host to a robust magmatic-hydrothermal system, with significant differences in the style and composition of venting at the cone and caldera sites. At the cone, the large shifts in pH, very high δ3He% values, elevated TDFe and TDFe/TDMn, and the H2S- and CH4-rich nature of the plume fluids, together with elevated Ti, P, V, S, and Al in hydrothermal particulates, indicates significant magmatic volatile ± metal contributions in the hydrothermal system coupled with aggressive acidic water-rock interaction. By contrast, the caldera has low TDFe/TDMn in hydrothermal plumes; however, elevated Al and Ti contents in caldera particulate samples, combined with the presence of alunite, pyrophyllite, sulfide minerals, and native sulfur in samples from Mussel Ridge suggest past, and perhaps recent, acid volatile-rich venting and active Fe sulfide formation in the subsurface. DEWEY : 553 ISSN : 0361-0128 En ligne : http://economicgeology.org/content/107/8/1669.short

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