Description: Clark volcano of the Kermadec arc, northeast of New Zealand, is a large stratovolcano comprised of two coalescing volcanic cones; an apparently younger, more coherent, twin-peaked edifice to the northwest and a relatively older, more degraded and tectonized cone to the southeast. High-resolution water column surveys show an active hydrothermal system at the summit of the NW cone largely along a ridge spur connecting the two peaks, with activity also noted at the head of scarps related to sector collapse. Clark is the only known cone volcano along the Kermadec arc to host sulfide mineralization. Volcano-scale gravity and magnetic surveys over Clark show that it is highly magnetized, and that a strong gravity gradient exists between the two edifices. Modeling suggests that a crustal-scale fault lies between these two edifices, with thinner crust beneath the NW cone. Locations of regional earthquake epicenters show a southwest-northeast trend bisecting the two Clark cones, striking northeastward into Tangaroa volcano. Detailed mapping of magnetics above the NW cone summit shows a highly magnetized “ring structure” ~350 m below the summit that is not apparent in the bathymetry; we believe this structure represents the top of a caldera. Oblate zones of low (weak) magnetization caused by hydrothermal fluid upflow, here termed “burn holes,” form a pattern in the regional magnetization resembling Swiss cheese. Presumably older burn holes occupy the inner margin of the ring structure and show no signs of hydrothermal activity, while younger burn holes are coincident with active venting on the summit. A combination of mineralogy, geochemistry, and seafloor mapping of the NW cone shows that hydrothermal activity today is largely manifest by widespread diffuse venting, with temperatures ranging between 56° and 106°C. Numerous, small (≤30 cm high) chimneys populate the summit area, with one site host to the ~7-m-tall “Twin Towers” chimneys with maximum vent fluid temperatures of 221°C (pH 4.9), consistent with δ34Sanhydrite-pyrite values indicating formation temperatures of ~228° to 249°C. Mineralization is dominated by pyrite-marcasite-barite-anhydrite. Radiometric dating using the 228Ra/226Ra and 226Ra/Ba methods shows active chimneys to be 〈20 with most 〈2 years old. However, the chimneys at Clark show evidence for mixing with, and remobilizing of, barite as old as 19,000 years. This is consistent with Nd and Sr isotope compositions of Clark chimney and sulfate crust samples that indicate mixing of ~40% seawater with a vent fluid derived from low K lavas. Similarly, REE data show the hydrothermal fluids have interacted with a plagioclase-rich source rock. A holistic approach to the study of the Clark hydrothermal system has revealed a two-stage process whereby a caldera-forming volcanic event preceded a later cone-building event. This ensured a protracted (at least 20 ka yrs) history of hydrothermal activity and associated mineral deposition. If we assume at least 200-m-high walls for the postulated (buried) caldera, then hydrothermal fluids would have exited the seafloor 20 ka years ago at least 550 m deeper than they do today, with fluid discharge temperatures potentially much hotter (~350°C). Subsequent to caldera infilling, relatively porous volcaniclastic and other units making up the cone acted as large-scale filters, enabling ascending hydrothermal fluids to boil and mix with seawater subseafloor, effectively removing the metals (including remobilized Cu) in solution before they reached the seafloor. This has implications for estimates for the metal inventory of seafloor hydrothermal systems pertaining to arc hydrothermal systems.

Description: 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.

Description: The geochemistry of pyroclasts sampled from four volcanoes along the Kermadec arc in the SW Pacific is used to investigate the genesis of silicic magmas in a young (〈2 Myr), archetypical intra-oceanic arc setting. Raoul, Macauley and Raoul SW volcanoes in the northern Kermadec arc, and Healy volcano in the southern Kermadec arc have all recently erupted dacitic to rhyolitic crystal-poor pumice. In addition to whole-rock analyses, we present a detailed study of mineral and glass chemistries to highlight the complex structure of the Kermadec magmatic systems. Major and trace element bulk-rock compositions mostly fall into relatively narrow compositional ranges, forming discrete groups by eruption for Raoul, and varying with relative crystal contents for Healy. In contrast, pumices from Macauley cover a wide range of compositions, between 66 and 72·5 wt % SiO 2 . At all four volcanoes the trace element patterns of pumice are subparallel to both those of previously erupted basalts and/or whole mafic blebs found both as discrete pyroclasts and as inclusions within pumices. Pb and Sr isotopic compositions have limited ranges within single volcanoes, but vary considerably along the arc, being more radiogenic in the southern volcanoes. Distinctive crystal populations and zonation patterns in pumices, mafic blebs and plutonic xenoliths indicate that many crystals did not grow in the evolved magmas, but are instead mixed from other sources including gabbros and hydrothermally altered tonalites. Such open-system mixing is ubiquitous at the four volcanoes. Oxygen isotope compositions of both phenocrysts (silicic origin) and xenocrysts or antecrysts (mafic origin) are typical for mantle-derived melts. Whole-rock, glass and mineral chemistries are consistent with evolved magmas being generated at each volcano through ~70–80% crystal fractionation of a basaltic parent. Our results are not consistent with silicic magma generation via crustal anatexis, as previously suggested for these Kermadec arc volcanoes. Although crystallization is the dominant process driving melt evolution in the Kermadec volcanoes, we show that the magmatic systems are open to contributions from both newly arriving melts and wholly crystalline plutonic bodies. Such processes occur in variable proportions between magma batches, and are largely reflected in small-scale chemical variations between eruption units.

Description: Sea-floor imagery, volcanic rock, massive sulfide, and hydrothermal plume samples ( 3 He, 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 3 He 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.

Description: 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 CO 2 and SO 3 ) 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.

Description: Clark volcano of the Kermadec arc, northeast of New Zealand, is a large stratovolcano comprised of two coalescing volcanic cones; an apparently younger, more coherent, twin-peaked edifice to the northwest and a relatively older, more degraded and tectonized cone to the southeast. High-resolution water column surveys show an active hydrothermal system at the summit of the NW cone largely along a ridge spur connecting the two peaks, with activity also noted at the head of scarps related to sector collapse. Clark is the only known cone volcano along the Kermadec arc to host sulfide mineralization. Volcano-scale gravity and magnetic surveys over Clark show that it is highly magnetized, and that a strong gravity gradient exists between the two edifices. Modeling suggests that a crustal-scale fault lies between these two edifices, with thinner crust beneath the NW cone. Locations of regional earthquake epicenters show a southwest-northeast trend bisecting the two Clark cones, striking northeastward into Tangaroa volcano. Detailed mapping of magnetics above the NW cone summit shows a highly magnetized "ring structure" ~350 m below the summit that is not apparent in the bathymetry; we believe this structure represents the top of a caldera. Oblate zones of low (weak) magnetization caused by hydrothermal fluid upflow, here termed "burn holes," form a pattern in the regional magnetization resembling Swiss cheese. Presumably older burn holes occupy the inner margin of the ring structure and show no signs of hydrothermal activity, while younger burn holes are coincident with active venting on the summit. A combination of mineralogy, geochemistry, and seafloor mapping of the NW cone shows that hydrothermal activity today is largely manifest by widespread diffuse venting, with temperatures ranging between 56° and 106°C. Numerous, small (≤30 cm high) chimneys populate the summit area, with one site host to the ~7-m-tall "Twin Towers" chimneys with maximum vent fluid temperatures of 221°C (pH 4.9), consistent with 34 S anhydrite-pyrite values indicating formation temperatures of ~228° to 249°C. Mineralization is dominated by pyrite-marcasite-barite-anhydrite. Radiometric dating using the 228 Ra/ 226 Ra and 226 Ra/Ba methods shows active chimneys to be 〈20 with most 〈2 years old. However, the chimneys at Clark show evidence for mixing with, and remobilizing of, barite as old as 19,000 years. This is consistent with Nd and Sr isotope compositions of Clark chimney and sulfate crust samples that indicate mixing of ~40% seawater with a vent fluid derived from low K lavas. Similarly, REE data show the hydrothermal fluids have interacted with a plagioclase-rich source rock. A holistic approach to the study of the Clark hydrothermal system has revealed a two-stage process whereby a caldera-forming volcanic event preceded a later cone-building event. This ensured a protracted (at least 20 ka yrs) history of hydrothermal activity and associated mineral deposition. If we assume at least 200-m-high walls for the postulated (buried) caldera, then hydrothermal fluids would have exited the seafloor 20 ka years ago at least 550 m deeper than they do today, with fluid discharge temperatures potentially much hotter (~350°C). Subsequent to caldera infilling, relatively porous volcaniclastic and other units making up the cone acted as large-scale filters, enabling ascending hydrothermal fluids to boil and mix with seawater subseafloor, effectively removing the metals (including remobilized Cu) in solution before they reached the seafloor. This has implications for estimates for the metal inventory of seafloor hydrothermal systems pertaining to arc hydrothermal systems.

Description: Little is known about the effects that subducting an oceanic large igneous province (LIP) has on the petrogenesis of submarine arc volcanoes and their geochemical composition. The southern Kermadec arc represents a rare example where an LIP—the Hikurangi Plateau—is currently subducting and where its effect on mantle composition, element recycling and arc volcanism can be studied. We present mineral chemistry and whole-rock major and trace element, and Sr–Nd–Pb isotope data from samples recovered from the southern Kermadec arc volcanoes Rumble II East and Rumble II West, together with shipboard gravity and magnetic measurements. The Rumble II volcanoes (including a volcanic cone ~10 km further west) form an ~23 km long arc–backarc transect located ~250 km north of New Zealand above the subducting Hikurangi Plateau. Although only a short distance apart, rocks from the two volcanoes have different mineral and whole-rock geochemical compositions. Lavas from Rumble II East are predominantly basaltic and contain primitive olivine phenocrysts (≤Fo 91 ), high-Mg# clinopyroxene (≤96) and anorthitic plagioclase (≤An 97 ). Geochemically these lavas are very diverse and cover a spectrum from low Th/Yb (〈0·15) at high Ba/Th (〉1014) to higher Th/Yb (〉0·15) at lower Ba/Th (〈844). This spectrum, together with 206 Pb/ 204 Pb and 143 Nd/ 144 Nd in the range of 18·74–18·83 and 0·51309–0·51298 respectively (at similar to slightly elevated 87 Sr/ 86 Sr), suggests a mantle wedge that has undergone previous melt extraction and significant fluid addition from the subducting Pacific Plate and that contains sediment and HIMU-type Hikurangi Plateau components. The geochemistry of the sediment–HIMU-type components is exemplified in an olivine pyroxenite (e.g. 206 Pb/ 204 Pb = 20·02; 87 Sr/ 86 Sr = 0·70516; 143 Nd/ 144 Nd = 0·5126). We propose that the olivine pyroxenite formed through melt or fluid–rock metasomatism and represents the first direct evidence of a near Moho arc mantle rock that shows the imprint from a subducting HIMU-type (Hikurangi) seamount. Conversely, lavas from Rumble II West and the cone ~10 km to the west are generally more silica rich than Rumble II East lavas and mainly contain plagioclase with less ortho- and clinopyroxene + olivine phenocrysts. The low Ba/Th (〈470) and 206 Pb/ 204 Pb (〈18·74), a range of 143 Nd/ 144 Nd (0·51297–0·51307) and elevated Th/Yb (0·13–0·39) in these lavas can best be explained by minor sediment input into a less depleted mantle wedge. In addition, the geochemical composition of the Rumble II West lavas does not require involvement of a Hikurangi component, placing a spatial limit on Hikurangi material influencing regional melt generation beneath the backarc. Supported by a gravity model requiring two distinct magma chambers, the different geochemical compositions of Rumble II East and West lavas are inconsistent with a shared magma plumbing system. The different geochemical compositions of lavas from the two Rumble II volcanoes furthermore demonstrate that across-arc geochemical heterogeneities can occur within a few kilometres and may originate from both a geochemically heterogeneous mantle wedge and Moho transition layer, recording inherited geochemical heterogeneities beneath the volcanoes.