ALBERT

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: 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: 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.

Description: 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 km 2 ; depth to caldera floor 1,590 m), which has formed within an older caldera some 84 km 2 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 3 He (≤358%), and noticeable H 2 S (up to 32 μ m), and CH 4 (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 H 2 S detected, pH shift of –0.06 pH units, CH 4 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 3 He 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 3 He% values, elevated TDFe and TDFe/TDMn, and the H 2 S- and CH 4 -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.