ALBERT

Description: Hydrothermal activity on submarine volcanic arcs in the western Pacific Ocean is known but mostly unexplored. In March 1999, the New Zealand American PLUme Mapping Expedition (NZAPLUME) cruise conducted the first systematic exploration of hydrothermal venting along a sizeable section of an intra-oceanic arc, visiting 13 volcanoes along 260 km of the southern Kermadec arc, just northeast of New Zealand. Conclusive evidence of hydrothermal plumes exists for seven of the 13 volcanoes; at two other volcanoes plume indications were weak and uncertain. The hydrothermal origin of the particle plumes was confirmed by positive anomalies in the ratios of sulphur, iron and copper to titanium relative to non-plume particles, in mass concentrations similar to particles collected from hydrothermal plumes over mid-ocean ridges. The spatial density of active sites along the southern Kermadec arc is at least 2.7 per 100 km (2.7/100 km), probably not significantly different from the weakly constrained value of c. 1/100 km on slow- and intermediate-rate mid-ocean ridges. An analysis of the number of hydrothermal fields produced for the magma delivery rate in each of these environments suggests that the southern Kermadec arc presently has relatively abundant hydrothermal activity. While this result cannot yet be generalized to other Pacific arcs, submarine volcanoes may contribute significantly to the global hydrothermal budget.

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.

Description: Near-bottom magnetic field data were collected using a towed magnetometer over selected parts of Palinuro and Marsili submarine volcanoes in the southern Tyrrhenian Sea, Italy. We obtained equivalent magnetizations maps at these sites by inverting the corresponding magnetic anomalies, highlighting the seafloor expression of hydrothermal alteration. Zones of reduced magnetization are interpreted as evidence for alteration related to hydrothermal processes; they are associated with water-column and seafloor observations of hydrothermal activity and altered host rocks. At Marsili volcano, a large elliptical area of reduced magnetization is located south of the summit cone and perpendicular to the trend of Marsili volcano itself. This confirms the presence of a large hydrothermal system centered over the more recent eastern volcanic ridge and its associated magma chamber. Similarly, areas of reduced magnetization over Palinuro caldera are consistent with hydrothermal venting occurring along the caldera walls, consistent with permeability structures related to caldera ring faults providing preferred pathways for the upflow of hydrothermal fluids.

Description: East Diamante is a submarine volcano in the southern Mariana arc that is host to a complex caldera ~5 x 10 km (elongated ENE-WSW) that is breached along its northern and southwestern sectors. A large field of barite-sulfide mounds was discovered in June 2009 and revisited in July 2010 with the R/V Natsushima , using the ROV Hyper-Dolphin . The mound field occurs on the northeast flank of a cluster of resurgent dacite domes in the central caldera, near an active black smoker vent field. A 40 Ar/ 39 Ar age of 20,000 ± 4000 years was obtained from a dacite sample. The mound field is aligned along a series of fractures and extends for more than 180 m east-west and 〉120 m north-south. Individual mounds are typically 1 to 3 m tall and 0.5 to 2 m wide, with lengths from about 3 to 8 m. The mounds are dominated by barite + sphalerite layers with the margins of each layer composed of barite with disseminated sulfides. Rare, inactive spires and chimneys sit atop some mounds and also occur as clusters away from the mounds. Iron and Mn oxides are currently forming small (〈1-m diam, ~0.5-m tall) knolls on the top surface of some of the barite-sulfide mounds and may also drape their flanks. Both diffusely and focused fluids emanate from the small oxide knolls. Radiometric ages of the layered barite-sulfide mounds and chimneys vary from ~3,920 to 3,350 years. One layer, from an outcrop of 10- to 100-cm-thick Cu-rich layers, is notably younger with an age of 2,180 years. The Fe-Mn oxides were 〈5 years old at the time of collection in 2009. Most mound, chimney, and layered outcrop samples are dominated by barite, silica, and sphalerite; other sulfides, in decreasing order of abundance, are galena, chalcopyrite, and rare pyrite. Anglesite, cerussite, and unidentified Pb oxychloride and Pb phosphate minerals occur as late-stage interstitial phases. The samples contain high Zn (up to 23 wt %), Pb (to 16 wt %), Ag (to 487 ppm), and Au (to 19 ppm) contents. Some layered outcrop samples are dominated by chalcopyrite resulting in ≤4.78 wt % Cu in a bulk sample (28 wt % for a single lens), with a mean of 0.28 wt % for other samples. Other significant metal enrichments are Sb (to 1,320 ppm), Cd (to 1,150 ppm), and Hg (to 55 ppm). The East Diamante mound field has a unique set of characteristics compared to other hydrothermal sites in the Mariana arc and elsewhere. The geochemical differences may predominantly reflect the distribution of fractures and faults and consequently the rock/water ratio, temperature of the fluid in the upper parts of the circulation system, and extensive and prolonged mixing with seawater. The location of mineralization is controlled by fractures. Following resurgent doming within the caldera, mineralization resulted from focused flow along small segments of linear fractures rather than from a point source, typical of hydrothermal chimney fields. Based on the mineral assemblage, the maximum fluid temperatures were ~260°C, near the boiling point for the water depths of the mound field (367–406 m). Lateral fluid flow within the mounds precipitated interstitial sphalerite, silica, and Pb minerals within a network of barite with disseminated sulfides; silica was the final phase to precipitate. The current low-temperature precipitation of Fe and Mn oxides and silica may represent rejuvenation of the system.

Description: Hydrothermal alteration processes involve mineralogical, chemical, and textural changes as a result of hot aqueous fluid-rock interaction under evolving boundary conditions. These changes affect the physico-chemical properties of the rocks, enabling high-resolution geophysical prospecting to be an important tool in the detection of seafloor hydrothermal alteration. Here we present the results of recent geophysical investigations of the Marsili and Palinuro volcanic complexes, southern Tyrrhenian Sea, during the 2010 TIR10 and 2011 MAVA2011 cruises by the R/V Urania . The new dataset includes a dense grid of multibeam bathymetry; seafloor reflectivity, magnetic and gravity lines; and high-resolution single (CHIRP) and multichannel seismic profiles. The surveys were focused on areas known to host intense hydrothermal alteration in order to provide a more detailed description of the Marsili and Palinuro hydrothermal systems. Ground-truthing was based on earlier discoveries of hydrothermal vents and their associated deposits, and on direct observations made by ROV dives. High-resolution morpho-bathymetry, sonar reflectivity, rock magnetization, and density distribution together enabled us to assess the extent of seafloor hydrothermal alteration and its relationship to local volcanic and tectonic structures. Hydrothermal alteration associated with the Marsili seamount is largely distributed along primary volcano-tectonic structures at the ridge crest. By contrast, at Palinuro hydrothermal alteration is mostly associated with secondary volcanic structures such as collapsed calderas and volcanism reactivation along ring faults. In particular, evidence for intense hydrothermal activity occurs at Palinuro where volcanotectonic features interact with regional tectonic structures.

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: Samples of chimneys (black smokers) and other volcanogenic massive sulfide (VMS) mineralization were collected from hydrothermal vent fields associated with submarine arc and back-arc volcanoes in the western Pacific. Each specimen was dated using radiometric techniques to establish the frequency and duration of hydrothermal activity at that site, information that is vital when establishing the time elapsed to amass an ore deposit. In each case, the radiometric dating relied on the radium isotopes 228 Ra and 226 Ra with half-lives of 5.75 and 1,600 years, respectively, which coprecipitate with barite and then decay. We began dating VMS mineralization 15 years ago, knowing that the 226 Ra/Ba decreases with time, with the Ba and Ra assumed to be from a common source such that the initial 226 Ra/Ba value, i.e., the value at the onset of mineralization, was constant. Thus, the age of an "old" chimney (up to 15,000 yr) was calculated by comparing its 226 Ra/Ba value with the initial value established from nearby "young" chimneys (〈35 yr) that were active or still contained 228 Ra. Since then, we have discovered that chimneys and other forms of sea-floor VMS mineralization can contain barite from two or more hydrothermal events. That is, older barite can be dissolved and reprecipitated (remobilized) from earlier mineralization deposited subseafloor, or earlier sea-floor mineralization overprinted (crosscut) by new. In many cases, the 226 Ra/Ba dating did not provide an absolute age for the older material but, rather, a difference in age between two samples that had incorporated varying proportions of older barite that was low or "dead" (i.e., 〉15,000 yr) with respect to 226 Ra, due to radioactive decay. Remobilizing of older barite is consistent with zone refining of metals in large VMS deposits, but we have only considered this recently. However, our investigations show that this can affect the initial 226 Ra/Ba values for VMS mineralization from arc and back-arc volcanoes, and also from mid-ocean ridges. Where sufficient samples contained 228 Ra, we estimated an age for the remobilized barite by plotting a 226 Ra/Ba versus 228 Ra/Ba mixing line for the VMS at the time of mineralization and found that hydrothermal activity has occurred at some arc volcanoes for 〉15,000 years. Such long histories of hydrothermal activity indicate the propensity for producing large metal-rich deposits on the seafloor.

Description: 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.

Description: Radiometric dating methods using 238 U and 232 Th 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, 228 Th/ 228 Ra, 228 Ra/ 226 Ra (Bq.Bq –1 ), and 226 Ra/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, 210 Pb/ 226 Ra, derived from 210 Pb/[Pb] versus 226 Ra/[Pb] isochron plots. Iron oxide crusts are dated using 210 Pb in combination with either As or 10 Be. The 228 Ra/ 226 Ra 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 228 Ra/ 226 Ra values for VMS are most likely due to excess 228 Ra (the daughter of 232 Th) 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 228 Ra (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.