ALBERT

Description: The Australian-Antarctic Ridge (AAR) is one of the largest unexplored regions of the global mid-ocean ridge system. Here, we report a multi-year effort to locate and characterize hydrothermal activity on two 1 st -order segments of the AAR: KR1 and KR2. To locate vent sites on each segment, we used profiles collected by Miniature Autonomous Plume Recorders on rock corers during R/V Araon cruises in March and December of 2011. Optical and oxidation-reduction-potential anomalies indicate multiple active sites on both segments. Seven profiles on KR2 found 3 sites, each separated by ∼25 km. Forty profiles on KR1 identified 13 sites, some within a few km of each other. The spatial density of hydrothermal activity along KR1 and KR2 (plume incidence of 0.34) is consistent with the global trend for a spreading rate of ∼70 mm/yr. The densest area of hydrothermal activity, named “Mujin”, occurred along the 20-km-long inflated section near the segment center of KR1. Continuous plume surveys conducted in January-February of 2013 on R/V Araon found CH 4 / 3 He (1-15 × 10 6 ) and CH 4 /Mn (0.01-0.5) ratios in the plume samples, consistent with a basaltic-hosted system and typical of ridges with intermediate spreading rates. Additionally, some of the plume samples exhibited slightly higher ratios of H 2 / 3 He and Fe/Mn than others, suggesting that those plumes are supported by a younger hydrothermal system that may have experienced a recent eruption. The Mujin-field was populated by Kiwa crabs and seven-armed Paulasterias starfish previously recorded on the East Scotia Ridge, raising the possibility of circum-Antarctic biogeographic connections of vent fauna. This article is protected by copyright. All rights reserved.

Description: The Woodlark Basin is one of the rare places on earth where the transition from continental breakup to seafloor spreading can be observed. The potential juxtaposition of continental rocks, a large magmatic heat source, crustal-scale faulting, and hydrothermal circulation has made the Woodlark Basin a prime target for seafloor mineral exploration. However, over the past 20 years, only two locations of active hydrothermalism had been found. In 2009 we surveyed 435 km of the spreading axis for the presence of hydrothermal plumes. Only one additional plume was found, bringing the total number of plumes known over 520 km of ridge axis to only 3, much less than at ridges with similar spreading rates globally. Particularly the western half of the basin (280 km of axis) is apparently devoid of high temperature plumes despite having thick crust and a presumably high magmatic budget. This paucity of hydrothermal activity may be related to the peculiar tectonic setting at Woodlark, where repeated ridge jumps and a re-location of the rotation pole both lead to axial magmatism being more widely distributed than at many other, more mature and stable mid-ocean ridges. These factors could inhibit the development of both a stable magmatic heat source and the deeply penetrating faults needed to create long-lived hydrothermal systems. We conclude that large seafloor massive sulfide deposits, potential targets for seafloor mineral exploration, will probably not be present along the spreading axis of the Woodlark Basin, especially in its younger, western portion.

Description: Magma mixing plays a prominent role in the origins of intermediate magmas in subduction zones. However, the conditions and timescales of magma mixing and how these are linked to subsequent eruption are unclear. Mount Tauhara is the largest dacitic volcanic complex in the Taupo Volcanic Zone, New Zealand. Dacites from Tauhara Volcano have a complex petrography (Qtz + Plag + Amph + OPx + CPx + Oxi ± Oli) that can only have been produced by magma mixing and offer an ideal opportunity to investigate the processes and timescales involved in assembling dacite magmas in a continental subduction zone. Here we present whole-rock and mineral-specific major and trace element and isotopic data for the Tauhara dacites in order to identify the magma mixing end-members, constrain the physical conditions of mixing, and estimate the timescales and relationships between magma mixing, ascent and eruption. These data reveal that four separate mixing events between crystal-rich rhyolites (77–80 wt.% SiO 2 ; 40 ppm Sr) and crystal-poor mafic magmas of basaltic (48 wt.% SiO 2 ; 1340 ppm Sr) to andesitic (55–59 wt.% SiO 2 ; 490–580 ppm Sr) composition occurred to produce the Tauhara dacites. Mixing took place in well-stirred magma chambers located at mid-crustal depths (8–13 km) at temperatures from 840 to 900ºC. The timescales of magma mixing obtained from Ti diffusion in quartz appear to be largely dependent on the temperature and viscosity contrast between the end-members as andesite and rhyolite magma mixed on timescales of 2–7 months, whereas basalt and rhyolite magmas mixed on timescales of 1–2 years. The short magma mixing timescales, combined with the physical properties (e.g., viscosity and density) of the mixed dacite magmas, as compared with those of the end-member magmas, facilitated the ascent and eruption of dacite magmas at Tauhara volcano.

Description: The creation of ocean crust by lava eruptions is a fundamental Earth process, involving immediate and immense transfers of heat and chemicals from crust to ocean. This transfer creates event plumes (“megaplumes”), massive ellipsoidal eddies with distinctive and consistent chemical signatures. Here we report the discovery of unique event plumes associated with a 2008 eruption on the Northeast Lau Spreading Center. Instead of a large plume hundreds of meters thick, we detected at least eight individual plumes, each ∼50 m thick and apparently only 1–3 km in diameter, yet still rising 200–1000 m above the eruption site. Low and uniform 3He/heat (0.041 × 10−17 mol/J) and dissolved Mn/heat (0.04 nmol/J) ratios in water samples were diagnostic of event plumes. High H2 concentrations (up to 9123 nM) and basalt shards confirmed extensive interactions between molten lava and event plume source fluids. Remote vehicle observations in 2009 mapped a new, small (1.5–5.8 × 106 m3) lava flow. Our results suggest that event plumes are more variable, and thus perhaps more common, than previously recognized. Small event plumes may be preferentially associated with small or sheet-flow eruptions, and massive event plumes with slowly extruding pillow mounds 25–75 m thick. Despite this correlation, and high H2 concentrations, existing theory and seafloor observations argue that cooling lava cannot transfer heat fast enough to create the buoyancy flux required for event plumes. The creation of event plumes under a broad range of eruption conditions provides new constraints for any theory of their formation.

Description: Indian Ocean ridges north of the Rodriguez Triple Junction remain poorly explored for seafloor hydrothermal activity, with only two active sites confirmed north of 25°S. We conducted water column surveys and sampling in 2007 and 2009 to search for hydrothermal plumes over a segment of the Carlsberg Ridge. Here we report evidence for two separate vent fields, one near 3°42′N, 63°40′E and another near 3°41.5′N, 63°50′E, on a segment that is apparently sparsely magmatic. Both sites appear to be located on off-axis highs at the top of the southern axial valley wall, at depths of ∼3600 m or shallower (∼1000 m above the valley floor). At the 63°40′E site, plume sampling found local maxima in light scattering, temperature anomaly, oxidation-reduction potential (ORP), dissolved Mn, and 3He. No water samples are available from the 63°50′E site, but it showed robust light-scattering and ORP anomalies at multiple depths, implying multiple sources. ORP anomalies are very short-lived, so the strong signals at both sites suggest that fluid sources lie within a few kilometers or less from the plume sampling locations. Although ultramafic rocks have been recovered near these sites, the light-scattering and dissolved Mn anomalies imply that the plumes do not arise from a system driven solely by exothermic serpentinization (e.g., Lost City). Instead, the source fluids may be a product of both ultramafic and basaltic/gabbroic fluid-rock interaction, similar to the Rainbow and Logatchev fields on the Mid-Atlantic Ridge.

Description: We present multiple lines of evidence for years- to decade-long changes in the location and character of volcanic activity at West Mata seamount in the NE Lau Basin over a 16 year period, and a hiatus in summit eruptions from early 2011 through at least September 2012. Boninite lava and pyroclasts were observed erupting from its summit in 2009 and hydroacoustic data from a succession of hydrophones moored nearby show near-continuous eruptive activity from January 2009 to early 2011. Successive differencing of seven multibeam bathymetric surveys of the volcano made in the 1996-2012 period reveal a pattern of extended constructional volcanism on the summit and northwest flank punctuated by eruptions along the volcano's WSW rift zone (WSWRZ). Away from the summit, the volumetrically largest eruption during the observational period occurred between May 2010 and November 2011 at ~2920 m depth near the base of the WSWRZ. The (nearly) equally long ENE rift zone did not experience any volcanic activity during the 1996-2012 period. The cessation of summit volcanism recorded on the moored hydrophone was accompanied or followed by the formation of a small summit crater and a landslide on the eastern flank. Water column sensors, analysis of gas samples in the overlying hydrothermal plume and dives with a remotely operated vehicle in September 2012 confirmed that the summit eruption had ceased. Based on the historical eruption rates calculated using the bathymetric differencing technique, the volcano could be as young as several thousand years.

Description: Four cruises between 2008 and 2012 monitored the continuing eruption of West Mata volcano in the NE Lau Basin as it produced plumes of chemically altered water above its summit. Although large enrichments in 3 He, CO 2, Fe, and Mn were observed in the plumes, the most notable enrichment was that of H 2 , which reached concentrations as high as 14843 nM. Strongly enriched H 2 concentrations in the water column result from reactions between seawater or magmatic water and extremely hot rocks. In 2008, the observation of elevated H 2 concentrations in the water column above West Mata pointed to vigorous ongoing eruptions at the volcano's summit. The eruption was confirmed by visual observations made by the ROV Jason 2 in 2009 and demonstrated that H 2 measurements are a vital instrument to detect ongoing volcanic eruptions at the seafloor. Elevated H 2 in 2010 showed that the eruption was ongoing, although at a reduced level given a maximum H 2 concentration of 4410 nM. In 2012, H 2 levels in the water column declined significantly, to a maximum of only 7 nM, consistent with visual observations from the Quest-4000 ROV that found no evidence of an ongoing volcanic eruption. Methane behaved independently of other measured gases and its concentrations in the hydrothermal plume were very low. We attribute its minimal enrichments to a mixture of mantle carbon reduced to CH 4 and biological CH 4 from diffuse flow sites. This study demonstrates that ongoing submarine volcanic eruptions are characterized by high dissolved H 2 concentrations present in the overlying water column.

Description: The northern Lau Basin hosts a complicated pattern of volcanism, including Tofua Arc volcanoes, several back-arc spreading centers and individual “rear-arc” volcanoes not associated with these structures. Elevated 3 He/ 4 He ratios in lavas of the NW Lau Spreading Center suggest the influence of a mantle plume, possibly from Samoa. We show that lavas from mid-ocean ridges, volcanic arcs, and hotspots occupy distinct, non-overlapping fields in a 3 He/ 4 He vs. C/ 3 He plot. Applied to the northern Lau Basin, this approach shows that most of Lau back-arc spreading systems have mid-ocean ridge 3 He/ 4 He - C/ 3 He characteristics, except the NW Lau spreading center, which has 3 He/ 4 He - C/ 3 He similar to “high 3 He” hotspots such as Loihi, Kilauea, and Yellowstone, but with slightly lower C/ 3 He. Niua seamount, on the northern extension of the Tofua Arc, falls squarely in the arc field. All of the NE Lau rear-arc volcanoes, including the recently erupting West Mata, also have arc-like 3 He/ 4 He - C/ 3 He characteristics. Ba-Nb-Ti contents of the lavas, which are more traditional trace element indicators of mantle source enrichment, depletion and subduction input, likewise indicate arc and hot spot influences in the lavas of the Northern Lau Basin, but in a more ambiguous fashion because of a complex prior history. This verifies that 3 He/ 4 He - C/ 3 He systematics are useful for differentiating between mid-ocean ridge, arc, and hotspot affinities in submarine volcanic systems, that all three of these affinities are expressed in the northern Lau Basin, and provides additional support for the Samoan plume influence in the region. This article is protected by copyright. All rights reserved.

Description: Multiple geological processes affect the distribution of hydrothermal venting along a mid-ocean ridge. Deciphering the role of a specific process is often frustrated by simultaneous changes in other influences. Here we take advantage of the almost constant spreading rate (65-71 mm/yr) along 2500 km of the Southeast Indian Ridge (SEIR) between 77°-99°E to examine the spatial density of hydrothermal venting relative to regional and segment-scale changes in the apparent magmatic budget. We use 227 vertical profiles of light backscatter and (on 41 profiles) oxidation-reduction potential along 27 1 st - and 2 nd -order ridge segments on and adjacent to the Amsterdam-St. Paul (ASP) Plateau to map p h , the fraction of casts detecting a plume. At the regional scale, venting on the five segments crossing the magma-thickened hotspot plateau is almost entirely suppressed ( p h = 0.02). Conversely, the combined p h (0.34) from all other segments follows the global trend of p h versus spreading rate. Off the ASP Plateau, multi-segment trends in p h track trends in the regional axial depth, high where regional depth increases and low where it decreases. At the individual segment scale, a robust correlation between p h and cross-axis inflation for 1 st -order segments shows that different magmatic budgets among 1 st -order segments are expressed as different levels of hydrothermal spatial density. This correlation is absent among 2 nd -order segments. Eighty-five percent of the plumes occur in eight clusters totaling ~350 km. We hypothesize that these clusters are a minimum estimate of the length of axial melt lenses underlying this section of the SEIR.

Description: s The complex geology and expansive axial valleys typical of slow-spreading ridges makes evaluating their hydrothermal activity a challenge. This challenge has gone largely unmet, as the most under-sampled MOR type for hydrothermal activity is slow-spreading (20-55 mm/yr). Here we report the first systematic hydrothermal plume survey conducted on the Central Indian Ridge (CIR, 8°-17°S), the most extensive such survey yet conducted on a slow-spreading ridge. Using a combined CTD/Miniature Autonomous Plume Recorder (MAPR) package we used 118 vertical casts along seven segments of the CIR (~700 km of ridge length) to estimate the frequency of hydrothermal activity. Evidence for hydrothermal activity (particle and methane plumes) was found on each of the seven spreading segments, with most plumes found between 3,000 and 3,500 m, generally 〈1,000 m above bottom. We most commonly found plumes on asymmetric ridge sections where ultramafic massifs formed along one ridge flank near ridge-transform intersections or non-transform offsets. The estimated plume incidence ( p h ) for axial and wall casts ( p h =0.30, 35 of 118 casts) is consistent with the existing global trend, indicating that the long-term magmatic budget on the CIR is the primary control on the spatial frequency of hydrothermal venting. Our results show that the tectonic fabric of the CIR strongly determines where hydrothermal venting is expressed, and that using only near-axial sampling might underestimate hydrothermal activity along slow- and ultraslow-spreading ridges. Serpentinization is a minor contributor to the plume inventory, based on 15 profiles with methane anomalies only, predominantly at depths above the local valley walls.