Heat flow and near-seafloor magnetic anomalies highlight hydrothermal circulation at Brothers volcano caldera, southern Kermadec arc, New Zealand

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 46(14), (2019): 8252-8260, doi: 10.1029/2019GL083517.Brothers volcano is the most hydrothermally active volcano along the Kermadec arc, with distinct hydrothermal fields located on the caldera walls and on the postcollapse volcanic cones. These sites display very different styles of hydrothermal activity in terms of temperature, gas content, fluid chemistry, and associated mineralization. Here we show the results of a systematic heat flow survey integrated with near‐seafloor magnetic data acquired using remotely operated vehicles and autonomous underwater vehicles. Large‐scale circulation is structurally controlled, with a deep (~1‐ to 2‐km depth) central recharge through the caldera floor and lateral discharge along the caldera walls and at the summits of the postcollapse cones. Shallow (~ 0.1‐0.2 km depth) circulation is characterized by small‐scale recharge zones located at a distance of ~ 0.1–0.2 km from the active vent sites.We thank the Captains and crews of the R/V Sonne, Thompson, and Tangaroa and the engineers from Wood Hole Oceanographic Institution and MARUM for the successful operation of ABE, Sentry, Quest 4000, and Jason. The heat flow data surveys were funded by NSF grant OCE‐1558356 (PI Susan Humphris) and a grant from the German Ministry for Education and Research BMBF, project no. 03G0253A (PI Andrea Koschinsky). Funding from the New Zealand Government (Ministry of Business, Innovation and Employment) helped enable this study. This paper was significantly improved by the comments from the Editor Rebecca Carey and from two unknown reviewers. The data used in this paper can be downloaded from the U.S. Lamont‐Doherty MGDS database.2020-01-1

High-resolution water column survey to identify active sublacustrine hydrothermal discharge zones within Lake Rotomahana, North Island, New Zealand

This paper is not subject to U.S. copyright. The definitive version was published in Journal of Volcanology and Geothermal Research 314 (2016): 142-155, doi:10.1016/j.jvolgeores.2015.07.037.Autonomous underwater vehicles were used to conduct a high-resolution water column survey of Lake Rotomahana using temperature, pH, turbidity, and oxidation–reduction potential (ORP) to identify active hydrothermal discharge zones within the lake. Five areas with active sublacustrine venting were identified: (1) the area of the historic Pink Terraces; (2) adjacent to the western shoreline subaerial “Steaming Cliffs,” boiling springs and geyser; (3) along the northern shoreline to the east of the Pink Terrace site; (4) the newly discovered Patiti hydrothermal system along the south margin of the 1886 Tarawera eruption rift zone; and (5) a location in the east basin (northeast of Patiti Island). The Pink Terrace hydrothermal system was active prior to the 1886 eruption of Mount Tarawera, but venting along the western shoreline, in the east basin, and the Patiti hydrothermal system appear to have been initiated in the aftermath of the eruption, similar to Waimangu Valley to the southwest. Different combinations of turbidity, pH anomalies (both positive and negative), and ORP responses suggest vent fluid compositions vary over short distances within the lake. The seasonal period of stratification limits vertical transport of heat to the surface layer and the hypolimnion temperature of Lake Rotomahana consequently increases with an average warming rate of ~ 0.010 °C/day due to both convective hydrothermal discharge and conductive geothermal heating. A sudden temperature increase occurred during our 2011 survey and was likely the response to an earthquake swarm just 11 days prior.Funding was provided by GNS Strategic Development Fund

Interpretation of gravity and magnetic anomalies at Lake Rotomahana: geological and hydrothermal implications

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in Journal of Volcanology and Geothermal Research 314 (2016): 84-94, doi:10.1016/j.jvolgeores.2015.07.002.We investigate the geological and hydrothermal setting at Lake Rotomahana, using recently collected potential-field data, integrated with pre-existing regional gravity and aeromagnetic compilations. The lake is located on the southwest margin of the Okataina Volcanic Center (Haroharo caldera) and had well-known, pre-1886 Tarawera eruption hydrothermal manifestations (the famous Pink and White Terraces). Its present physiography was set by the caldera collapse during the 1886 eruption, together with the appearance of surface activities at the Waimangu Valley. Gravity models suggest subsidence associated with the Haroharo caldera is wider than the previously mapped extent of the caldera margins. Magnetic anomalies closely correlate with heat-flux data and surface hydrothermal manifestations and indicate that the west and northwestern shore of Lake Rotomahana are characterized by a large, well-developed hydrothermal field. The field extends beyond the lake area with deep connections to the Waimangu area to the south. On the south, the contact between hydrothermally demagnetized and magnetized rocks strikes along a structural lineament with high heat-flux and bubble plumes which suggest hydrothermal activity occurring west of Patiti Island. The absence of a well-defined demagnetization anomaly at this location suggests a very young age for the underlying geothermal system which was likely generated by the 1886 Tarawera eruption. Locally confined intense magnetic anomalies on the north shore of Lake Rotomahana are interpreted as basalts dikes with high magnetization. Some appear to have been emplaced before the 1886 Tarawera eruption. A dike located in proximity of the southwest lake shore may be related to the structural lineament controlling the development of the Patiti geothermal system, and could have been originated from the 1886 Tarawera eruption.Science funding provided by GNS Science Strategic Development Fund

Submarine Hydrothermal Activity on the Aeolian Arc: New evidence from Helium Isotopes

In November 2007 we conducted a water-column and seafloor mapping study of the submarine volcanoes of the Aeolian Arc in the southern Tyrrhenian Sea aboard the R/V Urania. A total of 26 CTD casts were completed, 13 vertical casts and 13 tows. In addition to in situ measurements of temperature, conductivity, pressure and suspended particles, we also collected discrete samples for helium isotopes, methane, and trace metals. The helium isotope ratio, which is known to be an unambiguous indicator of hydrothermal input, showed a clear excess above background at 5 out of the 10 submarine volcanoes surveyed. We found the strongest helium anomaly over Marsili seamount, where the 3He/4He ratio reached maximum values of 3He = 23% at 610 m depth compared with background values of ~ 7%. We also found smaller but distinct 3He anomalies over Enerato, Eolo, Palinuro, and Secca del Capo. We interpret these results as indicating the presence of hydrothermal activity on these 5 seamounts. Hydrothermal venting has been documented at subsea vents offshore of the islands of Panarea, Stromboli, and Vulcano (Dando et al., 1999; Di Roberto et al., 2008), and hydrothermal deposits have been sampled on many of the submarine volcanoes of the Aeolian Arc (Dekov and Savelli, 2004). However, as far as we know this is the first evidence of present day hydrothermal activity on Marsili, Enerato, and Eolo. Samples collected over Filicudi, Glabro, Lamentini, Sisifo, and Alcioni had 3He very close to the regional background values, suggesting either absence of or very weak hydrothermal activity on these seamounts. Helium isotope measurements from the background hydrocasts positioned between the volcanoes revealed the presence of an excess in 3He throughout the SE Tyrrhenian Sea. These background profiles reach a consistent maximum of about 3He = 11% at 2300 m depth. Historical helium profiles collected in the central and northern Tyrrhenian Sea in 1987 and 1997 do not show this deep 3He maximum (W. Roether and B. Klein, private comm.). Furthermore, the maximum is too deep to be attributed to the volcanoes of the Aeolian Arc, which are active at <1000 m depth. We are currently conducting additional measurements to determine whether this deep 3He maximum is from a local hydrothermal source or is somehow related to the deep water mass transient which occurred in the eastern Mediterranean in the 1990’s

Early evolution of a young back-arc basin in the Havre Trough

Back-arc basins are found at convergent plate boundaries. Nevertheless, they are zones of significant crustal extension that show volcanic and hydrothermal processes somewhat similar to those of mid-ocean ridges. Accepted models imply the initial rifting and thinning of a pre-existing volcanic arc until seafloor spreading gradually develops over timescales of a few million years. The Havre Trough northeast of New Zealand is a unique place on Earth where the early stages of back-arc basin formation are well displayed in the recent geological record. Here we present evidence that, in this region, rifting of the original volcanic arc occurred in a very narrow area about 10–15 km wide, which could only accommodate minimal stretching for a very short time before mass balance required oceanic crustal accretion. An initial burst of seafloor spreading started around 5.5–5.0 million years ago and concluded abruptly about 3.0–2.5 million years ago, after which arc magmatism dominated the crustal accretion. The sudden transition between these different tectonomagmatic regimes is linked to trench rollback promoted by gradual sinking of the subducting lithosphere, which could have diverted the arc flux outside the region of seafloor spreading and induced the vertical realignment of surface volcanism with the source of arc melts at depth

Multiple hydrothermal sources along the south Tonga arc and Valu Fa Ridge

Quantifying hydrothermal venting at the boundaries of tectonic plates is an outstanding geoscience problem. Considerable progress has been made by detailed surveys along mid-ocean ridges (MORs), but until recently little was known about fluid venting along volcanic arcs. We present the first systematic survey for hydrothermal venting along the 425-km-long south Tonga arc and new chemistry data for particle and thermal plumes previously reported along an adjacent 88-km-long section of the back-arc Valu Fa Ridge (VFR). Eleven hydrothermal plumes, recognized by their anomalous light backscattering, Eh, temperature, pH, dissolved 3He, CH4, and total dissolvable Fe and Mn, were identified arising from seven volcanic centers along the arc. Five plumes on the VFR were characterized chemically. Vent field density for the south Tonga arc was 2.6 sites/100 km of arc front, comparable to that found by surveys of the Kermadec arc (1.9 to 3.8 sites/100 km) and to MORs in the eastern Pacific (average value for 2280 km of surveyed ridgecrest: 3.2 sites/100 km). A "vent gap" occurs along a 190 km section of the arc closest to the VFR, and a site density twice the average for MORs on the eastern edge of the Pacific plate was found on this part of the VFR (6.6 sites/100 km). We suggest magmas ascending under the adjacent south Tonga arc have been captured by the VFR. While chemical enrichments of plumes on the south Tonga arc were, in general, slightly less than those on the Kermadec arc, several instances of excessive anomalies in pH suggest a similar presence of fluids enriched in magmatic volatiles (CO2-SO2-H2S). Locally, venting on the VFR has contributed to accumulations of 3He, Fe, and Mn within the southern Lau basin. On a broader scale, our results provide considerable support for the notion that venting from intraoceanic arcs on the convergent margin of the Pacific plate adds significantly to the total hydrothermal input into the Pacific Ocean

Louisville seamount subduction and its implication on mantle flow beneath the central Tonga–Kermadec arc

Subduction of intraplate seamounts beneath a geochemically depleted mantle wedge provides a seldom opportunity to trace element recycling and mantle flow in subduction zones. Here we present trace element and Sr, Nd and Pb isotopic compositions of lavas from the central Tonga–Kermadec arc, west of the contemporary Louisville–Tonga trench intersection, to provide new insights into the effects of Louisville seamount subduction. Elevated 206Pb/204Pb, 208Pb/204Pb, 86Sr/87Sr in lavas from the central Tonga–Kermadec arc front are consistent with localized input of subducted alkaline Louisville material (lavas and volcaniclastics) into sub-arc partial melts. Furthermore, absolute Pacific Plate motion models indicate an anticlockwise rotation in the subducted Louisville seamount chain that, combined with estimates of the timing of fluid release from the subducting slab, suggests primarily trench-normal mantle flow beneath the central Tonga–Kermadec arc system

Subduction of the oceanic Hikurangi Plateau and its impact on the Kermadec arc

Large igneous province subduction is a rare process on Earth. A modern example is the subduction of the oceanic Hikurangi Plateau beneath the southern Kermadec arc, offshore New Zealand. This segment of the arc has the largest total lava volume erupted and the highest volcano density of the entire Kermadec arc. Here we show that Kermadec arc lavas south of B32°S have elevated Pb and Sr and low Nd isotope ratios, which argues, together with increasing seafloor depth, forearc retreat and crustal thinning, for initial Hikurangi Plateau—Kermadec arc collision B250 km north of its present position. The combined data set indicates that a much larger portion of the Hikurangi Plateau (the missing Ontong Java Nui piece) than previously believed has already been subducted. Oblique plate convergence caused southward migration of the thickened and buoyant oceanic plateau crust, creating a buoyant ‘Hikurangi’ me´lange beneath the Moho that interacts with ascending arc melts

Submarine Magmatic-Hydrothermal Systems at the Monowai Volcanic Centre, Kermadec Arc

Authors listed on this Accepted Manuscript vary slightly from those listed on the Version of Record. Harold L. Gibson is an additional author on the published version.The Monowai volcanic centre (MVC) is located at the mid-point along the ~2530 km long Tonga-Kermadec arc system, is probably the most hydrothermally active submarine volcanic system globally. The MVC is comprised of a large elongate caldera (Monowai caldera, 7.9 x 5.7 km; 35 km²; depth to caldera floor is 1590 m), which has formed within an older caldera some 84 km² in area. To the south of the nested caldera system is a large composite volcano, Monowai cone, which rises to within ~ 100 m of the sea surface and has been volcanically active for at least several decades. Despite the large size, mafic volcanic rocks dominate the MVC; basalts are the most common rock type recovered; less common are basaltic andesites and andesites. Hydrothermal plume mapping during the 2004 NZAPLUME III cruise showed at least three major hydrothermal systems associated with the caldera and cone. Monowai cone has hydrothermal venting from the summit. This summit plume is gas-rich and acidic; plume samples show a pH shift of -2.00 pH units, δ³He up to 358 ‰, H₂S concentrations up to 32 μM and CH₄ concentrations up to 900 nM. The summit plume is also metal-rich with elevated total dissolvable Fe (TDFe up to 4200 nM), TDMn (up to 412 nM), and TDFe/TDMn (up to 20.4). Monowai caldera has a major hydrothermal vent system with plumes extending from ~ 1000 to 1400 m depth. The caldera plume has lower values for TDFe, although ranges to higher TDMn concentrations than the summit plume, and is relatively gas-poor (no H₂S detected, pH shift of -0.06 pH units, CH₄ concentrations up to 26 nM). Hydrothermal vents have been observed associated with prominent basaltic andesite ridges (Mussel Ridge) proximal to the southwest wall of the caldera (1025 – 1171 m depth). However, the composition of the hydrothermal plumes in the caldera are different to the vents, indicating that the source of the caldera plumes is at greater depth and is more metal-rich and therefore likely higher temperature. Minor plumes detected as light scattering anomalies down the northern flank of Monowai caldera most likely represent resuspension of volcanic debris. Particulate samples from both the cone sites and the caldera site are enriched in Al, Ti, Ca, Mg, Si, and S, with the cone summit plume especially enriched in K, As, W and Cu, Pb, Zn. The elevated Ti and Al suggest acidic water-rock reactions and intense high-sulfidation alteration of the host volcanic rocks. Observations from submersible dives with Pisces V in 2005 and the remotely operated vehicle ROPOS in 2007 of Mussel Ridge indicate numerous low temperature vents (< 60°C), with a large biomass of vent-associated fauna, in particular large accumulations of the mussel Bathymodiolus sp. and the tubeworm Lamellibrachia sp. We interpret the Monowai volcanic centre as possessing a robust high-sulfidation magmatic-hydrothermal system, with significant differences in the style and composition of venting at the cone and caldera sites. At Monowai cone, the large shifts in pH, elevated TDFe and TDFe/TDMn, and H₂S-, CH₄- and ³He-rich nature of the plume fluids coupled with elevated Ti, P, V, S and Al in the particulates indicates significant magmatic volatile ± metal contributions to the hydrothermal system and aggressive acidic water-rock interaction. By contrast, Monowai caldera has low TDFe/TDMn in hydrothermal plumes; however, end-member vent fluid compositions, combined with presence of alunite, sulfide minerals and native sulfur in samples from Mussel Ridge suggest recent acid volatile-rich venting and active Fe-sulfide formation in the subsurface, and the potential for the presence of significant SMS mineralization