ALBERT

Description: Three active-source seismic refraction profiles are integrated with morphological and potential field data to place the first regional constraints on the structure of the Kermadec subduction zone. These observations are used to test contrasting tectonic models for an along-strike transition in margin structure previously known as the 32°S boundary. We use residual bathymetry to constrain the geometry of this boundary and propose the name Central Kermadec Discontinuity (CKD). North of the CKD, the buried Tonga Ridge occupies the forearc with VP 6.5–7.3 km s-1 and residual free-air gravity anomalies constrain its latitudinal extent (north of 30.5°S), width (110 ± 20 km) and strike (~005° south of 25°S). South of the CKD the forearc is structurally homogeneous down-dip with VP 5.7–7.3 km s-1. In the Havre Trough backarc, crustal thickness south of the CKD is 8-9 km, which is up-to 4 km thinner than the northern Havre Trough and at least 1 km thinner than the southern Havre Trough. We suggest that the Eocene arc did not extend along the current length of the Tonga-Kermadec trench. The Eocene arc was originally connected to the Three Kings Ridge and the CKD was likely formed during separation and easterly translation of an Eocene arc substrate during the early Oligocene. We suggest that the first-order crustal thickness variations along the Kermadec arc were inherited from before the Neogene and reflect Mesozoic crustal structure, the Cenozoic evolution of the Tonga-Kermadec-Hikurangi margin and along-strike variations in the duration of arc volcanism.

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 (≤Fo91), high-Mg# clinopyroxene (≤96) and anorthitic plagioclase (≤An97). 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 206Pb/204Pb and 143Nd/144Nd in the range of 18·74–18·83 and 0·51309–0·51298 respectively (at similar to slightly elevated 87Sr/86Sr), 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. 206Pb/204Pb = 20·02; 87Sr/86Sr = 0·70516; 143Nd/144Nd = 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 206Pb/204Pb (〈18·74), a range of 143Nd/144Nd (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: 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.

Description: 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.

Description: 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.

Description: Hydrothermal systems hosted by submarine arc volcanoes commonly include a large component of magmatic fluid. The high Cu-Au contents and strongly acidic fluids in these systems are similar to those that formed in the shallow parts of some porphyry copper and epithermal gold deposits mined today on land. Two main types of hydrothermal systems occur along the submarine portion of the Kermadec arc (offshore New Zealand): magmatically influenced and seawater-dominated systems. Brothers volcano hosts both types. Here, we report results from a series of drill holes cored by the International Ocean Discovery Program into these two types of hydrothermal systems. We show that the extent of hydrothermal alteration of the host dacitic volcaniclastics and lavas reflects primary lithological porosity and contrasting spatial and temporal contributions of magmatic fluid, hydrothermal fluid, and seawater. We present a two-step model that links the changes in hydrothermal fluid regime to the evolution of the volcano caldera. Initial hydrothermal activity, prior to caldera formation, was dominated by magmatic gases and hypersaline brines. The former mixed with seawater as they ascended toward the seafloor, and the latter remained sequestered in the subsurface. Following caldera collapse, seawater infiltrated the volcano through fault-controlled permeability, interacted with wall rock and the segregated brines, and transported associated metals toward the seafloor and formed Cu-Zn-Au–rich chimneys on the caldera walls and rim, a process continuing to the present day. This two-step process may be common in submarine arc caldera volcanoes that host volcanogenic massive sulfide deposits, and it is particularly efficient at focusing mineralization at, or near, the seafloor.

Description: Variations in trace metal contents and sulfur isotope ratios (δ34S) within pyrite, at the scale of individual mineral grains, preserves a record of temporal fluctuations in the source of metals and sulfur as well as changes in the chemical composition and temperature of hydrothermal fluid during the evolution of the Brothers volcano, Kermadec arc, New Zealand. In this study, we analyzed pyrite from drill core recovered from two geochemically distinct hydrothermal systems at the Brothers volcano, the seawater-influenced NW Caldera (Site U1530) and magmatic-volatile-dominated Upper Cone (Site U1528) during the International Ocean Discovery Program’s Expedition 376. At the NW Caldera site, from 189 m below the seafloor, a seawater-derived hydrothermal fluid forming chlorite-rich alteration overprints early pyrophyllite + illite alteration. Within ~ 30 m of the seafloor at this same site, pyrite contains zones of high As content with a variable δ34S signature that ranges from -4.5 to 3.4‰ (n = 26). Values for δ34S 〉 0‰ record shallow mixing of seawater with upwelling hydrothermal fluids. In deeper parts of the system, but still within the chlorite-rich alteration zone, δ34S values 〉 0‰ are absent, indicating that relatively more sulfur is contributed from magmatic volatile degassing and SO2 disproportionation. In the pyrophyllite-rich alteration zone, pyrite contains Co-enriched cores that correspond to sharp changes in δ34S values from -5.3‰ to 4.6‰ (n = 68). Cobalt enrichment occurs in response to the mixing of seawater-derived hydrothermal fluid with Co-rich magmatic brines. At the Upper Cone site, a relatively constant supply of a low-salinity magmatic fluid results in pyrite grains that rarely exhibit any internal zonation in trace metal content. In pyrite where zonation does exist, a correlation between Cu and Sb and uniformly low δ34S values (〈 0‰) indicates a link between metal enrichment, the pulsed degassing of magmatic volatiles, and SO2 disproportionation.

Description: https://doi.org/10 .1594/PANGAEA.920208

Description: research