Description: Fresh volcanic glasses from the extrusive section of the Troodos Ophiolite in Akaki Canyon are tholeiitic and basaltic to dacitic in composition. Compared to normal MORB they have extremely low fractionation corrected Na8, Fe8 and Ti8 and are enriched in fluid-mobile trace elements, including U, Ba, Rb, Sr and Pb, relative to non-fluid mobile elements of similar incompatibility. Trace element compositions of Akaki lavas define an array extending between ‘back-arc lava’-like compositions, and the field defined by Troodos boninites from the upper part of the lava sequence. Troodos lavas were derived from a mantle source that underwent early melt depletion, and later enrichment by both fluids and small degree melts. These processes can explain the unusual negative correlation of Pb/Ce with Zr/Nb and Ba/Nb in Troodos extrusives. Although some Troodos lavas are similar in composition to lavas from back-arc spreading centres, the boninites from the upper parts of the lava pile do not appear to have exact compositional equivalents among lavas from fore-arcs, back-arcs or other tectonic settings where similar rocktypes have been recovered. We suggest that the geochemical evolution inferred for the mantle source of Troodos lavas, together with geological evidence is most consistent with an origin for the Troodos Ophiolite at a spreading centre close to a ridge–trench–trench, or ridge–trench–transform triple junction, where highly depleted, subduction-modified, fluid-enriched mantle wedge material was able to upwell and decompress to shallow depths in a ‘fore-arc’ location. In such a tectonic setting, arc volcanism is captured by the spreading centre, explaining the lack of evidence for subaerial arc magmatism in Troodos. Rapid lateral migration of the triple junction could account for the similar ages of other Tethyan supra-subduction zone ophiolites.

Description: Highlights • Epi melts have experienced no disequilibrium modification by mixing or assimilation • Melts fractionate continuously while ascending, rather than stagnating • Magma ascent is through a complex system of dykes and sills • Epi situated between compressional and extensional regime on thick island arc crust • Structural features have impact on focusing and composition of island arc magmas Abstract We present here new bathymetric, petrological and geochemical whole rock, glass and mineral data from the submarine Epi volcano in the New Hebrides (Vanuatu) island arc. The structure has previously been interpreted to be part of a larger caldera structure but new bathymetric data reveal that the volcanic cones are aligned along shear zones controlled by the local tectonic stress field parallel to the recent direction of subduction. We aim to test if there is an interaction between local tectonics and magmatism and to what extent the compositions of island arc volcanoes may be influenced by their tectonic setting. Primitive submarine Epi lavas and those from the neighbouring Lopevi and Ambrym islands originate from a depleted mantle wedge modified by addition of subduction zone components. Incompatible element ratios sensitive to fluid input (e.g., Th/Nb, Ce/Yb) in the lavas are positively correlated with those more sensitive to mantle wedge depletion (e.g., Nb/Yb, Zr/Nb) amongst the arc volcanoes suggesting that fluids or melts from the subducting sediments have a stronger impact on the more depleted compositions of the mantle wedge. The whole rock, glass and mineral major and trace element compositions and the occurrence of exclusively normally zoned clinopyroxene and plagioclase crystals combined with the absence of inversely zoned crystals and water-bearing phases in both mafic and evolved lavas suggest that the erupted melt was relatively dry compared to other subduction zone melts and has experienced little disequilibrium modification by melt mixing or assimilation. Our data also imply that differentiation of amphibole is not required to explain the incompatible element patterns but may rather result from extensive clinopyroxene fractionation in agreement with petrographic observations. Thermobarometric calculations indicate that the melts fractionated continuously during ascent, contrasting with fractionation during stagnation in an established crustal magma reservoir. We interpret the occurrence of this fractional crystallisation end-member in a relatively thick island arc crust (~30 km thickness) to result from isolated and relatively rapid ascent of melts, most likely through a complex system of dykes and sills that developed due to the tectonic positioning of Epi in a complex tectonic zone between a compressional environment in the north and an extensional setting in the south. We can show that the alignment of the cones largely depends on the local tectonic stress field at Epi that is especially influenced by a large dextral strike-slip zone, indicating that structural features have a significant impact on the location and composition of volcanic edifices.

Description: Documenting the early tectonic and magmatic evolution of the Izu–Bonin–Mariana (IBM) arc system in the Western Pacific is critical for understanding the process and cause of subduction initiation along the current convergent margin between the Pacific and Philippine Sea plates. Forearc igneous sections provide firm evidence for seafloor spreading at the time of subduction initiation (52 Ma) and production of “forearc basalt”. Ocean floor drilling (International Ocean Discovery Program Expedition 351) recovered basement-forming, low-Ti tholeiitic basalt crust formed shortly after subduction initiation but distal from the convergent margin (nominally reararc) of the future IBM arc (Amami Sankaku Basin: ASB). Radiometric dating of this basement gives an age range (49.3–46.8 Ma with a weighted average of 48.7 Ma) that overlaps that of basalt in the present-day IBM forearc, but up to 3.3 m.y. younger than the onset of forearc basalt activity. Similarity in age range and geochemical character between the reararc and forearc basalts implies that the ocean crust newly formed by seafloor spreading during subduction initiation extends from fore- to reararc of the present-day IBM arc. Given the age difference between the oldest forearc basalt and the ASB crust, asymmetric spreading caused by ridge migration might have taken place. This scenario for the formation of the ASB implies that the Mesozoic remnant arc terrane of the Daito Ridges comprised the overriding plate at subduction initiation. The juxtaposition of a relatively buoyant remnant arc terrane adjacent to an oceanic plate was more favourable for subduction initiation than would have been the case if both downgoing and overriding plates had been oceanic.

Description: Iron isotopes in ocean floor basalts (OFB) away from convergent margins comprising mid-ocean-ridge and ocean island lavas show significant variation of 〉0.4‰ (expressed in the delta notation δ57Fe relative to IRMM-014), but processes responsible for this variation remain elusive. Bond-valence theory predicts that valence states (Fe3+ vs. Fe2+) control Fe isotopes during partial melting and crystal fractionation along the liquid line of descent and thus contribute substantially to this variation. Memory of past melt extraction or metasomatic re-enrichment in the source of OFB may further add to the observed variability, but systematic investigations to elucidate the respective contributions of these effects have been lacking. Submarine ridges and rifts in the Lau back-arc basin offer a unique opportunity to compare Fe isotopes in OFB from different melting regimes and variably depleted mantle sources. New Fe isotope data is presented for submarine lavas from the Rochambeau Ridges (RR) and the Northwest Lau Spreading Centre (NWLSC), and is compared with published data from the Central Lau Spreading Centre (CLSC). In line with first principle calculations and observations from a range of natural systems, crystal fractionation is identified as the dominant, controlling process for elevating δ57Fe in the lavas with olivine tentatively identified as the key driver. To compensate for the effect of crystal fractionation, olivine is mathematically added towards calculated primitive melt compositions (δ57Feprim). For this, we used a constant Ol-melt isotope fractionation factor based on published equilibrium partition functions adapted to decreasing temperature in a cooling melt. The degree of calculated Fe isotope fractionation through olivine crystal fractionation (monitored as Δ57Fe = δ57Femeasured − δ57Feprim) is positively correlated with increasing S and decreasing Ni content in the cooling lavas, fortifying the validity of the approach. Primitive lavas from individual Lau spreading centres and ridges vary to 0.1‰ in δ57Feprim, similar to primitive open-ocean MORB. However, the entire spread in Fe isotope variability in the primitive melts remains at 0.3‰, which we propose to be the extent of isotope heterogeneity in Earth’s upper mantle, with few extreme exceptions. The largest variability in δ57Feprim is observed for RR intra-plate lavas, which have been associated with the Samoan mantle plume and melting in an edge-driven convection scenario. Low, mid-ocean ridge-like 87Sr/86Sr in RR lavas excludes significant influence of isotopically heavy Samoan EM2-type components. However, co-variations with rare earth element pattern in some RR intra-plate lavas indicate garnet plays a role in elevating δ57Feprim during deeper melting. Excluding these deep-seated melts uncovers systematically decreasing δ57Feprim coupled to the degree of mantle source depletion, as recorded in Lu/Hf and Sm/Nd, in the back-arc basin basalts. This, however, holds only true for a comparison between sources of individual ridges, whereas no co-variation is observed within ridge segment data. This suggests that a process other than source depletion and crystal fractionation further adds to Fe isotope variability in the order of 0.1‰ on scales of individual ridge segments. This either marks the degree of Fe isotope variability below ridge segments, or is caused by secondary processes, such as melt-wallrock interaction or RTX (recharge and crystal fractionation) magma chambers.

Description: Highlights • Synthesis of timescales of magmatic processes at spreading centres. • Compilation of drilled MORB glass compositions, chemical stratigraphy of the oceanic crust. • No chemical difference between MORB sampled from active ridges or by drilling. • Chemical variations on timescales 〈 1 ka reflect changes in melt recharge relative to fractionation. • Changes in the composition of melt entering crust occur over timescales of 10 to 100 ka. Abstract Oceanic crust is continuously created at mid-ocean ridges by decompression melting of the upper mantle as it upwells due to plate separation. Decades of research on active spreading ridges have led to a growing understanding of the complex magmatic, tectonic and hydrothermal processes linked to the formation of new oceanic igneous crust. However, less is known about the timescales of magmatic processes at mid-ocean ridges, including melting in and melt extraction from the mantle, fractional crystallisation, crustal assimilation and/or magma mixing. In this paper, we review the timescales of magmatic processes by integrating radiometric dating, chemical and petrological observations of mid-ocean ridge basalts (MORBs) and geophysical models. These different lines of evidence suggest that melt extraction and migration, and crystallisation and mixing processes occur over timescales of 1 to 10,000 a. High-resolution geochemical stratigraphic profiles of the oceanic crust using drill-core samples further show that at fast-spreading ridges, adjacent flow units may differ in age by only a few 100 a. We use existing chemical data and new major- and trace-element analyses of fresh MORB glasses from drill-cores in ancient Atlantic and Pacific crust, together with model stratigraphic ages to investigate how lava chemistry changes over 10 to 100 ka periods, the timescale of crustal accretion at spreading ridges which is recorded in the basalt stratigraphy in drilled sections through the oceanic crust. We show that drilled MORBs have compositions that are similar to those of young MORB glasses dredged from active spreading ridges (lavas that will eventually be preserved in the lowermost part of the extrusive section covered by younger flows), showing that the dredged samples are indeed representative of the bulk oceanic crust. Model stratigraphic ages calculated for individual flows in boreholes, together with the geochemical stratigraphy of the drilled sections, show that at fast-spreading ridges, magma compositions vary over 〈 100 to 1000 a, likely due to variations in the relative rates of crystallisation and melt recharge. However, on longer timescales of 10 to 100 ka, variations in the composition of the primitive melt feeding the ridge lead to chemical variations in the erupted lavas, likely as a function of thermal and/or chemical heterogeneity of the mantle source. The further understanding of these temporal variations in magma composition, especially at shorter timescales of less than a few centuries, is a promising area for future research.

Description: Highlights • Glass inclusions record 11 Ma of early arc magma evolution. • Arc tholeiites succeed calc-alkalic magmas temporally. • Volcanic arc output directly linked to mantle wedge composition. • Dynamic slab control on arc magmatism following subduction initiation. Subduction initiation is a key process for global plate tectonics. Individual lithologies developed during subduction initiation and arc inception have been identified in the trench wall of the Izu–Bonin–Mariana (IBM) island arc but a continuous record of this process has not previously been described. Here, we present results from International Ocean Discovery Program Expedition 351 that drilled a single site west of the Kyushu–Palau Ridge (KPR), a chain of extinct stratovolcanoes that represents the proto-IBM island arc, active for ∼25 Ma following subduction initiation. Site U1438 recovered 150 m of oceanic igneous basement and ∼1450 m of overlying sediments. The lower 1300 m of these sediments comprise volcaniclastic gravity-flow deposits shed from the evolving KPR arc front. We separated fresh magmatic minerals from Site U1438 sediments, and analyzed 304 glass (formerly melt) inclusions, hosted by clinopyroxene and plagioclase. Compositions of glass inclusions preserve a temporal magmatic record of the juvenile island arc, complementary to the predominant mid-Miocene to recent activity determined from tephra layers recovered by drilling in the IBM forearc. The glass inclusions record the progressive transition of melt compositions dominated by an early ‘calc-alkalic’, high-Mg andesitic stage to a younger tholeiitic stage over a time period of 11 Ma. High-precision trace element analytical data record a simultaneously increasing influence of a deep subduction component (e.g., increase in Th vs. Nb, light rare earth element enrichment) and a more fertile mantle source (reflected in increased high field strength element abundances). This compositional change is accompanied by increased deposition rates of volcaniclastic sediments reflecting magmatic output and maturity of the arc. We conclude the ‘calc-alkalic’ stage of arc evolution may endure as long as mantle wedge sources are not mostly advected away from the zones of arc magma generation, or the rate of wedge replenishment by corner flow does not overwhelm the rate of magma extraction.

Description: Highlights • We identify Volcano F as the source of the August 2019 pumice raft in Tonga. • Satellite and seismic data give constraints on the timing of the submarine eruption. • 2.5–12.3*106 m3 estimated eruption volume, corresponding to VEI 2–3. • First report of the morphology and geology of Volcano F. Abstract In August 2019 a large raft of pumice appeared in the territorial waters of Tonga. As in many other cases, this pumice raft was the only surface expression of a major submarine volcanic eruption. Discolored water and reconstruction of the drift path of the pumice raft using satellite imagery points towards ‘Volcano F’ in the Tofua Arc NW of the island of Vava’u as the most likely volcanic source. Here we present imagery from ESA’s Sentinel-2 satellite that captured the start of the submarine eruption on 6 August 2019 and the waning of the eruption on 8 August, followed by observations of the drifting pumice raft until 14 August. This start time is consistent with T-phase records at the seismic stations on Niue Island and Rarotonga and the signal delay time of 733 s between the two stations is consistent with an origin at or at least near Volcano F. On 8 August, a 〉136.7 km2 large raft of pumice appears at the sea surface. The modelled minimum raft volume is 8.2–41.0*106 m3, which is equivalent to 2.5–12.3*106 m3 dense rock. The eruption thus corresponds to a volcanic explosivity index (VEI) 2–3 eruption in the submarine environment. Prior to the volcanic eruption, a series of earthquakes close to Volcano F was recorded. The series started on 5 August with a Mb 4.7 event, followed by at least six shallow earthquakes (Mb 〉3.9) on 6 August. In December 2018 and January 2019, we surveyed the seafloor around Volcano F with multibeam sonar. Combining our data with pre-existing information, we present the first comprehensive bathymetric map of the volcanic edifice and its geologic setting. We show that Volcano F represents a major arc volcanic complex that is situated in an extensional setting. The basal diameter of the volcanic apron is 〉50 km with a large central, 8.7 x 6 km caldera with a floor at ∼700 m water depth. The top of the post-caldera constructional cone complex had a summit depth of 35 m below sea level in 2004. The volcano shows geochemical differences to the adjacent arc volcanoes on Fonualei and Late islands. The volcano’s pristine volcanic morphology and two documented eruptions (2001 and 2019) indicate a highly active volcanic system that warrants further scientific attention.

Description: Highlights • Review of the critical processes controlling ore formation in the New Ireland Basin. • Combining geological knowledge of the on- and offshore areas. • New constraints on the origin, timing, and location of pathways for metal-rich melts and fluids. • Significance of microplate tectonics for gold endowment. Abstract The Southwest Pacific region, and Papua New Guinea in particular, is spectacularly endowed with mineral resources, including some of the youngest and richest porphyry Cu-Mo-Au deposits in the world. Among them is the giant porphyry-epithermal Ladolam Au deposit on Lihir Island in the Tabar-Lihir-Tanga-Feni (TLTF) island chain, northeast of New Ireland. Its setting within a former forearc basin is very different from most Southwest Pacific porphyry and epithermal deposits. Our synthesis of published and previously unreleased data from ship-based multibeam and seismic studies, satellite gravimetry, geochemistry and geochronology reveals a far more complex crustal structure and composition than is presently understood from the geology of the islands alone. We show that the unique regional Au endowment results from the alignment of various preconditions that are prolific to ore formation: i) hydrous and metal-rich metasomatic veins in the mantle source, ii) second-stage, low volume partial melting due to incipient rifting, iii) high volatile contents and oxygen fugacities of the melts due to preferential melting of hydrous phases in the metasomatic veins, and iv) in the specific case of Lihir, unroofing of the volcanic edifice that led to boiling and rapid metal deposition. This study shows that the location of the Ladolam deposit on Lihir is controlled by large-scale structures that can be traced offshore and are the site of continuing submarine volcanism and epithermal-style Au mineralization. The observed structural framework is dominated by the emergence of trans-lithospheric faults that provided pathways for the melts to the seafloor, near-surface structural focusing of the ascending melts and fluids, and a regional tectonic stress regime that stabilized the conditions over a significant period of time and/or repeatedly. Marine seismic data confirms the complex structure of the TLTF island chain. Each island group sits on tilted blocks that form horst structures separated by half grabens developed due to regional NW-SE-directed extension. Regional compression perpendicular to the extension continues as a result of the transition from subduction to collision at the leading edge of the Ontong Java Plateau. The protracted, transtensional motion between distinct crustal blocks controls the location and timing of magmatism and mineralization. A kinematic link between volcanism at the location of Lihir and the splitting of New Ireland by NE-directed propagation of seafloor spreading in the Manus Basin is suspected. By combining onshore and offshore geology, we propose a new model of the evolution of the New Ireland Basin, magmatism along the TLTF island chain and ultimately ore deposit formation. This study demonstrates the importance of integrating offshore geology and geophysics into models that aim to explain the structural, magmatic, and sedimentary evolution of marginal basins that are host to economic mineral deposits.