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Central aims of IODP Expedition 352 were to delineate and characterize the magmatic stratigraphy in the Bonin forearc to define key magmatic processes associated with subduction initiation and their potential links to ophiolites. Expedition 352 penetrated 1.2 km of magmatic basement at four sites and recovered three principal lithologies: tholeiitic forearc basalt (FAB), high-Mg andesite, and boninite, with subordinate andesite. Boninites are subdivided into basaltic, low-Si, and high-Si varieties. The purpose of this study is to determine conditions of crystal growth and differentiation for Expedition 352 lavas and compare and contrast these conditions with those recorded in lavas from mid-ocean ridges, forearcs, and ophiolites. Cr# (cationic Cr/Cr+Al) vs. TiO2 relations in spinel and clinopyroxene demonstrate a trend of source depletion with time for the Expedition 352 forearc basalt to boninite sequence that is similar to sequences in the Oman and other suprasubduction zone ophiolites. Clinopyroxene thermobarometry results indicate that FAB crystallized at temperatures (1142–1190 °C) within the range of MORB (1133–1240 °C). When taking into consideration liquid lines of descent of boninite, orthopyroxene barometry and olivine thermometry of Expedition 352 boninites demonstrate that they crystallized at temperatures marginally lower than those of FAB, between ~1119 and ~1202 °C and at relatively lower pressure (~0.2–0.4 vs. 0.5–4.6 kbar for FAB). Elevated temperatures of boninite orthopyroxene (~1214 °C for low-Si boninite and 1231–1264 °C for high-Si boninite) may suggest latent heat produced by the rapid crystallization of orthopyroxene. The lower pressure of crystallization of the boninite may be explained by their lower density and hence higher ascent rate, and shorter distance of travel from place of magma formation to site of crystallization, which allowed the more buoyant and faster ascending boninites to rise to shallower levels before crystallizing, thus preserving their high temperatures.

International audience; The composition of the intrusive gabbroic lower oceanic crust remains poorly characterized in comparison to the extrusive portion of the oceanic crust, especially for intermediate-fast spreading mid-ocean ridges. This is a consequence of limited exposures of extant lower oceanic crust or ophiolites similar to mid-ocean ridge crust. One of the best analogues for mid-ocean ridge crust is the southern Samail ophiolite that formed during a period of rapid seafloor spreading above a nascent subduction zone. Here, we focus on the geochemical stratigraphy (whole rock and mineral major and trace element compositions) of the 5,200 m-thick, lower crustal, Wadi Khafifah section of the Wadin Tayin massif in the southern Samail (Oman) ophiolite. Gabbros from the lowermost 3,700 m of this section (the “lower gabbros”) show no systematic changes in composition with height above the Mantle Transition Zone. In contrast, gabbros from the uppermost 1,500 m (the “upper gabbros”) display marked increases in incompatible trace element concentration with increasing height. Liquids in equilibrium with the lower gabbros have major and trace element compositions that overlap with those measured in the upper gabbros and sheeted dikes. Upper gabbros preserve mineral cores with primitive major element compositions that overlap with the range of lower gabbros; however, upper gabbro whole rock compositions are significantly more enriched in incompatible trace elements relative to the lower gabbros. Our data reveal that the upper gabbros are a composite of accumulated minerals derived from primitive melts and a large fraction of evolved melts derived from the fractionation of the lower gabbros. We propose a new “Full Sheeted Sills” model for the lower oceanic crust in which primitive magmas from the mantle are emplaced throughout the lower crust and crystallized in situ. After diking events, evolved magmas leave the lower gabbros and replenish the upper gabbros, thereby contributing to the higher incompatible trace element budget in the upper gabbros relative to the lower gabbros. Our reconstructed bulk compositions of the lower plutonic crust and the bulk oceanic crust from the Wadi Khafifah section yield a plausible primary mantle-derived magma composition in equilibrium with depleted mid-ocean ridge basalts mantle.

Boninites are rare, high-Si, high-Mg, low-Ti lavas that have considerable tectonic significance, especially for recognizing and interpreting episodes of subduction initiation in the geologic record. Formal identification and classification of boninites may be carried out using MgO-SiO2 and MgO-TiO2 diagrams to find compositions that satisfy modified International Union of Geological Sciences (IUGS) criteria of Si8 > 52 and Ti8 < 0.5, where Si8 and Ti8 refer to concentrations of the oxides at 8 wt% MgO. However, screening of highly metasomatized rocks and accurate classification require precautions, including normalization to a 100% volatile-free basis. The MgO-SiO2 diagram can also be used for subdivision into low-Si boninites (Si8 < 57) and high-Si boninites (Si8 > 57). Satisfying one but not both of the boninite criteria are rocks with Si8 > 52 but Ti8 ≥ 0.5 (siliceous high-magnesium basalts) and rocks with Si8 ≤ 52 but Ti8 < 0.5 (low-Ti basalts). We tested the classification methodologies using ∼100 low-Ti lava suites dating from the present-day back to the Eoarchean. We conclude that, of those classifying as “boninite series,” Izu-Bonin-Mariana arc–type subduction initiation terranes provide the dominant setting only back as far as ca. 2 Ga, which marks the maximum age of extensive clinopyroxene-undersaturated melting and eruption of high-Si boninites. From 2 to 3 Ga, most boninites formed in intraplate settings by melting of refertilized, depleted cratonic roots. Prior to 3 Ga, hot, depleted mantle plumes provided the main boninite sources. Nonetheless, arc-basin boninites, though rare, do extend back to 3.8 Ga, and, together with the inherited subduction component in intracratonic boninites, they provide evidence for some form of subduction during the Archean.

The association of deep-sea trenches—steeply angled, planar zones where earthquakes occur deep into Earth’s interior—and chains, or arcs, of active, explosive volcanoes had been recognized for 90 years prior to the development of plate tectonic theory in the 1960s. Oceanic lithosphere is created at mid-ocean ridge spreading centers and recycled into the mantle at subduction zones, where down-going lithospheric plates dynamically sustain the deep-sea trenches. Study of subduction zone initiation is a challenge because evidence of the processes involved is typically destroyed or buried by later tectonic and crust-forming events. In 2014 and 2017, the International Ocean Discovery Program (IODP) specifically targeted these processes with three back-to-back expeditions to the archetypal Izu-Bonin-Mariana (IBM) intra-oceanic arcs and one expedition to the Tonga-Kermadec (TK) system. Both subduction systems were initiated ~52 million years ago, coincident with a proposed major change of Pacific plate motion. These expeditions explored the tectonism preceding and accompanying subduction initiation and the characteristics of the earliest crust-forming magmatism. Lack of compressive uplift in the overriding plate combined with voluminous basaltic seafloor magmatism in an extensional environment indicates a large component of spontaneous subduction initiation was involved for the IBM. Conversely, a complex range of far-field uplift and depression accompanied the birth of the TK system, indicative of a more distal forcing of subduction initiation. Future scientific ocean drilling is needed to target the three-dimensional aspects of these processes at new converging margins.

The Li-O isotopes of olivine and pyroxene from the mantle sequence of the Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere in Tibet, reveal a series of magmatic processes and geochemical evolution in a nascent mantle wedge during subduction initiation. Olivine grains from the ophiolitic mantle sequence display δ18O values similar to those of normal mantle (+5.18 ± 0.2‰), but have large δ7Li variations, ∼+4‰ to +13‰ in harzburgites and ∼+1‰ to +11‰ in high-Cr chromitites. Clinopyroxene grains in cpx-bearing harzburgites have finger-like protrusions and LREE-depleted patterns comparable to those of clinopyroxene in abyssal peridotites, and also show high Li concentrations (∼2–8 μg/g) and negative δ7Li values (∼−15‰ to −8‰), which are beyond the ranges of normal upper mantle (

The Izu–Bonin–Mariana (IBM) forearc contains a record of a widespread and short-lived subduction initiation event in the early Eocene. We present new high-precision ages to determine the rates and length scales of ocean crust production following subduction initiation along the IBM convergent margin. We also explore the implications of these ages for potential causes of subduction initiation and its far-field effects. Zircons extracted from gabbros collected by Shinkai 6500 diving within the ophiolitic sequence exposed in the outer Bonin and Mariana forearcs give crystallization ages (±2σ) of 51.94 ± 0.13 Ma and 51.81 ± 0.03 Ma, respectively, consistent with a near-synchronous seafloor spreading after initiation of subduction along the entire length of the IBM system. 40Ar/39Ar ages of forearc basalts (51.34 ± 0.78 Ma) and boninites (51.27 ± 0.09 Ma to 50.33 ± 0.55 Ma) cored upslope of the Bonin sample gabbro during IODP Expedition 352 further reveal that the outer forearc comprises oceanic lithosphere with MORB to boninite compositions created by seafloor spreading during a remarkably short time period of 0.6–2 Myr at a full-rate of c. 70 mm/yr. This time period is consistent with those of some well-dated ophiolites. Thus, near-trench seafloor spreading to form basalt then boninite lithosphere is an extensive, rapid, but short-lived process after subduction initiation.

International audience

This article is a companion to Shervais et al. (2020), https://doi.org/10.1029/2020GC009093 .

LIP printing is a term adapted from forensic science to describe the use of geochemical proxies for tectonic and petrogenetic fingerprinting of Large Igneous Provinces (LIPs). Here, we investigate in detail the LIP printing of basic lavas, sills and dykes using two immobile element proxies: Th/Nb, a crustal input proxy, to monitor subduction-metasomatism and crustal assimilation and Ti/Yb, a residual garnet proxy, to monitor depth and degree of melting. The LIP printing diagram, a plot of Th/Nb against Ti/Yb for intraplate, plume-derived magmas, is characterised by two distinct arrays: a subduction-modified lithospheric mantle (SZLM) array and a MORB-OIB-OPB (plume) array (where OPB = oceanic plateau basalt). LIP basalt suites divide into three categories on this diagram: Type I plots entirely within the MORB-OIB-OPB array indicative of a significant plume source; Type II plots entirely within the SZLM array indicative of a significant sub-continental lithospheric mantle source, and Type III plots on a variety of trends between the two arrays indicative of significant plume-lithosphere interactions. Modelling demonstrates how the three LIP types, and the observed trends within and between individual LIPs, can be explained by differences in the compositions and relative contributions of lithospheric and asthenospheric (plume) mantle, in temperature and depth of melting and in the extent and nature of magma-crust interactions. This large genetic, and hence compositional, variability within and between LIPs relates to differences in geological and geodynamic setting and supports the forensic concept that ‘no two LIP prints are alike’. The potential applications of the LIP printing diagram are demonstrated here using four types of examples that highlight temporal and spatial LIP print diversity: flood basalt terranes related to Atlantic breakup (NAIP, Parana-Etendeka and CAMP); giant dyke swarms (Superior Craton Late Archean to Early Proterozoic dyke swarms and the Mackenzie dyke swarm); mineralization-related LIP terranes (Bushveld and Noril’sk); and early (c. 3.5 Ga) Earth and extraterrestrial lavas (terrrestrial Paleoarchean basalts and komatiites, lunar mare basalts and Martian shergottites).

We present a new comprehensive isotope and trace element dataset for north and central Tongan lavas including high-precision High Field Strength Element measurements. The emphasis of our study is on lavas from the northernmost volcanic islands of Tafahi and Niuatoputapu that exhibit unique compositions compared with other Tongan volcanoes, in particular elevated high-precision Nb/Ta ratios as high as similar to 28. These lavas are extremely depleted in incompatible, fluid-immobile elements such as Ti, Zr, Sc and Yb, suggesting about 6% depletion of their mantle wedge source relative to average upper mantle prior to 20-30% partial melting. The extremely depleted mantle wedge was re-enriched in highly incompatible elements including Nb and Ta from the subducting slab. Three slab components-fluids from altered oceanic crust, pelagic sediments, and Louisville Seamount Chain lavas-have contributed to the source region of the magmas. A fluid from the subducted Louisville lavas has been added after the other two slab components and has radiogenic Pb-206/Pb-204. Components from the altered oceanic crust and Louisville sea-mounts exhibit little Nb/Ta fractionation, as demonstrated by the nearly uniform and mid-ocean ridge basalt-like Nb/Ta ratios. We suggest that the apparently large slab fluid flux beneath Tafahi and Niuatoputapu results from focused flow through subducting Louisville Seamount crust that had no or little sediment cover, allowing the fluids to pass through the subducting lithosphere more efficiently. Thus, the high Nb/Ta ratios indicate that the Louisville-derived fluids had probably equilibrated with residual rutile. Our results imply that significant amounts of Nb and Ta can be mobilized from distinct subducting oceanic lithologies-a process that may probably also operate beneath other island arcs. We propose a model in which the appearance of the subducted seamount signatures some 3-4Myr after their actual subduction is explained by storage in shallow metasomatized lithospheric mantle. This relic Louisville seamount signature is subsequently reactivated and added to transiting magmas 4 Myr later. This reactivation may result from slab rollback of the subducting Pacific plate towards the east. Our new temporal and spatial model also provides a viable explanation for the conundrum of the occurrence of Louisville signatures even though the Louisville seamount chain is, at present, being subducted several hundred kilometres further south.

International audience; nternational Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

Objectives: Clinicians need to be aware of the growing list of defined monogenic etiologies of autoimmune diseases. This is particularly relevant when evaluating children, as these rare monogenic forms of autoimmunity tend to present very early in life. Methods and results: By harnessing the transformative power of next generation sequencing, we made the unifying diagnosis of RAS-associated autoimmune leukoproliferative disease (RALD), caused by the somatic gain -of function p.G13C KRAS mutation, in a boy with the seemingly unrelated immune dysregulatory conditions of Rosai-Dorfman and systemic lupus erythematosus (SLE). Conclusions: This case expands our understanding of the clinical phenotypes associated with the extremely rare condition of RALD, and emphasizes the importance of always considering the possibility of a monogenic cause for autoimmunity, particularly when the disease manifestations begin early in life and do not follow a typical clinical course. Crown Copyright (C) 2016 Published by Elsevier Inc All rights reserved

International audience; Subduction initiation is one of the least understood aspects of plate tectonics. In an effort to obtain the first in situ magmatic record of subduction initiation, the International Ocean Discovery Program Expedition 352 drilled at four sites in the inner trench wall of the Bonin Trench to recover 1.22 km of oceanic upper crust accreted within a few m.y. of subduction initiation. The two sites nearer to the trench (U1440 and U1441) yielded axial and off-axis fore-arc basalts (FAB), while those c. 15 km further from the trench (U1439 and U1442) yielded axial low-silica boninites and high-Mg andesites overlain by off-axis high-silica boninites. This study uses Hf–Nd–Sr–Pb isotope analyses from c. 50 stratigraphically representative core samples to trace the evolution of the mantle source during the brief period of FAB-through-boninite magmatism immediately following subduction initiation. Results show that: 1) the FAB have high εHf relative to εNd and were derived from variably depleted mantle of ‘Indian’ provenance with no detectable subduction input; 2) the axial boninites follow mixing trends between a residual FAB mantle source and a subduction component derived from shallow (amphibolite facies) melting of oceanic crust of ‘Pacific’ provenance; and 3) the off-axis boninites define mixing trends between a hybrid mantle wedge (residual mantle + slab melt) and an additional subduction component with lower εNd and higher 207Pb/204Pb that requires a significant contribution from pelagic sediment. This incoming of pelagic sediments may signify a change from an accretionary to non-accretionary margin as subduction evolves. The results thus indicate a rapidly evolving system in terms of geodynamics, magma genesis and crustal accretion immediately following subduction initiation.

The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO2, P2O5, Zr) and Ni compared to MORB, with Na2O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle., Key Points Fore‐arc basalts (FABs) formed by decompression melting in response to subduction initiation and differ from mid‐ocean ridge basaltsFABs form by two‐stage melting, with early garnet field melts extracted prior to spinel field melting, resulting in LREE/HREE depletionHighly depleted FAB may reflect both an older depletion event and higher ambient temperatures related to the Manus plume

Much of our understanding of ocean ridges has come from the collection and analysis of glasses recovered from ridge axes. However, applying the resulting methodologies to ophiolite complexes is not straightforward because ophiolites typically experience intense alteration during their passage from ridge to subduction zone to mountain belt. Instead, immobile element proxies for fractionation indices, alkalinity, mantle temperature, mantle flow and subduction addition may be used to classify ophiolite lavas and fingerprint the precise setting of the ridge at which an ophiolite formed. The results can help us recognise and interpret past spreading centres and so make plate tectonic reconstructions.

The central Scotia Sea, located between the South American and Antarctic plates, is an integral part of the marine conduit that permits eastward deep-water flow from the Pacific Ocean to the Atlantic Ocean. The geologic history of the central Scotia Sea is therefore critical for a full understanding of the initiation and subsequent evolution of the complete, deep Antarctic Circumpolar Current, widely believed to have been a key factor in the history of Antarctic glaciation. Here, we present new evidence on the nature and age of the central Scotia Sea floor. Multibeam surveys and the first dredged samples indicate that a now-submerged remnant volcanic arc may have formed a barrier to deep eastward oceanic circulation until after the mid-Miocene climatic optimum. Inception and development of a full deep Antarctic Circumpolar Current may therefore have been important, not in the drop in global temperatures at the Eocene-Oligocene boundary as long surmised, but in the subsequent late Miocene global cooling and intensification of Antarctic glaciation.

The earliest subaerially exposed magmatic products of the Fiji–Tonga–Kermadec (FTK) arc are preserved in the Yavuna Group of Viti Levu, Fiji, and cobbles from ‘Eua, Tonga. They are similar in age and magma types to the earliest rocks of the Izu–Bonin–Mariana (IBM) arc. In Fiji they include typical island arc tholeiitic (IAT), boninitic (BON), and MORB-like early arc tholeiitic (EAT) pillow lavas that are interpreted as products of flux- and decompression-melting which occurred simultaneously during subduction initiation. Although the oldest rocks in the southwest Pacific (FTK) and the northwest Pacific (IBM) arcs are generally similar, they differ in two important respects. First, all magma types erupted simultaneously in the SW Pacific whereas a similar assemblage may have erupted sequentially in IBM. Second, the primary mantle wedge was “Pacific” in isotopic character in the SW Pacific, but “Indian” in the NW Pacific.

In the study of geochemical mass balances at subduction zones, the composition of the mantle wedge prior to additions from the slab is a critically important yet poorly constrained parameter. Deconvolving the influence of ancient versus modern enrichments is particularly difficult, especially when considering elements that are highly mobile. Here we provide an alternative approach, using less mobile elements, and a filter to remove the effects of recent slab additions. We provide new Hf isotope data for 30 Mariana Trough (MT) backarc basin lavas. Once filtered, Hf and Nd isotope ratios are highly correlated, of Indian mid-oceanic ridge basalt character, and display variations similar to ocean ridges of comparable lengths. The isotopic variability observed in this “ambient mantle” provides a new paradigm for the interpretation of the varied volcanic products of the arc. Thus, shoshonites associated with the northern termination of the backarc basin rift axis reflect the interaction of a subducted sediment melt with an isotopically enriched mantle source. In contrast, the large volcanoes of the Central Island province have a consistent offset in Nd isotope compositions from the MT array resulting from fluid addition. Existing data for smaller edifices in the submarine portion of the arc have larger variations resulting from fluid addition on a more local scale. We suggest that the similar characterization of ambient mantle elsewhere may help to resolve many conflicting geochemical observations in arc lavas worldwide.

Subduction-zone metamorphism is considered to be a major chemical filter for both the arc magmatism and mantle compositional heterogeneity. To understand the element transport during this process, we conducted a petrographic and geochemical study of the bulk-rock blueschists and eclogites from the western Tianshan ultrahigh-pressure metamorphic belt, northwest China. By examining correlations among incompatible elements, we show that high field strength elements (HFSEs), rare earth elements (REEs), Th, and U are relatively immobile, whereas Pb and Sr are mobile in both basaltic and sedimentary protoliths during subduction-zone metamorphism. K, Rb, Cs, and Ba are mobile in rocks of basaltic protolith but immobile in rocks of sedimentary protolith because of the presence and persistent stability of white mica throughout their petrologic history. The commonly observed enrichment of some immobile elements (e.g., U, light [L] REEs) in arc magmas may thus not be caused by subduction-zone aqueous fluids. The lack of Rb/Sr-Sm/Nd correlation in these metamorphosed rocks is inconsistent with the observed first-order Sr-Nd isotope correlation in oceanic basalts, suggesting that the residual ocean crust that has undergone subduction-zone metamorphism cannot be the major source material for oceanic basalts, as widely believed, although it can contribute to mantle compositional heterogeneity in general.

New 40Ar-39Ar ages of 5.6 to 1.3 Ma for lavas from the fossil Phoenix Ridge in the Drake Passage show that magmatism continued for at least 2 Ma after the cessation of spreading at 3.3 ± 0.2 Ma. The Phoenix Ridge lavas are incompatible element-enriched relative to average MORB and show an increasing enrichment with decreasing age, corresponding to progressively decreasing degrees of partial melting of spinel peridotite after spreading stopped. The low-degree partial melts increasingly tap a mantle source with radiogenic Sr and Pb but unradiogenic Nd isotope ratios implying an ancient enrichment. The post-spreading magmas apparently form by buoyant ascent of enriched and easily fusible portions of the upper mantle. Only segments of fossil spreading ridges underlain by such enriched and fertile mantle show post-spreading volcanism frequently forming bathymetric highs. The Phoenix Ridge lavas belong to the Pacific, rather than the Atlantic, mantle domain in regional Sr-Nd-Pb space. Our new data show that the southern Pacific Ocean mantle is heterogeneous containing significant enriched portions that are preferentially tapped at low melt fractions. Isotopic mapping reveals that Pacific-type upper mantle flows eastward through Drake Passage and surrounds the subducting Phoenix Plate beneath the Bransfield Basin.

Rhyodacite lavas (Newcastle Volcanic Formation) from the Wagwater Basin in eastern Jamaica dated at 52.74 ± 0.34 Ma (2σ) have adakitic-like major element compositions, low Y and heavy rare Earth element (REE) concentrations and negative Nb and Ta anomalies on a normal mid-ocean ridge basalt normalised multi-element diagram. They also have lower Sr (< 400 ppm), MgO (≤ 2.0 wt.%), Ni (mostly ≤ 30 ppm) and Cr (mostly ≤ 40 ppm) concentrations compared to other modern adakites and middle-late Archaean (3.5–2.5 Ga) trondhjemite, tonalite and granodiorite/dacites (TTG/Ds). eNd(i) and eHf(i) values indicate that the Newcastle adakite-like lavas cannot be formed by assimilation and fractional crystallisation processes involving any other igneous rock in the area and so the composition of the lavas is largely the result of the residual mineralogy in the source region. Low Sr and Al2O3 contents indicate a fluid/vapour-absent source with residual plagioclase and REE systematics point to residual amphibole and garnet in the source region. Similarly, high silica values and constant Zr and P2O5 concentrations suggest residual quartz and accessory zircon and apatite. The plagioclase and garnet residue implies that the Newcastle magmas were derived from partially melting a metabasic protolith at 1.0–1.6 GPa, which would intersect the amphibole dehydration partial melt solidus at ~ 850–900 °C. Radiogenic isotopes along with the low MgO, Ni and Cr concentrations in the Newcastle lavas demonstrate that the garnet amphibolite source region cannot be part of (1) the lower Jamaican arc crust, (2) delaminated lower crust or (3) subducted Proto-Caribbean “normal” oceanic crust that may, or may not, have detached. This data, in addition to partial melting models involving a theoretical garnet-amphibolite source region for the Newcastle lavas, shows that the adakite-like rocks are derived from metamorphosed Caribbean oceanic plateau crust that underthrust Jamaica in the early Tertiary. The underplated oceanic plateau crust partially melted by either (1) influx of basaltic magma during lithospheric extension in the early Tertiary or (2) direct partial melting of the underthrusting (subducting) plateau crust. The Newcastle magmas ascended and erupted without coming into contact with a mantle wedge thus forming the low MgO, Ni and Cr contents. Most Cenozoic adakites have compositions similar to the middle-late Archaean TTG/D suite of igneous rocks. In contrast, early (> 3.5 Ga) Archaean TTG/D crustal rocks have lower Sr, MgO, Ni and Cr concentrations and prior to this study had no modern adakite analogue. However, the Newcastle adakites have similar compositions to the, early Archaean TTG/D. The discovery of these rocks has important implications for our understanding of the formation of the Earth's earliest continental crust and so it is proposed that the Newcastle lavas be classified as a unique subgroup of adakites: Jamaican-type adakite.

Miyashiro (1973) famously initiated a debate on the tectonic setting of ophiolite complexes by proposing that ‘the Troodos ophiolitic complex was probably formed in an island arc’. This paper evaluates and updates Miyashiro's work by: (a) using the Mehegan–Robinson set of 137 fresh volcanic glass analyses to sidestep the controversy over the effect of alteration on major element classification diagrams; (b) using the volcanic glass database for Troodos analogues such as back-arc basins, slab edges, forearcs and subduction initiation terranes; (c) revising Miyashiro's classifications by including the boninitic series and subdividing the tholeiitic series into high- and medium-Fe series; and (d) extending Miyashiro's methodologies to new developments in the interpretation of major element data. We conclude that the Troodos Massif is made up of oceanic crust built from a high-Si8, moderate-Fe tholeiitic magma, overlain by boninites. Its low K8/H8 is consistent with near-trench crustal accretion and its Na8–Fe8 systematics indicate that TP = c.1400 °C for the Lower Lavas, consistent with rapid slab roll-back and/or sideways influx of hot mantle. Overall, the geochemical characteristics and geological setting support models in which the Troodos Massif formed by slab roll-back following subduction initiation, probably near a slab edge.

Archean continental crust largely comprises the trondhjemite, tonalite, and granodiorite/dacite (TTG/D) suite of igneous rocks. Formation of the earliest Archean (>3.5 Ga) TTG/Ds is controversial, being attributed to either subduction zone processes with active plate tectonics or thermochemical mantle convection with no plate tectonic processes. A suite of Cenozoic adakite-like lavas in Jamaica has geochemical compositions comparable to early Archean TTG/D. The data indicate that the adakites were generated by underthrusting (or subducting) and partial melting of oceanic plateau crust beneath Jamaica. This setting is analogous to proposed plate tectonic processes in the early Archean where hot, thick, and more buoyant Archean oceanic crust underthrusts adjacent plates. The new adakite data imply that earliest Archean TTG/D continental crust could have formed above primitive subduction zones.

The accessory mineral chrome–spinel, [(Mg,Fe) (Cr,Al,Fe)2O4], is well-established as a useful, alterationresistant petrogenetic and tectonic setting indicator for mafic–ultramafic rocks. This study of unaltered spinels from 58 peridotites from oceanic and ophiolitic settings further develops the tectonic discrimination of peridotites from mid-ocean ridges (MOR) and supra-subduction zone (SSZ) settings. The geochemical data include the standard suite of elements, together with accurate ferric iron obtained by correcting electron microprobe data with Mossbauer spinel standards, and gallium (Ga) obtained by Laser Ablation-ICP-MS. The results show that discrimination using the established oxygen fugacity (fO2)–Cr# [Cr/(Cr+Al)] diagram remains effective, but does depend on melt–rock reaction and magmatic differentiation of the reacting melt, in addition to tectonic setting. In particular, the MOR–SSZ boundary for dunites is displaced to higher oxygen fugacities than that for residual harzburgites, with some partially reacted harzburgites proving hard to fingerprint. Ga–Ti–Fe3# [Fe3/(Fe3+Cr+Al)] systematics provide a potential way to improve the discrimination. Significantly, Ga and Fe3+ have similar ionic radii but only Fe3+ is redox-dependent, so Ga/Fe3# is largely independent of magmatic differentiation while retaining the capability to separate MOR peridotites from the higher Fe3# SSZ peridotites. Whereas Ga/Fe3# ratios decrease during melt–rock reaction, Ti/Fe3# ratios stay constant or increase according to the relative compositions of the mantle lithosphere and the interacting melt. In consequence, the Ti/Fe3# vs. Ga/Fe3# diagram requires only a single boundary to separate MOR harzburgites and dunites (with high ratios) from SSZ harzburgites and dunites (with low ratios). Podiform chromitites can only be fingerprinted if they share a similar history of olivine– spinel subsolidus re-equilibration as the peridotites. Unlike the oxygen fugacity plot, the Ti/Fe3# vs. Ga/Fe3# diagram can also fingerprint peridotites that have no co-existing olivine or host rock information, such as serpentinites and detrital spinels. Testing these methods using spinels in mantle peridotites from the northern Oman–U.A.E. ophiolite confirms independent indicators that the ophiolite records the switch in tectonic setting from MOR to SSZ and hence magma genesis during the initiation of subduction.

Two geochemical proxies are particularly important for the identification and classification of oceanic basalts: the Th–Nb proxy for crustal input and hence for demonstrating an oceanic, non-subduction setting; and the Ti–Yb proxy for melting depth and hence for indicating mantle temperature and thickness of the conductive lithosphere. For the Th–Nb proxy, a Th/Yb–Nb/Yb projection demonstrates that almost all oceanic basalts lie within a diagonal MORB–OIB array with a principal axis of dispersion along the array. However, basalts erupted at continental margins and in subduction zones are commonly displaced above the MORB–OIB array and/or belong to suites with principal dispersion axes which are oblique to the array. Modelling of magma–crust interaction quantifies the sensitivity of the Th–Nb proxy to process and to magma and crustal compositions. For the Ti–Yb proxy, the equivalent Ti/Yb–Nb/Yb projection features a discriminant boundary between low Ti/Yb MORB and high Ti/Yb OIB that runs almost parallel to the Nb/Yb axis, reflecting the fact that OIB originate by melting beneath thicker lithosphere and hence by less melting and with residual garnet. In the case of volcanic-rifted margins and oceanic plume–ridge interactions (PRI), where hot mantle flows toward progressively thinner lithosphere (often becoming more depleted in the process), basalts follow diagonal trends from the OIB to the MORB field. Modelling of mantle melting quantifies the sensitivity of the Ti–Nb proxy to mantle potential temperature and lithospheric thickness and hence defines the petrogenetic basis by which magmas plot in the OIB or MORB fields. Oceanic plateau basalts lie mostly in the centre of the MORB part of that field, reflecting a high degree of melting of fertile mantle. Application of the proxies to some examples of MORB ophiolites helps them to be further classified as C (contaminated)-MORB, N (normal)-MORB, E (enriched)-MORB and P (plume)-MORB ophiolites, which may add a useful dimension to ophiolite classification. In the Archean, the hotter magmas, higher crustal geotherms and higher Th contents of contaminants all result in widespread crustal input that is easy to detect geochemically with the Th–Nb proxy. Application of this proxy to Archean greenstones demonstrates that almost all exhibit a crustal component even when reputedly oceanic. This indicates, either that some interpretations need to be re-examined or that intra-oceanic crustal input is important in the Archean making the proxy less effective in distinguishing oceanic from continental settings. The Ti–Yb proxy is not effective for fingerprinting Archean settings because higher mantle potential temperatures mean that lithospheric thickness is no longer the critical variable in determining the presence or absence of residual garnet.

Many diagrams conventionally used to classify igneous rocks utilize mobile elements, which commonly renders them unreliable for classifying rocks from the geological record.The K2O^SiO2 diagram, used to subdivide volcanic arc rocks into rock type (basalts, basaltic andesites, andesites, dacites and rhyolites) and volcanic series (tholeiitic, calc-alkaline, high-K calc-alkaline and shoshonitic), is particularly susceptible to the effects of alteration.However, by usingTh as a proxy for K2O and Co as a proxy for SiO2 it is possible to construct a topologically similar diagram that performs the same task but is more robust for weathered and metamorphosed rocks. This study uses 41000 carefully filtered Tertiary^Recent island arc samples to construct aTh^Co classification diagram. A‘testing set’comprising data not used in constructing the diagram indicates a classification success rate of c. 80%.When applied to some hydrothermally altered, then tropically weathered Cretaceous volcanic arc lavas from Jamaica, the diagramdemonstrates the presence ofa tholeiitic volcanic arc series dominated by intermediate^acid lavas overlain by a calc-alkaline series dominated by basic lavas.

Pb isotope systematics have already been used successfully to demonstrate that the lavas of the arc-basin terrains of the SW Pacific are derived from two mantle domains, one of Pacific-like character and the other of Indian-like character. However, the mobility of Pb during subduction and alteration has mainly restricted the fingerprinting of domains to fresh lavas of MORB composition. We demonstrate that the less alteration-sensitive Hf–Nd isotope projection also discriminates successfully between ‘Pacific’ and ‘Indian’ domains, and thus enables us to extend mantle domain fingerprinting to the back-arc basin basalts and boninites of the Lau and North Fiji Basins and the volcanic arc lavas of the Kermadec, Tonga and Vanuatu arcs. Fingerprinting is facilitated by the observation that the Hf isotope ratio is independent of subduction-input parameters, indicating that Hf has been essentially conservative during the subduction process. Subducted Nd has been added to the mantle source, but subtracting this numerically using the magnitude of negative Hf anomalies filters out the subduction effect. The data show that the ‘Indian’ domain provides the source for magmas erupted at ridges, and arcs near these ridges, that have propagated southwards following the 12 Ma collision of the Ontong-Java Plateau with the Vitiaz Trench. This indicates that the ‘Indian’ domain is actually derived from SOPITA mantle (South Pacific Isotopic and Thermal Anomaly) — mantle modified by the Samoa and other plumes outboard of the trench which only entered the SW Pacific arc-basin system after the Ontong-Java Plateau collision removed the slab barrier at < 12 Ma. In the west, mantle flows beneath the network of south-propagating ridges in the North Fiji and NW Lau Basins, undergoing progressive depletion until the final loss of plume components produces an N-MORB mantle (Indian MORB Mantle) composition in the south North Fiji Basin and Central Lau Spreading Centre. In the east, newly-depleted Samoan plume mantle provides the source for the boninites and depleted arc tholeiites of the northern Tonga arc.

The objectives for Expedition 352 were to drill through the entire volcanic sequence of the Bonin fore arc to 1. Obtain a high-fidelity record of magmatic evolution during subduction initiation and early arc development, 2. Test the hypothesis that fore-arc basalt lies beneath boninite and understand chemical gradients within these units and across the transition, 3. Use drilling results to understand how mantle melting processes evolve during and after subduction initiation, and 4. Test the hypothesis that the fore-arc lithosphere created during subduction initiation is the birthplace of suprasubduction zone (SSZ) ophiolites. Expedition 352 successfully cored 1.22 km of igneous basement and 0.46 km of over-lying sediment, providing diverse, stratigraphically controlled suites of fore-arc basalts (FAB) and boninite related to seafloor spreading and earliest arc development. FAB were recovered at the two deeper water sites (U1440 and U1441) and boninites at the two sites (U1439 and U1442) drilled upslope to the west. FAB lavas and dikes are depleted in high-field strength trace elements such as Ti and Zr relative to mid-ocean-ridge basalt but have relatively diverse concentrations of trace elements bezcause of variation in degrees of melting and amount of subducted fluids involved in their genesis. All FAB magmas underwent significant crystal fractionation in a persistent magma chamber system. Holes U1439C and U1442A yielded entirely boninitic lavas. We defined three boninite differentiation series based on variations in MgO, SiO2, and TiO2 concentrations of the parental magmas. Lavas in both pairs of holes have compositions that generally become more primitive and have lower TiO2 concentrations upward. The presence of dikes at the base of the sections at Sites U1439 and U1440 provides evidence that boninitic and FAB lavas are both underlain by their own conduit systems and that FAB and boninite group lavas are likely offset more horizontally than vertically. We thus propose that seafloor spreading related to subduction initiation migrated from east to west after subduction initiation and during early arc development. Initial spreading was likely rapid, and an axial magma chamber was present. Melting was largely decompressional during this period, but subducted fluids affected some melting. As subduction continued and spreading migrated to the west, the embryonic mantle wedge became more depleted, and the influence of subducted constituents dramatically increased, causing the oceanic crust to be built of boninitic rather than tholeiitic magma. The general decrease in fractionation upward reflects the eventual disappearance of persistent magma chambers, either because spreading rate was decreasing with distance from the trench or because spreading was succeeded by off-axis magmatism trenchward of the ridge. The extreme depletion of the sources for all boninitic lavas was likely related to the incorporation of mantle residues from FAB generation. This mantle depletion continued during generation of lower silica boninitic magmas, exhausting clinopyroxene from the mantle such that the capping high-Si, low-Ti boninites were generated from harzburgite. Additional results of the cruise include recovery of Eocene to recent deep-sea sediment that records variation in sedimentation rates with time resulting from variations in climate, the position of the carbonate compensation depth, and local structural control. Three phases of highly explosive volcanism (latest Pliocene to Pleistocene, late Miocene to earliest Pliocene, and Oligocene) were identified, represented by 132 graded air fall tephra layers. Structures found in the cores and reflected in seismic profiles show that this area had periods of normal, reverse, and strike-slip faulting. Finally, basement rock P-wave velocities were shown to be slower than those observed during logging of normal ocean crust sites.

176Hf/177Hf isotopes provide information about the behaviour of so-called immobile elements in subduction environments. Early studies of Hf isotopes in subduction zones reached different conclusions regarding the mobility of high-field-strength elements during subduction-related processes. To test the behaviour of Hf during subduction, we have examined the young, intra-oceanic South Sandwich subduction system. Combined 176Hf/177Hf and trace element ratios reveal that Hf may behave as both immobile and mobile, depending upon the exact spatial relationship of the arc volcano to the slab. Throughout most of the arc, magmas show no detectable Hf transfer from the slab to the wedge, perhaps because enrichment of the wedge took place by Hf-deficient, fluid-dominated processes. On the basis of ΔeNd values, which describe the Nd isotope deviation from a local MORB-OIB array, we can discern that northern volcanoes of the arc require a source enriched by fluids that originated from the oceanic crust, whereas southern arc volcanoes have a source modified by a higher proportion of sediment-derived fluids. However, close to the southern slab edge and in rear-arc settings, arc magmas were derived from a source that had undergone Hf addition; we attribute this to element transfer via partial melts from sediment. This implies that Hf mobility from the slab is possible where temperatures are sufficiently high to induce sediment melting rather than fluid generation alone. The implication of this work, for the majority of the arc, is that sediment-derived fluids contribute to magmatism and that sediment-derived melt does not.

The Central Anatolian Crystalline Complex (CACC) contains Paleozoic-Mesozoic, medium-high grade metamorphic rocks overthrust by Upper Cretaceous ophiolitic units and intruded by a number of plutons (around 79.5 to 66.6 Ma). Central Anatolia exhibits good examples of calc-alkaline and alkaline magmatism of similar age in a collision-related tectonic setting. Igneous enclaves are widespread in these calc-alkaline plutonic rocks of the CACC. Two types of igneous enclave have been recognized in the calc-alkaline Behrekdag, Cefalikdag, and Celebi (BCC) (metaluminous I-type ranging from monzodiorite to granite) plutons of Central Anatolia: (1) fine-grained (Type-I); and (2) medium-grained to porphyritic with feldspar megacrysts (Type-II). Most abundant are Type-I enclaves with quartz dioritic to quartz monzodioritic compositions (SiO2 = 55 to 66 wt%) and finegrained margins indicating chilling against the host rocks. Less common are Type-II enclaves with quartz dioritic to gabbro compositions (SiO2= 42 to 59 wt%) and cumulate to porphyritic textures, reflecting accumulation of early precipitating phases. Both sets of enclave have mineral phases similar to that of the host rocks, but with different mineral proportions. The origin and evolution of the enclaves are strongly linked to those of their host rocks. For example, mafic to intermediate Type-I enclaves (originated from magma mixing/mingling) in the BCC plutonic rocks indicate interaction between silicic magmas and mantle-derived melts. Their presence could imply that the host plutonic rocks have undergone hybridization to a greater or lesser extent. In contrast, mafic Type-II enclaves (formed as cumulates of early crystallized minerals) provide information on the parental magma of the plutonic rocks.

Mantle preconditioning may be defined as the extraction of small melt fractions from mantle asthenosphere during its flow to the site of magma generation. Equations may be written for mantle preconditioning, assuming that the mantle comprises enriched ‘plums’ in a depleted matrix. The equations take into account variations in mass fraction of plums, the relative rate of melting of plums and matrix, the temperature and pressure of melt extraction, the mass fraction of melt extracted, the extent of chemical exchange between plums and matrix, and the efficiency of melt extraction. Monitoring mineralogical changes and variations in partition coefficients along the inferred P–T–t path of the mantle asthenosphere allows the equations to be correctly applied to the conditions under which melt extraction takes place. Numerical experiments demonstrate the influence of petrogenetic variables on the shape of melt extraction trajectories and provide new criteria for distinguishing between melt extraction and mixing as the cause of regional geochemical gradients. Representative examples of arc–back-arc systems (Scotia), continental break-up (Afar) and plume–ridge interaction (Azores) indicate that the compositions of the mantle sources of mid-ocean ridge basalts and island arc basalts may be determined, at least in part, by the melt extraction histories of their asthenospheric sources.

The intra-oceanic South Sandwich subduction system is distinctive in having a narrow slab with slab edges at its northern and southern ends. We present new geochemical data to investigate magma genesis beneath the parts of the arc and back-arc segments that lie close to the two slab edges: Kemp and Nelson seamounts at the southern edge of the South Sandwich arc, and segments E1 and E2 in the south, plus segments E9 and E10 in the north, of the East Scotia Sea. In the arc, Kemp and Nelson seamounts exhibit enhanced subduction fluxes compared to the remainder of the arc. The southernmost (Nelson) has the isotope (low Nd and high Sr isotope ratios) and elemental (ultra-high Th and Ba and high Hf/Nd ratios) characteristics of a sediment melt, or supercritical aqueous fluid, component. The more northerly (Kemp) has the same characteristics as the remainder of the arc (high Nd and slightly raised Sr isotope ratios, high Nd/Hf ratios, high Ba/Th ratios), indicative of a fluid component derived mainly from subducted crust, but has a greater mass fraction of that component than the rest of the arc. In the back-arc basin, the slab-edge segments are generally fed by more fertile mantle (E-MORB in all but E1) than the segments in the centre of the basin (N-MORB). At the edges, segments furthest from the trench (E2, E9) have small subduction components while those nearer to the trench (E1, E10) have larger subduction components and slightly more depleted mantle. We argue that several processes were important at the slab edges: roll-back of the slab, forcing sideways flow of relatively enriched mantle into the mantle wedge; convergence of the arc with the back-arc spreading centre, imparting a greater subduction component into the back-arc lavas; and anomalous heating of the subducting slab, increasing subduction fluxes and the contribution of sediment melts to the subduction component.

Neogene potassic lavas in northern and southern Tibet have different isotopic (eNd(i) north, −5·5 to −10·3; south −8·8 to −18·1) and major element signatures suggesting derivation from separate sub-continental lithospheric mantle (SCLM) sources. Inverse trace-element modelling shows that the southern Tibet magmas were derived by 1–2% partial melting of a phlogopite and amphibole peridotite, and that the northern samples were derived by 3–4% partial melting of a phlogopite peridotite. In both cases, melting is inferred to take place in the spinel stability field. Both sources show large ion lithophile element (LILE) enrichment relative to the high field strength elements (HFSE), and heavy rare earth element (HREE) depletion relative to primitive mantle. LILE/HFSE enrichment suggests subduction-related metasomatism; HREE depletion is indicative of prior melt extraction. Extension postdates the earliest magmatism in southern and north–central Tibet by 7 Myr and 5 Myr, respectively, which, in combination with the shallow depths of melting inferred for the Tibetan samples, supports geodynamic models invoking thinning of the SCLM. The northern Tibetan magmatism and extension can be explained by convective removal of the lower SCLM; the older ages and arcuate distribution of the southern magmas are most consistent with the SCLM erosion following slab break-off.

Central Anatolia exhibits good examples of calc-alkaline and alkaline magmatism of similar age in a collision-related tectonic setting (continent–island arc collision). In the Central Anatolia region, late Cretaceous post-collisional plutonic rocks intrude Palaeozoic–Mesozoic metamorphic rocks overthrust by Upper Cretaceous ophiolitic units to make up the Central Anatolian Crystalline Complex. In the complex, three different intrusive rock types may be recognised based on their geochemical characteristics: (i) calc-alkaline (Behrekdag, Cefalikdag, and Celebi); (ii) subalkaline-transitional (Baranadag); and (ii) alkaline (Hamit). The calc-alkaline and subalkaline plutonic rocks are metaluminous I-type plutons ranging from monzodiorite to granite. The alkaline plutonic rocks are metaluminous to peralkaline plutons, predominantly A-type, ranging from nepheline monzosyenite to quartz syenite. All intrusive rocks show enrichment in LILE and LREE relative to HFSE, and have high 87Sr/86Sr and low 143Nd/144Nd ratios. These characteristics indicate an enriched mantle source region(s) carrying a subduction component inherited from pre-collision subduction events. The tectonic discrimination diagram of Rb vs. (Y+Nb) suggests that the calc-alkaline, subalkaline, and alkaline plutonic rocks have been affected by crustal assimilation combined with fractional crystallisation processes. The coexistence of calc-alkaline and alkaline magmatism in the Central Anatolian Crystalline Complex may be attributed to mantle source heterogeneity before collision. The former carries a smaller intraplate component and pre-subduction enrichment compared to the latter. Either thermal perturbation of the metasomatised lithosphere by delamination of the thermal boundary layer (TBL), or removal of a subducted plate (slab breakoff) is the likely mechanism for the initiation of the post-collisional magmatism in the Complex.

The main plutonic complex of the Troodos ophiolite, north of the Arakapas Fault Zone, has been re-examined both from field and geochemical perspectives. Ion microprobe analyses of clinopyroxene crystal cores show that the range of melt compositions added to the lower crust far exceeds that of published lavas in the main Troodos massif. This suggests that the lower crust acted as a filter into which a large range of melt compositions were added and out of which a homogenised (and generally fractionated) derivative was extracted. This crustal-level aggregation homogenised diverse melt fractions from a broad range of degrees of melting. Depleted melts with U-shaped rare earth element (REE) patterns were a significant component of the melts added to the crust, but because of their low incompatible element abundances, mixing with less depleted melts prior to eruption masked their signature in the lavas. The discovery that highly depleted melts constituted a significant component of the melts added to the Troodos crust, but not of the lavas, demonstrates that the spatial distribution of lava-types is not necessarily a good indicator of where different parental melt compositions are generated within the mantle. Compared with normal mid-ocean ridge basalts, the Troodos parental melts were (1) generally depleted in immobile incompatible trace elements, (2) less depleted in light REE (LREE) than would be expected for the concomitant depletion in middle and heavy REE, (3) enriched in Sr with respect to the LREE and (4) more oxidised. Modelling of these characteristics suggests a mantle source that had previously lost a significant melt fraction under relatively reducing conditions. This was followed by remelting under more oxidising conditions in an environment in which Sr and LREE were added to the source consistent with previous models of a supra-subduction zone setting.