Trace-element and isotopic constraints on the source of magmas in the active volcano and Mariana island arcs, Western Pacific

Analytical results of the relative and absolute abundance of LIL-incompatible trace elements (K, Rb, Cs, Sr, and Ba) and isotopic compositions ( , , and ) are summarized for fresh samples from active and dormant volcanoes of the Volcano and Mariana island arcs. The presence of thickened oceanic crust (T 15–20 km) beneath the arc indicates that while hybridization processes resulting in the modification of primitive magmas by anatectic mixing at shallow crustal levels cannot be neglected, the extent and effects of these processes on this arc's magmas are minimized. All components of the subducted plate disappear at the trench. This observation is used to reconstruct the composition of the crust in the Wadati-Benioff zone by estimating proportions of various lithologies in the crust of the subducted plate coupled with analyses from DSDP sites. Over 90% of the mass of the subducted crust consists of basaltic Layers II and III. Sediments and seamounts, containing the bulk of the incompatible elements, make up the rest. Bulk Western Pacific seafloor has , δ ¹⁸O +7.2, K/Rb 510, K/Ba 46, and K/Cs 13,500. Consideration of trace-element data and combined systematics limits the participation of sediments in magmagenesis to less than 1%, in accord with the earlier results of Pb-isotopic studies. Combined data indicate little, if any, involvement of altered basaltic seafloor in magmagenesis. Perhaps more important than mean isotopic and LIL-element ratios is the restricted range for lavas from along over 1000 km of this arc. Mixtures of mantle with either the subducted crust or derivative fluids should result in strong heterogeneities in the sources of individual volcanoes along the arc. Such heterogeneities would be due to: (1) gross variations of crustal materials supplied to the subduction zone; and (2) lesser efficiency of mixing processes accompanying induced convection between arc segments (parallel to the arc) as compared to that perpendicular to the arc. The absence of these heterogeneities indicates that either some process exists for the efficient mixing of mantle and subducted material parallel to the arc or that subducted materials play a negligible role in the generation of Mariana-Volcano arc melts.Consideration of plausible sources in the mantle indicates that (1) an unmodified MORB-like mantle cannot have generated the observed trace-element and isotopic composition of this arc's magmas, while (2) a mantle similar to that which has produced alkali-olivine basalts (AOB) of north Pacific “hot spot” chains is indistinguishable in many respects spects from the source of these arc lavas.     

Setouchi high-Mg andesites revisited: geochemical evidence for melting of subducting sediments

In order to evaluate the mechanism of production of unusual high-Mg andesite (HMA) magmas, Pb–Nd–Sr isotopic compositions were determined for HMAs and basalts from the Miocene Setouchi volcanic belt in the SW Japan arc. The isotopic compositions of Setouchi rocks form mixing lines between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting sediment component to the HMA magma generation. Mixing calculations using compositions of an inferred original mantle and local oceanic sediments suggest that a sediment-derived melt, neither an H₂O-rich fluid nor an amphibolite/eclogite-derived melt, could have been produced first and served as a plausible metasomatic agent for the HMA magma source. The unusual tectonic setting, including subduction of a newly-borne hence hot plate, may be responsible for melting of subducting sediments.     

Possible contribution of the asthenosphere, below the subducted oceanic lithosphere, to the genesis of arc magmas: Geochemical evidence from the andean southern volcanic zone (33–46°S)

The application of the Sr/Ca-Ba/Ca systematics to volcanic rocks of the Andean Southern Volcanic Zone (33°S–46°S) has revealed a good correlation between the estimated degree of partial melting required to generate primary magmas and the projected extensions of the oceanic Nazca plate fracture zones under the continental South American plate. Magmas erupted at volcanic centers situated above these projections are thought to have been derived from primary magmas generated by relatively high degrees of melting, whereas those erupted at other centers are thought to have evolved from magmas produced by comparatively low degree of fusion. We interpret this relationship to reflect the facilitation of heat and mass transfer from the asthenosphere below the subducted oceanic lithosphere to the subarc mantle by the fracture zones. This contribution enhances the degree of melting of the subarc mantle source as well as the fraction of material derived from the subducted oceanic crust. This model predicts the predominance of basalts depleted in incompatible trace elements in centers located above the Nazca plate fracture zone extensions and of basalts enriched in incompatible trace elements in centers situated between boundaries of fracture extensions.     

himu-em: The French Polynesian connection

himu, em i andem ii are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteristichimu signature is found in Tubuai, Mangaia and Rurutu,em i is present in the source of Rarotonga and Pitcairn volcanics andem ii dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands.We suggest that the three components are not independent but are physically related in the mantle. Thehimu component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction.em i andem ii are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust.The trace element pattern and the isotopic composition ofhimu lavas can be quantitatively modelled using a mixture of 25% old recycledmorb crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of theU/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those ofem i andem ii are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlyingmorb oceanic crust. Pelagic sediments (μ 5 andκ 6) account for the Pb isotopic composition ofem i whereas terrigenous sediments (μ 10 andκ 4.5) evolve towards theem ii end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of thehimu component. This, together with the low probability of isolating oceanic crust in the mantle for 2 Ga, explains why the extremehimu composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.     

The Zealandia Volcanic Complex: Further evidence of a lower crustal “hot zone” beneath the Mariana Intra‐oceanic Arc,Western Pacific

This paper addresses formation of felsic magmas in an intra‐oceanic magmatic arc. New bathymetric, petrologic, geochemical, and isotopic data for Zealandia Bank and two related volcanoes in the south‐central Mariana arc is presented and interpreted. These three volcanoes are remnants of an older andesitic volcano that evolved for some time and became dormant long enough for a carbonate platform to grow on its summit before reawakening as a rhyodacitic volcano. Zealandia lavas are transitional between low‐ and medium‐K and tholeiitic and calc‐alkaline suites. They define a bimodal suite with a gap of 56–58 wt% SiO₂; this suggests that mafic and felsic magmas have different origins. The magmatic system is powered by mantle‐derived basalts having low Zr/Y and flat rare earth element patterns. Two‐pyroxene thermometry yields equilibration temperatures of 1000–1100 °C for andesites and 900–1000 °C for dacites. Porphyritic basalts and andesites show textures expected for fractionating magmas but mostly fine‐grained felsic lavas do not. All lavas show trace element signatures expected for mantle and crustal sources that were strongly melt‐depleted and enriched by subduction‐related fluids and sediment melts. Sr and Nd isotopic compositions fall in the normal range of Mariana arc lavas. Felsic lavas show petrographic evidence of mixing with mafic magma. Zealandia Bank felsic magmatism supports the idea that a large mid‐ to lower‐crustal felsic magma body exists beneath the south‐central Mariana arc, indicating that MASH (mixing, assimilation, storage, and homogenization) zones can form beneath intra‐oceanic as well as continental arcs.     

Insights into operation of the subduction factory from the oxygen isotopic values of the southern Izu–Bonin–Mariana Arc

Abstract Oxygen is the most abundant element in the earth, and isotopic analysis of this element in island arc lavas potentially provides sensitive constraints on the proportion of oxygen recycled from subducted material, relative to that extracted from the mantle. Here we report on 225 new oxygen isotopic analyses of whole‐rock and glass samples, and clinopyroxene separates, from lavas collected from the southernmost 1500 km of the Izu–Bonin–Mariana (IBM) convergent margin. Whole‐rock samples clustered around a mean of 6.11 ± 0.47‰, whereas Mariana Trough glasses and mafic melts, calculated to be in equilibrium with mafic phenocrysts, clustered narrowly around a mean of 5.7‰. These data demonstrate that unequivocal identification of magmatic oxygen requires analysis of fresh glass or mafic minerals, and that the source of southern IBM Arc melts is entirely, or almost entirely, in equilibrium with normal mantle oxygen. If the elemental enrichments characteristic of the subduction component originate in subducted materials, these oxygen isotopic data are most consistent with the interaction of a small amount of sediment melt (<4%; mostly less than 1%) with mantle peridotite to yield the hybrid mantle that melts to form IBM Arc magmas.     

The relationship between calc-alkaline volcanism and within-plate continental rift volcanism: evidence from Scottish Palaeozoic lavas

Lower Carboniferous lavas from the Midland Valley and adjacent regions of Scotland are mildly alkaline and intraplate in nature. The sequence is dominated by basalt and hawaiite, although mugearite, benmoreite, trachyte and rhyolite are also present. Basic volcanic rocks display the LIL element and LREE enrichment typical of intraplate alkali basalt terrains. Low initial⁸⁷Sr/⁸⁶Sr (0.7029–0.7046), high ε_Nd (−0.4 to +5.6) and moderately radiogenic²⁰⁶Pb/²⁰⁴Pb (17.77–18.89) ratios are also comparable with alkali basalts from other continental rifts and oceanic islands.When the Carboniferous lavas are compared with subduction-related lavas of Old Red Sandstone age, erupted in and around the Midland Valley ca. 50 Ma earlier (at 410 Ma) remarkable similarities are apparent. Significant overlap occurs in Nd and Pb isotopic compositions. Sr isotopic compositions are, however, more radiogenic in the older subduction-related lavas. This, combined with high K and Rb concentrations in ORS lavas may be explained by the incorporation of a sediment component derived from the subducted slab, which by Lower Carboniferous times had been lost from the mantle source region by convection. A pronounced negative Nb anomaly in the ORS subduction-related lavas may be explained by the retention of a Nb-bearing phase in the mantle during hydrous melting of the mantle wedge above the subduction zone.Allowing for the effects of the added component from the subducted slab, there appears to be no necessity to invoke separate mantle source regions for the two suites of lavas: both may have been derived from chemically similar portions of mantle. If volcanic arc lavas are derived from the mantle wedge, the implication is that such a source lies at relatively shallow depth within the upper mantle: the same may therefore apply to the Carboniferous continental rift basalts. This evidence, combined with the fact that there is no evident hot-spot trail across the Midland Valley despite a long period of within-plate volcanism and rapid plate movements during the Carboniferous, suggests that the alkali basalt magmatism is not the product of a deep-seated mantle plume. Rather, the volcanism appears to owe more to passive rifting and to diapiric upwelling from a source region within the uppermost mantle.     

Isotope characteristics of submarine lavas from the Philippine Sea: implications for the origin of arc and basin magmas of the Philippine tectonic plate

Igneous rocks from the Philippine tectonic plate recovered on Deep Sea Drilling Project Legs 31, 58 and 59 have been analyzed for Sr, Nd and Pb isotope ratios. Samples include rocks from the West Philippine Basin, Daito Basin and Benham Rise (40–60 m.y.), the Palau-Kyushu Ridge (29–44 m.y.) and the Parece Vela and Shikoku basins (17–30 m.y.). Samples from the West Philippine, Parece Vela and Shikoku basins are MORB (mid-ocean ridge basalt)-like with ⁸⁷Sr/⁸⁶Sr= 0.7026−0.7032, ¹⁴³Nd/¹⁴⁴Nd= 0.51300−0.51315, and ²⁰⁶Pb/²⁰⁴Pb= 17.8−18.1. Samples from the Daito Basin and Benham Rise are OIB (oceanic island basalt)-like with ⁸⁷Sr/⁸⁶Sr= 0.7038−0.7040, ¹⁴³Nd/¹⁴⁴Nd= 0.51285−0.51291 and ²⁰⁶Pb/²⁰⁴Pb= 18.8−19.2. All of these rocks have elevated ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb compared to the Northern Hemisphere Regression Line (NHRL) and have δ²⁰⁷Pb values of 0 to +6 and δ²⁰⁸Pb values of +32 to +65. Lavas from the Palau-Kyushu Ridge, a remnant island arc, have ⁸⁷Sr/⁸⁶Sr= 7032−0.7035, ¹⁴³Nd/¹⁴⁴Nd= 0.51308−0.51310 and ²⁰⁶Pb/²⁰⁴Pb= 18.4−18.5. Unlike the basin magmas erupted before and after them, these lavas plot along the NHRL and have Pb-isotope ratios similar to modern Pacific plate MORB's. This characteristic is shared by other Palau-Kyushu Arc volcanic rocks that have been sampled from submerged and subaerial portions of the Mariana fore-arc.At least four geochemically distinct magma sources are required for these Philippine plate magmas. The basin magmas tap Source 1, a MORB-mantle source that was contaminated by EMI (enriched mantle component 1 [31]) and Source 2, an OIB-like mantle source with some characteristics of EMII (enriched mantle component 2 [31]). The arc lavas are derived from Source 3, a MORB-source or residue mantle including Sr and Pb from the subducted oceanic crust, and Source 4, MORB-source or residue mantle including a component with characteristics of HIMU (mantle component with high U/Pb [31]). These same sources can account for many of the isotopic characteristics of recent Philippine plate arc and basin lavas. The enriched components in these sources which are associated with the DUPAL anomaly were probably introduced into the asthenosphere from the deep mantle when the Philippine plate was located in the Southern Hemisphere 60 m.y.b.p.     

Strontium isotopic composition and trace element data bearing on the origin of Cenozoic volcanic rocks of the central and southern Andes

The Cenozoic volcanic rocks of the southern Andes are characterized by low ⁸⁷Sr/⁸⁶Sr ratios (0.7040–0.7045), which are consistent with an origin in the downgoing slab of oceanic lithosphere or the overlying mantle. These values are distinctly lower than those from corresponding rocks of the central Andes.The calc-alkaline rocks of the central Andes exhibit higher Sr isotopic values (0.705–0.713) and variable Rb/Sr ratios. Different explanations are possible for this behaviour as well as for the positive correlation between ⁸⁷Sr/⁸⁶Sr and Rb/Sr expressed in an apparent isochron of 380 ± 50 m.y. It is postulated that these magmas result from a mixing process between a primary magma with basaltic affinities and crustal material of relatively young age.A model is proposed for the generation of the “andesitic” magmas of the central Andes by which crustal rocks of the upper part of the crust are added to the base of the crust by an accretionary process at the margin of the continent. Melts from these upper crustal rocks act as contaminants in “andesitic” magmas.The role of crustal material is still more significant in the generation of the ignimbritic magmas; they are considered to result from a two-stage melting process by which igneous rocks, belonging to a former stage of development of the Andes, are engulfed in the subduction zone, where they melt.     

143Nd/144Nd,87Sr/86Sr and trace element constraints on the petrogenesis of Aleutian island arc magmas

¹⁴³Nd/¹⁴⁴Nd,⁸⁷Sr/⁸⁶Sr and trace element results are reported for volcanic and plutonic rocks of the Aleutian island arc. The Nd and Sr isotopic compositions plot within the mantle array with ε_Nd values of from 6.5 to 9.1 and⁸⁷Sr/⁸⁶Sr ratios of from 0.70289 to 0.70342. Basalts have mildly enriched light REE abundances but essentially unfractionated heavy REE abundances, while andesites exhibit a greater degree of light to heavy REE fractionation. Both the basalts and andesites have significant large ion lithophile element to light rare earth element (LILE/LREE) enrichments. Variations in the isotopic compositions of Nd and Sr are not related to the spatial distribution of volcanoes in the arc, nor are they related to temporal differences. ε_Nd and⁸⁷Sr/⁸⁶Sr do not correlate with major element compositions but do, however, correlate with certain LILE/LREE ratios (e.g. Ba_N/La_N). Plutonic rocks have isotropic and trace element characteristics identical to some of the volcanic rocks. Rocks that make up the tholeiitic, calc-alkaline and alkaline series in the Aleutians do not come from isotopically distinct sources, but do exhibit some differing LILE characteristics.Given these elemental and isotopic constraints it is shown that the Aleutian arc magmas could not have been derived directly from homogeneous MORB-type mantle, or fresh or altered MORB subducted beneath the arc. Mixtures of partially altered MORB with deep-sea sediment can in principle account for the isotopic characteristics and most of the observed LILE/LREE enrichments. However, some samples have exceedingly high LILE/LREE enrichments which cannot be accounted for by sediment contamination alone. For these samples a more complex scenario is considered whereby dehydration and partial melting of the subducted slab, containing less than 8% sediment, produces a LILE-enriched (relative to REE) metasomatic fluid which interacts with the overlying depleted mantle wedge. The isotopic and LILE characteristics of the mantle are extremely sensitive to metasomatism by small percentages of added fluid, whereas major elements are not substantially effected, Major element compositions of Aleutian magmas are dominantly controlled by the partial melting of this mantle and subsequent crystal fractionation; whereas isotopic and LILE characteristics are determined by localized mantle heterogeneities.     

Element transport by dehydration of subducted sediments: Implication for arc and ocean island magmatism

High-pressure experiments on a natural pelite have been conducted at 2–11-GPa pressures in order to evaluate contributions of subducted sediments to arc and ocean island magmatism. Obtained phase relations suggest that, at least in modern subduction zones, subsolidus dehydration of chlorite and phengitic muscovite in the subducted sediments, rather than partial melting, is a predominant process in overprinting sediment components onto the magma source region. Trace element compositions of sediment-derived fluids are estimated based on dehydration experiments at 5.5 GPa and 900/1300°C. Pb is effectively transported by fluids relative to other elements. This results in the Pb enrichment for arc basalts by fluids, generated by the dehydration of subducted sediments, together with altered mid-ocean ridge basalt (MORB), and complementary depletion of Pb in subducted sediments. Inferred arc magma compositions obtained by model calculations based on the present experimental results agree well with a natural primitive arc basalt composition. A large increase in the U/Pb ratio in the subducted sediments at deeper levels than major dehydration depths results in a high Pb isotopic ratio through radioactive decay after long periods of isolation. Combined with other isotopic ratios such as Sr and Nd, it is possible to produce the EM II source, one of the enriched geochemical reservoirs for ocean island basalt magmas, by mixing of a small amount of subducted sediments with depleted or primitive mantle.     

Across-arc variation of lava chemistry in the Izu-Bonin Arc: identification of subduction components

In order to understand the role of the subducted lithosphere in producing the geochemical characteristics of arc magmas, major- and trace-element along with Sr- and Nd-isotope compositions have been determined for Quaternary volcanic rocks from the Izu-Bonin intra-oceanic arc. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios decrease away from the volcanic front of this arc and lie on mixing lines between the assumed isotopic compositions of fluid phases mainly derived from the basalt layer of the subducted lithosphere and upper-mantle materials in the sub-arc wedge. This across-arc variation can be explained through a simple sequence of processes involving initial release of fluid phases from the subducted oceanic crust to produce hydrous peridotite at the base of the mantle wedge. This hydrous peridotite is dragged downward with the slab and releases a second-stage metasomatizing fluid beneath the volcanic arc. The higher concentrations of both Sr and Nd in the fluid beneath the volcanic front than those beneath the back-arc side may be a possible cause of the observed across-arc variation in Sr-Nd isotopic ratios. The difference in compositions of fluid phases is attributed to the different hydrous phases which decompose in the hydrous peridotite layer; amphibole beneath the volcanic front and phlogopite beneath the back-arc side of the volcanic arc. The mineralogically controlled fluid addition may also be responsible for the across-arc variation in Rb/K and Rb/Zr ratios, increasing away from the volcanic front.     

A Jurassic-Cretaceous island arc in the Ladakh-Himalayas

The Ladakh Mesozoic ophiolite belt (western Himalaya) contains a pile of volcanic thrust sheets (Dras unit) which differ significantly in structure and composition from the ophiolitic mélange zones. The Dras unit is composed of pillow lavas, doleritic sills, very irregular basaltic (?basaltic andesites) and dacitic flows intercalated with pyroclastics, volcanoclastic sediments and radiolarian cherts. According to fossil evidence, this volcanism must have been active between Upper Jurassic and Upper Cretaceous.The presence of relict primary minerals, such as magnesiochromite, clinopyroxene, hastingsitic hornblende and Ti-magnetite as well as distinctive bulk chemistries, suggests that the volcanics belong to island arc tholeiite and to calc-alkaline rock series, typical of present island arcs in the Caribbean and Pacific.Model calculations incorporating probed phenocryst phases indicate that in addition to olivine, clinopyroxene and plagioclase, amphibole and titanomagnetite are crucial fractionating phases in the development of the dacites from a primitive tholeiitic melt. The latter process must have taken place at about 1000°C and at moderate depth of 5–15 km within or underneath the island arc. Today, hornblende-bearing mafic cumulates appear in the vicinity of Kargil within and close to the Dras volcanics.In a Sr-evolution diagram, the Dras volcanics have yielded a “pseudo-isochron” with a low initial ratio of 0.7035 ± 0.0003, which is in the same range as the mean of modern island arc volcanics. However, a geologically unrealistic age of 263 m.y., is obtained from the slope of this isochron.The upper mantle is regarded as the source material for the island arc tholeiitic magmas. Enrichment in K, Ba, Sr and LREE supports the involvement of components derived from dehydration or incipient melting of subducted Tethyan oceanic crust in the mantle.     

Magmatic response to early aseismic ridge subduction: the Ecuadorian margin case (South America)

A geochemical and isotopic study of lavas from Pichincha, Antisana and Sumaco volcanoes in the Northern Volcanic Zone (NVZ) in Ecuador shows their magma genesis to be strongly influenced by slab melts. Pichincha lavas (in fore arc position) display all the characteristics of adakites (or slab melts) and were found in association with magnesian andesites. In the main arc, adakite-like lavas from Antisana volcano could be produced by the destabilization of pargasite in a garnet-rich mantle. In the back arc, high-niobium basalts found at Sumaco volcano could be produced in a phlogopite-rich mantle. The strikingly homogeneous isotopic signatures of all the lavas suggest that continental crust assimilation is limited and confirm that magmas from the three volcanic centers are closely related. The following magma genesis model is proposed in the NVZ in Ecuador: in fore arc position beneath Pichincha volcano, oceanic crust is able to melt and produces adakites. En route to the surface, part of these magmas metasomatize the mantle wedge inducing the crystallization of pargasite, phlogopite and garnet. In counterpart, they are enriched in magnesium and are placed at the surface as magnesian andesites. Dragged down by convection, the modified mantle undergoes a first partial melting event by the destabilization of pargasite and produces the adakite-like lavas from Antisana volcano. Lastly, dragged down deeper beneath the Sumaco volcano, the mantle melts a second time by the destabilization of phlogopite and produces high-niobium basalts. The obvious variation in spatial distribution (and geochemical characteristics) of the volcanism in the NVZ between Colombia and Ecuador clearly indicates that the subduction of the Carnegie Ridge beneath the Ecuadorian margin strongly influences the subduction-related volcanism. It is proposed that the flattening of the subducted slab induced by the recent subduction (<5 Ma?) of the Carnegie Ridge has permitted the progressive warming of the oceanic crust and its partial melting since ca. 1.5 Ma. Since then, the production of adakites in fore arc position has deeply transformed the magma genesis in the overall arc changing from ‘typical’ calc-alkaline magmatism induced by hydrous fluid metasomatism, to the space- and time-associated lithology adakite/high-Mg andesite/adakite-like andesite/high-Nb basalts characteristic of slab melt metasomatism.     

Evidence for mantle metasomatism: an oxygen and strontium isotope study of the Vulsinian District, Central Italy

This paper presents new O and Sr isotope data for lavas from the northern part of the Roman perpotassic province. The samples comprise the tephritic leucititic to leucite phonolitic lavas and the saturated lavas from the Vulsinian District, the olivine leucite melilitite of San Venanzo, and the kalsilite diopside melilitite of Cupaello. Previous oxygen isotope work on the lavas of the Vulsinian District suggested crustal contamination of “normal” mantle-derived magmas. The new data cover the ranges previously found. O and Sr isotope ratios of evolved lavas of the undersaturated suite indicate assimilation in variable amounts of up to ca. 10% of continental crustal material. The saturated lavas probably assimilated large amounts (up to ca. 50%) of crust. Lavas chemically identified as corresponding to little modified mantle-derived liquids are high in both⁸⁷Sr/⁸⁶Sr andδ¹⁸O: 0.7103−0.7107, +7.8 to +9.4 (Vulsini), 0.7104, +12.3 (San Venanzo) and 0.7112, +14.4 (Cupaello). These high values are interpreted to have been inherited from a metasomatized parental mantle. Hydrous fluids enriched in large-ion lithophile elements and high inδ¹⁸O and⁸⁷Sr/⁸⁶Sr are thought to have mixed with mantle of “normal”δ¹⁸O and⁸⁷Sr/⁸⁶Sr. The fluids probably origi dehydration of continent-derived sediments, which were subducted beneath a mantle wedge in the continent-continent collision of the Corsica-Sardinia block and the Adriatic (Italian) plate. This hypothesis is supported by Pb and Nd isotopic evidence and is probably valid for the entire Roman Province.     

Flux versus decompression melting at stratovolcanoes in southeastern Guatemala

The major and trace element geochemistry of lavas erupted from four volcanic front (VF) stratovolcanoes in southeastern Guatemala show differences in the relative importance of flux and decompression melting in a continental arc setting. The VF stratovolcanoes exhibit a wide compositional range from basalt to dacite, although modern Pacaya erupts basaltic lavas. The VF basalts have relatively low MgO contents and plot outside the field of primary arc magmas defined by melting experiments on hydrous peridotite. After subtracting the effects of the fractionation, assimilation, and alteration of some VF lavas, separate partial melting and mixing trends were identified for Agua–Pacaya and Tecuamburro–Moyuta.The distinct chemical signatures of the hemipelagic and carbonate sediments subducted off Guatemala provide constraints on material transfer processes that occurred between the slab and mantle wedge. Model fluids and melts from the subducted slab were calculated using recently published mineral–aqueous fluid partition coefficients. Wide separation of the model fluid and melt compositions on a U/La versus Ba/Th diagram creates diagnostic mixing curves with an enriched mid-ocean ridge basalt source. Fluid from mature ocean crust has high U/La, fluid from carbonate sediment has high Ba/Th, and fluid and melt from hemipelagic sediments have both high U/La and Ba/Th. In a simple single-stage model, a mantle metasomatized by fluid originating largely from the oceanic crust with only minor sediment fluid contributions best explains the overall large ion lithophile element composition of the VF lavas. (Th/Rb)_N ratios of ∼1 in the VF lavas from southeastern Guatemala require a component of sediment melting. Therefore, a more realistic two-stage model to describe the Guatemalan arc data involves an initial hemipelagic sediment melt input to the wedge followed by minor fluid additions from the oceanic crust or sediments. Correlation between measures of slab input and extent of melting in the older VF lavas from Tecuamburro and Moyuta favors flux-dominated melting near the base of the mantle wedge. In sharp contrast, the lack of a relationship between slab additions and melting in younger lavas from Agua and Pacaya volcanoes implies a significant role for decompression melting closer to the top of the wedge. In this melting scenario, the rate of crustal extension determines the extent of melting.     

Causes of spatial compositional variations in Mariana arc lavas: Trace element evidence

New inductively coupled plasma mass spectrometry (ICP-MS) trace element data are presented on a suite of arc lavas from the northern Mariana and southern Bonin island arcs. The samples were dredged from seamounts in the Central Island Province (CIP), the Northern Seamount Province (NSP) and the Volcano Arc (VA), and they range in composition from low-K tholeiites to shoshonites. Previous studies on these samples concluded that the primary compositional control was two-component mixing between a fluid-metasomatized mid-ocean ridge basalt (MORB) source and an enriched, ocean island basalt (OIB)-like, mantle component, with subducted sediment material playing a secondary role. However, the new trace element data suggest that the compositional variations along the Mariana arc can be better explained by the addition of spatially varying subduction components to a spatially varying mantle source. The data suggest that the subduction component in the CIP and VA is dominated by aqueous fluids derived from altered oceanic crust and a pelagic sediment component, while the subduction component in the NSP is dominated by more silicic fluids derived from volcanogenic sediments as well as from pelagic sediment and altered oceanic crust. The mantle wedge in the CIP and VA is depleted relative to a normal mid-ocean ridge basalt source by loss of a small melt fraction, while the mantle wedge in the NSP is enriched either by possible gain of a small melt fraction or addition of a sediment-derived melt. Because the subduction of seamounts controls the arc and back-arc geometries, so the concomitant variation between subducted material and mantle composition may be no coincidence. The high field strength element (HFSE) data indicate a high degree of melting (∼ 25–30%) throughout the arc, ∼ 10% of which may be attributed to decompression and ∼ 20% to fluid addition.     

Enriched back-arc basin basalts from the northern Mariana Trough: implications for the magmatic evolution of back-arc basins

The composition of basalts erupted at the earliest stages in the evolution of a back-arc basin permit unique insights into the composition and structure of the sub-arc mantle. We report major and trace element chemical data and O-, Sr-, Nd-, and Pb- isotopic analyses for basalts recovered from four dredge hauls and one ALVIN dive in the northern Mariana Trough near 22°N. The petrography and major element chemistry of these basalts (MTB-22) are similar to tholeiites from the widest part of the Trough, near 18°N (MTB-18), except that MTB-22 have slightly more K₂O and slightly less TiO₂. The trace element data exhibit a very strong arc signature in MTB-22, including elevated K, Rb, Sr, Ba, and LREE contents; relatively lowK/Ba and highBa/La andSr/Nd. The Sr- and Nd- isotopic data plot in a field displaced from that of MTB-18 towards Mariana arc lavas, and the Pb-isotopic composition of MTB-22 is indistinguishable from Mariana arc lavas and much more homogeneous than MTB-18. Mixing of 50–90% Mariana arc component with a MORB component is hypothesized. We cannot determine whether this resulted from physical mixing of arc mantle and MORB mantle, or whether the arc component is introduced by metasomatism of MORB-like mantle by fluids released from the subducted lithosphere. The strong arc signature in back-arc melts from the Mariana Trough at 22°N, where the back-arc basin is narrow, supports general models for back-arc basin evolution whereby early back-arc basin basalts have a strong arc component which diminishes in importance relative to MORB as the back-arc basin widens.     

Major and trace element and Sr–Nd isotope signatures of lavas from the Central Lau Basin: Implications for the nature and influence of subduction components in the back-arc mantle

New major and trace element and Sr–Nd isotope data are presented for basaltic glasses from active spreading centers (Central Lau Spreading Center (CLSC), Relay Zone (RZ) and Eastern Lau Spreading Center (ELSC)) in the Central Lau Basin, SW Pacific. Basaltic lavas from the Central Lau Basin are mainly tholeiitic and are broadly similar in composition to mid-ocean ridge basalts (MORB). Their generally high ⁸⁷Sr/⁸⁶Sr ratios, combined with relatively low ¹⁴³Nd/¹⁴⁴Nd ratios are more akin to MORB from the Indian rather than Pacific Ocean. In detail, the CLSC, RZ and ELSC lavas are generally more enriched in large ion lithophile elements (Rb, Ba, Sr, and K) than average normal-MORB, which suggests that the mantle beneath the Central Lau Basin was modified by subducted slab-derived components. Fluid mobile/immobile trace element and Sr – Nd isotope ratios suggest that the subduction components were essentially transferred into the mantle via hydrous fluids derived from the subducted oceanic crust; contributions coming from the subducted sediments are minor. Compared to CLSC lavas, ELSC and RZ lavas show greater enrichment in fluid mobile elements and depletion in high field strength elements, especially Nb. Thus, with increasing distance away from the arc, the influence of subduction components in the mantle source of Lau Basin lavas diminishes. The amount of hydrous fluids also influences the degree of partial melting of the mantle beneath the Central Lau Basin, and hence the degree of melting also decreases with increasing distance from the arc.     

Petrogenesis of the late Cretaceous K‐rich volcanic rocks from the Central Pontide orogenic belt,North Turkey

Subduction‐related volcanic rocks are widespread in the Central Pontides of Turkey, and represented by the Hamsaros volcanic succession in the Sinop area to the north. The volcanic rocks display high‐K calc‐alkaline, shoshonitic and ultra‐K affinities. ⁴⁰Ar/³⁹Ar age data indicate that the rocks occurred during the Late Cretaceous (ca 82 Ma), and the volcanic suites were coeval. Primitive mantle‐normalized trace element patterns of all the lavas are characterized by strong enrichments in large ion lithophile elements (LILE) (Rb, Ba, K, and Sr), Th, U, Pb, and light rare earth elements (LREE; La, Ce) and prominent negative Nb, Ta, and Ti anomalies, all typical of subduction‐related lavas. There is a systematic increase in the enrichment of incompatible trace elements from the high‐K calc‐alkaline lavas through the shoshonitic to the ultra‐K lavas. In addition, the shoshonitic and ultra‐K lavas have significantly higher ⁸⁷Sr/⁸⁶Sr (0.70666–0.70834) and lower ¹⁴³Nd/¹⁴⁴Nd (0.51227–0.51236) initial ratios than coexisting high‐K calc‐alkaline lavas (⁸⁷Sr/⁸⁶Sr 0.70576–0.70613, ¹⁴³Nd/¹⁴⁴Nd 0.51245–0.51253). Geochemical and isotopic data show that the shoshonitic and ultra‐K rocks cannot be derived from the high‐K calc‐alkaline suite by any shallow level differentiation process, and point to a derivation from distinct mantle sources. The shoshonitic and ultra‐K rocks were derived from metasomatic veins related to melting of recycled subducted sediments, but the high‐K calc‐alkaline rocks from a lithospheric source metasomatized by fluids from subduction zone.