MORB Range Research Articles

Light stable Cr isotope compositions of mid-ocean ridge basalts: Implications for mantle source composition

Stable isotopes of chromium (Cr) and iron (Fe), both multivalent elements in silicate melts, are valuable proxies of redox conditions in magmatic systems. Partial melting and fractional crystallization have been identified as the dominant fractionation processes for Cr and Fe isotopes in high-temperature reservoirs that can lead to small but detectable isotope variations in igneous rocks. More recently, however, the source mineralogy has been suggested to play an important role, which can ultimately affect the magnitude of Cr and Fe isotopic fractionation during partial melting and thus the Cr and Fe isotopic composition of the originating melt.With the aim to investigate the role of magmatic processes and the source Cr and Fe isotope signatures on the isotope composition of these transition metals in normal mid-ocean ridge basalts (N-MORB), we analyzed 21 well-characterized MORB glasses from the southern East-Pacific Rise, Pacific-Antarctic Ridge and Mid-Atlantic Ridge. The Cr isotope compositions of N-MORB show a narrow range from −0.278 to −0.186 ‰ in δ53/52Cr (i.e., difference of a sample’s 53Cr/52Cr ratio relative to the international reference material NIST SRM979), with an average value of −0.237 ± 0.050 ‰ (2SD; n = 19). As such, the average N-MORB δ53/52Cr value is significantly lower than that of Bulk Silicate Earth of −0.12 ± 0.06 ‰ and the Cr isotope compositions reported for most ocean island basalts ranging from ∼ −0.23 to 0.00 ‰. Iron isotopic compositions of these MORB range from +0.032 to +0.137 ‰ in δ56/54Fe (i.e., difference of a sample’s 56Fe/54Fe ratio relative to the international reference material IRMM014) and are within the range of previously published MORB data. The lack of correlations between N-MORB Cr isotope signatures and indices of magmatic differentiation suggests a negligible control of this process on Cr stable isotopes. Partial melting modeling using pMELTS provides evidence that the significant offset towards lower δ53/52Cr values of N-MORB cannot be produced by decompression partial melting of a lherzolithic source such as the depleted upper mantle alone. Instead, we suggest that the lower δ53/52Cr values of MORB could be linked to intrinsic small-scale 53Cr-depleted pyroxene-rich domains in the upper mantle that influence the melt’s Cr isotopic budget during partial melting.

The heterogeneity of the Mexican lithospheric mantle: Clues from noble gas and CO2 isotopes in fluid inclusions

The abundance of mantle-derived rocks and lavas, in combination with its tectonic evolution, render Mexico a perfect laboratory to investigate the chemical and the isotopic heterogeneity of the lithospheric mantle. New data on the composition of noble gases and CO2in Mexican mantle xenoliths and lavas is reported. Our samples consist of six ultramafic nodules from the Durango Volcanic Field (DVF) and the San Quintin Volcanic Field (SQVF), monogenetic complexes belonging to the Mexican Basin and Range province; and four lavas from the Sierra Chichinautzin (SCN), a Quaternary monogenetic volcanic field located in the Mexican volcanic arc. Ne and Ar isotopes in fluid inclusions reveal mixing between atmospheric and MORB-like fluids (e.g.,40Ar/36Ar < 1,200). DVF and SQVF nodules record low40Ar/36Ar and4He/20Ne that confirm the existence of recycled atmospheric-derived noble gases in the local mantle. The averages of the Rc/Ra ratios (3He/4He corrected for atmospheric contamination) measured in Mexican localities are within the MORB-like range: DVF= 8.39 ± 0.24 Ra, SQVF = 7.43 ± 0.19 Ra and SCN lavas = 7.15 ± 0.33 Ra (1σ). With the aim of assessing the isotopic variability of the Mexican lithospheric mantle, the above results were compared with similar data previously obtained from ultramafic nodules found in the Ventura Espiritu Santo Volcanic Field (VESVF), another Quaternary monogenetic volcanic complex belonging the Basin and Range. The higher3He/4He ratios in DVF relative to those reported for the VESVF and the SQVF are explained as reflecting different ages of mantle refertilization, triggered by the retreating of the Farallon slab (∼40 Ma ago) and associated delamination slab processes. We propose that the DVF mantle was refertilized more recently (<10 Ma ago) than the mantle beneath the SQVF and VESVF (∼40–20 Ma ago). On the other hand, He-Ne-Ar compositions of SCN olivines share similarities with VESVF xenoliths, suggesting a relatively homogeneous lithospheric mantle in central Mexico. Finally, DVF and the SCN samples exhibit δ13C values within the MORB range (comparable to other values previously reported in fluid inclusions and fumaroles from Popocatépetl, Colima—Ceboruco volcanoes). While we explain the MORB-like carbon signatures of the DVF samples as the result of the above-mentioned refertilization process, the SCN signatures likely reflect either (i) trapping of isotopically fractionated CO2derived from magmatic degassing or (ii) a mantle source unaffected by subduction-related crustal carbon recycling.

3He/4He Signature of Magmatic Fluids from Telica (Nicaragua) and Baru (Panama) Volcanoes, Central American Volcanic Arc

Constraining the magmatic 3He/4He signature of fluids degassed from a magmatic system is crucial for making inferences on its mantle source. This is especially important in arc volcanism, where variations in the composition of the wedge potentially induced by slab sediment fluids must be distinguished from the effects of magma differentiation, degassing, and crustal contamination. The study of fluid inclusions (FIs) trapped in minerals of volcanic rocks is becoming an increasingly used methodology in geochemical studies that integrates the classical study of volcanic and geothermal fluids. Here, we report on the first noble gas (He, Ne, Ar) concentrations and isotopic ratios of FI in olivine (Ol) and pyroxene (Px) crystals separated from eruptive products of the Telica and Baru volcanoes, belonging to the Nicaraguan and Panamanian arc-segments of Central America Volcanic arc (CAVA). FIs from Telica yield air corrected 3He/4He (Rc/Ra) of 7.2–7.4 Ra in Ol and 6.1–7.3 in Px, while those from Baru give 7.1–8.0 Ra in Ol and 4.2–5.8 Ra in Px. After a data quality check and a comparison with previous 3He/4He measurements carried out on the same volcanoes and along CAVA, we constrained a magmatic Rc/Ra signature of 7.5 Ra for Telica and of 8.0 Ra for Baru, both within the MORB range (8 ± 1 Ra). These 3He/4He differences also reflect variations in the respective arc-segments, which cannot be explained by radiogenic 4He addition due to variable crust thickness, as the mantle beneath Nicaragua and Panama is at about 35 and 30 km, respectively. We instead highlight that the lowest 3He/4He signature observed in the Nicaraguan arc segment reflects a contamination of the underlying wedge by slab sediment fluids. Rc/Ra values up to 9.0 Ra are found at Pacaya volcano in Guatemala, where the crust is 45 km thick, while a 3He/4He signature of about 8.0 Ra was measured at Turrialba volcano in Costa Rica, which is similar to that of Baru, and reflects possible influence of slab melting, triggered by a change in subduction conditions and the contemporary subduction of the Galapagos hot-spot track below southern Costa Rica and western Panama.

The composition of fluids stored in the central Mexican lithospheric mantle: Inferences from noble gases and CO2 in mantle xenoliths

We present the first isotopic (noble gases and CO2) characterization of fluid inclusions coupled to Raman microspectroscopy analyses in mantle xenoliths from Central Mexico, a geodynamically complex area where the Basin and Range extension was superimposed on the Farallon subduction (terminated at 28 Ma). To characterize the isotopic signature of the Central Mexican lithospheric mantle, we focus on fluid inclusions entrapped in mantle xenoliths found in deposits of the Joya Honda maar (JH), a Quaternary monogenetic volcano belonging to the Ventura Espiritu Santo Volcanic Field (VESVF) in the state of San Luis Potosí (central Mexico). Thirteen ultramafic plagioclase-free xenoliths were selected, all exhibiting a paragenesis Ol > Opx > Cpx > > Sp, and being classified as spinel-lherzolites and harzburgites. All xenoliths bring textural evidence of interstitial glass veins bearing dendritic trails of secondary melt and fluid inclusions (composed of silicate glass ± CO2 ± Mg-Ca carbonates ± pyrite). These are related to pervasive mantle metasomatism driven by carbonate-rich silicate melt. The Ar and Ne systematics reflect mixing between MORB-like upper mantle and atmospheric fluids, the latter interpreted as reflecting a recycled air component possibly inherited from the Farallon plate subduction. The 3He/4He ratios vary between 7.13 and 7.68 Ra, within the MORB range (7–9 Ra), and the 4He/40Ar* ratios (0.4–3.11) are similarly close to the expected range of the fertile mantle (1–5). Taken together, these pieces of evidence suggest that (i) either the mantle He budget was scarcely modified by the Farallon plate subduction, and/or (ii) that any (large) crustal contribution was masked by a later metasomatism/refertilization episode, possibly during the subsequent Basin and Range extension. A silicate melt-driven metasomatism/refertilization (revealed by the association between glass veins and fluid inclusions) is consistent with calculated helium residence time for the Mexican lithospheric mantle (20 to 60 Ma) that overlaps the timing of the above geodynamic events. We propose that, after the refertilization event (e.g., over the last ~20 Ma), the lithospheric mantle has evolved in a steady-state, becoming slightly more radiogenic. We also estimated 3He fluxes (0.027–0.080 mol/g), 4He production rates (340–1000 mol/yr), and mantle CO2 fluxes (3.93 × 107 mol/yr to 1.18 × 108 mol/yr) using the helium isotopic values measured in JH mantle xenoliths. Finally, the JH xenoliths exhibit CO2/3He ratios comparable to those of the upper mantle (from 3.38 × 108 to 3.82 × 109) but more positive δ13C values (between −1.0 and − 2.7‰), supporting the involvement of a crustal carbonate component. We propose that the metasomatic silicate melts recycled a crustal carbonate component, inherited by the Farallon plate subduction.

Geochemistry and mantle source characteristics of the Itasy volcanic field: Implications for the petrogenesis of basaltic magmas in intra-continental-rifts

We present new major and trace element and Sr, Nd, Pb, Hf, and Os isotope data for Quaternary basaltic lavas and tephra from the Itasy volcanic field, in order to assess the origin and sources of recent volcanism in the central highlands of Madagascar. Our age determinations provide further evidence for Pleistocene volcanism on the island, with ages between ∼31 and 104 ka. Mafic magmas from Itasy exhibit significant geochemical heterogeneity, and they are chemically distinct from those of the neighboring Ankaratra volcanic complex. Trace element signatures of the Itasy volcanic field are very similar to those of ocean island basalts (OIB), with significant enrichment in high field strength elements (e.g. Nb and Ta) and depletion in large ion lithophile elements (e.g. Rb, Cs, and K), and relatively high Nb/U (>45) and Ce/Pb (>25) ratios. Significant intra-volcanic field isotopic heterogeneity is observed, with well-defined negative correlations of 87Sr/86Sr with 206Pb/204Pb and 143Nd/144Nd. These correlations, along with highly radiogenic Os isotopic signatures in many Itasy samples, could be attributed to assimilation of the Precambrian basement. However, with the exception of Os, the elemental and isotopic characteristics of the Itasy magmas are inconsistent with significant crustal assimilation, and instead are interpreted to represent features of their mantle sources. The Itasy ΔεHf and Δ7/4 values fall within the range of MORB and OIB, and indicate derivation primarily from an asthenospheric mantle source. The absence of significantly negative ΔεHf in the Itasy magmas argues against the involvement of ancient recycled oceanic crust or stranded lower crust in their sources, but the presence of a low µ (LOMU) component in some Itasy magmas can be attributed to pollution of the asthenosphere by delaminated subcontinental lithospheric mantle. A simple two-stage, three-component mixing model indicates that a mixture of mantle containing 90% depleted asthenosphere (DMM) and 10% foundered SCLM, which is further mixed with 30 to 50% of a plume-type mantle component, can reproduce the isotopic compositions of the Itasy magmas. Based on our model, it is possible that easily fusible superplume materials might be widely distributed in the upper mantle underneath the EARS, from the Indian Ocean to the Red Sea-Gulf of Aden, and mixed with shallow asthenosphere that has itself incorporated eroded SCLM. These components are heterogeneously distributed at a small spatial scale, similar to that observed recently in other mafic monogenetic volcanic fields in both intraplate and subduction settings.

Noble gas magmatic signature of the Andean Northern Volcanic Zone from fluid inclusions in minerals

Trace volatile elements like He are key for understanding the mantle source signature of magmas and to better constrain the relative roles of subduction and crustal processes to the variability of along-arc chemical and isotopic signatures of magmatic fluids. Here we report on noble gas abundances and isotopic data of Fluid Inclusions (FIs) in eruptive products and/or fumarolic gases from the Colombia-Ecuador segment of Andean Northern Volcanic Zone (NVZ). FIs in olivine phenocrysts from Ecuador (El Reventador, Cotopaxi and Tungurahua) yield air-normalized corrected 3He/4He ratios of 7.0–7.4 RA, within the MORB range (8 ± 1 RA). With exception of the Cotopaxi lavas (opx < <oliv.), these are indistinguishable of those obtained for their cogenetic orthopyroxene pairs and of gas emissions previously reported in literature. Olivine phenocrysts from Nevado del Ruiz fissure lavas also yield the highest 3He/4He (8.5 ± 0.3 RA) for this volcanic system, which is in the range of fumarolic gases for Galeras (previously reported as high as 8.8 RA and here measured to a maximum of 8.3 ± 0.1 RA). Our dataset highlights disparities between isotope signatures of eruptive products from Ecuador (avg. ~7.2 RA) and those reported for the Colombian portion of the NVZ (avg. ~8.5 RA). Previous studies on the geochemistry of erupted products put in evidence significant along-arc variations ascribed either to the involvement of different slab components, or to variable depths of evolution of arc magmas within the continental crust. However, the same variation is not discernible in the signature of noble gases, especially helium, from FIs and gas emissions analyzed in this study, with little inter-variation between Cotopaxi, Reventador and Tungurahua (all within 0.2 RA from the Ecuador average of 7.2) and Galeras and Nevado del Ruiz, whose maximum values differ by ~0.3 RA. We therefore suggest a homogenous MORB-like 3He/4He signature for the mantle wedge beneath this arc segment, whereby along-arc variations in crustal thickness (from <35 km at the northernmost part of the segment to ≥50 km at the Ecuadorian arc segment) may factor largely into the variability recorded on our data set. The first CO2/3He ratios obtained in FIs from Andean rocks support the hypothesis of increasing crustal contamination from Colombia to Ecuador, concomitant with increasing crustal thicknesses under the respective arc regions.

Mineral compositions and thermobarometry of basalts and boninites recovered during IODP Expedition 352 to the Bonin forearc

AbstractCentral 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.

Heterogeneous kimberlite metasomatism revealed from a combined He-Os isotope study of Siberian megacrystalline dunite xenoliths

The Siberian sub-continental lithospheric mantle (SCLM) is regionally heterogeneous due to the complex modification of ancient cratonic material by various metasomatic fluids and interaction with the Siberian plume at ∼250 Ma. Here, we assess the extent and origin of this heterogeneity by analysis of helium isotopes, rhenium-osmium isotopes, trace-element, and highly siderophile element [HSE: Os, Ir, Ru, Pt, Pd, Re] abundances in a suite of mantle-derived megacrystalline dunite xenoliths from the 360 Ma Udachnaya East kimberlite pipe, Siberia. This enables assessment of the style and extent of metasomatism acting to modify the volatile budget of the Siberian SCLM. The olivine megacrysts have 3He/4He values that range from 2.3 to 7.4 RA (where RA = the 3He/4He of air), outside the canonical range for the lithospheric mantle (6.1 ± 2.1 RA; Day et al., 2015). High [He] and low U + Th concentrations in these olivine megacrysts indicate that the Udachnaya megacrystalline dunite xenolith suite has undergone minimal post-eruptive modification of He isotopes by 4He recoil. We instead interpret He isotope variations to reflect pre- or syn-eruptive metasomatic signatures during kimberlite emplacement. The dunites can be divided into two groups, ultimately reflecting modification by two distinct metasomatic styles. Group 1 dunites are characterized by highly unradiogenic Os isotope compositions (γOs360Ma −16 to −13) and ancient melt depletion ages (∼3 Ga) typical of ancient cratonic lithospheric mantle; yet display a range of He isotope ratios (2.3–5.9 RA). This indicates that group 1 dunites have been modified by gas-rich metasomatic fluids acting to change He abundances and isotope ratios, while preserving unradiogenic Os isotope and HSE abundance systematics. Group 2 dunites extend to significantly more radiogenic Os isotope signatures and higher He isotope values (γOs360Ma > −5 to +53; 3He/4He from 3.7 to 7.4 RA). These two groups define a negative hyperbolic array between Os and He isotopes which we interpret to result from metasomatism by variable mixtures between: (1) a gas-rich and HSE-poor fluid (similar to the fluids acting to modify Group 1 dunites) with helium isotope compositions above the MORB range (>8 RA) derived from the asthenosphere, and: (2) strongly radiogenic fluids (∼2 RA; 187Os/188Os > 0.25) likely sourced from within the SCLM. The Udachnaya megacrystalline dunite xenolith suite provides insight into how mantle-plume derived fluids interact with metasomatic fluids within the SCLM.

Geochemistry of Noble Gases and CO2 in Fluid Inclusions From Lithospheric Mantle Beneath Wilcza Góra (Lower Silesia, Southwest Poland)

Knowledge of the products originating from the subcontinental lithospheric mantle (SCLM) is crucial for constraining the geochemical features and evolution of the mantle. This study investigated the chemistry and isotope composition (noble gases and CO2) of fluid inclusions (FI) from selected mantle xenoliths originating from Wilcza Gora (Lower Silesia, southwest Poland), with the aim of integrating their petrography and mineral chemistry. Mantle xenoliths are mostly harzburgites and sometimes bear amphiboles, and are brought to the surface by intraplate alkaline basalts that erupted outside the north-easternmost part of the Eger (Ohře) Rift in Lower Silesia. Olivine (Ol) is classified into two groups based on its forsterite content: (1) Fo88.9–91.5, which accounts for a fertile-to-residual mantle, and (2) Fo85.5–88.1, which indicates large interactions with circulating (basic) melts. This dichotomy is also related to orthopyroxene (Opx) and clinopyroxene (Cpx), which show two ranges of Mg# values (87–90 and 91–93, respectively) and clear evidence of recrystallization. CO2 predominates within FI, followed by N2. The δ13C of mantle CO2 varies between –4.7‰ and –3.1‰, which mostly spans the MORB range (−8‰ < δ13C < −4‰). The 3He/4He ratio is 6.7–6.9 Ra in Cpx, 6.3–6.8 Ra in Opx, and 5.9–6.2 Ra in Ol. These values are within the range proposed for European SCLM (6.3±0.3 Ra). The decrease in 3He/4He from Cpx to Ol is decoupled from the He concentration, and excludes any diffusive fractionation from FI. The chemistry of FI entrapped in Ol indicates that the mantle is depleted by variable extents of partial melting, while that of Opx and Cpx suggests the overprinting of at least one metasomatic event. According to Matusiak-Malek et al. (2017), Cpx, Opx, and amphiboles were added to the original harzburgite by carbonated hydrous silicate melt related to Cenozoic volcanism. This process resulted in entrapment of CO2-rich inclusions whose chemical and isotope composition resembles that of metasomatizing fluids. We argue that FI data reflect a mixing between two endmembers: (1) the residual mantle, resulting from partial melting of European SCLM, and (2) the metasomatic agent, which is strongly He-depleted and characterized by MORB-like features.

The Magmatic Gas Signature of Pacaya Volcano, With Implications for the Volcanic CO2 Flux From Guatemala

AbstractPacaya volcano in Guatemala is one of the most active volcanoes of the Central American Volcanic Arc (CAVA). However, its magmatic gas signature and volatile output have received little attention to date. Here, we present novel volcanic gas information from in‐situ (Multi‐GAS) and remote (UV camera) plume observations in January 2016. We find in‐plume H2O/SO2 and CO2/SO2 ratios of 2‐20 and 0.6‐10.5, and an end‐member magmatic gas signature of 80.5 mol. % H2O, 10.4 mol. % CO2, and 9.0 mol. % SO2. The SO2 flux is evaluated at 885 ± 550 tons/d. This, combined with co‐acquired volcanic plume composition, leads to H2O and CO2 fluxes of 2,230 ± 1,390 and 700 ± 440, and a total volatile flux of ∼3,800 tons/d. We use these results in tandem with previous SO2 flux budgets for Fuego and Santiaguito to estimate the total volcanic CO2 flux from Guatemala at ∼1,160 ± 600 tons/day. This calculation is based upon CO2/total S (St) ratios for Fuego (1.5 ± 0.75) and Santiaguito (1.4 ± 0.75) inferred from a gas (CO2/St ratio) versus trace‐element (Ba/La ratio) CAVA relationship. The H2O‐poor and low CO2/St ratio (∼1.0‐1.5) signature of Pacaya gas suggests dominant mantle‐wedge derivation of the emitted volatiles. This is consistent with 3He/4He ratios in olivine hosted fluid inclusions (FIs), which range between 8.4 and 9.0 Ra (being Ra the atmospheric 3He/4He ratio) at the upper limit of MORB range (8 ± 1 Ra). These values are the highest ever measured in CAVA and among the highest ever recorded in arc volcanoes worldwide, indicating negligible 4He contributions from the crust/slab.

Constraints on the noble gas composition of the deep mantle by bubble-by-bubble analysis of a volcanic glass sample from Iceland

Contamination of samples by air-derived noble gases is a well-known problem in noble gas geochemistry, and, as a result, determining the true neon and argon isotopic ratios of the mantle is not straightforward. Here, we directly target individual bubbles in an Icelandic volcanic glass, DICE 11, with a 193nm excimer laser in order to reduce air-contamination, and analyze He and Ar isotopes, plus 22Ne abundances, on a Helix SFT mass-spectrometer. The CO2 content of the bubbles was measured with a capacitance manometer. In addition, new He, Ne and Ar compositions obtained by crushing on similar samples (DICE 10 and DICE 11) are presented.Our analyses show that 3He/4He ratios are homogeneous in all the vesicles in this glass sample at 17.4±0.4 Ra and are consistent with analyses by crushing. Precise 40Ar/36Ar isotopic ratios were obtained on the largest vesicles only, due to high blank contributions to the smallest vesicles, and are 8600±700, i.e. the highest values measured so far in primitive basalts from Iceland. Considering that the Ar and He isotopic compositions obtained for individual vesicles (by laser ablation) are representative of the true mantle source values, not contaminated by air, we can precisely correct the Ne isotopic analyses obtained by crushing for air contamination; the corrected 20Ne/22Ne ratios are consistent with the presence of neon-B in the sub-Icelandic mantle, which is consistent with the hypothesis that neon in the OIB source region has an irradiated meteorite origin rather than purely solar. In contrast to He and Ar isotopic compositions (which are homogeneous), the relative He–Ne–Ar–CO2 abundances in the different bubbles are heterogeneous and show strong correlations which are compatible with equilibrium degassing of the magma. Because He and CO2 do not fractionate during this degassing, we can precisely constrain the CO2/3He ratio of the mantle source at 6.7±0.5∗108, at the lower end of the MORB range. Therefore the deep mantle below Iceland is not enriched in carbon compared to the upper mantle (via recycling, for example).

Chemical composition of hydrothermal fluids in the central and southern Mariana Trough backarc basin

We present a geochemical data set from hydrothermal fluids collected from the central and southern Mariana Trough during dive programs conducted in the 1990s. The fluid samples were collected from two hydrothermal fields: (1) the Alice Springs field (18°13′N, 144°42′E, water depth: 3600m) on the axial ridge crest of the central Marana Trough; and (2) the Forecast field (13°24′N, 143°55′E, water depth: 1470m) on Peak B seamount, one of a cluster of small volcanic seamounts which represents a unique tectonic setting in the southern Mariana Trough. Gas geochemistry of the Alice Springs fluid showed He and C isotope ratios in the range of MORB, whereas that of the Forecast fluid showed a higher CO2/3He ratio and 13C-enriched isotope ratio, both of which are commonly recognized in hydrothermal fluids from intraoceanic arc volcanoes. The major and minor element composition of the Alice Springs fluid is characterized by enrichment in mobile alkali elements such as K, Rb and Cs. This geochemical signature is in accordance with the well-known characteristics of basaltic lava collected from the same area, which show substantial enrichment in incompatible elements. As interpreted in previous petrological studies, this signature is considered to be an arc geochemical signal reflecting contribution from the subduction component. The B concentration of the Alice Springs fluid is also high and is another arc geochemical signal. The Forecast fluid, by contrast, did not show enrichment in the mobile alkali elements, although its B concentration is high. The Forecast fluid is characterized instead by enrichment in Ca and Sr coupled with depletion in Na and Li. As discussed in previous studies, this signature is attributed to hydrothermal reactions at rather low temperature due to the shallow water depth. We recognize two different arc geochemical signals in these two hydrothermal fluids: mobile alkali elements in the Alice Spring fluid and gaseous species in the Forecast fluid. This difference could reflect different behaviors of these species during magmatic processes in the mantle wedge beneath the Mariana Trough backarc spreading center.

Subduction-related halogens (Cl, Br and I) and H2O in magmatic glasses from Southwest Pacific Backarc Basins

Submarine magmatic glasses from the Manus, Woodlark, North Fiji and Lau backarc basins in the Southwest Pacific, and Volcano A on the volcanic front of the Tonga Arc adjacent to the Lau Basin, were investigated to characterise the Cl, Br and I elemental budgets in subduction systems. In particular we seek to determine the extent of variability in the Br/Cl and I/Cl ratios of backarc basin basalts (BABB) and evaluate if these ratios could improve constraints on the source of subducted volatile components in backarc basins worldwide. The selected glasses represent variably evolved melts of boninite, basalt, basaltic-andesite, dacite and rhyolite composition and were selected from spreading centres and seamounts located at varying distances from the associated arcs. In general the strongest subduction signatures (e.g. Ba/Nb of 100–370) occur in the samples closest to the arcs and lower more MORB-like Ba/Nb of <16 are found in the more distal samples. The glasses investigated have extremely variable halogen concentrations (e.g. 3–4200 ppm Cl), with the highest concentrations in enriched glasses with the most evolved compositions. As observed in previous studies, the K/Cl, Br/Cl and I/Cl ratios of glasses from individual settings do not vary as a function of MgO and are considered representative of the magma sources because these ratios are not easily altered by partial melting or fractional crystallisation. Systematic variations in these ratios between basins can therefore be related to mixing of halogens from different sources including: (i) the mantle wedge which has MORB-like Br/Cl and I/Cl; (ii) a subduction-derived slab fluid with estimated salinity of ∼4–10 wt% salts and variable I/Cl; and (iii) brines characterised by salinities of 55±15 wt% salts and Br/Cl slightly higher than seawater, that are sometimes assimilated in crustal magma chambers. The slab fluids enriching the Woodlark Basin, North Fiji Basin and the Fonualei Spreading Centre of the Lau Basin have MORB-like I/Cl and Br/Cl overlapping the lower end of the MORB range, indicating a probable source from dehydration of altered ocean crust (AOC). In contrast, slab fluids with I/Cl ratios of up to 10 times the MORB value were detected in BABB from Manus Basin, the Valu Fa Ridge and the Tonga Arc, and in these cases the elevated I/Cl ratios are most easily explained by the involvement of fluids released by breakdown of I-rich serpentinites. The data show that slab fluids vary in composition across the Tonga Arc and from north to south in the Lau Basin. However, the compositional range of subducted halogens overlaps with that of MORB indicating subduction could be a major source of halogens in the Earth's mantle.

Geochemical variations in the Central Southern Volcanic Zone, Chile (38–43°S): The role of fluids in generating arc magmas

We present new Sr–Nd–Pb–Hf–O isotope data from the volcanic arc (VA, volcanic front and rear arc) in Chile and the backarc (BA) in Argentina of the Central Southern Volcanic Zone in Chile (CSVZ; 38–43°S). Compared to the Transitional (T) SVZ (34.5–38°S; Jacques et al., 2013), the CSVZ VA has erupted greater volumes over shorter time intervals (Völker et al., 2011) and produced more tholeiitic melts. Although the CSVZ VA monogenetic cones are similar to the TSVZ VA samples, the CSVZ VA stratovolcanoes have higher ratios of highly fluid-mobile to less fluid-mobile trace elements (e.g. U/Th, Pb/Ce, Ba/Nb) and lower more- to less-incompatible fluid-immobile element ratios (e.g. La/Yb, La/Sm, Th/Yb, Nb/Yb), consistent with an overall higher fluid flux and greater degree of flux melting beneath the CSVZ stratovolcanoes compared to the CSVZ monogenetic centers and the TSVZ VA. The CSVZ monogenetic centers overlap the TSVZ in Sr and Nd isotopes, but the stratovolcanoes are shifted to higher Sr and/or Nd isotope ratios. The Pb isotopic composition of the CSVZ overlaps the TSVZ, which is clearly dominated by the composition of the trench sediments, but the CSVZ monogenetic samples extend to less radiogenic Pb isotope ratios. δ18Omelt from the CSVZ stratovolcano samples are below the MORB range, whereas the CSVZ monogenetic and the TSVZ samples fall within and slightly above the MORB range. The Nd and Hf isotopic ratios of the CSVZ VA extend to more radiogenic compositions than found in the TSVZ VA, indicating a greater contribution from a more depleted source. These correlations are interpreted to reflect derivation of fluids from hydrothermally altered oceanic crust and/or serpentinized upper mantle of the subducting plate. CSVZ BA basalts largely overlap TSVZ BA basalts, displaying less or no subduction influence compared to the VA, but some CSVZ BA basalts tap more enriched mantle, possibly subcontinental lithosphere, with distinctively lower Nd and Hf and elevated 207Pb/204Pb and 208Pb/204Pb isotope ratios.

The geochemistry of host arc volcanic rocks to the Co–O epithermal gold deposit, Eastern Mindanao, Philippines

Mindanao is the second largest island of the Philippines and is located in the southern part of the archipelago. It comprises the suture zone between the Eurasian and the Philippine plate, which is displayed in the Philippine Mobile Belt. Eastern Mindanao is part of the Philippine Mobile Belt and outcropping rocks are mainly Eocene to Pliocene in age related to episodes of arc volcanism alternating with sedimentation.New high-precision elemental analysis of the Oligocene magma series, hosting the Co-O epithermal Au deposit, which represents an arc segment in the central part of Eastern Mindanao, revealed dominantly calc-alkaline rocks ranging in composition between basalt and dacites. Major element trends (MgO vs. TiO2 and Fe2O3) are comparable to other magmas in Central and Eastern Mindanao as well as other SW Pacific Islands such as Borneo. Rare earth and trace element distribution patterns display typical island arc signatures highlighted by the conjunction of LILE-enrichment with troughs at Nb, Ta, and Ti. Ratios of Zr/Nb in basalts vary between 17 and 39, signifying a depleted subarc mantle wedge comparable to the range of MORB, and other Indonesian island arc basalts. In basalts, Nb/Ta and Zr/Sm ratios are 12–37 and 14–27 respectively indicative of deep melts of rutile-eclogite subducted slab, as well as fluids, infiltrating the mantle wedge source of basalts.Moderate large ion lithophile element contents and low Th/La and Th/Ce ratios suggest no significant slab-derived components such as sediment or crustal fragments. The comparatively low Ce and Yb values in basalts, but also andesites and dacites, are consistent with a thin arc crust related to an intraoceanic convergent margin setting. This is further supported by Nb contents in basalts that range between 1 and 3ppm and are within the range of modern oceanic convergent margin basalts. The range of HREE fractionation signifies that basaltic melts separated at deeper levels of the subarc wedge, possibly between the forearc and arc axis, followed by a calc-alkaline convergent margin magma suite involving shallower crustal AFC near the central arc sector.The analysed Oligocene arc segment is related to a potentially steep to intermediate dipping subduction zone in an extensional to neutral geotectonic regime. The large subduction accretion complex of the Philippine Mobile Belt provides an ideal setting for significant metal deposits during its entire evolution. This is evidenced in the Eastern Mindanao Ridge, which hosts substantial porphyry Cu and epithermal Au deposits.

Crustal contamination and mantle source characteristics in continental intra-plate volcanic rocks: Pb, Hf and Os isotopes from central European volcanic province basalts

We report new Os–Pb–Hf isotope data for a suite of alkaline to basaltic (nephelinites, basanites, olivine tholeiites to quartz-tholeiites) lavas from the Miocene Vogelsberg (Germany), the largest of the rift-related continental volcanic complexes of the Central European Volcanic Province (CEVP). 187Os/ 188Os in primitive (high-MgO) alkaline lavas show a much wider range than has been observed in alkaline basalts and peridotite xenoliths from elsewhere in the CEVP, from ratios similar to those in modern MORB and OIB (0.1260–0.1451; 58.9–168 ppt Os) to more radiogenic ratios (0.1908 and 0.2197; 27.6–15.1 ppt Os). Radiogenic Os is associated with high ε Hf and ε Nd, low 87Sr/ 86Sr and does not correlate with Mg ∗ or incompatible trace elements (e.g. Ce/Pb), suggesting the presence of a radiogenic endmember in the mantle rather than crustal contamination as the source of radiogenic Os. This contrasts with another high-Mg alkaline lava characterized by highly radiogenic 187Os/ 188Os (0.4344, 10.3 ppt Os), lower ε Hf and ε Nd, higher 87Sr/ 86Sr, and Pb isotope signatures than the other alkaline lavas with similar trace element composition suggestive of contamination with crustal material. Hafnium (ε Hf: +8.9 to +5.0) and Pb isotope compositions ( 206Pb/ 204Pb: 19.10–19.61; 207Pb/ 204Pb: 15.56–15.60) of the alkaline rocks fall within the range of enriched MORB and some OIB. The Vogelsberg tholeiites show even more diverse 187Os/ 188Os, ranging from 0.1487 in Os-rich olivine tholeiite (31.7 ppt) to ratios as high as 0.7526 in other olivine-tholeiites and in quartz-tholeiites with lower Os concentrations (10.3–2.0 ppt). Low- 187Os/ 188Os tholeiites show Pb–Hf isotope ratios ( 206Pb/ 204Pb:18.81; 207Pb/ 204Pb: 15.61; ε Hf: +2.7) that are distinct from those in alkaline lavas with similar 187Os/ 188Os and originate from a different mantle source. By contrast, the combination of radiogenic Os and low 206Pb/ 204Pb and ε Hf in the other tholeiites probably reflects crustal contamination. The association at Vogelsberg of primitive alkaline and tholeiitic lavas with a range of MORB- to OIB-like Os–Pb–Hf–Nd–Sr isotopic characteristics requires at least two asthenospheric magma sources. This is consistent with trace element modelling which suggests that the alkaline and tholeiitic parent magmas represent mixtures of melts from garnet and spinel peridotite sources (both with amphibole), implying an origin of the magmas in the garnet peridotite-spinel peridotite transition zone, probably at the asthenosphere–lithosphere interface. We propose that uncontaminated Vogelsberg lavas originated in ‘metasomatized’ mantle, involving a 3-stage model: (1) early carbonatite metasomatism several 10–100 Ma before the melting event (2) deposition of low-degree asthenospheric melts from carbonated peridotite at the lithosphere–asthenosphere thermal boundary produces hydrous amphibole-bearing veins or patches, and (3) remobilization of this modified lithospheric mantle into other asthenospheric melts passing through the same area later. In keeping with ‘metasomatized’ mantle models for other continental basalt provinces, we envisage that stage (2) is short-lived (few Ma), thus producing a prominent lithospheric trace element signature without changing the asthenospheric isotopic signatures. Models of this type can explain the peculiar mix of lithospheric (prominent depletions of Rb and K) and asthenospheric (OIB-like high 187Os/ 188Os, 143Nd/ 144Nd and 176Hf/ 177Hf) signatures observed in the Vogelsberg and many other continental basalt suites.

Lithium abundance and isotope composition of Logudoro basalts, Sardinia: Origin of light Li signature

Copyright © 2011 by The Geochemical Society of Japan. al., 2008). However, altered MORB becomes usually heavier in their Li isotopic ratios (δ7Li < +20.8‰) by seawater-alteration at low temperature. The δ7Li values of seafloor sediments are variable, ranging from 0 to +7‰ (Chan et al., 2002; Bouman et al., 2004). The Li isotopic composition might therefore be a useful tracer to investigate recycling of surface derived material into mantle if the isotopic signatures of subducted surface material are preserved during subduction and its storage in mantle. In spite of the variation of Li input in subduction, Li isotopic ratios of most OIB are constant and similar to those of MORB and normal mantle. Lithium isotopic compositions of Mauna Loa and Mauna Kea basalts from Hawaii range from +2.5 to +5.7‰ (Chan and Frey, 2003), thereby overlapping the range of MORB. The HIMU (Mangaia, Tubuai, Raivavae, and Rurutu) are characterized by Li isotopic ratios ranging from +5.0 to +7.9‰, and are slightly heavier than those of MORB (Nishio et al., 2005; Chan et al., 2009; Vlastelic et al., 2009). The Li isotopic ratios of Rapa lava, EM2-type, range from +3.7 to +5.9‰ (Chan et al., 2009). The few data available for EM1 lava samples, e.g., Pitcairn Island, (+3.3 to +4.1‰, Chan et al., 2006) display MORB-like values. Some basalts or xenoliths from continental settings also show EM1-like isotopic signatures. However, little data of Li isotopic composition have been reported on them. Lithium abundance and isotope composition of Logudoro basalts, Sardinia: Origin of light Li signature

Plume–ridge interaction along the Galapagos Spreading Center: discerning between gas loss and source effects using neon isotopic compositions and 4He– 40Ar ∗–CO 2 relative abundances

To document the plume–ridge interaction at the Galapagos Spreading Center (GSC), we present neon 3-isotope analyses performed on vesicles-trapped volatiles of MORB glasses dredged along the ridge axis between 86°W and 98°W. 4He– 40Ar ∗ –CO 2 relative abundances were also measured in order to study gas loss in this context and discern between source and degassing effects. Neon isotopic compositions are in the MORB range with 21Ne/ 22Ne ratios extrapolated to the 20Ne/ 22Ne mantle ratio of 12.5 varying between 0.053 and 0.072. Unradiogenic plume-like compositions were not measured. The 4He– 40Ar ∗ –CO 2 relative abundances are highly variable along the ridge, for example 4He/ 40Ar ∗ ratio varies between 3 and 433, but these variations can be fully explained by a simple model of Rayleigh distillation, with a single volatile source composition for the entire GSC. Magma fractional crystallisation, which increases in the plume influenced zone, seems to be the main motor for degassing. As other geochemical and geophysical studies indicate a significant plume influence on the GSC, these results suggest the plume component feeding the ridge is either degassed or else different from the plume core.

Mantle melting and magma supply to the Southeast Indian Ridge: The roles of lithology and melting conditions from U-series disequilibria

Intermediate-spreading Southeast Indian Ridge (SEIR) basalts display geographic gradients from 90°E to 118°E in Th/U, ( 230Th/ 232Th) and 238U– 230Th– 226Ra disequilibria ( 230Th-excesses of 1 to 21% and 226Ra-excesses of 0 to 160%). Highly correlated ( 238U/ 232Th) and ( 230Th/ 232Th) ( r 2 = 0.94) span nearly the entire global MORB range; basalts from three of four ridge morphologies form subparallel, vertically stacked arrays on an equiline diagram. ( 226Ra/ 230Th) is inversely correlated with ( 230Th/ 238U) within sub-regions of the study area. There is a convex ( 230Th/ 238U) along-axis pattern (lower 230Th-excesses in the west and east, at shallowest and deepest depths), such that ( 230Th/ 238U) does not correlate with axial depth as described elsewhere, despite the ~ 2300 m west-to-east axial depth increase. κ and κ Pb also vary along axis from κ < κ Pb in the west to κ ≅ κ Pb in the east. Variations in axial depth, crustal thickness, U-series disequilibria, κ and κ Pb are best explained by forward model scenarios in which the physical characteristics of melting (such as melting rate, degree of melting, porosity and melt initiation depth) vary along axis in response to an inferred long wavelength temperature gradient and small variations in the underlying source lithology (pyroxenite veins). This leads to decreased melt supply from west to east, a concomitant change in axial morphology and axial magma chamber depth, and systematic long wavelength geochemical gradients. Melting rate covaries with melt supply in the western and central regions, decreasing eastward along the SEIR, but supply and rate vary inversely from the central to the eastern regions, because cooler eastern mantle contains a few percent of fusible pyroxenite veins that melt productively to generate melts with high Th–U–Ra concentration but low 230Th-excess and 226Ra-excess. The veins contribute only a small proportion of the total melt volume from mantle that is > 95% peridotite. Th, Pb and He isotopic gradients are consistent with a changing proportion of enriched pyroxenite veins along axis having a roughly 0.5 to 1 Gyr reservoir age.

Controls on magmatism in an island arc environment: study of lavas and sub-arc xenoliths from the Tabar–Lihir–Tanga–Feni island chain, Papua New Guinea

The Tabar–Lihir–Tanga–Feni (TLTF) islands of Papua New Guinea mainly comprise high-K calc-alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction-related magmatism. Numerous sedimentary, mafic, and ultramafic xenoliths recovered from Tubaf seamount, located on the flank of Lihir Island, provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region. The sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with Pacific affinity. A majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the TLTF arc. Leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50% or more of elements such as Cu, Zn, Rb, U, Pb, and light REE (rare-earth elements), most likely introduced in the xenoliths via hydrous fluids released from a subducted slab. The leaching experiments demonstrated that the light REE enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit REE patterns that range from flat to light REE depleted. Sr–Nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87Sr/86Sr with decreasing 143Nd/144Nd ratios. The labile phases in the ultramafic xenoliths, represented by the leachates, show decoupling between Sr and Nd with distinctly more radiogenic 87Sr/86Sr than the residues. Both leachates and residues exhibit very wide range in their Pb isotopic compositions, indicating the involvement of three components in the mantle wedge under the TLTF islands. Two of the components can be identified as Pacific Oceanic mantle and Pacific sediments. Some of the ultramafic samples and clinopyroxene separates, however, exhibit relatively low 206Pb/204Pb at elevated 207Pb/204Pb suggesting that the third component is either Indian Ocean-type mantle or Australian subcontinental lithospheric mantle. Geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized, it did not significantly contribute to the isotopic and incompatible trace element compositions of TLTF lavas. Compared to the mantle samples, the TLTF lavas have very restricted Pb isotopic compositions that lie within the Pacific MORB range, indicating that magma compositions were dominated by melts released from a stalled subducted slab with Pacific MORB affinity. Interaction of slab melts with depleted peridotitic component in the mantle wedge, followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area.