Richard J. Arculus

Chemical characteristics of island-arc basalts: Implications for mantle sources

Abstract Major-element, trace-element and isotopic compositions of approximately 1200 basalts ( Mg ( Mg +Fe 2+ ) > 65 ] IAB and MORB are similar, but differ significantly from IPB. In general, IAB do not have higher Al2O3, lower TiO2 or a lack of Fe enrichment compared to primitive MORB but many do have greater K2O contents. Differences in major- and minor-element contents between more evolved IAB and MORB result from the dominance of plagioclase + olivine crystal fractionation in MORB magmas vs. clinopyroxene + olivine controlled fractionation in IAB suites. This difference in crystallization history may be related to the higher PH2O or greater depth of crystallization of IAB magmas compared to those inferred for MORB. IAB are characteristically enriched in large-ion-lithophile (LIL) elements and depleted in high-field-strength ions (e.g., Zr, Nb and Hf) relative to normal MORB (N-type) and IPB. The enrichment of some LIL elements (e.g., Sr, Rb, Ba and Pb) relative to the rare-earth elements in IAB is difficult to explain by simple partial melting alone and suggests a multistage petrogenesis involving an LIL-enriched component. Low abundances of high-field-strength ions in evolved IAB are explicable in terms of fractional crystallization, but the cause for consistently low abundances in primitive IAB remains problematic. Island-arc lavas contain greater concentrations of volatiles and have higher CO 2 H 2 O and Cl/F ratios than either MORB or IPB, suggesting involvement of a slab-derived volatile component. However, this is not consistent with 3 He 4 He data which indicate that only near-trench volcanics have been significantly affected by dehydration of the oceanic crust. Sr-, Nd-, Pb- and O-isotopic data, in conjunction with the trace-element data, clearly indicate that IAB are derived from heterogeneous, LIL-depleted mantle sources most similar to those which give rise to enriched MORB (E-type). The marked shift towards higher 87 Sr 86 Sr in IAB compared to oceanic lavas with similar 143 Nd 144 Nd values cannot be explained simply by the addition of radiogenic Sr from the slab. Variable degrees of contamination from a crustally-derived sedimentary component is consistent with the isotopic and trace-element data from a number of arcs. However, the lack of correlation between LIL/REE ratios and more radiogenic isotopic ratios suggests that this enrichment/contamination process is complex. A multi-stage petrogenetic model involving subducted oceanic crust (± sediments), dehydration/volatile transfer, and partial melting of metasomatized mantle beneath island arcs is considered the most reasonable, although least constrained, method to generate a variety of primitive IAB.

The redox state of subduction zones: insights from arc-peridotites

Spinel peridotites from a variety of island arcs have been utilised to calculate the redox state of the mantle wedge above subduction zones. Oxygen fugacities (fO2 values) calculated from the ferric iron content of spinels, measured by Electron Microprobe (EMP) using secondary standards [Wood, B.J., Virgo, D., 1989. Upper mantle oxidation state: ferric iron contents of lherzolite spinels by 57Fe Mossbauer spectroscopy and resultant oxygen fugacities. Geochim. Cosmochim. Acta, 53, 1277–1291.], yield values which range from 0.3 to 2.0 above the fayalite–magnetite–quartz (FMQ) buffer. These data provide further evidence that the mantle wedge is ubiquitously oxidised relative to oceanic and ancient cratonic mantle. There is no correlation between fO2 values and the presence of hydrous phases and, in fact, the most oxidised samples contain no hydrous phases. Within individual suites there is no correlation between fO2 and degree of depletion as indicated by spinel Cr#, except for a suite of reacted forearc-peridotites. However, when the data is viewed as a whole there is broad a positive correlation between fO2 and spinel Cr# suggesting that partial melting processes may influence the redox state of the mantle wedge. We suggest that the ultimate source of the oxygen which oxidises the mantle wedge is from the subducted slab. It is not clear whether this oxidising agent is a solute-rich hydrous fluid or a water-bearing silicate melt. However, our data does indicate that silicate melts are effective oxidisers of the depleted shallow upper mantle. Simple mass balance calculations based on the ferric iron content of primitive subduction zone magmas indicates that the source region must contain 0.6–1.0 wt.% Fe2O3. This amount of Fe2O3 in a fertile spinel peridotite yields an oxygen fugacity of 0.5–1.7 log units above FMQ in the IAB source. If water is the sole oxidising agent in the mantle wedge then 0.030–0.075 wt.% H2O is required which is considerably less than the 0.25% H2O envisaged by Stolper and Newman [Stolper, E.M., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett., 121, 293–325.], suggesting water is not necessarily an efficient oxidising agent. Alternatively, ferric iron may be added to the mantle wedge by addition of a ferric iron-rich sediment melt or more likely as a solute-rich hydrous fluid. This model would produce spinel, orthopyroxene or amphibole in the wedge with only a slight increase in fO2 of the source region. Although it is unclear which model is correct the maximum fO2 of the fertile mantle wedge is unlikely to be above FMQ+2 and therefore some decompression melting in the mantle wedge is required to explain the higher fO2 values of primitive arc lavas than arc-peridotites.

Aspects of magma genesis in arcs

Geochemical analysis of magmas developed above subduction zones in combination with chemical, phase equilibria and physical modeling of petrogenetic processes have shown that the mantle wedge overlying the subducted lithosphere is volumetrically the most significant source of the majority of island arc magmas. Locally and at distinct episodes in the history of some arcs, direct melts of the subducted lithosphere may be erupted essentially unmodified or become involved as components of wedge-derived magmas. A flux of hydrous fluid from the subducted lithosphere is the trigger for partial melting of the overlying wedge, a source of oxidation of the magmas produced, and a transporting agent for some trace elements that are typically enriched relative to elements of equal crystal-melt incompatibity but non-hydrophilic character, in arc magmas compared with ocean ridge settings. For some hydrophilic trace elements (e.g., B, Be), the subducted lithosphere is volumetrically the largest supplier in arc magmas. In addition however, leaching of some of these elements (e.g., Sr) from the wedge by fluid infiltration prior to melting significantly augments the eventual magmatic abundances. Wedge compositional variability can be demonstrated on a global basis. A number of processes have probably led to this variability including concurrent and/or past magma extraction in backarcs, localised past input of hot-spot/ocean island magmas, global variability of sediment input, and incorporation of ancient subcontinental mantle domains. In the case of the Bonin arc, a remarkably stable environment of magma generation has perisisted for the past ∼45 million years since arc inception, with no evidence of secular increases in alkalinity. Similarly, a consistent trend of increasing alkalinity with evolution of the Mariana arc is absent. In both examples, extension of the lithosphere concurrent with magmatism may have maintained vertically extensive melting domains between arc lithosphere and the subducted slab. There is no convincing support for production of greater arc crustal thicknesses than backarc crust for either the Bonins or Marianas, indicative of partial melting percentages in arcs⩽backarcs. For most estimates of the percentages of melting beneath arcs, mass balance of Fe2O3/FeO in the mantle wedge requires addition of oxygen (via H2O). Although the melting relations of amphibole are critical in arc magma genesis, it is likely that this phase does not persist to the mantle wedge domains where the majority of high-Mg island arc basalts with eruption temperatures ⩾ 1200°C are formed.

Release of gold-bearing fluids in convergent margin magmas prompted by magnetite crystallization

A relationship between convergent margin magmas and copper–gold ore mineralization has long been recognized. The nature of the genetic link is controversial, particularly whether the link is due to high-oxygen-fugacity (fO2) melts and fluids released from subducted slabs or to brine exsolution during magmatic evolution. For submarine, subduction-related volcanic glasses from the eastern Manus basin, Papua New Guinea, we here report abrupt decreases in gold and copper abundances, coupled with a switch in the behaviour of titanium and iron from concentration increases to decreases as SiO2 rises. We propose that the abrupt depletion in gold and copper results from concurrent sulphur reduction as a result of fO2 buffering, causing enhanced formation of copper–gold hydrosulphide complexes that become scavenged from crystallizing melts into cogenetic magmatic aqueous fluids. This process is particularly efficient in oxidized arc magmas with substantial sulphate. We infer that subsequent migration and cooling of exsolved aqueous fluids create links between copper–gold mineralization and arc magmatism in the Manus basin, and at convergent margins in general.

Resolution of the effects of crustal assimilation, sediment subduction, and fluid transport in island arc magmas: Pb Sr Nd O isotope geochemistry of Grenada, Lesser Antilles

Abstract New lead, strontium, neodymium, and oxygen isotope and chemical data are reported for forty-three samples from the picrite to low-CaO andesite differentiation series (the M-series) of Grenada, Lesser Antilles island arc. Revised isotopic data for the low-Mg, high-Ca C-series basalts show tight correlations with MgO that confirm published interpretations of C-series evolution by coupled assimilation-fractional crystallisation (AFC). δ18O determined by laser fluorination on phenocryst augite increases from mantle values (+5.6) to the highest values observed in Grenada (+6.2) over the small observed range in 143Nd/144Nd, consistent with AFC. The most magnesian C-series basalts can be generated by about 16% olivine fractionation from picrites similar in major elements to M-series picrites, but with greater large ion lithophile element (LILE) enrichment and more MORB-like isotopic ratios. The strong enrichment in LILE relative to rare earth elements (REE), and in REEs relative to Nb and Zr, implies fluid transport of these elements from subducted altered ocean crust. This is consistent with constant 87Sr/86Sr around 0.7045, but lower 143Nd/144Nd and higher Δ207Pband Th/U than MORB require a small (ca. 0.2%) additional subducted sediment contribution. Some LILE (Pb, Rb) are not strongly enriched and Ce/Pb ratios are close to those of MORB. This may be due to release of a substantial part of the slab Pb at shallower depth than the zone of magma generation. Grenada high-silica andesites (>58% SiO2) lie on extensions of C-series AFC correlations, reflecting assimilation with amphibole-dominated fractionation from C-series parents. AFC does not lead to strongly radiogenic Pb in the andesites despite an inferred contaminant With 206Pb/204Pb > 20. This is because Pb is much more incompatible than Nd due to apatite and hornblende fractionation, resulting in progressively slower 206Pb/204Pb increase as magmatic Pb contents rise. M-series picrites and basic andesites display evidence for three-component mixing on Sr Nd isotope, Nd Pb isotope, and Pb/Nd ratio— 143Nd/144Nd diagrams. One endmember has isotopic compositions similar to the uncontaminated C-series lavas, although it has less LILE enrichment from subduction fluid. Two groups of M-series picrites are recognized with lower and higher La/Y; these have respectively higher (ca. 0.51290) and lower (ca. 0.51280) 143Nd/144Nd, but similar ranges in 87Sr/86Srand206Pb/204Pb. The sample with the most extreme low 143Nd/144Nd (0.51256) has the lowest 206Pb/204Pb in the M-series: all aspects of the chemistry and isotope systematics of this sample are consistent with generation from a mantle source with a relatively high contribution (ca. 2%) from subducted local seafloor sediment (206Pb/204Pb ≈ 19.2). A progressively greater contribution from this sediment can successfully explain the neodymium isotopic shift from the C-series through the low-La/Y M-series picrites to the high-La/Y M-series picrites. The extent of fluid modification of the mantle source, as monitored by LILE enrichment relative to LREE, broadly decreases as the subducted sediment component increases. Basic andesites of the M-series also display low- and high-La/Y groups, and can be generated from the picrites by amphibole-dominated fractional crystallisation. The highest 206Pb/204Pb,87Sr/86Sr, and Pb/Nd in Grenada are found in low-La/Y basic andesites and suggest that fractionation of low-La/Y picrite was accompanied by some 6% high-level assimilation of crust with high Sr/Ndand206Pb/204Pb > 20. This hypothesis is strongly supported by the presence of 18O-enriched quartz xenocrysts of crustal origin in two of the three basic andesites. Smaller amounts (2–5%) of the same crustal component can satisfactorily model the variation of Pb/Nd,206Pb/204Pb,and87Sr/86Sr at constant 143Nd/144Nd in the picrites, an interpretation supported by correlations between isotope ratios and MgO in the high-La/Y picrites. AFC relationships in the C-series can be modelled using a similar crustal assimilant, but it must have much lower Pb/Nd ratios. The C-series assimilant cannot be M-series magma products, since M-series Pb/Nd ratios are too high, and M-series magmas themselves only achieve high 206Pb/204Pb through crustal assimilation. Lead isotope compositions of Grenada magmas, even picrites, are largely controlled by high-level crustal assimilation, mostly through AFC processes. Since these magmas include some of the most magnesian lavas found in arcs worldwide, we recommend caution in interpreting the lead isotope chemistry of arc magmas elsewhere to be the result of mantle and subduction processes.

Nd and Sr isotope geochemistry of island arc volcanics, Grenada, Lesser Antilles

87Sr/86Sr and 143Nd/144Nd ratios, REE, K2O, Rb and Sr are presented for a variety of basanitoids, alkalic and subalkalic basalts, and calc-alkalic rocks from the island of Grenada, Lesser Antilles. The 87Sr/86Sr ratios vary from 0.7039 to 0.7058 whereas, with one exception, the 143Nd/144Nd ratios cover a small range from 0.51282 to 0.51308. There is a reasonable correlation between Sm/Nd and 143Nd/144Nd, but not between Rb/Sr and 87Sr/86Sr. The ankaramites tend to have higher K2O and Sr contents, but lower 87Sr/86Sr ratios than the microphyric basalts. A broad negative correlation between Nd and Sr isotopes is observed but it is both displaced to relatively higher 87Sr/86Sr ratios and has a flatter slope than most of the available results on mid-ocean ridge and ocean island basalts. The high 87Sr/86Sr ratios are thought to reflect a contribution from seawater affected material in the subducted ocean crust and yet they are observed in SiO2-undersaturated rocks which are most unlikely to have been derived by melting of subducted ocean crust. This is reconciled in a model whereby the alkalic elements are preferentially released during dehydration of the subducted lithosphere and contaminate the overlying mantle source region of the arc magmas.

Geochemical window into subduction and accretion processes : Raspas metamorphic complex, Ecuador

The high-pressure, low-temperature ( P = 1.3–2 GPa; T is ≤ 600 °C) Raspas metamorphic complex is an exhumed fragment of the partially accreted, partially subducted Amotape-Chaucha terrane in southwest Ecuador. Comparative analysis of major and trace elements plus Sr, Nd, and Pb isotopes in bulk lithologies and individual crystalline phases shows that the complex includes one to three layers of ordinary oceanic crust and underlying mantle lithosphere together with oceanic plateau fragments. Subduction (and exhumation) of oceanic lithosphere resulted in selective bulk trace element geochemical changes: Rb, Ba, and Sr have been lost (in amounts from approximately 85%–50%) from the high- P , low- T metamorphosed pelites and basalts, whereas Pb is enriched in mafic rocks. During formation of the eclogite, U, Pb, and rare earth elements (REEs) were immobile. High- P , low- T metamorphosed terranes form the basement of active Ecuadorian arc volcanoes; partial melting of this basement by mantle-wedge–derived basalt is a likely source of adakitic components.

Enhanced mantle-to-crust rhenium transfer in undegassed arc magmas.

Variations in the 187Os/188Os isotopic signature of mantle and mantle-derived rocks have been thought to provide a powerful chemical tracer of deep Earth structure. Many studies have inferred from such data that a long-lived, high-rhenium component exists in the deep mantle (187Re is the parent isotope decaying to 187Os, with a half-life of ∼42 billion years), and that this reservoir probably consists of subducted oceanic crust. The interpretation of these isotopic signatures is, however, dependent on accurate estimates of rhenium and osmium concentrations in all of the main geochemical reservoirs, and the crust has generally been considered to be a minor contributor to such global budgets. In contrast, we here present observations of high rhenium concentrations and low Yb/Re ratios in arc-type melt inclusions. These results indicate strong enrichment of rhenium in undegassed arc rocks, and consequently the continental crust, which results in a crustal estimate of ∼2 p.p.b. rhenium, as compared to previous estimates of 0.4–0.2 p.p.b. (refs 4, 5). Previous determinations of rhenium in arc materials, which were largely measured on subaerially erupted samples, are likely to be in error owing to rhenium loss during degassing. High mantle-to-crust rhenium fluxes, as observed here, require a revaluation of geochemical models based on the 187Re-187Os decay system.

Intrinsic oxygen fugacity measurements: techniques and results for spinels from upper mantle peridotites and megacryst assemblages

A new technique for the determination of intrinsic oxygen fugacities (ƒO2s) of single and polyphase geological samples with solid ZrO2, oxygen-specific electrolytes is described. Essentially the procedure involves isolating the emf signal from the sample from that unavoidably imposed by the residual atmosphere inside the sample-bearing sensor. By varying the ƒO2 of the residual atmosphere, it is possible to determine a ‘plateau’ value of constant ƒO2 recorded from the sensor which represents a reversed intrinsic ƒO2 measurement for the sample alone, and where the extent of the plateau reflects the innate buffering capability of the sample. A measure of the precision and accuracy of the data obtained is the fact that identical ƒO2 values are obtained whether on a heating or cooling cycle of the sample + compatible atmosphere system. These techniques have been applied to measurements of the intrinsic ƒO2 of spinels from peridotites and megacryst assemblages from Australia, West Germany and the U.S.A. Oxidation states range from ∼- 0.25 log10 units more oxidized to 1 log10 unit more reduced than the iron-wustite (IW) buffer. The overall reduced nature of the spinels and the range of ƒO2s obtained are striking features of the data. One implication of the results is that the majority of mantle-derived magmas are initially highly reduced, and the relatively oxidized values observed at surface (∼- 4–5 log10 orders more oxidized than IW) reflect late-stage alteration, perhaps by H2 loss (Sato, 1978).

Rhenium systematics in submarine MORB and back-arc basin glasses: laser ablation ICP-MS results

Abstract Rhenium and other trace elements, including the moderately chalcophile elements Mo and Cu, were determined for 37 submarine basaltic glasses from the Lau and Coriolis Troughs (CT) back-arc basins and Woodlark marginal basin, as well as 30 mid-ocean ridge basalt (MORB) glasses from the Pacific and Atlantic Oceans, using laser ablation ICP-MS. Rhenium is strongly positively correlated with Yb for all these submarine basaltic glasses. Enriched (E-) and normal (N-) MORB as well as Kings Triple Junction samples show similar correlations with constant Yb/Re ratios, indicating that Re and Yb exhibit similar compatibility during melt evolution [Chem. Geol. 139 (1997) 185]. In contrast, samples from the East and Central Lau Spreading Centers have much higher ratios compared with MORB samples and form steeper arrays on Re–Yb variation diagrams, similar to komatiites. More incompatible element-depleted samples including those from the Lau and Woodlark Basin spreading centers and the more depleted (D-) MORB samples are also distinguished from E- and N-MORB and samples from Kings Triple Junction and Coriolis Troughs Basin on the basis of their higher Cu/Re ratios. These observed elemental systematics are interpreted to reflect progressive melting of depleted mantle, where previous melting events result in the elimination of sulfides in the source regions of the depleted samples. Using the determined Yb/Re and Ce/Mo ratios and assuming that the abundances of Yb and Ce are 10% and 40% reduced in the DMM compared to the primitive mantle (PM), average concentrations of 0.12 ppb for Re and 34 ppb for Mo are estimated for the DMM. The partition coefficients of the analysed moderately incompatible elements are in the order of Mo