Malcolm T. McCulloch

Geochemical and geodynamical constraints on subduction zone magmatism

Abstract The geochemical and geodynamical parameters that may influence the composition of island-arc basalts (IABs) are evaluated. Systematic correlations amongst high-field strength (HFS) elemental ratios (Zr/Nb, Sm/Nb and TiO2/Zr) relative to Nb abundances, indicate that HFS element systematics are not controlled by the presence of residual Nb-bearing phases in the slab. This provides confirmation of models whereby high-field strength (HFS) and HREE elements remain immobile during slab-fluxing processes and are thus derived from the mantle wedge without additional enrichments from the slab. In contrast enrichment of large-ion-lithophile elements (LIL) such as Rb, Cs, Ba, Sr, Pb, U and LREE (i.e., La, Ce) in IABs is consistent with slab involvement, with their relative enrichment, being due to a combination of both their high rock-melt incompatibility and slab-“fluid” mobility. As a consequence, the low abundances of HFS elements such as Nb, Ti, Zr, and Hf in IABs reflect a depleted (relative to MORB source) mantle wedge overlying the subduction slab. Depletion of the arc mantle wedge in HFS elements is attributed to previous melting events in the mantle wedge, and to geodynamic conditions associated with the formation and evolution of coupled island arcs and back-arc basins. These processes ensure a budgetary deficit in the HFS elements relative to those elements derived from the subducted slab (predominantly LILE and LREE). Thus, although in MORBs and OIBs, Nb has a similar incompatibility to U, in subduction zones the main factor controlling its abundance is its highly immobile character, particularly relative to elements like U which are mobile during prograde dehydration reactions in the slab. Based on these observations, a quantitative model has been developed for IAB petrogenesis with the transfer of trace elements from the slab to the mantle wedge being modelled with empirical slab-“fluid” partition coefficients whilst the mantle-wedge to arc-crust transfer is constrained by melt-solid partitioning. The empirically derived slab-“fluid” partition coefficients indicate that the enrichment factors characteristic of slab fluxing processes have a distinctive pattern particularly for the elements Nb, U, Th, and Sr.

Precise determination of SmNd ratios, Sm and Nd isotopic abundances in standard solutions☆

The methods used for precise calibrations of Sm/Nd ratios and the average isotopic abundances obtained for normal Sm and Nd are given. A mixed Sm-Nd normal solution with a precisely known ^(147)Sm/^(144)Nd ratio close to the nominal average chondritic value is described and the calibration discussed. Aliquots of this standard solution are available on request and may be useful for precise interlaboratory calibration of Sm and Nd.

Geochemical and NdSr isotopic composition of deep-sea turbidites: Crustal evolution and plate tectonic associations

Abstract Petrographic, geochemical, and isotopic data for turbidites from a variety of tectonic settings exhibit considerable variability that is related to tectonic association. Passive margin turbidites (Trailing Edge, Continental Collision) display high framework quartz (Q) content in sands, evolved major element compositions (high Si Al , K Na ), incompatible element enrichments (high Th Sc , La Sc , La Yb ), negative Eu-anomalies and variable Th U ratios. They have low 143 Nd 144 Nd and high 87 Sr 86 Sr (ϵNd = −26 to −10; 87 Sr 86 Sr = 0.709 to 0.734 ), indicating a dominance of old upper crustal sources. Active margin settings (Fore Arc, Continental Arc, Back Arc, Strike Slip) commonly exhibit quite different compositions. Th Sc varies from Eu Eu ∗ = 1.0 ) to Eu-depletions typical of post-Archean shales ( Eu Eu ∗ = 0.65 ). Active margin data are explained by mixtures of young arc-derived material, with variable composition and old upper crustal sources. Major element data indicate that passive margin turbidites have experienced more severe weathering histories than those from active settings. Most trace elements are enriched in muds relative to associated sands because of dilution effects from quartz and calcite and concentration of trace elements in clays. Exceptions include Zr, Hf (heavy mineral influence) and Tl (enriched in feldspar) which display enrichments in sands. Active margin sands commonly exhibit higher Eu Eu ∗ than associated muds, resulting from concentration of plagioclase during sorting. Some associated sands and muds, especially from active settings, have systematic differences in Th Sc ratios and Nd-isotopic composition, indicating that various provenance components may separate into different grain-size fractions during sedimentary sorting processes. Trace element abundances of modern turbidites, from both active and passive settings, differ from Archean turbidites in several important ways. Modern turbidites have less uniformity, for example, in Th Sc ratios. On average, modern turbidites have greater depletions in Eu (lower Eu Eu ∗ ) than do Archean turbidites, suggesting that the processes of intracrustal differentiation (involving plagioclase fractionation) are of greater importance for crustal evolution at modern continental margins than they were during the Archean. Modern turbidites do not display HREE depletion, a feature commonly seen in Archean data. HREE depletion ( Gd N Yb N > 2.0 ) in Archean sediments results from incorporation of felsic igneous rocks that were in equilibrium (or their sources were in equilibrium) with garnet sometime in their history. Absence of HREE depletion at modern continental margins suggests that processes of crust formation (or mantle source compositions) may have differed. Differences in trace element abundances for Archean and modern turbidites add support to suggestions that upper continental crust compositions and major processes responsible for continental crust differentiation differed during the Archean. Neodymium model ages, thought to approximate average provenance age, are highly variable (TDMND = 0–2.6 Ga) in modern turbidites, in contrast with studies that indicate Nd-model ages of lithified Phanerozoic sediment are fairly constant at about 1.5–2.0 Ga. This variability indicates that continental margin sediments incorporate new mantle-derived components, as well as continental crust of widely varying age, during recycling. The apparent dearth of ancient sediments with Nd-model age similar to stratigraphic age supports the suggestion that preservation potential of sediments is related to tectonic setting. Many samples from active settings have isotopic compositions similar to or only slightly evolved from mantle-derived igneous rocks. Subduction of active margin turbidites should be considered in models of crust-mantle recycling. For short-term recycling, such as that postulated for island arc petrogenesis, arc-derived turbidites cannot be easily recognized as a source component because of the lack of time available for isotopic evolution. If turbidites were incorporated into the sources of ocean island volcanics, the isotopic signatures would be considerably more evolved since most models call for long mantle storage times (1.0–2.0 Ga), prior to incorporation. Four provenance components are recognized on the basis of geochemistry and Nd-isotopic composition: 1. (1) Old Upper Continental Crust (old igneous/metamorphic terranes, recycled sediment); 2. (2) Young Undifferentiated Arc (young volcanic/plutonic source that has not experienced plagioclase fractionation); 3. (3) Young Differentiated Arc (young volcanic/plutonic source that has experienced plagioclase fractionation); 4. (4) MORB (minor). Relative proportions of these components are influenced by the plate tectonic association of the provenance and are typically (but not necessarily) reflected in the depositional basin. Provenance of quartzose (mainly passive settings) and non-quartzose (mainly active settings) turbidites can be characterized by bulk composition (e.g., Th Sc ) and Nd-isotopic composition (reflecting age).

A simple method for the precise determination of ≥ 40 trace elements in geological samples by ICPMS using enriched isotope internal standardisation

The combination of enriched isotopes and conventional elemental internal standards permits the precise determination of > 40 trace elements by ICPMS in a broad spectrum of geological matrixes. Enriched isotopes expand the suite of available reference isotopes spaced through the mass spectrum, so that the complex mass-dependent variations in sensitivity encountered during ICPMS analysis can be monitored and deconvolved. The method we have developed is straightforward, entailing simple sample preparation, instrument calibration, and data reduction procedures, as well as providing extended element coverage, improved precision, and both time and cost benefits compared to alternative analytical strategies. Analytical precision near or better than 1% RSD (relative standard deviation) is achieved for most elements with mass > 80 amu and between 1% and 4% (RSD) for elements with mass 80 amu and < 10 ng g−1 to 1 μg g−1 for elements with mass < 80 amu). The subtle geochemical differences which can be resolved using this method are demonstrated by analyses of Nb, Ta, Zr, and Hf in magmas from ocean islands and subduction zones. These data reveal significant departures from chondritic Zr/Hf and Nb/Ta values, and systematic trends which are consistent with greater incompatibility of Zr relative to Hf and also of Nb relative to Ta during melting of the upper mantle. The occurrence of significantly subchondritic Zr/Hf and Nb/Ta ratios in Nb-poor subduction zone magmas, supports the notion that the depletion of high-field strength elements in subduction magmas is due to their removal from the mantle wedge by prior melting events.

Reconciliation of late Quaternary sea levels derived from coral terraces at Huon Peninsula with deep sea oxygen isotope records

A major discrepancy between the Late Quaternary sea level changes derived from raised coral reef terraces at the Huon Peninsula in Papua New Guinea and from oxygen isotopes in deep sea cores is resolved. The two methods agree closely from 120 ka to 80 ka and from 20 ka to 0 ka (ka = 1000 yr before present), but between 70 and 30 ka the isotopic sea levels are 20–40 m lower than the Huon Peninsula sea levels derived in earlier studies. New, high precision U-series age measurements and revised stratigraphic data for Huon Peninsula terraces aged between 30 and 70 ka now give similar sea levels to those based on deep sea oxygen isotope data planktonic and benthic δ18O data. Using the sea level and deep sea isotopic data, oxygen isotope ratios are calculated for the northern continental ice sheets through the last glacial cycle and are consistent with results from Greenland ice cores. The record of ice volume changes through the last glacial cycle now appears to be reasonably complete.

Sm-Nd and Rb-Sr Chronology of Continental Crust Formation

Samarium-neodymium and rubidium-strontium isotopic systematics together with plausible assumptions regarding the geochemical evlution of continental crust material, have been used to ascertain the times at which segments of continental crust were formed. Analyses of composites from the Canadian Shield representing portions of the Superior, Slave, and Churchill structural provinces indicate that these provinces were all formed within the period 2.5 to 2.7 aeons. It has been possible to determine the mean age of sediment provenances, as studies of sedimentary rocks suggest that the samarium-neodymium isotopic system is not substantially disturbed during sedimentation or diagenesis.

Coral record of increased sediment flux to the inner Great Barrier Reef since European settlement

The effect of European settlement on water quality in the Great Barrier Reef of Australia is a long-standing and controversial issue. Erosion and sediment transport in river catchments in this region have increased substantially since European settlement, but the magnitude of these changes remains uncertain. Here we report analyses of Ba/Ca ratios in long-lived Porites coral from Havannah Reef—a site on the inner Great Barrier Reef that is influenced by flood plumes from the Burdekin river—to establish a record of sediment fluxes from about 1750 to 1998. We find that, in the early part of the record, suspended sediment from river floods reached the inner reef area only occasionally, whereas after about 1870—following the beginning of European settlement—a five- to tenfold increase in the delivery of sediments is recorded with the highest fluxes occurring during the drought-breaking floods. We conclude that, since European settlement, land-use practices such as clearing and overstocking have led to major degradation of the semi-arid river catchments, resulting in substantially increased sediment loads entering the inner Great Barrier Reef.

Geochemistry of loess, continental crustal composition and crustal model ages

Isotopic, major and trace element studies of loess deposits from America, China, Europe and New Zealand show general uniformity of composition. Silica, Zr and Hf are enriched relative to estimates of bulk composition of the upper continental crust. The REE data are indistinguishable from those of average shales, confirming the concept that these REE patterns (LaN/YbN = 9.5 Eu/Eu∗ = 0.66) represent the upper crustal average. Sm-Nd model ages are variable but <1700 m.y. They reflect derivation from younger elevated erogenic areas subject to Pleistocene glaciation. Although Sm-Nd model ages vary by a factor of two, the REE patterns remain constant. This indicates that processes responsible for formation of the upper crust have produced no secular change in composition since the mid-Proterozoic.

Geochemical and isotopic systematics in carbonatites and implications for the evolution of ocean-island sources

Geochemical and Sr, Nd, Pb, O and C isotopic data are reported for 13 carbonatites from Africa, Australia, Brazil, Europe and the United States. The carbonatites possess generally high Ba, Th, LREE, Sr and low Cs, Rb, K and HREE abundances. Some examples have low Ti, Nb, Ta, P, Zr, Hf and U concentrations which are consistent with variable fractionation of sphene, apatite, perovskite, monazite or zircon. The samples range in age from Proterozoic to Tertiary and possess a range of initial Sr isotopic compositions between 0.7020 and 0.7054, initial ϵNd values of −0.4 to +3.8 and (with the exception of the Brazilian Jacupiranga carbonatite) generally radiogenic initial Pb isotopic compositions. δ18OSMOW compositions of the intrusive carbonatites range from +5.5 to +12.4‰ Higher δ18OSMOW values of +14 and +17%. are found in the volcanically-emplaced Proterozoic Goudini complex of South Africa, suggesting the involvement of secondary alteration processes. δ13CPDB ranges from −0.5 to −6.6‰ with samples having near-primary δ18OSMOW (between +5.5 and +8%.) possessing δ13CPDB between −2.9 to −6.6‰. On the initial Sr-Nd isotope diagram, most carbonatites plot below the mantle array and below or within the field of many ocean-island basalts. The Pb isotopic compositions of carbonatites generally lie along the array defined by oceanic basalts. The characteristics of carbonatites from a number of continents and their isotopic similarity to some ocean-island basalts favour an asthenospheric mantle “plume” origin. This conclusion suggests that some ocean-island alkali basalts may have been derived from trace-element-depleted mantle sources which have been re-fertilised by low-viscosity, trace-element-rich carbonatitic melts. The common close spatial and temporal association and the overlap in trace-element geochemistry and isotopic characteristics of Group 1 (basaltic) kimberlites and carbonatites argues strongly for a genetic relationship. Although late-stage melt/vapour fractionation may play some role, the extreme LREE-enrichment typical of carbonatites requires their derivation by small degrees of melting (< ≈ 1%) from a garnet-rich eclogitic source. This source may originally have been CO2- and volatile-rich subducted oceanic lithosphere.

Nd isotopic characteristics of S- and I-type granites

The initial Nd and Sr isotopic composition has been determined in S- and I-type granites from the Paleozoic Berridale and Kosciusko Batholiths of southeast Australia. The Nd and Sr isotopic variations form a strongly covariant array with S-types granites having a relatively restricted range ineNd values from −6.1 to −9.8 but a large range in initial87Sr86Sr of from 0.7094 to 0.7184. These characteristics are indicative of an∼1400-m.y. sedimentary or metasedimentary source for S-types. I-types have variable initial Nd ranging from +0.4 to −8.9, and a more limited range in initial87Sr86Sr of from 0.70453 to 0.7119. These isotopic characteristics are consistent with a two-component mixing model whereby a depleted mantle-like component (DMC) witheNd = +6 and87Sr86Sr= 0.703, is mixed with a crustal component (CC) havingeNd = −9 and87Sr86Sr= 0.720. Although this two-component mixing model satisfies the isotopic constraints the source rock chemistry of the I-types is not compatible with the large proportion (up to 50%) of sedimentary material implied by the isotopic data. This indicates that more than two components are required to account for both the isotopic and chemical data. Both the chemical and isotopic data are consistent with I-type granites having been formed from source rocks of predominantly mantle derivation and obtained progressively from the mantle over a period of 1000 m.y. prior to granite formation.