Marc D. Norman

Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa

Abstract Laboratory experiments on natural, hydrous basalts at 1–4 GPa constrain the composition of “unadulterated” partial melts of eclogitized oceanic crust within downgoing lithospheric slabs in subduction zones. We complement the “slab melting” experiments with another set of experiments in which these same “adakite” melts are allowed to infiltrate and react with an overlying layer of peridotite, simulating melt:rock reaction at the slab–mantle wedge interface. In subduction zones, the effects of reaction between slab-derived, adakite melts and peridotitic mantle conceivably range from hybridization of the melt, to modal or cryptic metasomatism of the sub-arc mantle, depending upon the “effective” melt:rock ratio. In experiments at 3.8 GPa, assimilation of either fertile or depleted peridotite by slab melts at a melt:rock ratio ∼2:1 produces Mg-rich, high-silica liquids in reactions which form pyrope-rich garnet and low-Mg# orthopyroxene, and fully consume olivine. Analysis of both the pristine and hybridized slab melts for a range of trace elements indicates that, although abundances of most trace elements in the melt increase during assimilation (because melt is consumed), trace element ratios remain relatively constant. In their compositional range, the experimental liquids closely resemble adakite lavas in island-arc and continental margin settings, and adakite veins and melt inclusions in metasomatized peridotite xenoliths from the sub-arc mantle. At slightly lower melt:rock ratios (∼1:1), slab melts are fully consumed, along with peridotitic olivine, in modal metasomatic reactions that form sodic amphibole and high-Mg# orthopyroxene.

Growth of early continental crust by partial melting of eclogite

The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic (‘grey gneiss’) and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of ‘average’ Archaean TTG, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle, notably in intraoceanic arc settings in the forearc and back-arc regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking and tectonic accretion of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, ‘arc-like’ crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.

186Os–187Os systematics of Hawaiian picrites

Primitive Hawaiian picrites have 187Os/188Os as high as ∼0.145 and are more radiogenic than the depleted upper mantle, reflecting a time-integrated suprachondritic Re/Os ratio. The high Re/Os may be explained either by an ancient recycled crustal component or an evolved outer core component in the Hawaiian plume. New high precision 186Os/188Os measurements for these picrites, combined with previous analyses, show that the Hawaiian plume source has 186Os/188Os that range from chondritic mantle values of ∼0.119834 to more radiogenic values as high as ∼0.119848. The higher 186Os/188Os reflects long-term suprachondritic Pt/Os and is coupled with higher 187Os/188Os in all but the Koolau picrites. The latter have near-chondritic 186Os/188Os but with radiogenic 187Os/188Os. The Pt/Re of crustal materials that may make up ancient recycled slabs ranges from ∼0.1 to 33. Recycled slab material with such Pt/Re ratios evolved for 1–3 Ga and added to the plume source may explain the Koolau Os isotopic compositions. Pt/Re ratios of 88–100, however, are required for the ancient recycled crust to generate the coupled enrichments of 186Os/188Os and 187Os/188Os in picrites from Loihi and Hualalai. These high Pt/Re ratios do not occur in any known crustal materials, but they are consistent with the observed partition coefficients for Os>Re>Pt, during metal crystallization from an initially chondritic molten core (Pt/Re ∼21–24). Such partitioning may have produced an evolved outer core with suprachondritic Pt/Os, Re/Os and Pt/Re, resulting in the production of suprachondritic 186Os/188Os and 187Os/188Os over time. Small amounts of outer core metal (≤1.2%) mixed into the Hawaiian plume source can explain the coupled 186Os/188Os and 187Os/188Os enrichment in some of the Hawaiian picrites. In addition, the most radiogenic 186Os/188Os in the Hawaiian picrites is correlated with higher 3He/4He, consistent with an undegassed, and likely, lower mantle source. These data provide compelling geochemical evidence that the Hawaiian plume was generated at the core-mantle boundary.

Primitive magmas and source characteristics of the Hawaiian plume: petrology and geochemistry of shield picrites

A suite of tholeiitic picrites from eight of the younger (<2 Ma) Hawaiian shield volcanoes provides new information about the compositions of primitive magmas and source components in the Hawaiian plume. Olivine and bulk rock compositions show that parental melts at Hawaiian volcanoes have at least 13–17% MgO and ∼10% Al2O3. The picrites have bulk compositions ranging from 14 to 30% MgO, and although most of these lavas have accumulated olivine + spinel, several have compositions that may approximate primitive melts. Olivine and spinel compositions show that the phenocrysts are closely related to the melt fraction of these lavas and are not accidental xenocrysts. Diverse isotopic compositions (Pb, Os, Sr, Nd) of these picrites require multiple sources in the Hawaiian plume, but key trace element characteristics (La, Nb abundances normalized to 16% MgO, Sm/Nd, Lu/Hf, La/Yb, Zr/Nb) are consistent with variable degrees of melting of a common, garnet-bearing source for all of the volcanoes except Koolau. The trace element composition of this Hawaiian pyrolite plume source can be modelled as an incipiently depleted, nearly primitive mantle that has lost a very small melt fraction, but a more complex origin may be more realistic. The Koolau picrites are exceptional in having anomalously low Nb and Ti contents, and high Zr/Nb ratios that fall off the melting arrays defined by the other picrites, indicating a distinctive source component that is also expressed in major element and isotopic compositions. The nearly constant Sr/Pb, Sr/Y, and Ba/Th ratios of these isotopically variable picrites are inconsistent with formation of the plume source either by bulk recycling of oceanic crust into the mantle, or by addition of dacitic melts from entrained eclogite to plume-derived basaltic magmas. Alternatively, the Hawaiian plume may consist of variably depleted mantle that was enriched by small-degree melts, possibly during subduction or entrainment of lithospheric mantle. Radiogenic 186Os/188Os isotopic compositions of these picrites are consistent with transport of this material to the deep lower mantle and addition of a small amount of outer core to the plume source.

Quantitative analysis of trace element abundances in glasses and minerals: a comparison of laser ablation inductively coupled plasma mass spectrometry, solution inductively coupled plasma mass spectrometry, proton microprobe and electron microprobe data

Many geological, environmental and industrial applications can be enhanced through integrated microbeam and bulk geochemical determinations of major and trace element concentrations. Advantages ofin situ microanalysis include minimal sample preparation, low blanks, information about the spatial distribution of compositional characteristics and the ability to avoid microscopic inclusions of foreign material. In this paper we compare trace element data obtained by laser ablation ICP-MS, solution ICP-MS, electron microprobe analysis and proton microprobe analysis for a variety of silicate glasses and minerals. New determinations for 36 trace elements in BCR-2G, a microbeam glass standard, are presented. Results obtained by the various microbeam and solution methods agree well for concentrations ranging over several orders of magnitude. Replicate analyses of BCR-2G demonstrate an analytical precision of 2–8% relative (1σ) for all elements by laser ablation ICP-MS and ≤3% by solution ICP-MS, except for Li (5%). These data emphasize the utility of laser ablation ICP-MS as a quantitative microbeam technique capable of rapid, precise determinations of sub-ppm trace element abundances in a variety of targets.

Rhenium and platinum group element abundances correlated with mantle source components in Hawaiian picrites: sulphides in the plume

Core addition and crustal recycling models that seek to explain the radiogenic Os isotopic compositions of primitive Hawaii tholeiites predict distinctive geochemical consequences for chalcophile and siderophile element abundances in the mantle plume. To test these models and to improve our understanding of compositional variability in the Hawaiian plume, the platinum group element (PGE) and Re contents of primitive shield picrites from several Hawaiian volcanoes were measured. PGE abundances span a large range, from similar to MORB for a picrite from Koolau, to compositions similar to those of basaltic komatiites for picrites from Kilauea and Loihi. Re concentrations range from 0.25 to 0.95 ng/g and with a mean of 0.73 ng/g, higher than previously compiled global averages for ocean island basalts (OIB) (0.38 ng/g) and closer to average MORB (0.98 ng/g) than previously recognised. Some subaerial tholeiites, notably from Kilauea and Mauna Kea, have anomalously low Re abundances and high Cu/Re ratios, possibly reflecting Re loss upon eruption or during degassing of shallow magma chambers. These data show that the PGE and primary Re contents of primitive Hawaiian picrites are well correlated with isotopic compositions of these lavas, linking the PGE and Re characteristics directly with source features of the mantle plume. However, mixing models that describe the isotopic effects of core addition and crustal recycling do not account for the PGE and Re abundances. The range of PGE and Re contents in these lavas does not appear to reflect abundance variations in the plume components, but some aspect of the melting process that is linked to source characteristics of the plume. One possibility is that the PGE and Re characteristics of Hawaiian tholeiites may reflect variable amounts or compositions of residual sulphide during melting. In this scenario, the high PGE and Re contents of Kilauea and Loihi picrites may be indicating a relatively small amount of residual sulphide during melting, whereas the low PGE and Re contents of Koolau primitive magmas may be indicating greater amounts of residual sulphide in the plume. The systematic compositional variations of PGE and Re in primitive tholeiites must be accounted for by any model for the origin of the Hawaiian plume.

Routine quantitative multi-element analysis of sulphide minerals by laser ablation ICP-MS: Standard development and consideration of matrix effects

ABSTRACT The paper describes a calibration standard for quantitative in-situ multi-element analysis of sulphide minerals by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Standard STDGL2b2 is a mixture of 25% Zn concentrate and 75% pyrrhotite doped with a number of additional trace elements and fused into an X-ray fluorescence (XRF) glass disk. The homogeneity of the disk has been tested for the 55 elements of interest. All elements except Se, Tl, Au and Pt are homogenous (< 5% variation). Accurate analysis for the above four elements requires averaging multiple analyses of the standard. Element concentrations in STDGL2b2 were quantified by XRF and standard solution ICP-MS using a Finnigan Element and an Agilent 4500 mass-spectrometers. For the analysis of pyrite, pyrrhotite, chalcopyrite, galena and sphalerite, analytical errors caused by matrix-dependent fractionation have been evaluated by analysing five pressed-powder pellets.. The compositions of the powders have been analysed by XRF and solution ICP-MS. When Fe or Pb can be used as the internal standard, errors for most elements are < 15%, but reach up to 50% for W, Zn and Cd, requiring correction factors to be introduced. However, when Zn is used as the internal standard, significant correction factors are required for most elements. Comparison of the results obtained with two different laser microprobes, a solid state 213 nm and an excimer 193 nm, indicates that either is well suited for LA-ICP-MS analysis of sulphide minerals using STDGL2b2 as the calibration standard. Use of STDGL2b2 significantly improves accuracy of sulphide analysis by LA-ICP-MS compared to silicate reference materials, such as the NIST 600 series.

39Ar40Ar age and petrology of Chico: Large-scale impact melting on the L chondrite parent body

Our studies of the 105 kg Chico L chondrite show that it contains ∼60% impact melt and the largest volume of impact melt recognized in stony meteorites. We suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large, if not catastrophic, impact on the L chondrite parent body at about 0.5 Ga. Petrologic and 39Ar40Ar dating studies were made on several lithologies, including the massive melt zone, host chondrite, and melt-chondrite boundaries, for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the KAr chronometer. The chondritic host is shocked to stage S6 and contains pockets and veins of melt. There are no unmelted clasts in the interior of the melt; coalesced metal-troilite nodules reach up to 2 cm in size. Melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly. Metal-troilite textures suggest cooling rates of ∼0.1°C/s in the interior of the melt dike during crystallization. Secondary kamacite rims indicate cooling at 0.01–1°C/y over the range of 700-500°C, consistent with an impact-heated volume of up to a kilometer in thickness. Compositions of olivines and pyroxenes are generally similar in melt and chondritic host, reflecting rapid crystallization, not metamorphic equilibration. The interior melt shows an overall depletion in K, whereas the melt near the boundary is enriched in K. The 39Ar40Ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts, based on Ar diffusion properties and K/Ca ratios. The low-temperature, high K/Ca phase of both melt and host chondrite show ages of 0.54–0.78 Ga. Ages of the high-temperature, low K/Ca phase of the melt are comparable or higher, 0.61–1.35 Ga, whereas those of the host chondrite are even higher, 0.87–1.86 Ga, due to lesser degrees of degassing. Isochron plots for several melt samples suggest an age of ∼0.53 Ga and the presence of variable amounts of excess 40Ar not completely degassed by the impact. Even this age, however, is significantly higher than the previously reported RbSr isochron age of 0.467 ± .015 Ga. The apparent retention of radiogenic 40Ar in the Chico impact melt, in spite of its relatively large size, absence of clasts, and moderately slow cooling rate below 700°C, raises questions as to the reliability of using melts for 39Ar40Ar dating of meteoritic impact events.

Assimilation of seawater-derived components in an oceanic volcano: evidence from matrix glasses and glass inclusions from Loihi seamount, Hawaii

We report major element, H_(2)O, Cl, B, and Be analyses of matrix glass and olivine-hosted glass inclusions from two pillow lava samples dredged from 4200 m on the southern rift zone of Loihi seamount, Hawaii. Matrix glasses (MgO∼9 wt.%) have H_(2)O, Cl, and B contents considerably in excess of the values expected from mantle melting or fractional crystallization of parental Loihi magmas. Glass inclusions have H_(2)O, Cl, and B contents ranging from the high values of the matrix glasses to lower concentrations that are more typical of Hawaiian magmas. Concentrations of other incompatible elements (e.g., K_(2)O, P_(2)O_(5), and Be) in matrix glasses and glass inclusions are uncorrelated with their H_(2)O, Cl, and B contents. Glass inclusions show considerable scatter in major element compositions compared to matrix glasses, but except for H_(2)O, Cl, and B, the average glass inclusion composition corresponds well to the matrix glass compositions. We propose that the glass inclusions represent compositionally diverse liquids present within the magmatic plumbing system at Loihi that were mixed and homogenized to produce the liquid that quenched to the matrix glass on eruption. This range of liquid compositions present at depth was trapped by crystallizing olivine prior to blending and homogenizing and therefore preserves a compositional diversity not present in erupted whole rocks. The high H_(2)O, Cl, and B contents of matrix glasses and some glass inclusions, and the range of H_(2)O, Cl, and B concentrations in glass inclusions, are best explained by variable extents of assimilation by Loihi magmas of H2O–Cl–B-rich, seawater-derived components prior to eruption. The required assimilants range from material similar in composition to seawater to materials with Cl/H_(2)O and B/H_(2)O ratios much higher than seawater. Our preferred explanation (similar to that suggested for MORB by Michael and Schilling, 1989) [Michael, P.J., Schilling, J.-G., 1989. Chlorine in mid-ocean ridge magmas: Evidence for assimilation of seawater-influenced components. Geochim. Cosmochim. Acta, 53, pp. 3131–3143.] is that most of the assimilated materials were brines (or rocks containing brines in inclusions or along grain boundaries) enriched in Cl by high temperature phase separation of seawater in sub-sea-floor hydrothermal circulation systems. Addition of ∼1.0 wt.% of a 15 wt.% NaCl brine can explain the H_(2)O and Cl contents of the matrix glasses. Addition of altered basalt cannot readily account for the Cl and H_(2)O contents of matrix glasses and glass inclusions, but may be required to account for their elevated B contents. The enrichment in Cl and contamination with atmospheric noble gases observed in other samples from Loihi could also result from assimilation of Cl-enriched, seawater-derived components.

Tungsten isotope evidence that mantle plumes contain no contribution from the Earth's core

Osmium isotope ratios provide important constraints on the sources of ocean-island basalts, but two very different models have been put forward to explain such data. One model interprets 187Os-enrichments in terms of a component of recycled oceanic crust within the source material. The other model infers that interaction of the mantle with the Earths outer core produces the isotope anomalies and, as a result of coupled 186Os–187Os anomalies, put time constraints on inner-core formation. Like osmium, tungsten is a siderophile (‘iron-loving’) element that preferentially partitioned into the Earths core during core formation but is also ‘incompatible’ during mantle melting (it preferentially enters the melt phase), which makes it further depleted in the mantle. Tungsten should therefore be a sensitive tracer of core contributions in the source of mantle melts. Here we present high-precision tungsten isotope data from the same set of Hawaiian rocks used to establish the previously interpreted 186Os–187Os anomalies and on selected South African rocks, which have also been proposed to contain a core contribution. None of the samples that we have analysed have a negative tungsten isotope value, as predicted from the core-contribution model. This rules out a simple core–mantle mixing scenario and suggests that the radiogenic osmium in ocean-island basalts can better be explained by the source of such basalts containing a component of recycled crust.