James M. Brenan

Behaviour of boron, beryllium, and lithium during melting and crystallization: constraints from mineral-melt partitioning experiments

In order to provide a more substantial foundation for interpreting the behaviour of B, Be, and Li during the production and early crystallization of primitive igneous rocks, we have measured olivine-, clinopyroxene-, orthopyroxene-, and amphibole-melt partition coefficients for these elements involving broadly basaltic-andesitic melt compositions. Experiments were conducted at both one atmosphere and 1.0 -1.5 GPa and employed a time-temperature history that yielded large crystals with minimal compositional zoning. Experiment temperatures ranged from 1000 to 1350°C and were selected to minimize the total crystal fraction in a given experiment. Partition coefficients for olivine and clinopyroxene were found to be independent of run duration or total concentration of B, Be, or Li suggesting that crystal-liquid equilibrium was closely approached. Olivine-, orthopyroxene-, and clinopyroxene-melt partition coefficients decrease in the order: Li (0.1- 0.2) @ Be ; B (0.002- 0.03), whereas amphibole-melt partition coefficients for Be and Li are similar (;0.2) and larger than those for B (;0.02). Comparison of partition coefficients measured in this study with previous determinations yields good agreement, with the exception of some of our mineral-melt values for B, which are uniformly lower (up to 10 times) than values determined at similar conditions of pressure and temperature. The latter discrepancy could be due to mineral or melt compositional effects, but this hypothesis is currently untestable owing to the absence of reported mineral compositions in previous studies. Partition coefficients for olivine and clinopyroxene have been found to vary as a function of mineral and melt composition, and with the exception of B partitioning into clinopyroxene, this variation can be modeled using simple exchange reactions involving the trace element and a substituent element, such as Na, Mg, or Al. Partition coefficients measured in this study were combined with simple models of melting and crystallization to evaluate how accurately element ratios such as B/Be, B/K, B/Nb, Be/Nd, Li/V, and Li/Yb in primitive magmas reflect that of their source. These models further confirm that the source regions of IAB magmas are enriched in B/Be, B/Nb, and Li/Yb relative to the MORB source, thus lending further support to the notion of metasomatic enrichment of the IAB source by slab-derived fluids. Moreover, our modeling also indicates that the low B/Be and B/Nb in primitive OIB magmas is indicative of similarly low values in OIB sources, which is consistent with the hypothesis that OIB sources contain a B-depleted component, such as subducted, dehydrated oceanic crust. Partial melting models have also been constructed to explore the possibility of using the Li/V ratio in MORB and IAB as a monitor of redox conditions in their source-regions. Models indicate that this ratio does not uniquely constrain source fO2 without a priori knowledge of the degree of melting. However, the small amount of dispersion in MORB Li/V is consistent with (1) the small variation in source-region fO2 inferred for MORB by independent means and (2) degrees of melting close to clinopyroxene exhaustion. The very large dispersion in Li/V ratios in the IAB suite can be reconciled by melt generation under more oxidising conditions than that for MORB, in addition to variation in source composition resulting from metasomatism involving a Li-rich component. Copyright

The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction: experiments and models

Abstract The low atomic mass elements B, Be, and Li are viewed as sensitive tracers of the involvement of subducted materials in the genesis of island arc magmas. In order to better assess the role of dense aqueous fluids in the slab-to-mantle transfer of these elements during subduction, measurements have been made of partition coefficients for B, Be, and Li between aqueous fluid and minerals likely to be present in the basaltic portion of the downgoing slab, namely clinopyroxene and garnet. Experiments at 900°C and 2.0 GPa reveal that the average clinopyroxene-fluid partition coefficient for Be (∼2) exceeds that for either Li (∼0.2) or B (∼0.02) and values are 100× (B,Li) to 1000× (Be) larger than partition coefficients for garnet. Clinopyroxene-fluid partition coefficients were found to vary with the alumina content of run-product clinopyroxenes, but this variation is interpreted to reflect the specific exchange reaction that governs the incorporation of these elements into the pyroxene structure, and not mineral-fluid disequilibrium. The element pairs B-Be, B-Nb, and Li-Yb are considered to be essentially unfractionated during the partial melting process, as evidence by their coherent behaviour in apparently cogenetic lavas and the similarity in their measured mineral-melt partition coefficients. A comparison of clinopyroxene-fluid partition coefficients for these elements with clinopyroxene-silicate melt values reveals that B/Be, B/Nb, and Li/Yb ratios will be significantly fractionated in coexisting aqueous fluid with respect to the residual solid. The elevated ratios of B/Be, B/Nb, and Li/Yb in island arc lavas relative to MORB are thus considered to be consistent with an origin by fluid-mediated slab-to-mantle transport. A quantitative model of slab dehydration accompanied by progressive water loss and changes in residual mineral mode reveals that source regions with B/Be and B/Nb appropriate for producing the Izu and Kurile IAB suites can be generated using available estimates for the composition of altered oceanic crust, although B abundances at the high end of published values are required. Because the highest values of B/Be and B/Nb are produced in the mantle wedge at relatively shallow depths, some additional process, such as subduction-induced flow of a hydrated mantle wedge, is required in order to transfer enriched material to depths appropriate for the formation of magmas beneath the volcanic front. Calculations indicate that by the time the slab reaches a depth of 200 km, B/Be and B/Nb in the dehydration residue has been reduced to ∼5–12% of initial values. Thus, the preferential loss of B during dehydration is viewed as a viable mechanism to prevent the excess B acquired during near-surface alteration of oceanic crust from being cycled into the mantle, thereby maintaining the distinction in B/Be and B/Nb for mantle and crustal reservoirs.

Experimental constraints on the partitioning of rhenium and some platinum-group elements between olivine and silicate melt

Abstract We have performed partitioning experiments to assess the role of olivine in controlling the behavior of rhenium and the platinum group elements (PGEs) during basalt petrogenesis. Olivines were crystallized from an iron-bearing basalt at 1 bar (105 Pa) and log fO2 of −2.6, −4.9 and −7.4 (FMQ +4.3, +2 and −0.5, respectively). In situ analyses of olivine and glass by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) reveal a homogeneous distribution of Ru, Rh, Pd, Re, and Pt, but significant Os heterogeneity at the μm scale. This latter behavior arises from the presence of undissolved Os micronuggets suspended in the melt, and included in olivine crystals. Olivine–melt partition coefficients (Ds) for Re and the PGEs follow the order: DRh>DRu≫DPd∼DRe∼DPt. With decreasing fO2, Rh and Ru become more compatible, with maximum partition coefficients of ∼2.6 and ∼2, respectively, at log fO2 of −4.9. In contrast, D values for Pd become smaller with decreasing fO2, to a value of ∼0.006 at log fO2 of −7.4. Olivine–melt partitioning of Rh, Ru, Pd, Re and Pt derived from our experiments is confirmed by the behavior of these elements in lavas that have evolved by olivine fractionation. An elastic strain model predicts the olivine–melt partitioning of these elements, excepting our measured value of DPt, which is much lower. The fO2 dependence on partitioning implies that at higher fO2 some portion of PGEs exist in higher valence states than predicted from their solubility.

Diffusion of osmium in pyrrhotite and pyrite: implications for closure of the Re–Os isotopic system

Abstract In order to better constrain the extent to which common sulfide minerals will retain their osmium isotopic composition subsequent to crystallization, we have conducted experiments to quantify the diffusion behavior of osmium in pyrite and pyrrhotite. Experiments consisted of either (1) isothermal soaking of diffusion couples consisting of natural pyrite or pyrrhotite crystals packed against powdered Os-bearing Fe-sulfide or (2) ‘relaxation’ of initially high near-surface osmium concentrations produced in the latter experiments (pyrite only). Osmium penetration into samples was characterized by depth profiling using Rutherford backscattering spectroscopy (RBS) (pyrite) or electron microprobe analyses across sectioned run products (pyrrhotite). Results of the first type of diffusion experiment involving pyrite show only limited osmium penetration into sample surfaces, with the extent of penetration uncorrelated with run duration. Images of pyrite samples using atomic force microscopy show roughening of initially smooth surfaces as a consequence of step formation and suggest that osmium incorporation into the near-surface occurred by solute uptake during step growth and not by volume diffusion. Prolonged (1000+ h) ‘relaxation’ experiments revealed no additional osmium penetration into pyrite surfaces and based on the depth resolution for RBS, a maximum diffusion coefficient of 2.5×10−23 m2/s at 500°C was calculated. Experiments involving pyrrhotite over the temperature range of 950–1100°C showed extensive osmium uptake and osmium concentration gradients that conform with Fickian diffusion behavior. We found that pyrrhotite Fe/S could be varied by changes in the composition of the starting material and osmium source and over the range of Fe/S produced in experiments (molar Fe/S=0.83–0.90), we observed no systematic variation in the osmium diffusion coefficient. Diffusion coefficients measured parallel to the a crystallographic axis were on average ∼1.4× higher than values measured parallel to c and regression of the c-axis data yielded the Arrhenius relation: D(T)=1.31±0.16×10 −5 m 2 / s exp −211.5±14.7 kJ / mol RT The application of these diffusion data to simple models of diffusive exchange during static or polythermal time–temperature histories is used to assess the conditions under which radiogenic osmium will be retained. During isothermal annealing, calculations indicate that the cores of millimeter-sized spherical pyrrhotite crystals undergoing diffusive exchange with an external osmium reservoir will have their initial compositions perturbed in ≤0.5 Ma at temperatures exceeding 400°C. Pyrite undergoing the same process at 500°C requires in excess of 10 Ma before crystal cores are affected. The relatively short ‘core retention’ time-scales for pyrrhotite indicates that this mineral may be prone to isotopic resetting following relatively brief crustal thermal events, thus possibly accounting for the scatter that commonly occurs in Re–Os isochrons generated from massive sulfide samples. Calculated closure temperatures (Tc) for osmium exchange in pyrrhotite yielded values of 300–400°C for grain sizes ranging from 10 to 1000 μm. These values of Tc are similar to those calculated for Ar retention in biotite, and considerably lower than for Sr in apatite and plagioclase, for example. Such low closure temperatures for pyrrhotite suggest this mineral will date the final stage in the cooling of a magmatic system and possibly be susceptible to open system osmium exchange in the presence of late-stage hydrothermal fluids. This latter result infers that caution be applied when interpreting elevated initial osmium isotopic ratios as a product of crustal assimilation at the magmatic stage.

Extreme crustal oxygen isotope signatures preserved in coesite in diamond

The anomalously high and low oxygen isotope values observed in eclogite xenoliths from the upper mantle beneath cratons have been interpreted as indicating that the parent rock of the eclogites experienced alteration on the ancient sea floor. Recognition of this genetic lineage has provided the foundation for a model of the evolution of the continents whereby imbricated slabs of oceanic lithosphere underpin and promote stabilization of early cratons. Early crustal growth is thought to have been enhanced by the addition of slab-derived magmas, leaving an eclogite residuum in the upper mantle beneath the cratons. But the oxygen isotope anomalies observed in eclogite xenoliths are small relative to those in altered ocean-floor basalt and intermediate-stage subduction-zone eclogites, and this has hindered acceptance of the hypothesis that the eclogite xenoliths represent subducted and metamorphosed ocean-floor basalts. We present here the oxygen isotope composition of eclogitic mineral inclusions, analysed in situ in diamonds using an ion microprobe/secondary ion mass spectrometer. The oxygen isotope values of coesite (a polymorph of SiO2) inclusions are substantially higher than previously reported for xenoliths from the subcratonic mantle, but are typical of subduction-zone meta-basalts, and accordingly provide strong support for the link between altered ocean-floor basalts and mantle eclogite xenoliths.

The solubility of ruthenium in sulfide liquid: implications for platinum group mineral stability and sulfide melt–silicate melt partitioning

To more completely assess the primary magmatic origin of Ru–Os–Ir (IPGE) alloys, we conducted experiments to evaluate the effects of T, fO2, fS2 and melt composition on the solubility of Ru in molten Fe–Ni–sulfide. Fe–Ni–S melt+Ru were held in olivine crucibles, and experiments were done in a vertical-tube gas-mixing furnace at 1200–1400 °C for 1–5 days. At constant fO2 and fS2, Ru solubility increases with T, and a similar result is obtained if fO2 is varied parallel to the fayalite–magnetite–quartz buffer (FMQ), with fS2 levels to maintain sulfide liquid saturation. At log fS2 of −1.7, Ru solubility decreases from ∼11 wt.% at log fO2 of −10.8, to ∼0.3 wt.% at log fO2 of −8.1. At a log fO2 of −8.6, a similar reduction in Ru solubility occurred as log fS2 decreased from −0.9 to −3.0. Substitution of Ni for Fe in the sulfide results in an increase in Ru solubility, with values ranging from ∼3 wt.% at Fe/Ni of 36 to ∼10 wt.% at Fe/Ni of 6 (log fO2, fS2 of −9.1, −1.7, respectively). Dilution of Ru with a 1:1 mix of Os+Ir results in a 4- and 10-fold decrease in melt Ru content for alloys with ∼60 and 35 mol% Ru, respectively. For the fO2–fS2 conditions required for sulfide liquid saturation in natural basaltic magmas, pure Ru solubility in molten sulfide is expected to exceed 10 wt.%, and dilution by Os+Ir is still likely to require wt.% levels of Ru for IPGE alloy saturation. Since Ru abundances of ore-grade massive sulfide is <50 ppm, our results would preclude IPGE alloy saturation in the presence of immiscible sulfide liquid. Activity–composition relations determined for Ru in ternary Ru–Os–Ir alloy suggest, however, that the concentration of Ru in molten silicate required for alloy saturation is at or below levels in natural, high Mg igneous rocks, implying such alloys could form in sulfide-undersaturated systems. The negative fO2 dependence of Ru solubility in sulfide liquid is opposite that for Ru (and other PGEs) in silicate melt, suggesting that a decrease in fO2 will favor the partitioning of Ru into the sulfide liquid. However, it is not clear how much this effect will be offset by the concomitant reduction in fS2 required to maintain saturation in immiscible sulfide liquid. Comparison of our Ru solubility data with values determined in silicate melt yields apparent sulfide–silicate melt partition coefficients that exceed 107, which is more than 1000× larger than direct measurements on coexisting sulfide–silicate compositions. Possible reasons for this discrepancy are that (1) measured sulfide–silicate partition coefficients are inaccurate due to incomplete phase separation, (2) non-Henryian activity–composition relations at high Ru concentrations, (3) Ru solubilities in silicate melt measured at both high fO2 and in the absence of sulfur cannot be accurately extrapolated to the low fO2 and sulfur-bearing conditions of previous two-liquid partitioning experiments.

Effects of fO2, fS2, temperature, and melt composition on Fe-Ni exchange between olivine and sulfide liquid: implications for natural olivine–sulfide assemblages

The apparent equilibrium constant for the exchange of Fe and Ni between coexisting olivine and sulfide liquid (KD = (XNiS/XFeS)liquid/(XNiSi12O2/XFeSi12O2)olivine; Xi = mole fraction) has been measured at controlled oxygen and sulfur fugacities (fO2 = 10−8.1 to 10−10 and fS2 = 10−0.9 to 10−1.7) over the temperature range 1200 to 1385°C, with 5 to 37 wt% Ni and 7 to 18 wt% Cu in the sulfide liquid. At log fO2 of −8.7 ± 0.1, and log fS2 of −0.9 to −1.7, KD is relatively insensitive to sulfur fugacity, but comparison with previous results shows that KD increases at very low sulfur fugacities. KD values show an increase with the nickel content of the sulfide liquid, but this effect is more complex than found previously, and is greatest at log fO2 of −8.1, lessens with decreasing fO2, and KD becomes independent of melt Ni content at log fO2 ≤ −9.5. The origin of this variation in KD with fO2 and fS2 is most likely the result of nonideal mixing of Fe and Ni species in the sulfide liquid. Such behavior causes activity coefficients to change with either melt oxygen content or metal/sulfur ratio, effects that are well documented for metal-rich sulfide melts. Application of these experimental results to natural samples shows that the relatively large dispersion that exists in KD values from different olivine + sulfide-saturated rock suites can be interpreted as arising from variations in fO2, fS2, and the nickel content of the sulfide liquid. Estimates of fO2 based on KD and sulfide melt composition in natural samples yields a range from fayalite–magnetite–quartz (FMQ)–1 to FMQ-2 or lower, which is in good agreement with previous values determined for oceanic basalts that use glass ferric/ferrous ratios. Anomalously high KD values recorded in some suites, such as Disko Island, probably reflect low fS2 during sulfide saturation, which is consistent with indications of low fO2 for those samples. It is concluded that the variation in KD values from natural samples reflects olivine–sulfide melt equilibrium at conditions within the T-fO2-fS2 range of terrestrial mafic magmas.

Re–Os fractionation in magmatic sulfide melt by monosulfide solid solution

In order to better evaluate the effect of monosulfide solid solution (MSS) crystallization on Re–Os fractionation in magmatic sulfide ore systems, MSS–sulfide melt partition coefficients for Re and Os have been measured in experiments in the Fe–Ni–Cu–S system at 1100°C. Samples were encapsulated in evacuated silica tubes, run for durations of 13.5–61 h, and the compositions of coexisting MSS–melt pairs were characterized by electron microprobe. Average MSS–sulfide melt partition coefficients for Re and Os are 2.5 and 3.8, respectively, and are independent of experiment duration and MSS metal/sulfur ratio. Comparison of DOs measured in this study with values determined by Fleet et al. [Contrib. Mineral. Petrol. 115 (1993) 36–44] also suggests DOs is constant for melt Os contents ranging from 0.2 to 2000 ppm. Combining these data with a simple model of closed-system MSS fractionation reveals that the Re and Os content of coexisting MSS–residual sulfide melt pairs decreases with increased differentiation (as gauged by melt or MSS Cu abundances), whereas Re/Os ratios increase. This model reproduces the overall Re–Os–Cu trends observed in the zoned massive sulfide ore bodies at Noril’sk (Russia), although in detail, modeled Re variation is better predicted than Os or Re/Os variation. This latter result could suggest that the variation in ore Os content is not completely controlled by primary magmatic processes. In general, such results lend further support for the role of in situ differentiation in producing zoned ore bodies, but they also allow a clearer distinction to be made between primary vs. post-solidification processes in affecting the Re–Os isotopic system in sulfide-saturated igneous rocks.

Fe–Ni exchange between olivine and sulphide liquid: implications for oxygen barometry in sulphide-saturated magmas

Abstract In order to better understand the behaviour of nickel in magmatic processes, we have measured the apparent equilibrium constant ( K D ) for the exchange of Fe and Ni between coexisting olivine and sulphide liquid at controlled oxygen and sulphur fugacities ( f O 2 = 10 −8 –10 −10 and f S 2 = 10 −2 –10 −4 ) over the temperature range 1200 to 1400°C and with 5 to 50 wt.% nickel in the sulphide liquid. Measured values of K D are independent of temperature and sulphur fugacity, but increase linearly with the nickel content of the sulphide liquid, and follow a power-law increase with oxygen fugacity; behaviour that is consistent with previous measurements of K D under controlled conditions of f O 2 and f S 2 . The variation of K D with melt nickel content and f O 2 is most likely the result of nonideal mixing in the sulphide liquid, which results in a decrease in γ NiS /γ FeS with melt metal/sulphur ratio. As a consequence of the systematic dependence of K D on f O 2 , a new oxygen barometer is proposed for estimating oxygen fugacity in igneous rocks that were cosaturated in olivine and sulphide liquid. Application of the experimental results to natural samples shows that the relatively large variations that exist in K D values from different olivine + sulphide-saturated rock suites can be interpreted as arising from variations in f O 2 and/or the nickel content of the sulphide liquid. Oxygen fugacities calculated for oceanic basalt samples using the proposed Fe–Ni exchange oxybarometer are found to be relatively high (10 −8.5 –10 −10.4 ) which is in accord with the range of values determined using glass ferric/ferrous ratios. Moreover, the very low f O 2 (∼10 −14 ) calculated for the mafic dike from Disko Island is consistent with the presence of native iron in these samples and is in quantitative agreement with indicators of f O 2 based on chromite- and olivine-melt partitioning of vanadium. Consideration of the f O 2 exhibited by olivine + sulphide-saturated intrusive suites reveals a range from relatively oxidized (i.e., similar to oceanic basalts) to values at or below that of Disko Island. Field, petrographic, and geochemical evidence suggests that the production of the low f O 2 suite of samples is probably the result of reduction accompanying the assimilation of carbonaceous country rock.

The Platinum-Group Elements: “Admirably Adapted” for Science and Industry

The platinum-group elements (PGE) tend to exist in the metallic state or bond with sulfur or other Group Va and VIa ligands, and often occur as trace accessory minerals in rocks. Combined with three isotopic systems that contain the PGE, these elements afford a unique view of early solar system evolution, planet formation and differentiation, and biogeochemical cycling. Initial purification of the PGE was accomplished in the late 1700s, at which time their unique properties, including high melting point, chemical inertness, and ability to catalyze chemical reactions, became apparent. This led to enormous industrial demand, most notably for fuel production and engine emission control, which combined with scarcity in crustal rocks, has made the PGE a highly valued commodity.