Hans-Michael Seitz

The distribution of lithium in peridotitic and pyroxenitic mantle lithologies — an indicator of magmatic and metasomatic processes

Lithium concentrations in orthopyroxene, clinopyroxene, olivine, garnet or spinel from equilibrated spinel peridotite, garnet peridotite and garnet pyroxenite xenoliths and from metasomatised peridotites and pyroxenites were measured using the ion-microprobe (SIMS). With respect to changes in physical and chemical parameters, we present new data on lithium contents in mantle minerals and its partitioning behaviour in (1) equilibrated and (2) metasomatised samples. Lithium is preferentially incorporated into olivine, ranging between 1 and 2 ppm in equilibrated unmetasomatised peridotites. Pyroxenes from peridotitic xenoliths have Li concentrations on the order of several hundred ppb up to 1.3 ppm, while pyroxenes from pyroxenites have somewhat higher abundances (1–3 ppm with a maximum of 19 ppm). The following partitioning relationships have been established: ol>cpx≥opx≫grt or sp for garnet and spinel peridotites, respectively, and cpx≥opx>grt for garnet pyroxenites. The intercrystalline partitioning of Li is independent of T, P and bulk composition (for ultramafic to mafic compositions), making Li a suitable tracer element for chemical processes such as metasomatism. We estimate a bulk Li content of 1.0–1.5 ppm for fertile to moderately depleted lithospheric mantle. Low Li abundances in mantle peridotites and pyroxenites emphasise its incompatibility during partial melting and fractional crystallisation. However, elevated Li concentrations are present in some pyroxenites, presumably due to complete crystallisation of trapped partial melts. Metasomatised samples from peridotite massifs (Pyrenees and Ivrea Zone) and mantle nodules from two Victorian volcanic fields (Australia) clearly show enrichment of Li in both olivine and clinopyroxene, whereby the distribution of Li between olivine and clinopyroxene has generally not achieved equilibrium. Disequilibrium is manifested by preferential Li enrichment in either olivine or clinopyroxene depending on the type of metasomatic agent involved (carbonatite vs. mafic silicate melt). Differences in absolute Li abundances and in its partitioning behaviour allow the identification not only of metasomatic overprints, but also of magmatic processes such as partial melting, crystal fractionation and accumulation. The sensitivity of Li as such a chemical tracer gives an additional criterium for recognising cryptic metasomatism.

U-Pb zircon dating of mafic dykes and its application to the Proterozoic geological history of the Vestfold Hills, East Antarctica

The Vestfold Hills, one of several Archaean cratonic blocks within the East Antarctic Shield, comprises a high-grade metamorphic basement complex intruded by at least nine generations of Early to Middle Proterozoic mafic dykes. Extensive U-Pb ion microprobe (SHRIMP) analyses of zircons, derived predominantly from late-stage felsic differentiates of the mafic dykes, provide precise crystallisation ages for several dyke generations. These new ages enable constraints to be placed on both the history of mafic magmatism in the Vestfold Hills and the timing of the various interspersed Proterozoic deformation events. In addition to demonstrating the utility of zircons derived from felsic late-stage differentiates for the dating of co-genetic mafic dykes, this study also places doubt on previous wholerock Rb-Sr dating of mafic dyke suites in this and other areas of East Antarctica. The 207Pb/206Pb zircon ages of 2241±4 Ma and 2238±7 Ma for the Homogeneous and Mottled Norites, respectively, provide a younger emplacement age for associated group 2 High-Mg tholeiite dykes than the whole-rock Rb-Sr date (2424±72 Ma) originally interpreted as the age of all high-Mg intrusives in the Vestfold Hills. Zircon ages of 1754±16 Ma and 1832±72 Ma confirm the previously defined Rb-Sr age of the group 2 Fe-rich tholeiites. Two later dyke generations, the group 3 and 4 Fe-rich tholeiites, are distinguished on the basis of field orientations and cross-cutting relationships, and yield zircon emplacement ages of 1380±7 Ma and 1241±5 Ma which also define minimum ages for two suites of lamprophyre dykes. Xenocrystic zircons within both felsic segregations and mafic dykes yield zircon ages of 2478±5 Ma to ∼2740 Ma, indicating the presence of Archaean crustal source rocks of this antiquity beneath the Vestfold Hills.

Partitioning of transition elements between orthopyroxene and clinopyroxene in peridotitic and websteritic xenoliths: new empirical geothermometers

The partitioning of transition elements (Sc, Ti, V, Cr, Mn, Co, Ni) between orthopyroxene (opx) and clinopyroxene (cpx) in carefully selected garnet peridotite, spinel peridotite and garnet websterite xenoliths was determined by electron probe microanalyses (EPMA) and secondary ion mass spectrometry (SIMS). Xenoliths studied cover a wide compositional range and equilibrated under variable upper mantle conditions at temperatures between about 760 and 1370°C (two-pyroxene thermometer based on the enstatite-diopside solvus) and pressures between about 0.8 and 3.6 GPa (Al-in-opx and Ca-in-olivine barometers). We found that the partitioning of transition elements between opx and cpx (expressed as DM = concentration of element M in opx [cations per formula unit]/concentration of M in cpx [cations per formula unit]) is mainly controlled by temperature and to a much lesser degree by pressure. Variations in major element compositions of pyroxenes (e.g., variable XMg, AlIV or Na) have no influence on DM. For Sc, V, Cr, Mn, and Co, our data result in good correlations between ln DM and reciprocal absolute temperature, with correlation coefficients (r) between 0.950 and 0.981. It is therefore possible to use the partitioning of these elements between opx and cpx from peridotites and websterites as geothermometers. On the basis of our data, we suggest the following empirical thermometer equations: TSc = [(17.64 · P + 5663)/(3.25 − ln DSc)], TV = [(18.06 · P + 3975)/(2.27 − ln DV)], TCr = [(11.00 · P + 2829)/(1.56 − ln DCr)], TMn = [(−0.20 · P − 2229)/(−1.37 − ln DMn)], TCo = [(−4.31 · P − 2358)/(−0.98 − ln DCo)], where T is the absolute temperature in Kelvin and P the pressure in kilobars. For Ti and Ni observed correlations between ln DM and 1/T are less well defined.

Continental mafic dyke swarms as tectonic indicators: an example from the Vestfold Hills, East Antarctica

Abstract The two main types of magma that were emplaced as mafic dyke swarms in the Vestfold Hills are high-Mg tholeiite (HMT) and Fe-rich tholeiite (FRT). The former magma type is less dense than the Vestfold Hills felsic crust, whereas the latter is more dense at deep to intermediate crustal levels. Therefore, FRT dyke emplacement requires a higher deviatoric extensional stress, as is reflected in a more constant orientation. Palaeoproterozoic HMT-FRT bimodal dyke swarms were emplaced around 2.4 Ga and 2240 Ma. The geometry and orientation of 2240 Ma HMT dykes imply a low deviatoric extensional stress field, inconsistent with a mantle plume origin for magmatism. Current petrogenetic models for HMT magmatism suggest generation at active (convergent) plate boundaries. Although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2.5 Ga, the exact mechanisms for melting and the generation of crustal extensional stress are unknown. A possible scenario involves the sinking of gravitationally unstable oceanic crust. A major Mesoproterozoic magmatic—extensional event in the Vestfold Hills started with normal faulting, followed by lamprophyric dyke emplacement and by two distinct episodes of FRT dyke emplacement at 1380 and 1245 Ma. A consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift. By comparison with other mafic dyke swarms, this is interpreted to be due to a sheet of upwelling asthenosphere (elongated plume) around 1.4 Ga, rather than a low-angle detachment. This upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 Ma. Erosion during a long period of uplift prior to 1380 Ma was followed by sedimentation during thermal subsidence, as suggested by segmentation of 1245 Ma dykes. The lithospheric structure inherited from the Mesoproterozoic magmatic—extensional event was probably reactivated during two high-grade tectono-metamorphic events around 1.0 and 0.5 Ga. The Vestfold Hills formed part of a relatively stable craton, peripheral to the mobile belt that was formed during these events.

The potential of stable Cu isotopes for the identification of Bronze Age ore mineral sources from Cyprus and Faynan: results from Uluburun and Khirbat Hamra Ifdan

Copper isotope ratios differ between hypogene sulfidic, supergene sulfidic and oxidized ore sources. Traditional lead isotope signatures of ancient metals are specific to deposits, while Cu isotope signatures are specific to the types of ore minerals used for metal production in ancient times. Two methodological case studies are presented: First, the mining district of Faynan (Jordan) was investigated. Here, mainly oxidized copper ores occur in the deposits. The production of copper from Fayan’s ore sources is confirmed by the measurement of the Cu isotope signature of ingots from the Early Bronze Age metal workshop from Khirbat Hamra Ifdan. Based on our results illustrating differences in the Cu isotope composition between the ore mineralizations from Timna (Israel) and Faynan, it is now possible to determine these prehistoric mining districts from which copper artifacts originated by combining trace elements and Pb isotopes with Cu isotopes. The second case study presents data on Late Bronze Age copper production in Cyprus. Oxhide ingots from the shipwreck of Uluburun (Turkey) were tested for their lead isotope signatures and assigned to Cypriot deposits in the recent decades. The oxhide ingots from Uluburun show a Cu isotope signature which we also found for oxidized copper ores from Cyprus, while younger oxhide ingots as well as metallurgical slag from the Cypriot settlements Kition and Enkomi show a different signature which might be due to the use of sulfidic ore sources from a greater depth of deposits. We assert that there could be a chronological shift from oxidized to sulfidic ore sources for the copper production in Cyprus, requiring different technologies. Therefore, Cu isotopes can be used as a proxy to reconstruct mining and induced smelting activities in ancient times.

Metasomatic Processes Revealed by Trace Element and Redox Signatures of the Lithospheric Mantle Beneath the Massif Central, France

Trace element compositions of clinopyroxene and oxidation state measurements are both useful parameters for constraining metasomatic interactions within the subcontinental lithospheric mantle (SCLM). Laser ablation inductively coupled plasma mass spectrometry analyses of trace elements in clinopyroxene in combination with oxidation state data from the same samples provide a fuller picture of the metasomatic processes that occurred within the SCLM below the French Massif Central (FMC). A total of 89 spinel peridotite xenoliths from 39 localities across the FMC document variable degrees of partial melting and metasomatism at both regional and local scales. According to the degree of enrichment in light rare earth elements (LREE) and middle rare earth elements (MREE) in clinopyroxene, four main groups (A, B, C and D) are identified. Variations in other trace element concentrations are consistent with these groups. LREE enrichment correlates with a tendency to higher oxygen fugacity (fO2). These characteristics, along with their Ti/Eu–Zr/Hf systematics, are useful in assessing the different metasomatic agents that have interacted with the FMC SCLM. The behaviour of Lu and Hf additionally provides a means for distinguishing secondary clinopyroxene grains from those that have been overprinted. Most samples have been affected by cryptic metasomatism and different signatures indicate interaction with different types of metasomatic agent. Modal metasomatism is apparent by the presence of amphibole or secondary clinopyroxene. The lack of any notable trace element or redox signature related to the occurrence of amphibole suggests that it formed from small amounts of dilute H2O-rich fluids. Most samples from the northern sector of the FMC are fairly refractory and clinopyroxene exhibits a ‘tick-shaped’ REE signature (group D), consistent with metasomatism by subduction-related fluids during Variscan times. Such fluids were oxidizing (Δlog fO2 values > FMQ + 1, where FMQ is the fayalite–magnetite–quartz buffer) and responsible for LREE enrichment in clinopyroxene and low concentrations of the less mobile elements such as Ti, Eu, Zr and Hf. Clinopyroxene in several samples has very low Ti/Eu, elevated (La/Yb)N and suprachondritic Zr/Hf ratios, reflecting local carbonatite metasomatism. These samples also record oxidizing conditions, with Δlog fO2 values > FMQ + 1. Many other samples have clinopyroxenes with higher Ti/Eu and approximately chondritic Zr/Hf ratios, consistent with having undergone silicate melt metasomatism. Their chondrite-normalized REE element patterns vary from LREE-depleted to relatively flat (group A), indicating small but variable metasomatic enrichments in the more mobile elements, which can be attributed to chromatographic effects during reactive flow. The oxidation states of these samples record Δlog fO2 ∼ FMQ + 0·4 ± 0·5. Some samples have clinopyroxene with negative Zr and Hf anomalies and distinctly low Ti/Eu (<2000), but not as low as expected for carbonatite metasomatism. These may have interacted with carbonated-silicate melts, which also imposed somewhat elevated oxidation states (Δlog fO2 values up to ∼FMQ + 0·9). Some samples exhibit evidence for having been overprinted by more than one metasomatic event, in part indicated by two populations of geochemically distinct clinopyroxene. These samples, among others, also demonstrate that immobility of heavy REE (HREE) cannot be assumed when modeling the degree of melt extraction of strongly metasomatized peridotites. Addition of HREE by refertilization lowers the apparent degree of melting compared with calculations based upon Cr/(Cr + Al) in spinel. In other cases, a lowering of HREE concentrations via interaction with a low-HREE metasomatic agent produces higher apparent degrees of melting.

Ferropericlase inclusions in ultradeep diamonds from Sao Luiz (Brazil): high Li abundances and diverse Li-isotope and trace element compositions suggest an origin from a subduction mélange

The most remarkable feature of the inclusion suite in ultradeep alluvial and kimberlitic diamonds from Sao Luiz (Juina area in Brazil) is the enormous range in Mg# [100xMg/(Mg + Fe)] of the ferropericlases (fper). The Mg-richer ferropericlases are from the boundary to the lower mantle or from the lower mantle itself when they coexist with ringwoodite or Mg- perovskite (bridgmanite). This, however, is not an explanation for the more Fe-rich members and a lowermost mantle or a “D” layer origin has been proposed for them. Such a suggested ultra-deep origin separates the Fe-rich fper-bearing diamonds from the rest of the Sao Luiz ultradeep diamond inclusion suite, which also contains Ca-rich phases. These are now thought to have an origin in the uppermost lower mantle and in the transition zone and to belong either to a peridotitic or mafic (subducted oceanic crust) protolith lithology. We analysed a new set of more Fe-rich ferropericlase inclusions from 10 Sao Luiz ultradeep alluvial diamonds for their Li isotope composition by solution MC-ICP-MS (multi collector inductively coupled plasma mass spectrometry), their major and minor elements by EPMA (electron probe micro-analyser) and their Li-contents by SIMS (secondary ion mass spectrometry), with the aim to understand the origin of the ferropericlase protoliths. Our new data confirm the wide range of ferropericlase Mg# that were reported before and augment the known lack of correlation between major and minor elements. Four pooled ferropericlase inclusions from four diamonds provided sufficient material to determine for the first time their Li isotope composition, which ranges from δ7Li + 9.6 ‰ to −3.9 ‰. This wide Li isotopic range encompasses that of serpentinized ocean floor peridotites including rodingites and ophicarbonates, fresh and altered MORB (mid ocean ridge basalt), seafloor sediments and of eclogites. This large range in Li isotopic composition, up to 5 times higher than ‘primitive upper mantle’ Li-abundances, and an extremely large and incoherent range in Mg# and Cr, Ni, Mn, Na contents in the ferropericlase inclusions suggests that their protoliths were members of the above lithologies. This mélange of altered rocks originally contained a variety of carbonates (calcite, magnesite, dolomite, siderite) and brucite as the secondary products in veins and as patches and Ca-rich members like rodingites and ophicarbonates. Dehydration and redox reactions during or after deep subduction into the transition zone and the upper parts of the lower mantle led to the formation of diamond and ferropericlase inclusions with variable compositions and a predominance of the Ca-rich, high-pressure silicate inclusions. We suggest that the latter originated from peridotites, mafic rocks and sedimentary rocks as redox products between calcite and SiO2.