Thomas Pettke

Trace element signature of subduction-zone fluids, melts and supercritical liquids at 120–180 km depth

Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into the mantle wedge, facilitate melting and ultimately lead to prolific subduction-zone arc volcanism. The nature and composition of the mobile phases generated in the subducting slab at high pressures have, however, remained largely unknown. Here we report direct LA-ICPMS measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth of 120–180 km and temperatures of 700–1,200 °C. The resultant liquid/mineral partition coefficients constrain the recycling rates of key elements. The dichotomy of dehydration versus melting at 120 km depth is expressed through contrasting behaviour of many trace elements (U/Th, Sr, Ba, Be and the light rare-earth elements). At pressures equivalent to 180 km depth, however, a supercritical liquid with melt-like solubilities for the investigated trace elements is observed, even at low temperatures. This mobilizes most of the key trace elements (except the heavy rare-earth elements, Y and Sc) and thus limits fluid-phase transfer of geochemical signatures in subduction zones to pressures less than 6 GPa.

Quantitative multi-element analysis of minerals, fluid and melt inclusions by laser-ablation inductively-coupled-plasma mass-spectrometry

Laser-ablation ICPMS has become widely accessible as a powerful and efficient multi-element microanalytical technique. One of its key strengths is the ability to analyse a wide concentration range from major (tens of wt.%) to trace (ng/g) levels in minerals and their microscopic inclusions. An ArF excimer laser system (λ = 193 nm) with imaging optics for controlled UV ablation and simultaneous petrographic viewing was designed specifically for representative sampling and quantitative multi-element analysis of microscopic fluid, melt and mineral inclusions beneath the sample surface. After a review of the requirements and recent technical developments, results are presented which together document the reliability and reproducibility of quantitative microanalysis of complex samples such as zoned crystals or fluid and melt inclusions in various host minerals. Analytical errors due to elemental fractionation are reduced to the typical precision achieved by quadrupole LA-ICPMS in multi-element mode (2–5% RSD). This progress is largely due to the small size of aerosol particles generated by the optimized UV optical system. Depth profiling yields representative and accurate concentration results at a resolution of ∼0.1 μm perpendicular to the ablation surface. Ablation is largely matrix-insensitive for different elements, such that silicate and borate glasses, silicates and oxide minerals, or direct liquid ablation can be used interchangeably for external standardization of any homogeneous or heterogeneous material. The absolute ablation rate is material dependent, however, so that quantitative LA-ICPMS analysis requires an internal standard (i.e., an independent constraint such as the absolute concentration of one element). Our approach to quantifying fluid and melt inclusion compositions is described in detail. Experiments with synthetic fluid inclusions show that accurate results are obtained by combining the LA-ICPMS analysis of element concentration ratios with a microthermometric measurement of the NaCl equivalent concentration and an empirical description of the effect of major cations on the final melting temperatures of ice, hydrohalite or halite. Expected calibration errors for NaCl-H2O-dominated fluids are smaller than the typical analytical scatter within an assemblage of simultaneously trapped fluid inclusions. Analytical precision is limited by representative ablation of all phases in heterogeneous inclusions and the integration of transient ICPMS signals, to typically ±10 to 20% RSD. Element concentrations in devitrified and even coarsely crystallized silicate melt inclusions can be reconstituted from LA-ICPMS signals. Deconvolution of inclusion and host signals with internal standardization automatically corrects for sidewall crystallization after melt entrapment at high temperature. A test using melt inclusions in a midocean ridge basalt, a summary of published geochemical studies and a new application to REE analysis of coexisting fluids and mineral phases in carbonatite-related veins illustrate the versatility and some of the strengths and limitations of LA-ICPMS, in comparison with other microanalytical techniques.

APPLICATIONS OF MULTIPLE COLLECTOR-ICPMS TO COSMOCHEMISTRY, GEOCHEMISTRY, AND PALEOCEANOGRAPHY

Multiple collector-inductively coupled plasma mass spectrometry (MC-ICPMS) is a new technique for the measurement of isotopic compositions at high precision, and is of great relevance to planetary, earth, ocean, and environmental sciences. The method combines the outstanding ionization efficiency of the ICP source with the superior peak shapes achievable from the ion optical focal plane of a large dispersion magnetic sector mass spectrometer, utilizing simultaneous multiple collection to achieve the most precise isotopic measurements yet made for many elements—particularly those with high first ionization potential. The addition of a laser facilitates studies for which spatial resolution is required. This method is still in its infancy, yet diverse applications have already led to a number of important scientific developments. Here we review some of these accomplishments and the potential for further work. The Lu-Hf isotopic system, for many years considered analytically challenging, is now relatively straightforward and offers great promise in fields as diverse as garnet geochronology, hydrothermal fluxes to the oceans, and crustal evolution. The age of the Earth’s core, the Moon, and Mars have been measured using a new short-lived chronometer 182Hf-182W. Other such new chronometers will follow. High precision isotope dilution measurements of the Earth’s inventory of many poorly understood elements such as In, Cd, Te, and the platinum group elements are providing tests for models for the accretion of the inner solar system. The small natural isotopic variations in elements such as Cu and Zn, produced by mass dependent fractionation, are now measurable at high precision with this method, and entirely new fields of stable isotope geochemistry can be developed. Similarly, measurements of small nucleosynthetic isotopic anomalies should be made easier for some elements. Measurements of U and Th isotopic compositions at very high sensitivity and reproducibility are now possible, allowing the development of higher resolution Quaternary geochronology. Finally, using laser ablation, the first precise in situ Sr, Hf, W, and Pb isotopic measurements have been made in natural materials, opening up a range of microanalytical isotopic studies in petrology and marine geochemistry. MC-ICPMS offers exciting times ahead in areas well beyond the bounds of geochemistry. Indeed, MC-ICPMS is likely to become the method of choice for many isotopic measurements because it is a more user friendly and efficient method for the acquisition of high precision data. It is also much more versatile, permitting elements to be measured that were previously considered intractable, and allowing the acquisition of data in situ, all with a single mass spectrometer. The limiting factor on the sensitivity is the transmission which is ≤2% for all instruments thus far designed. If it is found possible to improve the transmission still further, thermal ionization mass spectrometry, the technique that has, thus far, provided the high precision measurements necessary for most of the vast field of radiogenic isotope geochemistry, may be relegated to specialized applications.

Major to trace element analysis of melt inclusions by laser-ablation ICP-MS : Methods of quantification

Current techniques for the quantification of melt inclusion chemistry require that inclusions are compositionally homogeneous and that post-entrapment devitrification or crystallization onto the inclusion walls could be reversed by appropriate re-melting. Laser-ablation ICP-MS provides a technique by which single heterogeneous inclusions can be analysed, thus avoiding the above prerequisites. Because host mineral is ablated with the inclusion, quantification of the melt composition necessitates deconvolution of the mixed signal by an internal standard. This can be obtained in various ways, including: (1) a fixed, pre-determined, concentration of a given element in the melt; (2) whole rock differentiation trends in a given igneous suite; (3) a constant, measured, distribution coefficient between the host and the inclusion melt and (4) determination of the volume ratios between the inclusion and total ablated volume. These four approaches were tested on a large set of cogenetic inclusions from a single plagioclase crystal in a rhyodacitic intrusion. Results suggest that quantification through whole rock differentiation trends is the most widely applicable, the most accurate and the least time-consuming technique, provided that the resulting data are critically interpreted with regard to the underlying assumptions. Uncertainties on the calculated element concentrations in the inclusions depend on the mass ratio between the melt inclusion and the host for a given ablation. They are of the order of 10% if the melt inclusion contributes more than 20% to the bulk analytical signal of a particular element. Calculated limits of detection for spherical 10 μm melt inclusions are of the order of a few ppm for elements strongly enriched in the melt relative to the host crystal. Concentrations in the melt inclusions can be determined even for elements enriched in the host mineral, but in this case uncertainties and calculated limits of detection increase with the concentration in the host. The uncertainty on the melt composition from a set of cogenetic inclusions can be commonly decreased by calculating of an uncertainty-weighted average of the concentration and their uncertainty.

Dust production and deposition in Asia and the north Pacific Ocean over the past 12 Myr

The silicate fractions of recent pelagic sediments in the central north Pacific Ocean are dominated by eolian dust derived from central Asia. An 11 Myr sedimentary record at ODP Sites 885/886 at 44.7°N, 168.3°W allows the evaluation of how such dust and its sources have changed in response to late Cenozoic climate and tectonics. The extracted eolian fraction contains variable amounts (>70%) of clay minerals with subordinate quartz and plagioclase. Uniform Nd isotopic compositions (ϵNd=−8.6 to −10.5) and Sm/Nd ratios (0.170–0.192) for most of the 11 Myr record demonstrate a well-mixed provenance in the basins north of the Tibetan Plateau and the Gobi Desert that was a source of dust long before the oldest preserved Asian loess formed. ϵNd values of up to −6.5 for samples <2.9 Ma indicate ≤35 wt% admixture of a young, Kamchatka-like volcanic arc component. The coherence of Pb and Nd in the erosional cycle allows us to constrain the Pb isotopic composition of Asian loess devoid of anthropogenic contamination to 206Pb/204Pb=18.97±0.06, 207Pb/204Pb=15.67±0.02, 208Pb/204Pb=39.19±0.11. 87Sr/86Sr (0.711–0.721) and Rb/Sr ratios (0.39–1.1) vary with dust mineralogy and provide an age indication of ∼250 Ma. 40Ar/39Ar ages of six dust samples are uniform around 200 Ma and match the K–Ar ages of modern dust deposited on Hawaii. These data reflect the weighted age average of illite formation. Changes from illite≥smectite with significant kaolinite to illite- and chlorite-rich, kaolinite-free assemblages since the late Pliocene document changes in the intensity of chemical weathering in the source region. Such weathering evidently did not disturb the K–Ar systematics, and only induced scatter in the Rb–Sr data. We propose that when smectite forms at the expense of illite, K and Ar are quantitatively lost from what becomes smectite, but are quantitatively retained in adjacent illite layers. 40Ar/39Ar age data, therefore, are insensitive to smectite formation during chemical weathering but date the diagenetic growth of illite, the major K-bearing phase in the dust. Over the past 12 Myr, the dust flux to the north Pacific increased by more than an order of magnitude, documenting a substantial drying of central Asia. This climatic change, however, did not alter the ultimate source of the dust, and neoformational products of chemical weathering always remained subordinate to assemblages reworked by mechanical erosion in dust deposited in eastern Asia and the Pacific Ocean.

Refertilization of mantle peridotite in embryonic ocean basins: trace element and Nd isotopic evidence and implications for crust–mantle relationships

Many mantle peridotites exhumed along ancient and present-day magma-poor passive continental margins, along (ultra-) slow spreading ridges and fracture zones are plagioclase-bearing and generally too fertile to be the residue of partial melting processes alone. Likewise, the associated gabbroic and basaltic rocks are not a priori genetically linked to the underlying mantle rocks. Trace element and Nd isotopic studies in the eastern Central Alps peridotites in eastern Switzerland and northern Italy provide evidence for near-fractional melting and depletion at high pressure in Permian time followed by refertilization of subcontinental mantle by ascending melts at low pressure in Jurassic time. These results suggest regional-scale modification of ancient subcontinental mantle by melt infiltration and melt–rock reaction during incipient opening of oceanic basins. The similar Nd isotopic composition of plagioclase peridotite (ϵNd160: 7.4–10.6) and associated mafic crust (ϵNd160: 7.3–9.6) indicates that the liquids, which reacted with the peridotites derived from similar N-MORB type mantle sources. Plagioclase peridotites in magma-poor passive margins may predominantly form as a consequence of diffuse porous flow of melt in the thermal boundary layer above an upwelling asthenosphere and probably represent modified ancient subcontinental mantle. Thus, plagioclase peridotites exhumed in passive margins and possibly in (ultra-) slow spreading ridges may represent magma-poor periods where liquids stagnate in the thermal boundary layer and react with the surrounding peridotites. Once the effects of conductive heat loss dominate over advection of heat from below, diffuse porous flow of melt becomes less important and might be replaced by the formation of gabbro bodies.

The magmatic-hydrothermal evolution of two barren granites: a melt and fluid inclusion study of the Rito del Medio and Cañada Pinabete plutons in northern New Mexico (USA)

The magmatic-hydrothermal evolution of two barren granites associated with the Tertiary Questa Caldera in northern New Mexico was reconstructed on the basis of microthermometric and laser-ablation inductively coupled plasma mass spectrometry analysis of fluid and melt inclusions in quartz of magmatic and hydrothermal origin. During progressive crystallization and fluid exsolution, the Cs content of the residual melt increased from 1 ppm to values as high as 5500 ppm, which requires an increase in crystallinity by at least 99.98%. In conjunction with a Rayleigh fractionation model simulating the melt evolution at fluid-saturated vs. fluid-undersaturated conditions, the melt inclusion data can be used to determine the crystallinity at which fluid saturation was reached. Our results suggest that both plutons attained fluid saturation before 30% crystallization and that evolved residual melts accumulated in their roof zones. At ∼90% crystallization, the exsolving fluids were of low salinity (∼5 wt.% NaClequiv) and in the one-phase field, in accordance with phase relations at the reconstructed P and T conditions (∼1.1 to 1.3 kbar, 700 to 720°C). Fluid-melt partition coefficients for a range of metals determined on assemblages of coeval melt and fluid inclusions were generally too low to allow efficient metal extraction from the melt (DX, fluid-melt < 22). As a result, the metal concentrations in both the residual melt and the coexisting fluid increased with progressive crystallization. Absolute metal contents in the fluids exsolving from barren systems appear low, however, when compared with mineralized systems. It is concluded that the absence of mineralization in the Rito del Medio and Canada Pinabete plutons primarily stems from a low salinity of the exsolving fluids, resulting in a less efficient metal extraction from the melt.

Majoritic garnets monitor deep subduction fluid flow and mantle dynamics

The ultradeep mantle rocks of western Norway display three generations of majoritic garnet. The first two derive from incompatible element–depleted transition-zone mantle and exsolved pyroxene components during Archean upwelling, accretion to subcratonic lithosphere (M1 stage), and isobaric cooling until the Middle Proterozoic (M2). A subsequent Scandian (430–390 Ma) subduction cycle initiated diamond crystallization (M3). Here we report a third majoritic garnet crystallized at grain boundaries and in microfractures, and stable with pyroxene, phlogopite, and spinel in the M3 assemblage. The trace element signatures of M3 minerals indicate crustal metasomatism, phlogopite being the main large ion lithophile element repository. These features imply majorite crystallization from crust-derived subduction fluids at 200 km depth. Our finding fixes the deepest occurrence of free subduction fluid phases and indicates that garnet is a reliable monitor of deep mantle evolution and fluid-mediated chemical recycling.

Ophiolitic Peridotites of the Alpine-Apennine System: Mantle Processes and Geodynamic Relevance

Ophiolites exposed in the Alpine-Apennine mountain range represent the oceanic lithosphere of the Ligurian Tethys, a small oceanic basin separating the Europe and Adria plates during Mesozoic time. Most of the peridotites represent former subcontinental mantle which was: (a) isolated from the convective mantle at different times (from Proterozoic to Permian); and (b) accreted to the thermal lithosphere, where it cooled along a conductive geothermal gradient under spinel-peridotite facies conditions. These peridotites record two magmatic cycles: (1) early diffuse porous flow percolation and impregnation by single-melt increments, focused percolation in dunite channels, and intrusion of MORB-type melts; and (2) late intrusion and extrusion of magmas deriving from aggregated MORB liquids. The early lithosphere/asthenosphere interaction by melt percolation induced significant depletion/refertilization and heating of mantle peridotites, leading to the thermochemical erosion of lithospheric mantle. Plagioclase-bearing peridotites of the Alpine-Apennine ophiolites were derived from melt impregnation, whereas part of the depleted spinel peridotites resulted from reactive percolation of depleted melts, rather than being refractory residua after near-fractional melting. The presence of large areas of impregnated peridotites indicates that significant volumes of melts were trapped in the lithospheric mantle; subsequently, asthenospheric melts reached the surface, both intruding as MORB gabbroic bodies or extruding as MORB lava flows. Our results provide a mechanism to explain nonvolcanic and volcanic stages during rift evolution of the Ligurian Tethys, and might be equally applicable to modern slow-spreading ridges, which are characterized by variable magmatic (volcanic) and amagmatic (nonvolcanic) stages.

Crystallographic texture and microstructure of terebratulide brachiopod shell calcite : An optimized materials design with hierarchical architecture

Abstract We analyzed the microstructure, microchemistry, and microhardness variations across the architectural elements of the shells of the brachiopod species Megerlia truncata and Terebratalia transversa with scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS), and Vickers microhardness indentation (VMHI). The brachiopod valves consist of two principal layers of distinct calcite biomineralization: a thin, nanocrystalline, outer, hard protective layer with VMHI values exceeding 200 HV and a much thicker, inner, secondary layer of a hybrid organic-inorganic fiber composite material. The secondary layer is further structured into two sublayers, an outer part with VMHI values varying between 110 and 140 HV, and a softer inner part (70 < HV < 110). Whereas the size of the calcite crystals within the primary layer varies between a few tens of nanometers and 2 μm, calcite crystals within the secondary layer are fibrous, commonly reaching lengths exceeding 150 μm. Cross sections of these fibrous crystals are spade shaped, their dimensions being about 5 × 20 μm. The fibers are aligned parallel to each other. They are single crystals with their morphological fiber axes pointing almost parallel to the shell vault. The crystallographic orientation of the morphological fiber axes, however, is arbitrary within the a-b plane of the calcite lattice, whereas the c-axis (hexagonal unit-cell setting) is perpendicular to the morphological fiber axes and thus parallel to the radial vector of the valve vault. This morphology strongly indicates that fibrous growth is controlled by confinement within a cell in an organic matrix and not by attachment of biomolecules to specific crystallographic faces. We observe inhomogeneous Sr2+ and Mg2+ concentrations in the shell calcite within the 0.1.0.9 wt% range. Design of the shell appears to be highly optimized for mechanical performance. Crystal morphology and orientation as well as incorporated organic matter are structured hierarchially at different length levels forming a hybrid organic-inorganic fiber composite architecture.