ALBERT

Description: Fluid-rock interaction has profound effects on the dynamics of the lithosphere. This Perspectives article describes the catalytic effects of water on the kinetics of mineral reactions and on the strength of rocks in terms of coupled dissolution and precipitation mechanisms on a macro- and nano-scale. The length scale of coupling between the dissolution and precipitation steps depends on the fluid composition at the mineral-fluid interface and also on differential stress. Stress-induced mass transport, dependent on the generation of porosity by mineral reactions, results in dissolution-precipitation creep as the principal mechanism of rock deformation in the lithosphere. The heterogeneous distribution of fluid infiltration into dry, strong rocks in the deep crust leads to weak rocks within strong, stressed host rock and the possibility of significant local variations in pressure. Fluid-rock interaction mechanisms are discussed in terms of recrystallisation reactions whereby mass transport and the most favourable nucleation sites determine the distribution and texture of the resultant assemblages. Metamorphic differentiation is a natural consequence of dissolution-precipitation mechanisms under deviatoric stress and similar mechanisms may apply to general pattern formation in rocks.

Description: The origin of the eclogites that reside in cratonic mantle roots has long been debated. In the classic Roberts Victor kimberlite locality in South Africa, the strongly contrasting textural and geochemical features of two types of eclogites have led to different genetic models. We studied a new suite of 63 eclogite xenoliths from the former Roberts Victor Mine. In addition to major- and trace-element compositions for all new samples, we determined 18O/16O for garnet from 34 eclogites. Based on geochemical and textural characteristics we identify a large suite of Type I eclogites (n = 53) consistent with previous interpretations that these rocks originate from metamorphosed basaltic-picritic lavas or gabbroic cumulates from oceanic crust, crystallised from melts of depleted MORB mantle. We identify a smaller set of Type II eclogites (n = 10) based on geochemical and textural similarity to eclogites in published literature. We infer their range to very low δ18O values combined with their varied, often very low Zr/Hf ratios and LREE-depleted nature to indicate a protolith origin via low-pressure clinopyroxene-bearing oceanic cumulates formed from melts that were more depleted in incompatible elements than N-MORB. These compositions are indicative of derivation from a residual mantle source that experienced preferential extraction of incompatible elements and fractionation of Zr-Hf during previous melting.

Description: This paper explores the unusual sulphide-graphite association of a selection of Beni Bousera garnet clinopyroxenites that initially equilibrated within the diamond stability field. Compared to common graphite-free garnet pyroxenites analysed so far, these rocks display tenfold S enrichment with concentrations up to 5550 μg/g. Fe-Ni-Cu sulphides (up to 1.5 wt.%) consist of large (up to 3 mm across), low-Ni pyrrrhotite (

Description: Peridotites from the Tonga Trench are some of the deepest-derived and freshest ever obtained from the seafloor. This study reports new bulk-rock major-, trace-, highly siderophile-element (HSE) abundance and 187Os/188Os data, as well as major- and trace-element abundances of mineral phases for NOVA88D dredge peridotites. The samples are harzburgites that experienced varying degrees of serpentinisation, recorded in their loss on ignition values, from zero to 16.7%. Degree of serpentinisation in samples is correlated with Na, B, K, Sr, Ca, Rb and U, and weakly correlated with W, Fe, Pb, Cs and Li abundances, but is uncorrelated with other lithophile elements, most especially the rare earth elements. Serpentinisation had no systematic effect on the HSE abundances or 187Os/188Os compositions in the harzburgites. NOVA88D harzburgites record 〉18% melt depletion which has resulted in heterogenous distribution of the HSE within the rocks, likely due to retention of these elements within sub-micron sized alloy or sulphide phases. Time of rhenium depletion (TRD) ages, recorded by Os isotopes, average ~0.7 ±0.4 Ga and can be as ancient as 1.5 Ga. Some harzburgite compositions are consistent with minor melt infiltration processes modifying incompatible trace element compositions and Re abundances, with a possible melt infiltration event at ~120 Ma based on 187Re-188Os, prior to the inception of subduction at the Tonga Trench at ~52 Ma. Evidence for ancient melt depletion, combined with limited melt processing since inception of subduction suggests that NOVA88D harzburgites represent melt residues incorporated into the Tonga arc, rather than their geochemical signatures being produced beneath the recent arc. Estimates of fO2 (~-0.4 ±0.4 ΔFMQ) and olivine-spinel equilibration temperatures for the Tonga Trench samples (830 ±120 ̊C) are similar to abyssal peridotites and some Izu-Mariana-Bonin peridotites. These values are unlikely to relate directly to recorded degrees of melt depletion and melt depletion ages in the rocks. Refractory residues from prior melt depletion events are probably common in the convecting mantle, and those with high degrees of melt depletion (〉18%) and relatively ancient melt depletion ages (

Description: The Bishop Tuff (BT), erupted from the Long Valley caldera in California, displays two types of geochemical gradients with temperature: one is related to magma mixing, whereas the other is found in the high-SiO2 rhyolite portion of the Bishop Tuff and is characterized by ≤ twofold concentration variations in minor and trace elements that are strongly correlated with temperature. It is proposed that the latter zonation, which preceded phenocryst growth, developed due to mineral–melt partitioning between interstitial melt and surrounding crystals in a parental mush, from which variable melt fractions were segregated. To test this hypothesis, trends of increasing vs. decreasing element concentrations with temperature (proxy for melt fraction), obtained from published data on single-clast pumice samples from the high-SiO2 rhyolite portion of the Bishop Tuff, were used to infer their relative degrees of incompatibility vs. compatibility between crystals and melt in the parental mush. Relative compatibility values (RCVi) for all elements, defined as the concentration slope with temperature divided by average concentration, are shown to be linearly correlated with their respective bulk partition coefficients (bulk Di). Mineral–melt partition coefficients from the literature were used to constrain the average stoichiometry of the crystallization/melting reaction in the parental mush: 32% quartz + 34% plagioclase + 31% K-feldspar + 1.60% biotite + 0.42% titanomagnetite + 0.34% ilmenite + 0.093% allanite + 0.024% zircon + 0.025% apatite = 100% liquid. The proportions of tectosilicates in the reaction (i.e., location of eutectic) are consistent with depths of melt segregation of ~400–550 MPa and an activity of H2O of ~0.4–0.6. Temperatures of

Description: Garnet commonly accommodates high contents of Mn+Y+H (heavy)-REEs (rare earth elements) that follow Rayleigh fractionation during garnet early growth, with the exception of overstepping nucleation (late crystallization due to reaction overstepping). Because of this, as the garnet porphyroblasts form mostly in equilibrium with the surrounding matrix, the concentration of these elements continuously decreases towards the porphyroblast rims. Yet, rapid changes in the reaction progress of a rock during garnet growth, namely the resorption–dissolution of minerals with high concentrations of Y+REEs, may create an anomaly or peak in the mantle or rim parts of garnet grains. In this study we present an example of the resorption of garnet cores and formation of atoll garnet textures in eclogite from the Krušné hory Mountains (in the Saxothuringian tectonic zone of the Bohemian Massif). Based on textural relations, we show that the atoll garnet grains in the studied rocks were formed during the prograde stage from blueschist to eclogite facies metamorphism. Preliminary observations showed that the full (non-atoll) garnet grains had compositionally different cores (interior, or garnet I) and rims (ring, or garnet II) that were separated by a Y+H(M, or medium) REEs concentration peak. The ring garnet II indicated an elevated concentration of Mn in comparison to the marginal parts of the interior garnet I. Therefore, minor elements that were less vulnerable to diffusion than major elements and strongly sensitive to the broad spectrum of geochemical processes, such as Y+REEs, were used to track possible mineral reactions during the whole garnet growth path. Thermodynamic modelling indicated the formation of garnet by the breakdown of chlorite and lawsonite/zoisite, and peak-pressure phases were represented by garnet, omphacite, quartz, amphibole, rutile, and talc. To quantify the sources of high Mn concentrations in garnet II and of the Y+H(M)REEs sharp peaks, the sequences of mineral reactions and dissolution of garnet I leading to the formation of the atoll structure were investigated. In addition to thermodynamic modelling and pressure–temperature path constraints, mass-balance calculations of trace elements were also performed. The results combined with the observed compositional and textural relations indicate that the concentrations of Mn+Y+H(M)REEs in garnet II and the concentration peaks at the interface of the two garnet types were controlled by a complex mechanism that included the dissolution of garnet I during the formation of the atoll texture, stepwise growth of garnet during increasing pressure and temperature, and decomposition of phases with high concentrations of trace elements, such as zoisite/epidote or lawsonite.

Description: Phase equilibria and trace-element modeling using two previously reported basaltic bulk-rock compositions (samples D11 and 104-16), were carried out in this study, in order to better understand mechanism of low-pressure (LP) partial melting of mafic rocks and associated melt compositions. The T–MH2O pseudosections for both samples at three pressures (i.e. 0.5, 1.0 and 2.0 kbar) display that the H2O-stability field gradually increased with decreasing pressure within the T–MH2O range of 600–1100 °C and 0–12 mol.%. The H2O contents of 10, 5.0, and 0.5 mol.% were selected on the basis of the T–MH2O pseudosections to calculate P–T pseudosections over a P–T window of 0.1–3 kbar and 600–1100 °C, so that the reactions of both the H2O-fluxed and -absent meltings at LP conditions can be investigated. The solidus displays a negative or near-vertical P–T slope, and occurs between 710 and 900 °C at pressure between 0.1 and 3.0 kbar. LP melting of metabasites is attributed to the reactions of the hydrous mineral (hornblende and/or biotite) melting and anhydrous mineral (plagioclase, orthopyroxene, and augite) melting. The hydrous mineral melting is gradually replaced by anhydrous mineral melting as pressure decreasing, as the stability of hornblende decreases with falling pressure. With increasing temperature at a given pressure, the modeled melt compositions are expressed as progressions of the granite-granodiorite-gabbroic diorite fields for sample D11and granite-quartz monzonite-monzonite-gabbroic diorite fields for sample 104-16 on the total alkali–silica diagram. The modeled melts produced through the H2O-fluxed melting display higher Al2O3, CaO, MgO, and lower SiO2 and K2O than those formed by H2O-absent melting at the same P–T conditions. Furthermore, the modeled melts formed by H2O-absent melting, become richer in Al2O3, CaO, MgO, FeO, Na2O, but poorer in SiO2 and K2O as increasing water content. The results of trace-element modeling suggests that the nearly flat REE patterns of modeled bulk-rock composition are inherited by all the modeled melts, and the negative Eu anomalies and Sr depletion of the modeled melts gradually decrease as melting degree increases. Combined with the geochemical characteristics of natural oceanic plagiogranites, which have low K2O contents and flat or slightly LREE-depleted REE patterns, our results imply that a bulk-rock composition with low K2O (

Description: Reconstructed whole-rock and mineral major- and trace-element compositions, as well as new oxygen isotope data, for 22 mantle eclogite xenoliths from the Catoca pipe (Kasai Craton) were used to constrain their genesis and evolution. On the basis of mineralogical and major-element compositions, the Catoca eclogites can be divided into three groups: high-alumina (high-Al) (kyanite-bearing), low-magnesian (low-Mg#), and high-magnesian (high-Mg#) eclogites. The high-Al Catoca eclogites contain kyanite and corundum; high Al2O3 contents in rock-forming minerals; rare earth element (REE) patterns in garnets showing depleted LREEs, positive Eu anomalies (1.03–1.66), and near-flat HREEs; and high Sr contents in garnets and whole-rock REE compositions. All of these features point to a plagioclase-rich protolith (probably gabbro). Reconstructed whole-rock compositions (major elements, MREEs, HREEs, Li, V, Hf, Y, Zr, and Pb) and δ18O of 5.5–7.4‰ of the low-Mg# Catoca eclogites are in good agreement with the compositions of picrite basalts and average mid-ocean ridge basalt (MORB). The depleted LREEs and NMORB-normalised Nd/Yb values of 0.07–0.41 indicate that the degree of partial melting for the majority of the low-Mg# eclogites protolith was ≥30%. The narrow δ18O range of 5.5–7.4‰ near the ‘gabbro–basalt’ boundary (6‰) obtained for the high-Al and low-Mg# Catoca eclogites reflects the influence of subduction-related processes. This case shows that mantle eclogites represented by two different lithologies and originating from different protoliths — plagioclase-rich precursor, presumably gabbro (for high-Al eclogites), and basalt (low-Mg# eclogites) — can provide similar and overlapping δ18O signatures on account of the influence of subduction-related processes. Chemical compositions of the high-Mg# eclogites indicate a complicated petrogenesis, and textural signatures reveal recrystallisation. The presence of Nb-rich rutile (8–12 wt% of Nb2O5) enriched with HFSE (Zr/Hf of 72.6–75.6) and multiple trace-element signatures (including reconstructed whole-rock NMORB-normalised Ce/Yb of 3.9–10.6 and Sr/Y of 5.8–9.6, MgO contents of 15.7–17.9 wt%, and high Ba and Sr) provide strong evidence for deep metasomatic alteration. High Cr contents in clinopyroxene (800–3740 ppm), garnet (430–1400 ppm), and accessory rutile (700–2530 ppm), together with extremely low Li contents of 1.0–2.4 ppm in clinopyroxene, may indicate hybridisation of the eclogites with peridotite. Comparison of the chemical compositions (major and trace elements) of (1) unaltered fresh cores of coarse-grained garnets from the low-Mg# eclogites, (2) secondary garnet rims (ubiquitous in the low-Mg# eclogites), (3) proto-cores in the coarse-grained garnet (high-Mg# eclogites), and (4) homogeneous recrystallised fine-grained garnets (high-Mg# eclogites) suggests that the high-Mg# eclogites formed through recrystallisation of low-Mg# eclogite in the presence of an external fluid in the mantle. Four of the five high-Mg# samples show that mantle metasomatism inside the Kasai craton mantle beneath the Catoca pipe occurred at a depth range of 145–160 km (4.5–4.8 GPa).

Description: We present a detailed petrologic study of rhyolites from seven eruptions spanning the full (∼190 ky) history of rhyolitic volcanism at Krafla volcano, northeast Iceland. The eruptions vary widely in size and style, but all rhyolites are crystal-poor (