Contributions to Mineralogy and Petrology最新文献

The flow law for diffusion creep of chromite spinel has been determined from uniaxial compressive creep experiments at temperatures of 1400° to 1600 °C and a pressure of 0.1 MPa on polycrystalline samples of Mg(Cr,Al)₂O₄ with grain sizes of ~ 10 μm and chromium mole fractions, X_Cr, of 0.25, 0.50, and 0.75. Under our experimental conditions, creep rate is dominated by grain boundary diffusion of Cr. The dependence of strain rate on composition is manifest through the compositional dependence of the activation energy. The activation energy for grain boundary diffusion can be described in terms of melting temperature, T_m, as a function of composition in the form (Q_{{text{Cr}}}^{gb} = g_{{text{Cr}}}^{gb} {text{R}}T_m (X_{{text{Cr}}} )), where (g_{{text{Cr}}}^{gb}) is a material-dependent scaling parameter. The value obtained for this scaling parameter of (g_{{text{Cr}}}^{gb}) = 19.7 ± 0.5 yields activations energies of approximately 405, 416, and 427 kJ/mol for samples with X_Cr = 0.25, 0.50, and 0.75, respectively. Extrapolation of our results to conditions appropriate for chromitite pods enveloped by peridotites from the Oman ophiolite indicates that lattice diffusion, rather than grain boundary diffusion, largely dominates plastic deformation of chromite spinel, which is significantly stronger than olivine.

Pyroxenite veins are commonly observed in the mantle section of ophiolites, reflecting a variety of melts that percolate through the mantle, react, and finally crystallize in the lithosphere. To better understand the formation mechanism of the pyroxenite veins and associated peridotites, we conducted an integrated petrological and geochemical study of a suite of orthopyroxenite, websterite, and composite clinopyroxenite–orthopyroxenite veins in residual peridotites from the Dazhuqu ophiolite (Southern Tibet, China). Both orthopyroxene and clinopyroxene in pyroxenites are characterized by high Mg#, low Al₂O₃ concentrations, and depleted patterns of incompatible trace elements. This suggests that parental melts of the pyroxenites could be formed by re-melting of a previously depleted mantle source. We observed systematic variations in the major and trace element compositions of clinopyroxene and orthopyroxene across the pyroxenite–harzburgite sequences. Through trace element modeling, we have established a model for the formation of pyroxenite veins and associated harzburgites. According to this model, hydrous and aggregated melts were expelled from the tip of the dunite channels and subsequently injected into the lithosphere via fractures. The infiltrating melts then underwent diffusional loss of water into the residual harzburgites within the asthenosphere, which in turn promoting partial melting of the harzburgite. The pyroxenite veins were formed by mixing the infiltrating channel melt and matrix melt derived from partially molten harzburgite. This formation model can be applied to explain the pyroxenite–harzburgite and dunite–pyroxenite–harzburgite sequences in the Yarlung-Tsangbo ophiolite and other ophiolites around the world.

Melilitite, nephelinite, basanite, and alkali basalt, along with phonolite differentiates, form the Freemans Cove Complex (FCC) in the south-eastern extremity of Bathurst Island (Nunavut, Canada). New ⁴⁰Ar/³⁹Ar chronology indicates their emplacement between ~ 56 and ~ 54 million years ago within a localized extensional structure. Melilitites and nephelinites, along with phonolite differentiates, likely relate to the beginning and end phases of extension, whereas alkali basalts were emplaced during a main extensional episode at ~ 55 Ma. The melilitites, nephelinites, and alkali basalts show no strong evidence for significant assimilation of crust, in contrast to some phonolites. Partial melting occurred within both the garnet- and spinel-facies mantle and sampled sources with He, O, Nd, Hf, and Os isotope characteristics indicative of peridotite with two distinct components. The first, expressed in higher degree partial melts, represents a relatively depleted component (“A”; ³He/⁴He ~ 8 R_A, ε_Ndi ~  + 3 ε_Hfi ~  + 7, γ_Osi ~ 0). The second was an enriched component (“B” ³He/⁴He < 3 R_A, ε_Ndi < – 1 ε_Hfi <  + 3, γ_Osi >  + 70) sampled by the lowest degree partial melts and represents carbonate-metasomatized peridotite. Magmatism in the FCC shows that rifting extended from the Labrador Sea to Bathurst Island and reached a zenith at ~ 55 Ma, during the Eurekan orogeny. The incompatible trace-element abundances and isotopic signatures of FCC rocks indicate melt generation occurred at the base of relatively thin lithosphere at the margin of a thick craton, with no mantle plume influence. FCC melt compositions are distinct from other continental rift magmatic provinces worldwide, and their metasomatized mantle source was plausibly formed synchronously with emplacement of Cretaceous kimberlites. The FCC illustrates that the range of isotopic compositions preserved in continental rift magmas are likely to be dominated by temporal changes in the extent of partial melting, as well as by the timing and degree of metasomatism recorded in the underlying continental lithosphere.

The composition of fluid from carbonate- and chlorine-bearing pelite has been studied at 5.5 GPa, 850–1030 °C and at 7.8 GPa, 940–1090 °C using the diamond trap method. The run products include a residue composed of an eclogitic assemblage (garnet, coesite, clinopyroxene, and kyanite ± phengite and accessory pyrite/pyrrhotite, rutile, and zircon) and fluid solutes captured in the trap. The new data show that the pelite-derived supercritical fluid, with nearly equal amounts of solutes and H₂O + CO₂ [at the weight ratio H₂O/(H₂O + CO₂) from 0.8 to 0.9], is stable at the applied P–T conditions. At higher temperatures, the amount of solutes in the supercritical fluid increases only slightly, apparently, due to the presence CO₂ and 0.4–0.5 wt% Cl in the fluid. The reconstructed supercritical fluid composition includes components decreasing in the series SiO₂ > Al₂O₃ > K₂O > Na₂O ≈ CaO ≈ MgO ≈ FeO. At 7.8 GPa, phengite becomes unstable, and K₂O in the supercritical fluid increases from 3 to 8 wt% (on hydrous basis) while Al₂O₃ decreases from 8 to 5–6 wt%. Among the elements that fractionated into the fluid, B, Sr, Rb and P, as well as K at 7.8 GPa and 1090 °C, are the least compatible. The fluid-residue Sr partition coefficient varies from 4 to 10 and is notably lower at higher temperatures. Thus, supercritical fluids can form in carbonate- and chlorine-bearing sediments under subduction back arc-conditions, in cases of fluid-fluxing of the slab. Such supercritical fluids penetrating into the mantle together with H₂O and CO₂ can transport large amounts of major elements, B, Sr, Rb and P. The formation of potassium-rich silicic supercritical fluids is possible during subduction of pelite to ~ 250 km depths. They can be important agents in metasomatism of lithospheric mantle, with its composition reconstructed from data on micro-inclusions from fibrous diamond.

We present the geochemical and Sr–Nd–Pb isotopic data and the petrological evolution of primitive basaltic lavas that erupted from the Miocene to Quaternary in the East Anatolia Collision Zone to understand the geodynamic conditions and the change in the lithospheric mantle over time. Major trace element abundances, Sr–Nd–Pb isotopic compositions and petrological models show that the primitive basaltic samples were not affected by crustal contamination and fractional crystallization. They are derived from a depleted MORB mantle modified by melts derived from subducted sediments. The primitive melts of the Miocene and Quaternary series were derived from an amphibole-bearing garnet lherzolitic mantle and an amphibole-bearing garnet–spinel lherzolite mantle source, respectively. In contrast, the Pliocene basaltic melts were formed by mixing melts originating from both an amphibole-bearing spinel and garnet lherzolite. Our thermodynamic calculations indicate that the lithosphere–asthenosphere boundary (LAB) is about 30 km shallower in the Pliocene than in the Miocene.This may be explained by lithospheric delamination in the Early Pliocene. In contrast, the LAB in the Quaternary is approximately 9 km deeper than in the Pliocene, which can be explained by relamination of the mantle lithosphere. Thermal calculations have shown that about 5–11 km of the relamination can occur within 5–6 Ma and that asthenospheric melts can relaminate the base of the thinned lithospheric mantle by cooling, and the presence of the relaminated mantle lithosphere is documented throughout the whole EACZ.

Lamprophyres are mantle-derived rocks characterized by enrichment of incompatible elements and volatiles; however, the origin of their enriched sources remains enigmatic. Here we present zinc isotopic data for contemporary Mesoproterozoic (~ 1.1 Ga) lamprophyres from three localities of the Eastern Dharwar Craton. The Mudigubba and Kadiri lamprophyres with island-arc basalt (IAB)-like trace element features, such as positive Pb and negative Nb–Ta, Zr–Hf and Ti anomalies, have mid-ocean ridge basalt (MORB)-like δ⁶⁶Zn values ranging from + 0.22‰ to + 0.29‰. In contrast, the Udiripikonda lamprophyre shows higher-than-MORB δ⁶⁶Zn values of + 0.39‰ to + 0.48‰ and elemental features of intra-plate magmas with a lack of pronounced Nb–Ta-negative anomalies. Based on the covariations between Zn isotopes and trace element ratios, we infer that the Mudigubba and Kadiri lamprophyres with MORB-like Zn isotopes and high Ba/La, K/Nb and low Nb/La, Ce/Pb ratios are inherited from sub-continental lithospheric mantle metasomatized by fluids derived from a subducted slab. On the contrary, the higher-than-MORB Zn isotopic compositions, with low Ba/La, K/Nb and high Nb/La, Ce/Pb, K/U and Ba/Th ratios for the Udiripikonda lamprophyre are inferred to derive from lithospheric mantle enriched by carbonatitic melts or subducted carbonate-bearing sediments within the mantle transition zone. Hence, our study suggests that contemporary lamprophyres can be derived from disparate enriched mantle sources.

Reaction rims contain a wealth of information that can be used to decipher the P-T-t-X history of metamorphic and metasomatic rocks. One of the most important parameters that controls reaction rim growth is the presence of volatiles, which can affect rim thicknesses, phase stabilities and the development of rim microstructures. In this study, reaction rim growth experiments were performed between periclase and quartz at anhydrous to water-saturated conditions at 3–4 kbar and 1100–1300 °C. Controlled minute amounts of water were added through OH-doped periclase, which enabled us to perform experiments at controlled water-undersaturated conditions. At anhydrous conditions, no reaction rim formed at all implying that water acts as a catalyst, and a minimum fluid threshold is needed to initiate metamorphic reactions. At water-undersaturated conditions extremely small variations in water content are sufficient to change reaction rim growth rates by multiple orders of magnitude. This implies that reaction rims have the potential to monitor variations in the amount of water at those grain boundaries that serve as fast pathways for component transport at water-undersaturated conditions during metamorphic and metasomatic reactions in natural systems, allowing them to be used as sensitive “geohygrometers”. Additionally, the effect of water on relative layer thicknesses may provide an application for reaction rim microstructures to be used as new physico-chemical gauges that will allow us to discriminate between water-undersaturated and water-saturated conditions during metamorphic events.

Despite over 400 occurrences of kimberlites and related rocks in India, mantle-derived xenoliths are known only from a few occurrences. This paucity of mantle-derived xenoliths in Indian kimberlites has hampered investigations of the subcontinental lithospheric mantle (SCLM). Using a valuable selection of the rare xenolith inventory, we here report Fe³⁺/ΣFe measurements for garnets using the electron microprobe (EPMA) flank method, targeting six mantle eclogite xenoliths (KL2 pipe) and fourteen peridotitic garnet xenocrysts (P9 and P10 hypabyssal intrusions) from the Wajrakarur kimberlite field (WKF) on the Eastern Dharwar craton (EDC). These data provide some of the first direct constraints on the oxygen fugacity (fO₂) of the lithospheric mantle beneath the Indian subcontinent. The measured Fe³⁺/ΣFe ratios vary between 0.02 and 0.05 (± 0.01) for the eclogite xenoliths and between 0.02 and 0.10 (± 0.01) for the peridotitic garnets. Calculated ΔlogfO₂ values for the KL2 eclogites show a wide range from FMQ-3.9 to FMQ-0.9 (± 0.6), straddling the boundary between the diamond and carbonate stability fields. In terms of redox compositions, it appears that the KL2 eclogites are able to host diamond, which is consistent with the diamondiferous nature of this particular WKF locality and the presence of eclogitic garnet inclusions in diamonds from the nearby TK4 kimberlite body. The peridotitic garnet xenocrysts from the P9 and P10 kimberlite bodies, which were entrained between ~ 125 and 170 km depth, reveal ΔlogfO₂ values between FMQ-4.5 and FMQ-2.6 (± 0.9). Garnet xenocrysts with ‘normal’ REE patterns exhibit higher Fe³⁺/ΣFe ratios compared to garnets with ‘sinusoidal’ REE patterns. Importantly, the Fe³⁺/ΣFe ratios of garnet xenocrysts with ‘normal’ REE patterns (~ 125–160 km depth) correlate with metasomatic Ti–Y–Zr–V enrichment, which suggests metasomatism-driven oxidation of the cratonic mantle at mid-lithospheric depths. Such melt-related mantle metasomatism was probably diamond-destructive within the otherwise diamond-fertile lithospheric keel. The observed wide range of ΔlogfO₂ values for the Dharwar cratonic mantle lithosphere allows for stabilization of various metasomatic phases (e.g., amphiboles, micas, carbonates) that may have formed (or concentrated in) distinctly different metasome assemblages within the continental root that underpins Peninsular India. Changing the relative contributions from such highly diverse volatile-rich metasomes may explain the spatiotemporal association of kimberlites and various diamond-bearing potassic magma types such as orangeites, ultramafic lamprophyres and lamproites, a scenario that is influenced by the redox composition of the Dharwar craton root.