Contributions to Mineralogy and Petrology最新文献

The dynamics and magma transport at the boundary between the upper and lower oceanic crusts (i.e., the dike–gabbro transition) are crucial for understanding the crustal accretion beneath mid-ocean ridges, which however have been studied at quite a few sites such as the East Pacific Rise and ophiolites like Troodos and Oman. Here we present detailed geological, petrological, and geochemical data for the dike–gabbro transition and associated basalts in the Yunzhug ophiolite, central Tibet, to constrain the complex magmatic processes in this specific horizon. The Yunzhug ophiolite contains a large (~ 20 km²) well-exposed sheeted dike complex, which is rooted in a dike–gabbro transition that consists of diverse lithologies, including diabase, gabbro, and minor porphyritic diabase. Petrographically, the Yunzhug gabbros could be grouped into the dominant Plg (plagioclase)-euhedral gabbros (euhedral–subhedral plagioclases enclosed in clinopyroxene oikocrysts) and a small amount of Cpx (clinopyroxene)-euhedral gabbros (with abundant euhedral clinopyroxenes). Plagioclases and their equilibrated melts of the two types of gabbros are similar, whereas clinopyroxenes and their equilibrated melts of the Cpx-euhedral gabbros are more primary and depleted than those of the Plg-euhedral gabbros. These petrographic and geochemical features suggest an earlier crystallization of clinopyroxene for the Cpx-euhedral gabbros, which is best explained by occasional water input in the magmatic system. Nevertheless, the modeled equilibrated melts of the two types of gabbros have compositions indistinguishable from the whole rock compositions of diabases and basalts, indicating a direct genetic linkage between these rocks. The unusual porphyritic diabases, on the other hand, provide evidence supporting for plagioclase accumulation and aggregation during magma upward migration, thus may have served as a unique way for magma to transport from the lower to upper crust. Studies of the Yunzhug ophiolite thus provide some key constraints on the complex magmatic processes in the oceanic dike–gabbro transition, regarding its dynamic accretion and magmatic plumbing mechanisms.

The aluminous calcium-ferrite type phase (CF) and new aluminous phase (NAL) are thought to hold the excess alumina produced by the decomposition of garnet in MORB compositions in the lower mantle. The respective stabilities of CF and NAL in the nepheline-spinel binary (NaAlSiO(_{4})–MgAl(_{2})O(_{4})) are well established. However with the addition of further components the phase relations at lower mantle conditions remain unclear. Here we investigate a range of compositions around the nepheline apex of the nepheline-kalsilite-spinel compositional join (NaAlSiO(_{4})–KAlSiO(_{4})–MgAl(_{2})O(_{4})) at 28–78 GPa and 2000 K. Our experiments indicate that even small amounts of a kalsilite (KAlSiO(_{4})) component dramatically impact phase relations. We find NAL to be stable up to at least 71 GPa in potassium-bearing compositions. This demonstrates the stabilizing effect of potassium on NAL, because NAL is not observed at pressures above 48 GPa on the nepheline-spinel binary. We also observe a broadening of the CF stability field to incorporate larger amounts of potassium with increasing pressure. For pressures below 50 GPa only minor amounts ((<0.011(1)frac{K}{K+Na+Mg})) of potassium are soluble in CF, whereas at 68 GPa, we find a solubility in CF of at least (0.088(3)frac{K}{K+Na+Mg}). This indicates that CF and NAL are suitable hosts of the alkali content of MORB compositions at lower mantle conditions. For sedimentary compositions at lower mantle pressures, we expect K-Hollandite to be stable in addition to CF and NAL for pressures of 28–48 GPa, based on our simplified compositions.

Slab-derived supercritical liquids separate into aqueous fluids and hydrous melts during their migration. Separated aqueous fluids further release melt components that cannot be dissolved during ascent. During these processes, elemental partitioning occurs, which may contribute to the geochemical evolution of subduction-zone fluids. Here, we report new experimental results of partition coefficients between a hydrous dacitic melt and Cl-free or Cl-rich aqueous fluids (D^fluid/melt) for 26 elements at a temperature of 1100°C and pressures of 0.3 and 0.7 GPa using internally-heated pressure vessels. Our results reveal that high-field strength elements (HFSE), except Th, are hardly partitioned into aqueous fluids, regardless of their salinity and pressure conditions. In contrast, the partitioning of other elements varies depending on the fluid salinity. D^fluid/melt of large-ion lithophile elements (LILE) and U increases with salinity, whereas that of rare earth elements (REE) and Th decreases. These results predict that slab-derived aqueous fluids can evolve to become richer in LILE and U and poorer in HFSE and REE by separating melt components, which explains the LILE- and U-rich and HFSE- and REE-poor characteristics of subduction-zone magmas. This also explains the higher LILE/HFSE and LILE/REE ratios in frontal-arc basalts than in rear-arc basalts: frontal-arc basalts can be generated by the addition of aqueous fluids that sufficiently separate the melt components at shallower depths, whereas rear-arc basalts are generated by the addition of supercritical liquids or aqueous fluid that insufficiently separate the melt components at greater depths. Such separation of melt components from ascending slab-derived fluid can determine the geochemical signature and across-arc compositional variation of subduction-zone magmas.

The mobility of trace elements in supercritical fluid-melt derived from pelite rich in volatiles has been studied experimentally at pressures from 3.0 to 7.8 GPa and temperatures from 750 to 1090 °С using the diamond trap method. The experiments simulate the conditions of warm and hot subduction, in which pelite either retains the whole inventory of volatiles or releases a fluid in three successive devolatilization steps. The 3.0 GPa and 750 °С runs with pelite rich in volatiles yield a supercritical fluid (SCF) which attains equilibrium with an eclogitic residue bearing phengite and accessory rutile, zircon, and monazite. At ≥5.5 GPa and ≥850 °С, above the second critical endpoint, the SCF transforms into a supercritical fluid-melt (SCFM) which acquires higher concentrations of almost all incompatible trace elements while the mineral assemblage of the equilibrium eclogitic residue remains the same but lacks monazite. The trace-element enrichment of SCFM is most prominent for Ba, Sr, LREE, Th, and U. At the hot subduction conditions, the fluid-melt likewise contains more K, Rb, Zr, and Hf, though LREE contents become lower. The negative Nb anomaly persists in all cases. SCFM has its trace-element composition generally similar to that of hydrous melt derived from oceanic sediments, but contains more REEs and water. Partitioning of LILE, HFSE, and LREE between the SCFM and residue phases mainly depends on the fluid-melt fraction and stability of host phengite, monazite, zircon, and rutile. Thus, sediment-derived SCFM can carry both fluid-mobile and sediment-melt elements to regions of arc- and back-arc magma generation and can translate the negative Nb anomaly inherited from sediment into the magmas. Early devolatilization of pelite increases the stability of monazite and phengite in the residue and provides efficient LREE, K and Rb transport to the mantle depths of ~ 250 km. Effective LREE and Th depletion of UHP metamorphic rocks is possible by SCFM release near peak metamorphic conditions.

U–Pb ages were determined by split-stream LA-SF/MC-ICPMS in garnets from UHT granulite xenoliths (Star mine, South Africa; 124 Ma). They give a considerable age range of 400 million years with well-defined maximas at 3.09, 3.01 and 2.75 Ga. The oldest peak overlaps with the changeover from tonalites to K-granites at 3.14–3.04 Ga and with zircon ages of the mid-crustal granulites of the Vredefort dome (3.1 Ga) in the wake of the 3.2 Ga collision of three terrains that compose the Witwatersrand block. Subduction (or sagduction) of the uppermost crust in an ultrahot orogen setting brought shales and greenstones to the lower crust. Ultrahigh temperature (UHT) conditions are the result of high mantle potential temperatures and self- heating by the radioactive inventory of the subducted lithologies. Metamorphism, anatexis to very high degrees and melt extraction left UHT granulites as residue. Rejuvenation of UHT conditions was brought about by Dominion Group magmatism between 3.0 and 2.95 Ga. Magmatic uprise caused intense shearing in the lower crust followed by recrystallisation of the shear zones to generate the younger garnet age group. Ventersdorp flood basalt volcanism caused similar processes at around 2.72 Ga and generated the third garnet age group. Zircon gives U–Pb ages mainly around 2.72 Ga (both literature and our own data) i.e. zircon adjusted or newly crystallized at the youngest UHT event. Only few zircon grains retained older ages up to 2.94 Ga. Still unconstrained, but very high closure temperatures (≥ 1100 °C) for the U–Pb system in garnet keep the memory of the oldest ages in UHT granulites. Such ages can only be reset by recrystallization. This way, garnet records a prolonged high-temperature history of the lower crust of the Kaapvaal craton.

The analysis of olivine-hosted melt inclusions (MIs) from the whole sub-alkaline and alkaline magmatic suites of Mt. Etna provides fundamental information about the composition of undifferentiated magmas and their pristine volatile content. Olivine crystals (Fo_88-66) were selected for Secondary Ion Mass Spectrometry (SIMS) analysis of volatile species (H₂O, CO₂, F, Cl and S) contained in their host MIs, after preliminary high-pressure/high-temperature re-homogenization, which allowed to reduce the developing of cracks in the host olivine and diffusion-driven outgassing of volatiles from the melt inclusions. This permitted to explore the compositional variability of volatiles of undifferentiated melts and the degassing behavior through the feeding system. The studied MIs show significant major elements compositional heterogeneities (44.57–52.37 wt% SiO₂; 3.60–7.51 wt% Na₂O + K₂O). Fractionation modelling was performed with Rhyolite-MELTs under variable fO₂ regimes (∆FMQ + 1.5 to + 3), starting from the less evolved MIs compositions and ultimately reproducing most of the observed compositional trends. Mantle melting modelling was used to replicate the observed MIs composition, starting from a spinel-lherzolitic source, accounting for the alkalinity and Fe content of reproduced melts by varying the eutectic contribution of Amph/Phlog and Opx/Cpx respectively. Although most of the studied MIs were degassed in an open-conduit regime, the observed range of volatile concentration in MIs (2.42–6.14 wt% H₂O; 308–8474 ppm CO₂; 132–697 ppm F; 221–1766 ppm Cl and 16–1992 ppm S) is correlated with a slight decrease in the molar H₂O/(H₂O + CO₂) ratio from early tholeiites to the recent 2015 alkaline products. This observation allows to estimate a minimum 12,250 ppm CO₂ and a maximum of 6.14 wt% H₂O in primary melts of the current activity.

In mafic systems where primary mineral assemblages have witnessed moderate- to high-temperature hydrous overprinting and deformation, little is known about the retentivity of the Lu–Hf isotopic system in apatite. This study presents apatite laser-ablation Lu–Hf and U–Pb geochronology, zircon geochronology, and detailed petrological information from polymetamorphic mafic intrusions located in the central-western Gawler Craton in southern Australia, which records an extensive tectonometamorphic history spanning the Neoarchaean to the Mesoproterozoic. Zircon records magmatic crystallisation ages of c. 2479–2467 Ma, coinciding with the onset of the c. 2475–2410 Ma granulite-facies Sleafordian Orogeny. The amphibole-dominant hydrous assemblages which extensively overprint the primary magmatic assemblages are hypothesised to post-date the Sleafordian Orogeny. The Lu–Hf and U–Pb isotopic systems in apatite are used to test this hypothesis, with both isotopic systems recording significantly younger ages correlating with the c. 1730–1690 Ma Kimban Orogeny and the c. 1590–1575 Ma Hiltaba magmatic event, respectively. While the early Mesoproterozoic apatite U–Pb ages are attributed to thermal re-equilibration, the older Lu–Hf ages are interpreted to reflect re-equilibration facilitated primarily by dissolution-reprecipitation, but also thermally activated volume diffusion. The mechanisms of Lu–Hf isotopic resetting are distinguished based on microscale textures and trace element abundances in apatite and the integration of apatite-amphibole textural relationships and temperatures determined from the Ti content in amphibole. More broadly, the results indicate that at low to moderate temperatures, apatite hosted in mafic rocks is susceptible to complete recrystallisation in rocks that have weak to moderate foliations. In contrast, at higher temperatures in the absence of strain, the Lu–Hf system in apatite is comparatively robust. Ultimately, the findings from this study advance our understanding of the complex role that both metamorphism and deformation play on the ability of mafic-hosted apatite to retain primary Lu–Hf isotopic signatures.

Modeling the behavior of trace elements during lunar magma ocean solidification is important to further our understanding of the chemical evolution of the Moon. Lunar magma ocean evolution models rely on consistent datasets on how trace elements partition between a lunar silicate melt and coexisting minerals at different pressures, temperatures, and redox conditions. Here we report new experimental trace element partition coefficients (D) between clinopyroxene (cpx), pigeonite, orthopyroxene, plagioclase, olivine (ol), and silicate melt at conditions relevant for the lunar magma ocean. The data include D^cpx−melt at ambient and high pressures (1.5 GPa and 1310 °C), and partition coefficients at ambient pressure for pig, opx, ol, and pl. Overall, clinopyroxene is a phase that may control the fractionation of key geochemical trace element ratios, such as Lu/Hf and Sm/Nd, during the evolution of the lunar magma ocean. We explore the impact of the new silicate D^{mineral−melt} on the trace element evolution of the lunar magma ocean and we find that accessory phosphate minerals, such as apatite or whitlockite are of critical importance to explain the observed trace element and isotopic signature of the KREEP reservoir on the Moon. The new partition coefficients were applied to calculate the trace element evolution of the residual melts of the crystallizing lunar magma ocean and we propose a new trace element composition for the urKREEP reservoir. The new data will be useful for future thermo-chemical models in order to adequately predict the duration of the lunar magma ocean and the age of the Moon.

The breakdown of omphacite plays an important role in the exhumation and retrogression of eclogites. Additionally, metamorphic reactions associated with grain size reduction have the potential to significantly impact deformation mechanisms and the rheology of crustal rocks. We analyze the breakdown reaction omphacite → diopsidic clinopyroxene + plagioclase ± amphibole and associated microstructures by electron backscatter diffraction. The reaction results in the formation of (diopsidic) clinopyroxene-plagioclase symplectites. Samples were chosen from localities on Holsnøy (western Norway) and Lofoten (northern Norway), that are representative of vermicular symplectites, partly recrystallized symplectites, and deformed symplectites. Interphase misorientation analysis based on the electron backscatter diffraction results reveals that the nucleation of (diopsidic) clinopyroxene-plagioclase symplectites was crystallographically controlled, with the diopside copying the lattice orientation of the omphacite, and the plagioclase growing along diopside planes with favorable, i.e., similar, interplanar spacing. Deformation of the (diopsidic) clinopyroxene-plagioclase symplectites occurred by fracturing, transitioning into grain boundary sliding accommodated by diffusion creep. The results indicate that the formation of vermicular symplectites is not associated with enhanced permeability and fluid flow. Subsequent recrystallisation and grain-size sensitive deformation of the symplectites facilitates fluid redistribution and weakening of the retrogressed eclogites.