Contributions to Mineralogy and Petrology最新文献

Geophysical observations have revealed the existence of low-velocity zones that potentially coincide with the ongoing downward flow of mantle material through the globally recognized sharp discontinuity at a depth of 670 km. The leading interpretation for these heterogeneities is related to dehydration induced incipient partial melting. Here, we have employed an iterative method to achieve equilibrium between melt pools and undifferentiated pyrolite at geotherm conditions, allowing for a precise determination of partitioning of the major components (SiO₂-MgO-Al₂O₃-FeO-CaO) between the melt and minerals. The melt holds 5.5 (0.4) wt% H₂O and is relatively mafic compared to its source material, with considerable enrichment in CaO and FeO. For the (shallow) lower mantle in hydrous conditions, the solidus phase is davemaoite, followed by ferropericlase, and bridgmanite becomes the liquidus phase. Density models show that such magma is positively buoyant in the shallow lower mantle. Thus, it is expected to travel upward and possibly freeze in the transition zone. This process implies that, over time, continuous dehydration of the downgoing slab and subsequent hydrous melt extraction from the lower mantle wedge limit the deep water cycle to the upper 670 km depth of the mantle.

The knowledge of Nitrogen speciation in the C-O-H-N fluid phase has important implications for the efficiency of Nitrogen flux and recycling in the mantle. The composition of nitrogen-rich C-O-H-N fluids is studied experimentally at pressures from 3.0 to 7.8 GPa, temperatures from 750 to 1090°С, and oxygen fugacity (fO₂) at the iron-wȕstite (IW) to fayalite-magnetite-quartz (FMQ) buffers simulating the conditions of a slab subducted to mantle depths from 100 to 250 km. Gas chromatography-mass spectrometry of the experimental quenched fluids reveals ammonia (NH₃), N₂, H₂O, CO₂, CH₄, as well as minor amounts of light alkanes and oxygen-bearing organics, while the NH₃/(N₂ + NH₃) ratio ranges from 0.01 to 0.97. Most of the experimental C-O-H-N fluids contain low fractions of NH₃ (≤ 0.12), while higher values are possible only at very low fO₂ unrealistic for subduction zones at depths < 250 km. A semi-empirical model, with the equilibrium constant K*(K*=(:{X}_{{NH}_{3}}^{2})/(:{X}_{{N}_{2}})), is suggested to predict nitrogen speciation for a sediment-derived mobile phase with reduced H₂O contents for typical P-T-fO₂ ranges. As inferred previously, the fraction of NH₃ can exceed the N₂ fraction ((:{X}_{{NH}_{3}}^{})/(:{X}_{{N}_{2}}) >1) only in diluted aqueous fluids generated at temperatures of warm subduction and at a pressure of ~3.0 GPa, but it remains negligible in the hot subduction conditions. The transformation of the aqueous fluid into a supercritical fluid-melt (with reduced H₂O fraction) in warm or hot slabs subducted to depths equivalent to P≥3.5-5.5 GPa, is demonstrated to decrease significantly the fraction of NH₃ in the mobile phase, the redox conditions being invariable. In general, the aqueous fluid-to-SCFM transformation is expected to reduce the concentrations of nitrogen species compatible with phengite. Correspondingly, the coefficient of nitrogen partitioning between the mobile phase and phengite becomes lower with increasing pressure, which reduces the flux of nitrogen carried by both warm and hot slabs to depths >150 km.

Elastic thermobarometry has been rarely applied to quartz inclusions entrapped in garnet (QuiG) in granulite and igneous terranes, in part, because there is uncertainty about the reliability of the thermobarometric results arising from the quartz inclusions being subject to tensile strain and stress when examined at room conditions. Here, we present QuiG results from high-temperature metapelites from the Adirondacks, NY, USA and piston-cylinder experiments that give insight into the deformation behavior of quartz inclusions under tension. Measured remnant pressures (P_inc) of experimental and natural samples calculated using the quartz phonon mode Grüneisen tensor are too tensile with respect to the expected P_inc values based on experimental and petrologic constraints. We show that these discrepancies are not related to non-elastic deformation nor inaccuracies in the quartz equation of state. Evaluation of previous density functional theory (DFT) results shows that the structural response of quartz is non-linear with increasing tensile strain. Therefore, because the available quartz phonon mode Grüneisen tensor was determined with a linear fit optimized for compressive strains, obtained tensile strains using this tensor are too large in magnitude. P_inc values obtained using the hydrostatic calibrations of the 128 and 464 cm⁻¹ peaks have better agreement with the expected values and return entrapment conditions that are consistent with petrologically constrained or known experimental pressures. P_inc values obtained through hydrostatic calibrations must nonetheless be treated with caution because the behavior of Raman phonon modes under tension has not been calibrated experimentally.

Here we study silicate inclusion-bearing diamonds from the Voorspoed Group II kimberlite (or carbonate-rich olivine lamproite—CROL) in the central Kaapvaal Craton. Based on major and trace elements, along with Sm–Nd isotope characteristics, the peridotitic garnet and clinopyroxene inclusions define three compositional groups that are contemporaneous with lithospheric refertilisation and re-healing related to 2.79–2.68 Ga Ventersdorp-related plume magmatism across the central Kaapvaal craton. Three harzburgitic (G10) inclusions define an isochron of 2877 ± 249 Ma, with a low initial ε_Nd =  − 15 ± 10 that overlaps the oldest Kaapvaal enriched components as represented by crust in the 3.66–3.22 Ga Ancient Gneiss Complex. This finding suggests that harzburgitic diamond formation occurred in the oldest > 3.2 Ga Kaapvaal lithospheric mantle. Five lherzolitic (G9) garnet and 11 Cr-diopside inclusions with La_N > 1 (where N refers to chondrite normalisation) and low ¹⁴⁷Sm/¹⁴⁴Nd (0.11–0.41), define a 2606 ± 95 Ma Sm–Nd isochron with an initial ε_Nd =  − 6.9 ± 5.4 that is consistent with diamond formation in enriched > 3.0 Ga lithospheric mantle. Four lherzolitic (G9) garnet and five Cr-diopside inclusions with La_N < 1 and higher ¹⁴⁷Sm/¹⁴⁴Nd (0.45–1.51), define a 2560 ± 51 Ma Sm–Nd isochron with an initial ε_Nd = 9.8 ± 7.8. This initial ratio overlaps with depleted mantle indicating rapid re-healing of the lithospheric mantle after 2.79–2.68 Ga Ventersdorp-related plume magmatism. Sixteen eclogitic inclusions (10 garnets and six omphacites) define an isochron of 2196 ± 61 Ma, with initial ε_Nd =  − 9.3 ± 3.9. Growth of Voorspoed eclogitic diamonds is interpreted to be related to magmatism and failed rifting at the time of the 2.25–2.23 Ga Hekpoort LIP and associated magmatism across the Transvaal Basin. Low-pressure protoliths for the eclogite host rocks are supported by the presence of omphacite that is not stable as a high-pressure liquidus phase, along with Eu anomalies in the majority of eclogitic inclusions. The likely host rocks for eclogitic diamond formation are subduction-related eclogites in the lithospheric mantle that may be similar to those sampled nearby at Lace. The Voorspoed peridotitic and eclogitic diamonds document multiple episodes of melt infiltration and modification of older lithospheric mantle, and have allowed us to place diamond formation and preservation in the context of the long history of plume impingement on the Kaapvaal lithospheric mantle.

Magma mixing in mush zones constitutes a common eruption trigger and modulates the composition of erupted lavas and their hazard potential. Plagioclase is an excellent archive of mixing processes, however in ocean island basalts, the mineral crystallizes late and is typically restricted to matrix microcrysts. The 2021 Tajogaite eruption (La Palma, Canary Islands) included rare macrocrysts of plagioclase with disequilibrium textures that afford investigation of mush interactions during volcanism. The 85-day-long eruption produced initial clinopyroxene-amphibole-bearing tephrites followed by dominant clinopyroxene-olivine-rich basanites. From days 13–35, some basanite lavas contained macrocrysts of plagioclase with strong compositional contrasts between cores (evolved andesine antecrysts; An_22–52) and rims (labradorite; An_50–64). The An-rich rims are chemically similar to microcrysts in the matrix (labradorite; An_54–66), cogenetic with the erupted basanite liquid. Trace element compositions change abruptly between antecryst cores (3352–5139 ppm Sr) and rims and matrix microlites (1624–3097 ppm Sr). ⁸⁷Sr/⁸⁶Sr ratios are similar for antecryst cores and rims (0.70313–0.70322), more radiogenic than the host matrix (0.70308–0.70314) in several samples. Such contrasts suggest recycling of distinctly evolved, Sr-radiogenic plagioclase-bearing mush by recharging basanite magma prior to, or during the eruption, as new inputs of basanite melt led to eruption of more fluidal and rapid lavas. Evolved plagioclase compositions have been reported in tephriphonolites from previous eruptions, interpreted to form in magma accumulation zones in the lower oceanic crust beneath La Palma. Our work provides new evidence of evolved magma mush developing upon ponding of basanite melts around Moho depths below ocean island basalts.

The North China Craton (NCC) consists of a destroyed eastern part underlain by a sharply thinned lithosphere and an intact western part supported by a deep mantle root. Lithospheric structure under the transitional belt remains poorly constrained but critical for a thorough understanding on the mechanism for the differential destruction of the craton. Here, we explore this problem by lithospheric tomography using newly found kimberlite-borne garnet xenocrysts in the Hebi area, central NCC. The garnets include a high-Mg# (> 85.5, atomic Mg/(Mg + Fe^T) *100) group mainly equilibrated at high temperatures (1040–1160 °C) and restricted pressures (24–26 kbar), and a low-Mg# (< 85.5) group with a wide range of equilibration temperatures (790–1000 °C) and pressures (20–30 kbar). The inverted thermochemical mantle profile suggests the presence of a ~ 120–130 km thick lithosphere with an overheated and refractory mantle (olivine Mg# > 92) interior. The elevated heavy rare-earth element (HREE, from Ho to Lu) contents of the high-Mg# garnets (∑HREE > 20 ppm) relative to the low-Mg# garnets (∑HREE < 15 ppm) further suggest an element re-distribution due to the heating-induced garnet breakdown within the lithosphere. The selective heating of the refractory lithospheric interior likely reflects preferential asthenospheric upwelling through fossil lithospheric weak zones possibly related to the amalgamation of craton nuclei. Together with the knowledge on the eastern and western NCC, we suggest that the unevenly distributed lithospheric weak zones and the circum-craton tectonics should have jointly shaped the uneven loss of deep root of the craton.

Resorption features formed on diamonds provide key insights into diamond-destructive mantle metasomatism and also offer a novel proxy for constraining kimberlitic fluid composition. However, the surface features of fibrous diamonds remain poorly documented, limiting our understanding of their post-growth history. Here, the etch features, impurity characteristics, and compositions of high-density fluids (HDFs) in the outermost fibrous growth layers of coated diamonds from the Mbuji-Mayi kimberlite, the Democratic Republic of the Congo (DRC) are investigated. These diamonds containing silicic–low-Mg carbonatitic HDFs exhibit negative stepped point-bottomed (p/b), stepped curved-bottomed (c/b), and ‘pure’ c/b trigons without stepped walls, together with positive and truncated trigons on {111} faces. The correlations among different types of trigons, along with the co-occurrence of other surface features, reveal three distinct resorption events–one in the mantle and two during the kimberlite ascent. Comparisons with previous experimental and natural etch features suggest that carbonate-rich melts in the mantle were responsible for stepped p/b trigons on coated diamonds, whereas ‘pure’ c/b trigons and positive trigons both resulted from interaction with CO₂-rich fluids, but at different depths during kimberlite ascent. This study presents the first systematic dataset of resorption features on fibrous diamonds, advancing our understanding of the formation mechanism of etch features on diamonds under natural settings and extending metasomatic records of fibrous diamonds beyond their crystallization stage. These results suggest a carbonate-rich melt environment after the growth of fibrous diamonds in the lithospheric mantle beneath the Congo craton, and confirm the CO₂-rich nature of fluids exsolved from the Mbuji-Mayi kimberlite magmas.