Contributions to Mineralogy and Petrology最新文献

Serpentine minerals have received a lot of attention because of their unique crystal structures, their wide occurrence in orogenic belts and their potential role in contributing seismic anisotropy in subducting slabs. Several studies have investigated crystal preferred orientation (CPO) in high temperature antigorite serpentinites from Japan, the Alps, Spain, Cuba and Tibet, documenting significant crystal alignment. However, only a limited number of lower grade serpentines have been explored to date. Mainly because of submicroscopic microstructural heterogeneities CPO cannot be measured with conventional methods such as optical microscopy and EBSD. In this study 15 serpentinites from different tectonic settings in California, the Central Alps and Northern Spain have been investigated, mainly with high energy synchrotron X-ray diffraction, to quantify bulk crystal alignment. We find that CPO is strong on sheared surfaces of fractured blocks and secondary veins but the bulk of most serpentinite samples, except high-grade recrystallized antigorite serpentinite, show only weak crystal alignment. Correspondingly calculated seismic anisotropy based on CPO is not very significant. This is supported by very heterogeneous microstructures as documented with SEM and TEM analyses.

The study area (Rampur domain) is situated to the east of the Eastern Ghats Boundary Shear Zone (EGBSZ) and encompasses portions of the granulite facies rocks of the exhumed Proterozoic Eastern Ghats Province (EGP), India. The EGP is characterized by a diverse array of rock types, featuring a wide variety of mineral parageneses and chemical compositions, including charnockite, mafic granulite, Mg-Al granulite, felsic granulites, amphibolite, khondalite and anorthosite. In this study, we report for the first time evidence of ultra-high temperature (UHT) metamorphism within the mafic granulites of the relatively unexplored Rampur domain of the Eastern Ghats Province, using the two-pyroxene assemblage. The stable mineral assemblage present during peak metamorphism typically includes garnet, orthopyroxene₁, clinopyroxene, hornblende₁, quartz, and plagioclase₁. The consumption of garnet observed in different reaction textures, alongside the formation of striking orthopyroxene₂–plagioclase₂ and hornblende₂–plagioclase₂ symplectites, represent the later phases of metamorphism. By applying TWQ calculation procedures to the mineral core compositions, we have determined peak metamorphic conditions of approximately 970 °C at a pressure of 10.5 kbar. Zircon dating results from LA-HR-ICP-MS indicate upper intercept ages of 2509.9 ± 21.7 Ma and 2479.9 ± 21.0 Ma for the protolith, while lower intercept ages of 965.7 ± 40.7 Ma and 979.8 ± 18.1 Ma correspond to the metamorphic age of the analyzed samples E-185 and E-186, respectively. Based on the textural relationship, derived zircon ages, fluid-P-T constraints, and P-T pseudosection model, we propose a decompressional evolutionary P-T-t path that supports the Neo-Proterozoic assembly of the Indo-Antarctic region.

At the pressure and temperature conditions of the lower crust, quartz undergoes a displacive phase transition from a trigonal ((alpha )) to a hexagonal phase ((beta )). At room pressure, the (alpha )–(beta ) quartz transition occurs at 574.1 °C and it is associated with large changes in the thermodynamic and elastic properties. For that reason, it is interpreted as the cause of significant seismic velocity contrasts in the crust seen by seismic tomography. Existing thermodynamic models and Equations of State (EoS) of quartz are mostly constrained by data collected at room pressure (or at high pressure and room temperature). In this work we characterized the (alpha )–(beta ) quartz transition experimentally at simultaneous HP–HT conditions using synchrotron X-ray diffraction and acoustic measurements, and derived values of (V_p), (V_s), the adiabatic bulk modulus ((K_s)) and the shear modulus (G). The data collected in the (alpha ) field agree with the models from the literature, so entrapment pressures of (alpha )-quartz inclusions calculated via elastic barometry with these EoS should be reliable. However, our measured (V_p), (V_s), and (K_s) are significantly lower than those predicted for (beta )-quartz. Whatever the cause of this discrepancy, interpretations of seismic data in terms of the properties of (beta )-quartz in the lower crust and calculations of entrapment conditions of quartz inclusions in the stability field of (beta )-quartz should be treated with caution.

Archean and Proterozoic layered intrusions represent an important portion of the igneous rock archive and their parental magma composition may provide crucial insights into the Earth’s magmatic and geodynamic evolution. Both komatiitic and boninitic parental magmas have been suggested for several major Archean layered intrusions, which could imply different tectonic settings for their formation. We studied the ~ 3.2 Ga Ujaragssuit Nunât layered ultramafic body from southern West Greenland (Ujaragssuit ultramafic body), which contains some of Earth’s oldest chromitites. Spinel major and trace elements, and whole-rock platinum group element compositions in massive chromitites from the Ujaragssuit ultramafic body, largely preserve primary igneous compositions. In contrast, spinels from most silicate-dominated ultramafic rocks were altered by metamorphic and metasomatic events. We collated a large spinel dataset to investigate variations in their parental magma compositions and tectonic settings using multivariate statistical analysis. Both the massive chromitites from the Ujaragssuit ultramafic body and chromitites from other Archean and Proterozoic ultramafic layered intrusion show high Cr/(Cr + Al) and Ti/V ratios in spinel, and high whole-rock Ir and Ru contents, which are consistent with those of komatiitic spinel. The compositions of chromitites suggest that the parental magmas of the Ujaragssuit ultramafic body are komatiitic, implying that the formation of these layered intrusions was related to mantle plumes. Our recognition of a komatiitic ultramafic body in North Atlantic Craton, where no komatiite has previously been reported, suggests that komatiitic magmas were a common feature among cratons.

Boron abundances and B isotopic compositions of well-characterized blueschists and eclogites from the Raspas Complex (Ecuador) were analyzed to improve the use of boron as a tracer for recycling at convergent margins. The MORB-type eclogite interacted with internally-derived fluids released from metabasalt during the transition from blueschist to eclogite, with input from sediments. During metasomatism, B was gradually leached from the MORB-type eclogites (decrease from 6(upmu )g/g to 1.5(upmu )g/g), and their B isotopic composition was driven to isotopically heavier values in the range of (-)7.4(permille ) to (-)3.4(permille ). The B isotopic composition of the metasomatic fluid is estimated between (-3) and +1(permille ). The isotopic composition of the least metasomatized MORB-type eclogite samples (({-7.4pm 0.7}{permille })) is considered close to the B isotopic composition of the dehydrated AOC in the case of Raspas at the stage of deepest subduction and most extensive dehydration. This constitutes a decrease in (delta ^{11}text {B}) of approximately 10(permille ) from its likely pre-subduction AOC protolith. The blueschist experienced a type of high-pressure metasomatism that is distinct from the one that affected the MORB-type eclogites. The metasomatic fluids were internally-derived and released by metabasalt as well, but with more input from sediments. The metasomatic fluid had a B isotope signature of approximately (-)5.2(permille ). The zoisite eclogite samples show a very distinct mineralogical and geochemical composition that records the highest degree of high-pressure metasomatic overprint. Their elemental and isotopic composition was thereby set to (text {[B]}={2.1pm 0.3}upmu hbox {g/g}) and (delta ^{11}text {B}={-5.8pm 1.8}{permille }). As demonstrated in previous studies, the high-pressure metasomatic fluid that caused the metasomatic overprint was mainly derived from– or interacted with– serpentinite, but had admixed components from metabasalts and metasediments. The B isotopic composition of the respective fluid is estimated at ({-2.6} {permille }), which overlaps with the composition of most volcanic arc basalts. This study, therefore shows, that metasomatic fluids that migrated through the Raspas slab at a depth of 50–70km had a B isotopic composition between (-5.2) to +1(permille ) and were, thus, significantly heavier than that of the mantle.

Upper Cretaceous to Miocene continental volcanism in NE Brazil spans 350 km in a N–S direction and 60 km in width, forming the Macau-Queimadas alignment (MQA). This study combines fieldwork, petrography, geochemistry, and Sr–Nd–Pb isotopes to explore its origin and evolution. The MQA consists of volcanic and hypabyssal mafic rocks intruding Cretaceous and Precambrian basement rocks, divided into two groups: (i) alkaline (foidite to trachy-basalt); and (ii) subalkaline (basalt and basaltic andesite). Both are sodic and LREE-enriched, with distinct La/Yb ratios. The alkaline group reflects an asthenospheric source (Nd model age of 1.1–0.4 Ga), while the subalkaline group incorporates an older lithospheric component (Nd model age of 2.1–1.2 Ga). These magmas originated from picritic parental melts, with < 15% melting for the alkaline group and ~ 25–30% melting for the subalkaline group, derived from spinel- to garnet-bearing peridotite. Differentiated series formed by successive small melt volumes, with some samples undergoing crustal fractional crystallization of clinopyroxene + olivine + plagioclase (alkaline group), and clinopyroxene + orthopyroxene + Ca-plagioclase (subalkaline group). The persistence of basaltic magmatism over ~ 90 Myr indicates sustained upper mantle melting. The alignment of volcanics, its association with a positive geoid anomaly, and its parallelism with the Mid-Atlantic Ridge suggest the MQA may represent an aborted ridge that never progressed to an oceanic stage.

The Marion Rise, located in the central portion of the Southwest Indian Ridge (SWIR), marks a relief high but is overall covered with a thin crust, and thus is inferred to be supported by depleted buoyant mantle. However, direct evidence of the regional mantle compositions from abyssal peridotites are still rare for such a hypothesis. This study presents whole rock and mineral compositions of 34 abyssal peridotites dredged from 7 sites between the Discovery and Indomed fracture zones on the Marion Rise. The samples are divided into low-Cr# (Cr# = 0.23–0.33) and high-Cr# (Cr# = 0.40–0.57) groups. The high-Cr# group samples display highly refractory characteristics (whole rock Al₂O₃ contents down to 0.52 wt%), which are reinforced by the depleted pyroxene compositions that indicate partial melting of up to > 18%. Nonetheless, the overall high extents of melting indicated by the peridotites are inconsistent with the regional thin crust, hence require an inherited origin of the melting signatures. Moreover, the Ti and Yb (Y) concentrations of clinopyroxenes (orthopyroxenes) in the high-Cr# group are too depleted to be residues of anhydrous melting at mid-ocean ridges, but instead suggest for a hydrous melting history near subduction zones. Collectively, we fill in a piece of the puzzle of mantle heterogeneity beneath the SWIR, by providing solid evidence for the existence of a highly refractory mantle beneath the Marion Rise. These mantle components carry subduction-modified characteristics, and very likely have a recycled mantle wedge origin.

The recent eruptions at Kīlauea Volcano (Hawai‘i) raised some fundamental questions on the longevity and the preservation of eruptible magma batches left over from previous eruptions. Fingerprinting magma batches at Kīlauea through time with bulk and glass compositions is challenging. Narrow compositional changes (e.g., Nb/Y ratio) in matrix glasses occur over time because of repeated magma mixing, and residence timescales of stored evolved magmas in the lower East Rift Zone are underconstrained. To evaluate the diversity in composition and the minimum residence timescales in Rift Zone magmas, we analyzed major and trace elements in plagioclase and matrix glasses from selected samples that erupted in the first weeks of the 2018 Kīlauea eruption. Plagioclase crystals in these samples represent mixed populations with a range in composition spanning An_30-80, corresponding to rhyodacitic to basaltic compositions and temperatures from 950 to 1200 °C. Diffusion modeling of Mg in these plagioclase crystals indicate minimum crystal residence timescales that range from < 1 to ~ 480 years. The complex zoning patterns in plagioclase (and resorptions) together with the protracted storage timescales from diffusion modeling imply that magmas from the East Rift Zone accumulated various plagioclase populations recording magma mixing events that occurred a few years to a few centuries before the 2018 eruption. The diversity of the magma batches (observed with An-Mg compositions in plagioclase) erupted in a single eruption offers research pathways to potentially estimate the frequency, volume and eruptibility of these evolved magmas, thereby refining the risk in the region.

The pressure and temperature conditions of the transition from spinel to garnet as the stable aluminous phase in peridotite lithologies of the upper mantle is integral to elucidating the tectonic significance of the ‘garnet signature’ in basalts. It provides an essential constraint on models of mantle partial melting and oceanic crust formation. Existing experimental results on the univariant phase transition in the simple systems MgO-Al₂O₃-SiO₂ (MAS) and CaO-MgO-Al₂O₃-SiO₂ (CMAS) are mutually inconsistent. To resolve this, we have re-determined the P-T coordinates of the univariant transition in both synthetic systems by running experiments containing both systems simultaneously in the piston-cylinder apparatus, along with the MgO-ZnO pressure sensor. These experiments show a ~ 0.4 GPa difference in the pressure of the spinel/garnet phase transition between the two chemical systems at 1400 ºC, double that inferred from a compilation of existing experimental data. Absolute pressure in these experiments can be verified using the MgO-ZnO sensor. The results imply that the thermodynamic data used in recent mineral equations of state based on the Holland-Powell thermodynamic dataset are substantially correct.