Contributions to Mineralogy and Petrology最新文献

We experimentally investigated plagioclase nucleation and growth in anhydrous arc basaltic andesite at 1 atm and Ni-NiO equilibrium. After equilibration at 1190 °C (15 °C above the liquidus) for 24 h, experiments were cooled at 1, 3, or 9 °C/h and quenched at 1175–1000 °C. New plagioclase grains nucleated near the liquidus, followed by minor amounts of Fe–Ti oxides and pyroxene below 1120 and 1050 °C, respectively. Plagioclase shapes varied from 2D tabular/elongated (1 and 3 °C/h) to hopper and swallowtail textures (9 °C/h), suggesting a transition from interface- to diffusion-controlled growth. Crystal shapes and sizes were correlated, with the smallest and largest having equant/elongated and tabular/bladed 3D shapes, respectively. To identify the most suitable method for inferring storage timescales in natural magmas, we calculated nucleation (J) and growth rates (G) with different methods: G_max from the average size of the 10 biggest crystals, G_mean from the entire crystal population, J_batch and G_batch from the number and proportion of plagioclase estimated by point counting, and J_CSD and G_CSD from the crystal size distribution (CSD). J and G were greatest near the liquidus and decreased during cooling; the decrease was minimal at slow cooling rates, making G nearly constant. G decreased with decreasing cooling rates (from 10⁻⁷ to 10⁻⁹ cm/s at 9 and 1 °C/h, respectively), stabilizing after ~ 20 h of cooling. These variations of G principally resulted from differences in experimental conditions, more than the calculation method considered. Given the uncertainties of CSD theory in closed systems and the size and crystallographic axis-dependence of growth rates, combining Gₘₑₐₙ and Gₘₐₓ appears to be the most effective method for experimentally determining growth rates. However, the batch method (J_Batch) still provides a good estimate of J.

The Lower Critical (LCZ)–Upper Critical Zone (UCZ) boundary of the Rustenburg Layered Suite is an intrusion-wide, major stratigraphic transition from intercumulus plagioclase in the LCZ to cumulus plagioclase in the UCZ. No consensus exists regarding the nature of this boundary, with some regarding the attainment of cumulus status by plagioclase at this level of the intrusion due to continued fractionation of the resident magma, whilst others argue for the addition of compositionally distinct magma(s) at this level of the intrusion. Here we report in-situ Sr-isotopic compositions for plagioclase along with whole-rock major- and trace element geochemical and mineral chemical data across the LCZ–UCZ boundary as intersected by borehole BH6958 on the farm Forest Hill in the eastern Bushveld Complex. Major and trace element data across the LCZ–UCZ boundary (e.g. the Cr content of orthopyroxene) support the notion that no compositionally distinct magma was added at this level of the intrusion. Sr- and Nd-isotopic data, however, point to open-system behaviour. The isotopic excursion cannot be explained through mixing between resident (B1) magma and other proposed parental magmas (e.g. B2 or B3 magmas). Modelling suggests that the observed isotopic excursion may be explained through mixing of resident (B1) magma with small amounts of lower crustal melts. Whether such mixing would have resulted in plagioclase stabilisation remains unclear. The observed isotopic excursion can also be explained through mixing of resident (B1) magma with small amounts of crustal fluids. In this case, the introduction of these fluids appears to have happened gradually, with ⁸⁷Sr/⁸⁶Sr_i in plagioclase being higher in LCZ rims than cores, and higher yet in the lower UCZ. We argue on the basis of thermodynamic modelling that when plagioclase joined the crystallising assemblage, the system contracted at a rate higher than that preceding plagioclase stabilisation, with fluids from the surrounding hydrothermal system entering the magma chamber to counter the volume loss experienced by the cooling system.

The bulk of the Icelandic crust is generated along the extensively studied neovolcanic zones. The subglacially formed Fjallgarðar Volcanic Ridge (FVR) in Northeast Iceland represents extensive near-rift volcanism and covers a large area east of the Northern Rift Zone, yet its magma plumbing system remains poorly understood. This study presents new geochemical data from basaltic pillow lavas and hyaloclastites sampled along the entire FVR, including major element compositions of whole-rock, groundmass glass, and minerals, trace element compositions of whole-rock and glass, and oxygen isotope data for whole-rocks and glasses. Our results suggest that two types of relatively evolved tholeiitic basalts (4.4 to 7.8 wt% MgO) are present along the entire, 200 km-long FVR: one group (Zr/Y = 2.5–4.3) is compositionally similar to basalts erupted in the Northern and Eastern Rift Zone, whereas a second group (Zr/Y = 4.7–5.4) is more enriched and overlaps compositionally with basalts from Kverkfjöll, a volcanic system adjacent to the southern tip of the FVR. These distinct magma types, which are generally similar to fissure basalts erupted in the rift axis, were stored at similar pressures and temperatures (2.2 ± 1.4 kbar, 1108 ± 45 °C), and experienced extensive crystal fractionation and some degree of crustal assimilation. We conclude that the crustal processes did not generate the compositional bimodality but instead, the two groups reflect the chemical variability of their parental mantle-derived melts. We propose that the high-Zr/Y magmas were injected in the FVR via long-distance (> 100 km) lateral dyke propagation from magma reservoir(s) from its southern distal end, and that the low-Zr/Y magmas were supplied from a series of more proximal magma reservoirs distributed along the FVR.

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.