Plagioclase Grains Research Articles

EH6 enstatite chondrites Northwest Africa 7976 and Northwest Africa 12945: Implications for EH chondrite metamorphism

AbstractThe enstatite chondrite class is known to have complex thermal histories, often interpreted to include impact melting and shock metamorphism. Highly equilibrated (type 6) EH group enstatite chondrites are rare and thought to have formed through collisional heating. We studied two EH6 chondrites, NWA 7976 and NWA 12945, for their textural, chemical, and mineralogical characteristics. The samples we studied contain subhedral to anhedral grains of enstatite and plagioclase, suggesting solid‐state recrystallization. They show low degrees of shock and no evidence of shock melting. Additionally, the ubiquitous occurrence of daubréelite exsolution lamellae in troilite and the Ni content of schreibersite suggest slow cooling at greater burial depths in the parent body, rather than rapid cooling as a result of an impact event. Based on the characteristics and scarcity of type 6 EH chondrites, and the ubiquitous shock effects and melt rocks in the enstatite chondrite class, we conclude that the unshocked NWA 7976 and NWA 12945 were formed by heat derived from impact melt sheets, analogous to contact metamorphism.

Petrogenesis of magmatic charnockite-biotite granite suite from parts of the Chotanagpur Granite Gneissic Complex (CGGC), eastern Indian shield: Implication for the break down of the Columbia Supercontinent

A rare porphyritic charnockite that is girdled by and mineralogically grades to biotite granite occurs as a part of the Chotanagpur Granite Gneissic Complex (CGGC) in and around Massanjore, Jharkhand, India. Preservation of certain textural features, including (a) euhedral to subhedral grains of orthopyroxene, (b) low dihedral angle subtended by orthopyroxene and plagioclase grains, and (c) relict intergranular and porphyritic textures, are consistent with the view that orthopyroxene in the studied rocks has a magmatic origin. Preserved magmatic features and other petrological attributes of these rocks do not support any significant mass change beyond a few tens of microns during the overprinting high-grade metamorphism. The geochemical variation of the felsic rock suite (porphyritic charnockite and biotite granite) indicates that they are cogenetic and are derived from a ferroan A-type granitoid magma by crystal fractionation. The observed geochemical trend of the studied felsic rock suites has been simulated by phase equilibria modelling in the open system using the system components Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2. The observed mineralogical and geochemical attributes and the results of the modelling study are consistent with a petrogenetic process in which high magma temperature (> 900 °C), low pressure, and low water activity in the parental melt favoured the separation of an orthopyroxene bearing cumulate assemblage (orthopyroxene + plagioclase + quartz + K-feldspar + ilmenite + magnetite) in the initial part of the magmatic differentiation. Removal of this anhydrous cumulitic assemblage raised the bulk H2O content in the residual melt. Orthopyroxene became unstable with respect to biotite in the evolved melt that eventually crystallised minerals that formed the biotite granite. An increase in magma fO2 also restricts the orthopyroxene stability in felsic magma. Taken together all the petrological and geochemical attributes, we demonstrate that fractionation of an orthopyroxene-bearing crystal cumulate from the melt is essential to form the charnockites, and that the biotite granite forms from the evolved melt after the fractionation. The charnockite-biotite granite association of the studied area was formed in an extensional tectonic setting, presumably during the breakdown of the Columbia Supercontinent.

Late Jurassic High-Pressure Metamorphism of Variscan I-Type Granitoids in the Northern Part of the Pelagonian Unit (Republic of North Macedonia)

Abstract The high P/T metamorphic Pelagonian Unit in the Republic of North Macedonia comprises (1) a Variscan basement consisting of gneisses, schists and minor meta-mafic rocks, which are all intruded by I-type granitoids and rare related dikes; (2) a metamorphosed sedimentary sequence of Permian to Lower Triassic age, and (3) a sequence of calcite and dolomite marbles resulting from Late Triassic to Middle Jurassic carbonate sediments. All these rocks underwent a common high-P/T metamorphism of Late Jurassic age. This paper deals with the metamorphism of the Variscan I-type granitoids which contained the igneous mineral assemblage plagioclase I + alkali feldspar I + quartz I + biotite I + titanite I + allanite I + zircon I + apatite I ± magnetite I. During Late Jurassic high-P/T metamorphism, these undeformed granitoids were thoroughly metamorphosed under isotropic pressure conditions as documented by undeformed granitic textures that are overgrown by metamorphic minerals such as garnet II, epidote II, and phengite II. Various, eventually metasomatic mineral reactions took place in different textural positions: (1) Former igneous plagioclase grains became completely transformed to Na-rich plagioclase IIa (An09–14) containing numerous small grains of epidote IIa and phengite IIa. Either this transformation was an allochemical one and was accompanied by the syn-metamorphic introduction of an aqueous fluid phase containing Fe, Mg and K or, alternatively, the more Ca-rich parts of plagioclase I became considerably sericitized before high-P/T metamorphism, and the resulting mixture of more Na-rich relic plagioclase with its sericite-rich domains became later metamorphosed under high-P/T conditions. In the first case, an aqueous phase is needed during metamorphism, while in the second case high-P/T metamorphism might have proceeded under H2O-undersaturated conditions; (2) igneous alkali feldspar I was changed to albite-poor orthoclase II or microcline II; (3) igneous Ti-rich biotite I reacted with plagioclase to metamorphic garnet II + Ti-poorer biotite II + titanite II + phengite II + quartz II ± epidote II ± rutile II, which is rimmed by Ttn II. At textural positions, where igneous plagioclase I was not available, igneous biotite I was transformed to Ti-poorer biotite II + titanite II ± ilmenite-hematite II; (4) during uplift, high-P/T metamorphic rutile II became marginally overgrown by titanite II ± ilmenite II; (5) igneous allanite I grains stayed unaltered, but when located near to former plagiocase I, they became partially rimmed by metamorphic epidote II. Equilibrium phase diagram calculations showed that the observed metamorphic paragenesis (plagioclase II + K-rich feldspar II + biotite II + garnet II + epidote II + phengite II + garnet II + quartz II + rutile II + titanite II) is only stable under H2O-unsaturated conditions. The I-type granitoids and their metamorphic country rocks were metamorphosed under high-P/T conditions of 1.3 to 1.5 GPa and 560 to 590 °C.

Fracturing, comminution and grain-size-sensitive creep as a record of coseismic loading in the middle-crust: Insights from the Urtiga mylonitic pluton (NE Brazil)

Evidence for mid-crustal seismic slip is scarce due to the limited capacity of downward propagation of rupture damage imposed by the confining pressure. Nevertheless, structures indicative of elevated stresses occur throughout the crustal profile. To further investigate this setting, we have studied the fabrics of the Urtiga pluton, emplaced at the south Patos shear zone (northeast Brazil), a major Neoproterozoic crustal boundary. K-feldspar and plagioclase porphyroclasts are fractured, with fine-grained K-feldspar + plagioclase mixtures filling cracks. The edges of the clasts are rimmed by fine feldspar grains that form a fine-grained matrix. Quartz ribbons are parallel to the mylonitic foliation and show microstructures and a fabric indicating deformation via dislocation creep. Chemical compositions of feldspars are typically similar between porphyroclasts, fractures and matrix, with plagioclase grains locally being more albitic within fractures. Crystallographic preferred orientations (CPO) in grains within fractures are host-controlled by the adjacent porphyroclast, while fine grains rimming the clasts show a weak CPO and are mostly strain-free. These characteristics suggest that grain size reduction in the Urtiga mylonitic pluton occurred through fracturing and subsequent grain-size-sensitive creep under mid-crustal conditions, which were possibly attained via downward propagation of seismic rupture from the overlying seismogenic zone during transient, seismic slip episodes, giving rise to spatially related pseudotachylytes at the boundaries of the southern Patos shear zone.

Extended Laterals and Hydraulic Fracturing Redevelop Tight Fractured Carbonates

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 216292, “Redevelopment of Tight Fractured Carbonates Through Extended Laterals and Hydraulic Fracturing,” by Antonio Buono, Cameron Taylor, and Alyssa Dordan, SPE, ExxonMobil, et al. The paper has not been peer reviewed. _ In the complete paper, the authors compare development scenarios in a fractured carbonate play between historic vertical and short horizontal development and modern hydraulically fractured extended lateral development. Because of its long production history and recent redevelopment efforts, the Austin Chalk was chosen as a natural laboratory to test how recent artificial stimulation techniques can lead to additional production from a wider range of pore systems. Development of the Austin Chalk In recent years, application of modern unconventional multistage hydraulic fracturing techniques, coupled with adding proppant to support induced fracture networks, mitigated the steep decline seen in historic production profiles. These improvements were exemplified in a recent Austin Chalk redevelopment where modern completions led to an increase in estimated ultimate recovery (EUR) by 250% on average. In the targeted area of development, historic, short Austin Chalk laterals without modern completions exhibited a wide range of well performance. Some outlier wells achieved high recoveries from accessing an existing natural fracture network with the original completion, whereas others, after only a few months of economic production, were unable to achieve continuous flow without a propped stimulation. The differences in performance partially can be explained by the fact that the reservoir quality of different intervals within the Austin Chalk is likely highly variable. This is exemplified in the B-2 Zone, which contains a well-developed vertical fracture network with variable lengths. Data suggest that the natural fractures are confined within the B-2 and it is geomechanically less-susceptible to wellbore collapse than zones with higher clay concentrations. The B-2 is likely a stiffer interval than the E Zone. This implies that differences exist in the properties of these units that are caused by changes in mineralogy and cementation. The authors aim to characterize reservoir quality and hydrocarbon distributions of the fractured relatively clean zones compared with the hydraulically stimulated reservoir in relatively “dirty” chalky zones, and evaluate geomechanical properties and production expectations from each zone. Depositionally, these units can be characterized broadly as carbonate-rich with subordinate siliciclastic detritus composed of clay minerals and silt-sized quartz and plagioclase grains. These units all contain distinctive stylolite seams. The most-favorable hydrocarbon shows typically are in the lower members of the Austin Chalk. Production data show that natural-fracture-only production (heritage) generally has lower total EUR with highly variable well-to-well production profiles vs. fracture and matrix production, which the authors write that they believe is achieved when unconventional technology is applied to these reservoirs. To project from heritage chalk production to expected EUR using modern completions, the authors used a risked EUR scaling factor of 1.5×. Consequently, the potential uplift in economic viability was recognized through a multiwell Austin Chalk appraisal to assess well-to-well communication with Eagle Ford codevelopment. The authors’ appraisal evaluated reservoir-quality differences between the main landing zones in the Austin Chalk with those from the E Zone of the Austin Chalk and Upper Eagle Ford, respectively, to demonstrate how significant economic uplift can be realized in mature, tight carbonate fields with unconventional technology.

The impact of tectonic inversion on diagenesis and reservoir quality of the Oligocene fluvial sandstones: The upper Huagang formation, Xihu Depression, East China Sea Shelf Basin

Inversion tectonics are of paramount importance for forming a range of hydrocarbon traps and reservoirs for petroleum exploration. Also, the uplift and exhumation of strata, the activation and creation of faults and leakage or breaching of hydrocarbon traps are often attributed to multiple inversion. Changes in fluid activity and the geochemical system are frequently observed during such tectonic inversions. However, the impacts of tectonic inversion on fluid flow and potential diagenetic alterations within deeply buried sandstone reservoirs are not fully understood, posing challenges to accurately assessing reservoir quality and furthering exploration efforts. The Oligocene fluvial sandstones in the Yuquan anticline of the Xihu Depression were investigated by XRD, optical microscope and SEM-EDS-CL along with and petrophysical data to illustrate the impact of tectonic inversion on the evolution of diagenesis and reservoir quality. The two representative exploration wells, C-1 and C-2, originating from the same provenance and sedimentary background in Yuquan anticline, exhibit significant diagenetic alterations attributed to varying degrees of tectonic inversion. Notably, well C-1 has experienced more intense alterations and dissolution of detrital grains compared to well C-2 (average secondary pore, well C-1: 4.8%, well C-2: 2.9%), particularly the complete dissolution of plagioclase grains. Moreover, the content of authigenic kaolinite and quartz cement of well C-1 is higher than that of well C-2, averaging 7.2%, 4.5% and 5%, 1.7% respectively. This diagenetic difference is attributed to the fact that the activity of early NE-NNE oriented faults and the development of late nearly E-W oriented faults are stronger in the northern well C-1 field than in the central well C-2 field. The structural characteristics resulting from tectonic inversion led to enhanced efficiency in both organic acid supply from underlying source rocks in well C-1 field and export of dissolution products within the diagenetic system when compared with Well C-2 field. Regional tectonic inversion plays a predominant role in improving reservoir quality than sedimentary factors in the study area. While the well C-1 has a higher porosity than well C-1 (well C-1: average 18%, well C-2: average 13.1%), the permeability of well C-1 is worse than that of well C-2, with an average of 14.7 mD and 27.9 mD respectively. The alteration and dissolution associated with tectonic inversion significantly enhance the porosity of sandstone reservoirs. However, the formation of secondary pores and dissolution products can complicate the pore system of reservoirs, thereby reducing the reservoir permeability. Our research aims to provide a better understanding of fluid-rock interactions and genetic mechanisms in gas-bearing sandstone reservoirs under varying degrees of tectonic inversion. This will support pre-drill evaluations of reservoir quality in deeply buried formations as well as other analogous targets.

A Tall Tale of a Tiny Pluton: The L’Enfer Norite, Grenville Province, Québec

Abstract We present new field observations and mineral and whole-rock geochemical data for the L’Enfer norite, a small pluton in the central Grenville Province of Québec. Although tiny, its plagioclase-rich and noritic character suggested that it might hold clues (possible parental magma?) to the origin of closely associated andesine anorthosites in this part of Québec. The pluton is characterized by spectacular coarse-grained diabasic textures and varies from relatively massive to locally layered varieties. Both coarse- and medium-grained types occur and may be present together in some outcrops. Rare occurrences of snowflake-textured plagioclase are also present. The mafic mineralogy is dominated by orthopyroxene and Fe-Ti oxides (ilmenite and magnetite) with minor clinopyroxene, amphibole (Ti-pargasite), and Ti-rich biotite. Olivine occurs in one of the norites and in a single occurrence of an ultramafic rock that is also enriched in Fe-Ti oxides. Plagioclase compositions range from An32 to An48, but can be divided into two groups on the basis of texture: in one subset of samples, the plagioclase contains exsolution lamellae of orthoclase (i.e., antiperthitic), whereas plagioclase in the remaining samples lacks such lamellae and has not exsolved. Despite these textural contrasts, whole-rock compositions of the two groups are similar. Surprisingly, the compositions of pyroxene, biotite, and ilmenite reveal two non-overlapping compositional groups that correlate with those defined by plagioclase texture. Specifically, mafic minerals in the group with unexsolved plagioclase are consistently enriched in minor elements compared to those in the group with antiperthitic plagioclase. We interpret these contrasts to reflect differences in cooling rate for at least two injections of noritic magma. Moreover, the snowflake plagioclase grains are consistent with some degree of undercooling and supersaturation brought about by magma mixing. Although the norite shares some mineralogical and geochemical features with nearby anorthosites, it does not appear to represent a viable parent magma, primarily due to a pronounced difference in Fe-Ti oxide assemblage (hemo-ilmenite in the anorthosites versus ilmenite + magnetite in L’Enfer).

Nanophase magnetite in matrix of anomalous EL3 chondrite Northwest Africa (NWA) 8785

AbstractNWA 8785 is a remarkable EL3 chondrite with a high abundance (~34 vol%) of an Fe‐rich matrix. This is the highest matrix abundance known among enstatite chondrites (ECs) and more similar to the matrix abundances in some carbonaceous and Rumuruti chondrites. X‐ray diffraction and TEM data indicate that the fine‐grained portion of the NWA 8785 matrix consists of nanoscale magnetite mixed with a noncrystalline silicate material and submicron‐sized enstatite and plagioclase grains. This is the first report of magnetite nanoparticles in an EL3. The Si content of the metal (0.7 wt%), presence of ferroan alabandite, and its O isotopic composition indicate NWA 8785 is EL3‐related. Having more abundant matrix than in other ECs, and that the matrix is rich in magnetite nanoparticles, which are not present in any other EC, suggest classification as an EL3 anomalous. Although we cannot completely exclude any of the mechanisms or environments for formation of the magnetite, we find a secondary origin to be the most compelling. We suggest that the magnetite formed due to hydrothermal activity in the meteorite parent body. Although ECs are relatively dry and likely formed within the nebular snow line, ices may have drifted inward from just beyond the snow line to the region where the EL chondrites were accreting, or more likely the snow line migrated inward during the early evolution of the solar system. This may have resulted in the condensation of ices and provided an ice‐rich region for accretion of the EL3 parent body. Thus, the EL3 parent body may have had hydrothermal activity and if Earth formed near the EC accretion zone, similar bodies may have contributed to the Earth's water supply. NWA 8785 greatly extends the range of known characteristics of ECs and EC parent body processes.

Petrogenesis of Chang'E‐5 young mare low‐Ti basalts

AbstractThe regolith samples returned by the Chang'E‐5 mission (CE‐5) contain the youngest radiometrically dated mare basaltic clasts, which provide an opportunity to elucidate the magmatic activities on the Moon during the late Eratosthenian. In this study, detailed petrographic observations and comprehensive geochemical analyses were performed on the CE‐5 basaltic clasts. The major element concentrations in individual plagioclase grain of the CE‐5 basalts may vary slightly from core to rim, whereas pyroxene has clear chemical zonation. The crystallization sequence of the CE‐5 mare basalts was determined using petrographic and geochemical relations in the basaltic clasts. In addition, both fractional crystallization (FC) and assimilation and fractional crystallization models were applied to simulate the chemical evolution of melt equilibrated with plagioclase in CE‐5 basalts. Our results reveal that the melt had a TiO2 content of ~3 wt% and an Mg# of ~45 at the onset of plagioclase crystallization, suggesting a low‐Ti parental melt of the CE‐5 basalts. The relatively high FeO content (>14.5 wt%) in melt equilibrated with plagioclase could have resulted in extensive crystallization of ilmenite, unlike in Apollo low‐Ti basalts. Furthermore, our calculations showed that the geochemical evolution of CE‐5 basaltic melt could not have occurred in a closed system. On the contrary, the CE‐5 basalts could have assimilated mineral, rock, and glass fragments that have higher concentrations of KREEP elements (potassium, rare earth elements, and phosphorus) in the regolith during magma flow on the Moon's surface. The presence of the KREEP signature in the CE‐5 basalts is consistent with literature remote sensing data obtained from the CE‐5 landing site. These KREEP‐bearing fragments could originate from KREEP basaltic melts that may have been emplaced at the landing site earlier than the CE‐5 basalts.

Source-to-sink controls on modern fluvial sands in the Pantanal back-bulge basin (Brazil)

Sedimentological processes are underexplored in retro-arc basin segments of source-to-sink systems, particularly in the distal back-bulge depozone. The extant Pantanal Basin of Latin America is a large sub-tropical continental back-bulge for which the boundary conditions on riverine sedimentation (tectonics, climate, phytogeography) are reasonably understood. Here, we use a new database of riverbed sand samples (n = 97) to investigate the modern provenance of the Pantanal Basin. Petrographic analysis of grain-mounted thin sections and Gazzi-Dickinson point counting were used to assess the framework composition of the sands, and a pour point analysis based on geographic information systems (GIS) defined the hinterland geological environment for each sample. Six provenance domains were defined: (a) lowlands, (b) Amazon craton, (c) Rio Apa craton, (d) plateau, (e) South Paraguay Belt, and (f) North Paraguay Belt. Most Pantanal samples are quartzose non-orogenic detritus (Qt88\F5\L7). Rivers draining the Paraguay Belt highlands carry more lithic grains derived from carbonate parent rocks. The occurrence of plagioclase grains is rare. However, river sands where >30% of the total assemblage was K-feldspar were present downstream of foliated metamorphic outcrops of the Rio Apa craton. Sand relatively rich in fine K-feldspar grains in the medial Taquari River megafan were attributed to channel avulsion, incision, and reworking of floodplain deposits. Statistical analysis suggests that bedrock lithology and average annual rainfall are important controls on fluvial sands in the Pantanal Basin, with watershed slope, temperature, and dilution playing secondary roles in the spatial distribution of dominant grain types. This study provides new actualistic data that aim to improve interpretations of the geological record.

Mn‐Precipitates Found in a Martian Crustal Rock

AbstractAlteration minerals in one of the Martian meteorite nakhlites, Yamato (Y) 000802, were studied to understand the alteration process and conditions. Mn‐precipitates are discovered between altered plagioclase grains in Y 000802. Mn‐precipitates consist of hausmannite (), manganite (γ‐Mn3+OOH), rhodochrosite (Mn2+CO3), and a trace amount of Mn4+O2 mineral. Jarosite ) is also found. Mn2+ dissolved from olivine contributes to the formation of Mn‐precipitates. A weakly acidic‐neutral fluid containing a trace amount of altered the olivine, and Mn2+ was dissolved into the fluid. The fluid also reacted with plagioclase and probably induced dealkalization of plagioclase, causing a local strong alkaline environment. Plagioclase was altered to ferroan saponite‐nontronite + amorphous SiO2 under alkaline conditions. Simultaneously, Mn2+/3+‐precipitates were formed from the Mn2+‐containing fluid in the interstices between the altered plagioclase grains under the strong alkaline reducing environment. These alterations occurred in the deep part of the nakhlite body, where they are isolated from Martian subsurface water, including strong oxidants. The formation of Mn2+/3+‐precipitates may have been triggered by the melting of permafrost caused by an impact event around ∼633 Ma. Later, the nakhlite body was probably excavated by another impact, making it susceptible to water including strong oxidants. Pyrrhotite was dissolved and a highly acidic oxidizing fluid was formed, which would induce the formation of jarosite and the Mn4+O2 mineral between ∼633 Ma and ∼11 Ma.

Crustal architecture of Neoproterozoic terrains of western Gondwana revealed by integrated geophysical and structural data on the eastern Borborema Province, NE Brazil

The deformation processes (e.g. accretionary or intraplate) occurring within the Borborema Province have been widely discussed, however the eastern part of the central subprovince lacks further investigation in such aspect. In this contribution, we present integrated airborne gamma ray and magnetometric geophysical data, as well as meso- and microstructural markers of the Archean-Paleoproterozoic Alto Moxotó and Neoproterozoic Rio Capibaribe terrains (central Borborema Province), providing evidence for hidden accretionary episodes in the region. Fourteen radiometric and ten magnetic domains were described and fitted with most of the geological units and mapped structures, suggesting geological contrasts between the studied terrains. For instance, the Alto Moxotó terrain presents high-to-medium eTh and eU radiometric signatures, spatially distributed along folded geometry, whereas the lithospheric record of the Rio Capibaribe terrain is characterized by K (%) enrichment and a dominant NE-SW sheared crust. In addition, the obtained Euler solutions show good correlations with the main structures and delineated shallow to intermediate sources of the Congo-Cruzeiro do Nordeste strike-slip shear zone system, the proposed structural boundary. This large set of shear zones are characterized by large mylonitic corridors with characteristic vertical foliation planes with moderately-plunging mineral lineations. Associated microstructures include bulging, subgrain rotation and recrystallization that, in association with the well-developed S–C fabrics and myrmekites indicate deformation at moderate temperature values (∼400–500 °C). Also, conspicuous fracture networks seen in plagioclase grains demonstrate that deformation also took place at the later developed brittle-ductile transition. Hence, the described contrasting rock ages and nature, geophysical signatures and deformational patterns is herein interpreted as the result of crustal agglutination during the Neoproterozoic, in which the Congo-Cruzeiro do Nordeste shear zone system would represent relics of a terrane boundary developed during the assembly of western Gondwana.

Fluid-enhanced diffusive mass transfer combined with GBS as an important process for protracted weakening in the middle-lower crust

Whether dislocation creep or diffusion creep dominates the flow of the middle-lower crust remains an enigma. In this study, integrated micro-analyses on laminated ultramylonites along the Wulian detachment fault zone, North China, were performed to unravel the dominant deformation mechanisms in naturally sheared granitic rocks in the middle-lower crust. The ultramylonites consist of interlayers of quartz ribbons and polymineralic layers of feldspars and quartz. The quartz ribbons are characterized by widespread intracrystalline low-angle grain boundaries and obvious crystallographic preferred orientations (CPOs), indicating the activity of dislocation creep. In contrast, random CPOs and well-mixed phases compatible with the operation of GBS characterize the polymineralic layers. Cathodoluminescence observations reveal zoning structures in the plagioclase and K-feldspar grains from such polymineralic layers. Meanwhile, chemical composition analysis shows that K-feldspar and plagioclase grains within the layers commonly possess potassium- and calcium-enriched rims, respectively. Furthermore, no evidence for intracrystalline distortion is detected in the plagioclase and K-feldspar grains. Together with interstitial K-feldspar grains, the evidence therefore shows that fluid-enhanced diffusive mass transfer played a dominant role in accommodating GBS in the polymineralic layers. In addition, mixed phases due to cavitation-sealing and geometric phase mixing inhibit grain growth and further ensure the maintenance of GBS during progressive deformation. Consequently, we would argue that operation of fluid-enhanced diffusive mass transfer combined with GBS leads to protracted weakening of the middle-lower crust.

Oriented secondary magnetite micro-inclusions in plagioclase from oceanic gabbro

Abstract Plagioclase-hosted submicrometer to micrometer-sized oriented needle- and lath-shaped magnetite micro-inclusions with their elongation direction aligned parallel to the plagioclase [001] (PL[001]) direction were investigated using correlated optical, scanning electron, and scanning transmission electron microscopy. The PL[001] magnetite micro-inclusions formed from older generations of differently oriented magnetite micro-inclusions by recrystallization during hydrothermal alteration. Six orientation variants of PL[001] magnetite micro-inclusions occur, and they share the same shape orientation but differ in their crystallographic orientation relationships to the plagioclase host. The magnetite-plagioclase interfaces are facetted. High-resolution scanning transmission electron microscopy revealed that interface facets are aligned parallel to low-index lattice planes corresponding to oxygen layers of either magnetite or plagioclase. In addition, the linkage between prominent crystal structure elements of magnetite and plagioclase across the interfaces and accommodation mechanisms minimizing misfit between the two crystal structures were discerned. Combined evidence suggests that the shape and shape orientation, as well as the crystallographic orientation relationships between the magnetite micro-inclusions and the plagioclase host, are crystallographically controlled. The close crystal-structural link between magnetite precipitates and plagioclase host ensures a low-energy configuration driving recrystallization of older generations of differently orientated magnetite micro-inclusions into those that are aligned parallel to PL[001] and facilitates the underlying reaction kinetics. Due to their single to pseudo-single domain characteristics, the plagioclase-hosted magnetite micro-inclusions are particularly robust carriers of natural remanent magnetization. Recrystallization of differently oriented preexisting magnetite micro-inclusions into magnetite micro-inclusions with uniform shape orientation parallel to PL[001] has interesting consequences for the magnetic anisotropy of magnetite-bearing plagioclase grains.

Crystal Structure Controls on Oriented Primary Magnetite Micro-Inclusions in Plagioclase From Oceanic Gabbro

Abstract Oriented needle-, lath- and plate-shaped magnetite micro-inclusions in rock forming plagioclase from mafic intrusive rocks, were investigated using correlated optical microscopy and scanning transmission electron microscopy. The magnetite micro-inclusions were analysed on cuts parallel and perpendicular to the inclusion–elongation directions. The crystal structures of the two phases are in direct contact along the interfaces. The shape, shape orientation and crystallographic orientation relationships between the magnetite micro-inclusions and the plagioclase host appear to be controlled by the tendency of the system to optimise lattice match along the interfaces. The elongation direction of the inclusions ensures good match between prominent oxygen layers in the magnetite and plagioclase crystal structures across the interfaces bounding the inclusions parallel to their elongation direction. In cross-section, additional modes of lattice match, such as the commensurate impingement of magnetite and plagioclase lattice planes along the interfaces, the parallel alignment of the interfaces to low-index lattice planes of magnetite or plagioclase, or the parallel alignment to low index lattice planes of both phases are observed, which appear to control the selection of interface facets, as well as the shape and crystallographic orientation relationships between magnetite micro-inclusions and plagioclase host. The systematics of the inclusion cross-sectional shapes and crystallographic orientation relationships indicate recrystallisation of magnetite with potential implications for natural remanent magnetisation of magnetite-bearing plagioclase grains.

Corona-Type Textures in Ultrabasic Complexes of the Serpentinite Belt, Kola Peninsula, Russia

For the first time, corona-type textures are described in ultrabasic rocks in three complexes of the Serpentinite Belt on the Kola Peninsula in the northeastern Fennoscandian Shield. Three variants of the corona texture formed at different stages during the crystallization of a komatiitic, Al-undepleted melt emplaced in a subvolcanic setting. The first type crystallized at an early stage (Mg# Ol = 87) in a fine-grained harzburgite of the Chapesvara-I sill, with the following order in the corona: Ol → Opx → Cpx → Pl → Amp (aluminous sodic-calcic). The second type displays the sequence Opx → Cpx → Amp → Pl → Qz, which is observed in the orthopyroxenite zone in the Lotmvara-I sill. The third type involves a symplectitic corona in a plagioclase-bearing orthopyroxenite in the Lyavaraka complex, in which the inferred order is: Cpx → Amp (aluminous hornblende) + symplectitic Qz, formed in direct contact with grains of Pl. The corona-type textures occur in fresh rocks and are not related to regional metamorphism. They likely formed as consequences of two important factors: (1) rapid cooling, leading to unsteady conditions of crystallization in a shallow setting; and (2) an intrinsic enrichment in H2O and other volatiles in the parental magma, giving rise to fluid-saturated environments at advanced stages of crystallization. This was followed by a deuteric deposition of Amp rims as a result of the accumulation of H2O and reaction of H2O-bearing fluid with early grains of pyroxene and late plagioclase. The likely existence of a close relationship is suggested by the drusites of the Belomorian complex, which are coeval. In addition, unusual occurrences of lamellar inclusions of phlogopite and Al2SiO5 are documented, hosted by interstitial grains of plagioclase in the orthopyroxenite zone of the Lotmvara-I sill. These are attributed to crystallization from late portions of remaining melt enriched in Al, K, Na, H2O, and Cl, which is indicated by the recorded occurrence of chlorapatite in this association. Thus, our findings indicate the presence and abundance of intrinsic volatiles, Cl, F, CO2, and especially magmatic H2O, which were important to lower the liquidus, decrease the density and viscosity of the highly magnesian melt of Al-undepleted komatiite, thus enabling its transport from the mantle to a shallow level in the crust.

Petrology of Palaeoarchaean mafic–ultramafic rock suites of the western Iron Ore Group, Singhbhum Craton, eastern India, using the chemistry of minerals

AbstractThe petrogenesis of Palaeoarchaean mafic–ultramafic rock suites from the western Iron Ore Group, Singhbhum Craton, eastern India, has been evaluated based on the chemistry of constituent mineral phases. The rock suites include basaltic rocks and mafic (gabbro) to ultramafic (serpentinized peridotite) intrusive rocks, which occur in host rocks covering phyllite, ferruginous shale, banded haematite quartzite and jasper. The constituent clinopyroxene shows dominant uralitization, whereas plagioclase grains are generally saussuritized, being marked by relatively tiny granular aggregates of albite, chlorite, epidote and K-feldspar. The ultramafic intrusive rocks are overwhelmingly serpentinized. Clinopyroxene compositions are augitic, whereas relict plagioclase is typically bytownite. Amphiboles of the investigated rock suites are divisible into the ‘uralite’ type (occurring peripherally to clinopyroxene) and the ‘completely changed-over amphibole’ type (with no traces of initial clinopyroxene). Both the amphibole types belong to the ‘calcic group’, showing a compositional spectrum from actinolite–magnesiohornblende–ferrohornblende–ferroactinolite. Opaque minerals include magnetite (both Cr-magnetite and Al-magnetite), chromite (Al-chromite) and ilmenite, whereas serpentine (belonging to the ultramafic intrusive rocks) corresponds to lizardite. Looking at the mineral compositions of the pyroxene, glass and amphibole, the studied rock suites show a wide equilibration temperature–pressure domain range (∼750 °C to ∼1400 °C at ∼0.26 kbar to ∼21 kbar), which corresponds to an ascending magma that underwent a ‘hydration event’ at a shallow level. The assessment of the clinopyroxene and spinel chemistry characteristically suggests an arc setting for the parent magma that has undergone both equilibrium and fractional crystallization in the course of magmatic evolution. During differentiation, the magmatic density remains almost constant, with variable oxygen fugacity.

Petrochemical and geochronological data of Permian-Lower Triassic clastic sedimentary rocks in the northwestern Junggar basin, NW China: Implications for provenance, tectonism and paleoclimate

The Permian clastic sediments in the Mahu sag offer outstanding conditions to test the control of tectonism and paleoclimate on sedimentation in a rift basin. A synthesis of petrological and geochemical analyses of sandstones, zircon U–Pb geochronology of granitic conglomerates, and principal component analysis have been used to investigate the source composition and chemical weathering indices. The rare earth and trace element compositions of 16 sandstone samples are consistent with first cycle sources from the Carboniferous–Lower Permian mafic and felsic igneous rocks in West Junggar. Lower Permian sandstones contain abundant volcanic lithic clasts and plagioclase grains, and have high Na2O, CaO, Sr and V, suggesting a coeval proximal volcanic source in Central West Junggar during the syn-rift stage. Middle Permian sandstones consist of trachyte and rhyolite clasts, and quartz and albite grains, and show a gradual increase of Th, U and SiO2, indicating an expansion of source areas to more distant Southern and Northern West Junggar during the post-rift stage. In the Lower Triassic, an abrupt increase in the percentages of quartz and K-feldspar, as well as the zircon U–Pb geochronology of granite-clast conglomerates (290–330 Ma), all indicate regional uplift and unroofing of Carboniferous–Permian granitic plutons in West Junggar during the tectonic inversion stage at the Permian-Triassic boundary. Various chemical weathering indices indicate cold and arid climate conditions during the early Permian, followed by a transition to warm and humid climatic conditions. Comprehensive analysis of petrography and geochemistry sheds a new light for studying the source-to-sink evolution of a rift basin, and has further implication for paleoclimate fluctuation and change.

Controls on determining the bulk water content of the Moon

AbstractThe Moon is much wetter than previously thought. The estimated bulk H2O concentrations based on the analyses of H2O in lunar materials show a wide range from 5 to 1650 ppm. To better constrain bulk H2O in the lunar magma ocean (LMO), we model LMO crystallization and vary DH (concentration of H2O in LMO mineral/concentration of H2O in melt), interstitial melt fraction, and initial LMO depth. We take the highest and lowest values of DH reported in the literature for the LMO minerals. We assess the bulk H2O content required in the initial magma ocean to satisfy two observational constraints: (1) H2O measured in plagioclase grains from ferroan anorthosites and (2) crustal mass from GRAIL. We find that the initial bulk LMO H2O that best explains the H2O content in crustal plagioclase is strongly dependent on DH rather than interstitial melt fractions or initial LMO depths, with a drier magma ocean (10 ppm H2O) being favored with higher DH and a wetter magma ocean (100–1000 ppm H2O) with lower DH. This underscores the importance of constraining DH specific to lunar conditions in future studies. We also demonstrate that crustal mass is not an effective hygrometer.

In situ crystallization of non-cotectic and foliated igneous rocks on a magma chamber floor

Layered mafic intrusions commonly contain non-cotectic, foliated igneous rocks that are traditionally attributed to processes involving settling, transport, and redeposition of crystals. Here we examine the chemistry of magnetitite layers of the Bushveld Complex using a portable XRF spectrometer on drill core and dissolution ICP-MS analysis on pure magnetite separates. While magnetitites contain foliated plagioclase grains in non-cotectic proportions, the magnetite is characterized by a regular upwards-depletion of Cr which is best explained by in situ crystallization. We suggest that plagioclase nucleation in thin residual compositional boundary layers atop a solidification front causes in situ growth of plagioclase in proportions much lower (<10%) than those expected from cotectic crystallization (±85%). Crystallization in such a boundary layer also favours lateral growth of the plagioclase, producing the foliation. We suggest that some non-cotectic, foliated rocks that are commonly interpreted to arise from gravity-induced sedimentary processes may instead be produced by in situ crystallization.