Petrology and geochemistry of feldspathic impact‐melt breccia Abar al' Uj 012, the first lunar meteorite from Saudi Arabia

Abar al' Uj (AaU) 012 is a clast‐rich, vesicular impact‐melt (IM) breccia, composed of lithic and mineral clasts set in a very fine‐grained and well‐crystallized matrix. It is a typical feldspathic lunar meteorite, most likely originating from the lunar farside. Bulk composition (31.0 wt% Al₂O₃, 3.85 wt% FeO) is close to the mean of feldspathic lunar meteorites and Apollo FAN‐suite rocks. The low concentration of incompatible trace elements (0.39 ppm Th, 0.13 ppm U) reflects the absence of a significant KREEP component. Plagioclase is highly anorthitic with a mean of An_96.9Ab_3.0Or_0.1. Bulk rock Mg# is 63 and molar FeO/MnO is 76. The terrestrial age of the meteorite is 33.4 ± 5.2 kyr. AaU 012 contains a ~1.4 × 1.5 mm² exotic clast different from the lithic clast population which is dominated by clasts of anorthosite breccias. Bulk composition and presence of relatively large vesicles indicate that the clast was most probably formed by an impact into a precursor having nonmare igneous origin most likely related to the rare alkali‐suite rocks. The IM clast is mainly composed of clinopyroxenes, contains a significant amount of cristobalite (9.0 vol%), and has a microcrystalline mesostasis. Although the clast shows similarities in texture and modal mineral abundances with some Apollo pigeonite basalts, it has lower FeO and higher SiO₂ than any mare basalt. It also has higher FeO and lower Al₂O₃ than rocks from the FAN‐ or Mg‐suite. Its lower Mg# (59) compared to Mg‐suite rocks also excludes a relationship with these types of lunar material.     

Tectonometamorphic evolution of the Atbashi high‐P units (Kyrgyz CAOB,Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. ⁴⁰Ar/³⁹Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during T_max conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the T_max (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.     

P–T–t estimation of deformation in low‐grade quartz‐feldspar‐bearing rocks using thermodynamic modelling and 40Ar/39Ar dating techniques: example of the Plan‐de‐Phasy shear zone unit (Briançonnais Zone,Western Alps)

Pressure–Temperature–time (P–T–t) estimates of the syn‐kinematic strain at the peak‐pressure conditions reached during shallow underthrusting of the Briançonnais Zone in the Alpine subduction zone was made by thermodynamic modelling and ⁴⁰Ar/³⁹Ar dating in the Plan‐de‐Phasy unit (SE of the Pelvoux Massif, Western Alps). The dated phengite minerals crystallized syn‐kinematically in a shear zone indicating top‐to‐the‐N motion. By combining X‐ray mapping with multi‐equilibrium calculations, we estimate the phengite crystallization conditions at 270 ± 50 °C and 8.1 ± 2 kbar at an age of 45.9 ± 1.1 Ma. Combining this P–T–t estimate with data from the literature allows us to constrain the timing and geometry of Alpine continental subduction. We propose that the Briançonnais units were scalped on top of the slab during ongoing continental subduction and exhumed continuously until collision.     

Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite facies mica 40Ar/39Ar ages in the Longmen Shan (eastern Tibet)

Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim ⁴⁰Ar/³⁹Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The ⁴⁰Ar/³⁹Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between ⁴⁰Ar/³⁹Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology.     

Modeling coseismic displacements resulting from SAR interferometry and GPS measurements during the 1997 Umbria-Marche seismic sequence

In this study we analyse coseismic GPS displacements and DInSAR data to constrain a dislocation model for the three largest earthquakes of the 1997 Umbria-Marche seismic sequence. The first two events, which occurred on September 26 at 00:33 GMT (Mw 5.7) and 09:40 GMT (Mw 6.0) respectively, are investigated using both GPS displacements and DInSAR interferograms. We discuss and compare the results of previous studies which separately modeled a smaller subset of geodetic data. We provide a dislocation model for these two earthquakes which fits well both GPS and DInSAR data and agrees with the results of seismological and geological investigations. The first event consists of a unilateral rupture towards the southeast with a uniform dislocation. The strike, rake and dip angles are those resulting from the CMT solution. The second event consists of an unilateral rupture towards the northwest and a variable slip distribution on the fault plane. The strike and the rake are consistent with the CMT solution, but the dip angle has been slightly modified to improve the simultaneous fit of GPS and DInSAR data. While the second rupture (09:40 GMT) arrived very close to the surface, the fit to geodetic data shows that the first rupture (00:33 GMT) is deeper (2 km), despite the more evident surface geological effects. The analysis of new SAR interferograms allows the identification of a 5–6 cm additional displacement caused by the October 3 (Mw 5.2) and 6 (Mw 5.4) seismic events.We use data from a new DInSAR interferogram to model the displacement field of the Sellano earthquake of October 14, 1997. For this event significant GPS measurements were not available. We tested two different fault plane geometries: a blind, planar fault (top depth = 2.4 km), and a curved (listric) fault reaching the surface. The two models provide a generally similar fit to the data, and show that most of the slip was released at depths greater than 2.4 km along a gently dipping (40^°–45^°) fault surface. They also show that a unilateral rupture does not allow fitting the interferometric fringes since there is evident surface deformation to the northwest of the hypocenter. Moreover, we suggest that the concentration of high residuals in the southern part of our uniform slip model may in fact indicate a certain slip variability in this area.We conclude that, despite the moderate magnitudes and the lack of significant surface faulting, the space geodetic data allowed to constrain dislocation models giving new insights in the rupture process of the three largest events of the sequence.     

Long-term fluid circulation in extensional faults in the central Catalan Coastal Ranges: P–T constraints from neoformed chlorite and K-white mica

The neoformation of chlorite and K-white mica in fault rocks from two main faults of the central Catalan Coastal Ranges, the Vallès and the Hospital faults, has allowed us to constrain the P–T conditions during fault evolution using thermodynamic modeling. Crystallization of M1 and M2 muscovite and microcline occured as result of deuteric alteration during the exhumation of the pluton (290 °C > T > 370 °C) in the Permian. After that, three tectonic events have been distinguished. The first tectonic event, attributed to the Mesozoic rifting, is characterized by precipitation of M3 and M4 phengite together with chlorite and calcite C1 at temperatures between 190 and 310 °C. The second tectonic event attributed to the Paleogene compression has only been identified in the Hospital fault with precipitation of low-temperature calcite C2. The shortcut produced during inversion of the Vallès fault was probably the responsible for the lack of neoformed minerals within this fault. Finally, the third tectonic event, which is related to the Neogene extension, is characterized in the Vallès fault by a new generation of chlorite, associated with calcite C4 and laumontite, formed at temperatures between 125 and 190 °C in the absence of K-white mica. Differently, the Hospital fault is characterized by the precipitation of calcite C3 during the syn-rift stage at temperatures around 150 °C and by low-temperature fluids precipitating calcites C5, C6 and PC1 during the post-rift stage. During the two extensional events (Mesozoic and Neogene), faults acted as conduits for hot fluids producing anomalous high geothermal gradients (50 °C/km minimum).     

Trace element mapping by LA-ICP-MS: assessing geochemical mobility in garnet

A persistent problem in the study of garnet geochemistry is that the consideration of major elements alone excludes a wealth of information preserved by trace elements, particularly the rare-earth elements (REEs). This is despite the fact that trace elements are generally less vulnerable to diffusive resetting, and are sensitive to a broader spectrum of geochemical interactions involving the entire mineral assemblage, including the growth and/or dissolution of accessory minerals. We outline a technique for the routine acquisition of high-resolution 2D trace element maps by LA-ICP-MS, and introduce an extension of the software package XMapTools for rapid processing of LA-ICP-MS data to visualise and interpret compositional zoning patterns. These methods form the basis for investigating the mechanisms controlling geochemical mobility in garnet, which are argued to be largely dependent on the interplay between element fractionation, mineral reactions and partitioning, and the length scales of intergranular transport. Samples from the Peaked Hill shear zone, Reynolds Range, central Australia, exhibit contrasting trace element distributions that can be linked to a detailed sequence of growth and dissolution events. Trace element mapping is thus employed to place garnet evolution in a specific paragenetic context and derive absolute age information by integration with existing U–Pb monazite and Sm–Nd garnet geochronology. Ultimately, the remarkable preservation of original growth zoning and its subtle modification by subsequent re-equilibration is used to ‘see through’ multiple superimposed events, thereby revealing a previously obscure petrological and temporal record of metamorphism, metasomatism, and deformation.     

Iterative thermodynamic modelling—Part 1: A theoretical scoring technique and a computer program (Bingo-Antidote)

This paper introduces the software solution Bingo-Antidote for thermodynamic calculations at equilibrium based on iterative thermodynamic models. It describes a hybrid strategy combining the strength of Gibbs energy minimization (GEM) and inverse thermobarometry models based on the comparison between the modelled and observed mineral assemblage, modes and compositions. The overall technique relies on quantitative compositional maps acquired by electron probe micro-analyser for obtaining a mutually consistent set of observed data such as bulk rock and mineral compositions. Thus it offers the opportunity to investigate metamorphic rocks on a microscale. The scoring part Bingo integrates three statistical model quality factors for the assemblage, for the mineral modes, for the mineral compositions combined in a global evaluation criterion that quantifies how the model reproduces the observations for the investigated volume. The input parameters of GEM affecting the model quality such as pressure, temperature and eventually some components of the bulk composition (e.g. the molar amount of hydrogen, carbon or oxygen) or activity variables of fluids and gases (e.g. , , f(O₂)) can be optimized by inversion in Antidote using several mapping stages followed by a direct search optimization. Examples of iterative models based on compositional maps processed with Bingo-Antidote demonstrate the utility of the program. In contrast to the qualitative interpretation of phase diagrams, the inversion maximizes the benefits of GEM and permits the derivation of statistically ‘optimal’ pressure–temperature conditions for well-equilibrated samples. In addition, Bingo-Antidote opens new avenues for petrological investigations such as the generation of chemical potential landscape maps.     

Joint analysis of SAR interferometry and electrical resistivity tomography surveys for investigating ground deformation: the case-study of Satriano di Lucania (Potenza, Italy)

We have jointly applied microwave remote sensing imaging and ground-based geophysical methodologies for investigating ground deformation. In particular, the Differential Interferometric Synthetic Aperture Radar (DInSAR) technique referred to as Small BAseline Subset (SBAS) algorithm and the Electrical Resistivity Tomography (ERT) method have been applied. The former is a tool for investigating large areas, allowing us to detect deformation phenomena and to analyse their spatial patterns and temporal evolution; the latter permits us to reconstruct, with high spatial resolution, the geometry of near-surface geological structures and to estimate the electrical resistivity of outcropping terrains. The starting point of our analysis has been a DInSAR survey carried out on an area of about 400 km² centred around the town of Potenza (Basilicata region, Italy), for which SAR data acquired by the ERS-1/2 sensors in the time interval 1992–2000 were available. The DInSAR analysis allowed us to detect an unknown ground deformation effect which involves the urban area of Satriano di Lucania, located close to Potenza. An on-site ERT survey was then carried out in order to characterize in detail the physical properties of the terrains involved in the main deformation pattern detected via the DInSAR processing. The integration of the DInSAR and ERT measurements permitted us to investigate areas where the deformation have space-variant characteristics and allowed us to formulate a hypothesis on the origin of the detected displacement processes that, although not fully conclusive, is consistent with the overall DInSAR and ERT analysis.