A compilation and characterisation of lithics in kimberlite and common maar-diatremes and tephra ring deposits

Maar-diatreme volcanoes are the second-most common type on land, occurring in volcanic fields within all major tectonic environments. Their deposits typically contain an abundance of lithic fragments quarried from the substrate, and many contain large, deep-sourced lithic fragments that were erupted to the surface. Primary volcaniclastic deposits fill the diatreme structure formed during eruption. There is negligible inelastic deformation of diatreme-adjacent country rock, indicating that country rock is removed to create the diatreme structures, either by being shifting downward below observable levels, ejected upward to contribute to surficial deposits, or dissolved and hidden in magma erupted or intruded at depth. No previous study has systematically reviewed and analysed the reported lithic fragments of maar-diatreme systems. We present a comprehensive compilation from published work of lithic characteristics in maar ejecta rings and in diatreme deposits of both common and kimberlite maar-diatremes. For maar-diatremes and their tephra ring deposits, we find no correlations among lithic clast sizes, shapes, depositional sites, and excavation depths. This is difficult to reconcile with models involving systematic diatreme deepening coupled with tephra-ring growth, but consistent with those involving chaotic explosions and mixing. Larger amounts of data are needed to further examine how these types of volcanoes operate.

A Compilation and Characterisation of Lithic Populations in Common and Kimberlite Maar-Diatremes and their Tephra Ring Deposits

Maar-diatreme volcanoes are the second-most common type on land, occurring in volcanic fields within all major tectonic environments. Their deposits typically contain an abundance of lithic fragments quarried from the substrate, and many contain large, deep-sourced lithic fragments that were erupted to the surface. Primary volcaniclastic deposits fill the diatreme structure formed during eruption. There is negligible inelastic deformation of diatreme-adjacent country rock, indicating that country rock is removed to create the diatreme structures, either by being shifting downward below observable levels, ejected upward to contribute to surficial deposits, or dissolved and hidden in magma erupted or intruded at depth. No previous study has systematically reviewed and analysed the reported lithic fragments of maar-diatreme systems. We present a comprehensive compilation from published work of lithic characteristics in maar ejecta rims and in diatreme deposits of both common and kimberlite maar-diatremes. For maar-diatremes and their tephra ring deposits, we find no correlations among lithic clast sizes, shapes, depositional sites, and excavation depths. This is difficult to reconcile with models involving systematic diatreme deepening coupled with tephra-ring growth, but consistent with those involving chaotic explosions and mixing. Larger amounts of data are needed to further examine how these types of volcanoes operate.

The Cr-Zr-Ca armalcolite in lunar rocks is loveringite: Constraints from electron backscatter diffraction measurements

“Cr-Zr-Ca armalcolite” is a mineral originally found in Apollo samples five decades ago. However, no structural information has been obtained for this mineral. In this study, we report a new occurrence of “Cr-Zr-Ca armalcolite” and its associated mineral assemblage in an Mg-suite lithic clast (Clast-20) from the brecciated lunar meteorite Northwest Africa 8182. In this lithic clast, plagioclase (An = 88–91), pyroxene (Mg#[Mg/(Mg+Fe)] = 0.87–0.91) and olivine (Mg# = 0.86–0.87) are the major rock-forming minerals. Armalcolite and “Cr-Zr-Ca armalcolite” are observed with other minor phases including ilmenite, chromite, rutile, fluorapatite, merrillite, monazite, FeNi metal, and Fe-sulfide. Based on 38 oxygen atoms, the chemical formula of “Cr-Zr-Ca armalcolite” is (Ca0.99Na0.01)Σ1.00(Ti14.22Fe2.06Cr2.01 Mg1.20Zr0.54Al0.49Ca0.21Y0.05Mn0.04Ce0.03Si0.03La0.01Nd0.01Dy0.01)Σ20.91O38. Electron backscatter diffraction (EBSD) results reveal that the “Cr-Zr-Ca armalcolite” has a loveringite R3 structure, differing from the armalcolite Bbmm structure. The estimated hexagonal cell parameters a and c of “Cr-Zr-Ca armalcolite” are 10.55 and 20.85 Å, respectively. These structural and compositional features indicate that “Cr-Zr-Ca armalcolite” is loveringite, not belonging to the armalcolite family. Comparison with “Cr-Zr-Ca armalcolite” and loveringite of other occurrences implies that loveringite might be an important carrier of rare earth elements in lunar Mg-suite rocks. The compositional features of plagioclase and mafic silicate minerals in Clast-20 differ from those in other Mg-suite lithic clasts from Apollo samples and lunar meteorites, indicating that Clast-20 represents a new example of diverse lunar Mg-suite lithic clasts.

The Polychnitos ignimbrite of Lesvos island

Miocene volcanism on Lesvos was particularly explosive giving rise to two extensive pyroclastic formations, the Sigri pyroclastics to the west and the Polychnitos ignimbrite to the east of the island. The Polychnitos ignimbrite at 17.2±0.5 Ma (Borsi et al.1972) is part of the shoshonitic succession on Lesbos which ranges in composition from basalt to rhyolite and is both underlain and overlain by calcalkaline volcanic rocks (Pe-Piper and Piper 1993) resting on a late Paleozoic metamorphic basement which has acted as an impediment to the free flow of the ignimbrite. The Polychnitos ignimbrite consists of eight lithological units, six of which are presumed to be facies of the same ignimbrite sheet ("PK", "PU", "MGF I, II, III", "Z"). Ignimbrite deposition at elevated temperatures is advocated by its columnar jointing, eutaxitic texture, gas escape structures and glassy zones of intense welding. The typical mineral assemblage of all Polychnitos ignimbrite units consists of plagioclase, Kfeldspar and biotite. It displays phenocryst microtextures indicative of magma mixing. Magma mixing is corroborated of glasses of two discrete compositions. Lithic clast measurements indicate a northeasterly trending fissure vent passing from the northeastern corner of the Kalloni Gulf.