Fingerprints of forearc element mobility in blueschist-facies metaconglomerates,Catalina Schist,California

ABSTRACT

Metaconglomerates in the lawsonite–blueschist facies unit of the Catalina Schist (California) contain gabbroic and dioritic clasts exhibiting evidence for extensive metasomatism during high-P/T metamorphism. We performed whole-rock and in situ analyses of these metaconglomerate clasts to better constrain the composition of infiltrating fluids and to elucidate the history of chemical alteration. Petrographic evidence for this alteration includes replacements of plagioclase by phengite and sodic amphibole rims developed on igneous hornblende. These observations regarding mineral replacement are reinforced by corresponding shifts in chemical compositions. Relative to compositions of presumed protoliths, whole-rock compositions of the metaconglomerate clasts show enrichments in elements that are relatively mobile in aqueous fluids (LILE: K, Rb, Cs, and Ba; Li, B, N), and elevated δ¹⁵N, and show depletions in Ca and Sr. Electron and ion microprobe data, and analyses of mineral separates, show that phengite and sodic amphibole are enriched in LILE and Li and B, respectively, relative to the igneous phases they have replaced. Oxygen and C isotope compositions of finely disseminated calcite in the clasts, and of calcite in veins cross-cutting or mantling the clasts, are consistent with crystallization from fluids previously equilibrated with metasedimentary rocks within the same unit. The same fluids are implicated as the source for the Li, B, N, and LILE enrichments. These metaconglomerate clasts provide unique records of forearc metasomatism due to the presumed extremely low and well-constrained concentrations of fluid-mobile elements in their protoliths and the previously published, larger-scale fluid–rock context into which the observed metasomatic changes can be placed.     

Spatial distribution of micrometre‐scale porosity and permeability across the damage zone of a reverse‐reactivated normal fault in a tight sandstone: Insights from the Otway Basin,SE Australia

Knowledge of the permeability structure of fault‐bearing reservoir rocks is fundamental for developing robust hydrocarbon exploration and fluid monitoring strategies. Studies often describe the permeability structure of low porosity host rocks that have experienced simple tectonic histories, while investigations of the influence of faults with multiple‐slip histories on the permeability structure of porous clastic rocks are limited. We present results from an integrated petrophysical, microstructural, and mineralogical investigation of the Eumeralla Formation (a tight volcanogenic sandstone) within the hanging wall of the Castle Cove Fault which strikes 30 km NE–SW in the Otway Basin, southeast Australia. This late Jurassic to Cenozoic‐age basin has experienced multiple phases of extension and compression. Core plugs and thin sections oriented relative to the fault plane were sampled from the hanging wall at distances of up to 225 m from the Castle Cove Fault plane. As the fault plane is approached, connected porosities increase by ca. 10% (17% at 225 m to 24% at 0.5 m) and permeabilities increase by two orders of magnitude (from 0.04 mD at 225 m to 1.26 mD at 0.5 m). Backscattered Scanning Electron Microscope analysis shows that microstructural changes due to faulting have enhanced the micrometre‐scale permeability structure of the Eumeralla Formation. These microstructural changes have been attributed to the formation of microfractures and destruction of original pore‐lining chlorite morphology as a result of fault deformation. Complex deformation, that is, formation of macrofractures, variably oriented microfractures, and a hanging wall anticline, associated with normal faulting and subsequent reverse faulting, has significantly influenced the off‐fault fluid flow properties of the protolith. However, despite enhancement of the host rock permeability structure, the Eumeralla Formation at Castle Cove is still considered a tight sandstone. Our study shows that high‐resolution integrated analyses of the host rock are critical for describing the micrometre‐scale permeability structure of reservoir rocks with high porosities, low permeabilities, and abundant clays that have experienced complex deformation.     

Geochemistry and petrology of howardite Miller Range 11100: A lithologically diverse piece of the Vestan regolith

The howardite‐eucrite‐diogenite (HED) clan of meteorites, which most likely originate from the asteroid Vesta, provide an opportunity to combine in‐depth sample analysis with the comprehensive remote‐sensing data set from NASA's recent Dawn mission. Miller Range (MIL) 11100, an Antarctic howardite, contains diverse rock and mineral fragments from common HED lithologies (diogenites, cumulate eucrites, and basaltic eucrites). It also contains a rare pyroxferroite‐bearing lithology—not recognized in HED until recently—and rare Mg‐rich (Fo_86‐91) olivine crystals that possibly represent material excavated from the Vestan mantle. Clast components underwent different histories of thermal and impact metamorphism before being incorporated into this sample, reflecting the diversity in geological histories experienced by different parts of Vesta. The bulk chemical composition and petrography of MIL 11100 suggest that it is akin to the fragmental howardite meteorites. The strong lithological heterogeneity across this sample suggests that at least some parts of the Vestan regolith show heterogeneity on the mm‐scale. We combine the outcomes of this study with data from NASA's Dawn mission and hypothesize on possible source regions for this meteorite on the surface of Vesta.