Diagenesis in salt dome roof strata: barite - calcite assemblage in Jebel Madar, Oman

Halokinesis causes a dynamic structural evolution with the development of faults and fractures, which can act as either preferential fluid pathways or barriers. Reconstructing reactive fluid flow in salt dome settings remains a challenge. This contribution presents for the first time a spatial distribution map of diagenetic phases in a salt dome in northern Oman. Our study establishes a clear link between structural evolution and fluid flow leading to the formation of diagenetic products (barite and calcite) in the salt dome roof strata. Extensive formation of diagenetic products occurs along NNE-SSW to NE-SW faults and fractures, which initiated during the Santonian (Late Cretaceous) and were reactivated in the Miocene, but not along the E-W fault, which was generated during Early Paleocene time. We propose that the diagenetic products formed by mixing of a warm (100 °C) saline (17 wt% NaCl eq.) 87Sr enriched (87Sr/86Sr: 0.71023) fluid with colder (35 °C) meteoric fluid during Miocene to Pleistocene. The stable sulphur and strontium isotope composition and fluid inclusion data indicate that a saline fluid, with sulfate source derived from the Ara Group evaporite and Haima Supergroup layers, is the source for barite formation at about 100 °C, predominantly at fault conjunctions and minor faults away from the main graben structure in the dome. In the Miocene, the saline fluid probably ascended along a halokinesis-related fault due to fluid overpressure (due to the rising salt and impermeable layers in the overlying stratigraphic sequence), and triggered the formation of barite due mixing with barium-rich fluids, accompanied by a drop in temperature. Subsequently, evolving salt doming with associated fault activity and erosion of the Jebel allows progressively more input of colder meteoric fluids, which mix with the saline warmer fluid, as derived from stable isotope data measured in the progressively younger barite-associated calcite, fault zone calcite and macro-columnar calcite. The reconstructed mixing model indicates a 50/50 to 90/10 meteoric/saline fluid mixing ratio for the formation of fault zone calcite, and a 10 times higher concentration of carbon in the saline fluid end member compared to the meteoric fluid end member. The presented mixing model of salt-derived fluids with meteoric fluids is suggested to be a general model applicable to structural diagenetic evolution of salt domes world wide

Federal Depository Library Program: Issues for Congress

A history and overview of current issues regarding the Federal Depository Library Program (FDLP)

High level triggers for explosive mafic volcanism: Albano Maar, Italy

Colli Albani is a quiescent caldera complex located within the Roman Magmatic Province (RMP), Italy. The recent Via dei Laghi phreatomagmatic eruptions led to the formation of nested maars. Albano Maar is the largest and has erupted seven times between ca 69-33ka. The highly explosive nature of the Albano Maar eruptions is at odds with the predominant relatively mafic (SiO2=48-52wt.%) foiditic (K2O=9wt.%) composition of the magma. The deposits have been previously interpreted as phreatomagmatic, however they contain large amounts (up to 30%vol) of deep seated xenoliths, skarns and all pre-volcanic subsurface units. All of the xenoliths have been excavated from depths of up to 6km, rather than being limited to the depth at which magma and water interaction is likely to have occurred, suggesting an alternative trigger for eruption. High precision geochemical glass and mineral data of fresh juvenile (magmatic) clasts from the small volume explosive deposits indicate that the magmas have evolved along one of two evolutionary paths towards foidite or phonolite. The foiditic melts record ca. 50% mixing between the most primitive magma and Ca-rich melt, late stage prior to eruption. A major result of our study is finding that the generation of Ca-rich melts via assimilation of limestone, may provide storage for significant amounts of CO2 that can be released during a mixing event with silicate magma. Differences in melt evolution are inferred as having been controlled by variations in storage conditions: residence time and magma volume. © 2013

The “problem” of teacher quality: exploring challenges and opportunities in developing teacher quality during the Covid-19 global pandemic in England

Teachers and teacher education are often presented as “problems” to be solved, with policy solutions that focus on ways to make teachers “better” and improve teacher “quality” by introducing prescriptive strategies. We investigate the ways Covid-19-related changes to university and school-based facets of Initial Teacher Education (ITE) in England influence teacher quality in relation to both student teachers and early career teachers, working in secondary schools. Drawing on 34 interviews with school leaders, school mentors and ITE tutors, we critically explore the ways in which teacher quality was developed through key aspects of teachers’ pedagogy and practice during the pandemic crisis when schools were closed and teaching moved online. Our findings show that the pandemic crisis has highlighted the different facets of teacher quality which arguably disrupt narrow and prescriptive understandings of what constitutes “quality” in policy terms. Although there were many instances of challenge in the development of new and student teachers, our data also shows how ITE tutors, school mentors and leaders responded creatively to the crisis. Participants highlighted the opportunities afforded by the pandemic to develop diverse and innovative pedagogies and practice, enhance students’ subject knowledge, as well as overcome some of the challenges in other areas of pedagogy and practice. Furthermore, the study shows that teacher quality was not substantially reduced despite the challenges arising from the pandemic and concerns that pre-service teachers would not be ready and prepared for a career in the classroom

Collaborative identity development during a global pandemic: exploring teacher identity through the experiences of pre-service high school teachers in England

Since early 2020, COVID-19 has had a substantial impact on teacher education. We consider novel aspects of how pre-service teachers have collaboratively developed their professional identities during the pandemic. Drawing on findings from forty-five interviews with pre-service high school teachers working in England during September 2020 – June 2021, we share how collaborative identity development was central and occurred in a variety of spaces, communities and modes. Collaborative identity development featured in how pre-service teachers saw themselves making a positive contribution to society through education and, in strong subject connections. Reflection that is collaborative, personalised, iterative, and separate from notions of formal progression enables positive identity work. Notions of identity are absent from international policy initiatives in ITE (Initial Teacher Education). This case study provides insights for policy makers in and beyond England who aim to support teachers at the beginning of their career so that they are retained

Two possible source regions for Central Greenland last glacial dust

Dust in Greenland ice cores is used to reconstruct the activity of dust-emitting regions and atmospheric circulation. However, the source of dust material to Greenland over the last glacial period is the subject of considerable uncertainty. Here we use new clay mineral and <10 µm Sr–Nd isotopic data from a range of Northern Hemisphere loess deposits in possible source regions alongside existing isotopic data to show that these methods cannot discriminate between two competing hypothetical origins for Greenland dust: an East Asian and/or central European source. In contrast, Hf isotopes (<10 µm fraction) of loess samples show considerable differences between the potential source regions. We attribute this to a first-order clay mineralogy dependence of Hf isotopic signatures in the finest silt/clay fractions, due to absence of zircons. As zircons would also be absent in Greenland dust, this provides a new way to discriminate between hypotheses for Greenland dust sources

Precambrian olistoliths masquerading as sills from Death Valley, California

Olistolith production and magmatism are processes commonly associated with extensional tectonic settings, such as rift basins. We present a cautionary exemplar from one such Precambrian basin, in which we reinterpret metabasite bodies, previously documented as sills, to be olistoliths. We nevertheless demonstrate that, on the basis of field observation alone, the previous but erroneous sill interpretation is parsimonious. Indeed, it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed. We present new data from metabasites and metasedimentary strata of the Kingston Peak Formation (Cryogenian) and Crystal Spring Formation (Mesoproterozoic) of Death Valley, USA. These include field observations, U?Pb apatite ages, U?Pb zircon ages (detrital and igneous) and whole-rock geochemistry. These data also provide a new maximum age for the base of the Pahrump Group and suggest that the Crystal Spring Diabase was more tholeiitic than previously thought. Similar sill/olistolith misinterpretations may have occurred elsewhere, potentially producing erroneous age and tectonic-setting interpretations of surrounding strata. This is particularly relevant in Precambrian rocks, where fossil age constraints are rare. This is illustrated herein using a potential example from the Neoproterozoic literature of the Lufilian belt, Africa. We caution others against Precambrian olistoliths masquerading as sills.publishersversionPeer reviewe