New constraints from Central Chile on the origins of enriched continental compositions in thick-crusted arc magmas

Magmas from continental arcs built on thick crust have elevated incompatible element abundances and “enriched” radiogenic isotope ratios compared to magmas erupted in island and continental arcs overlying thinner crust. The relative influence of the slab, mantle, and upper plate on this variability is heavily debated. The Andean Southern Volcanic Zone (SVZ; 33-46° S) is an ideal setting to investigate the production of enriched continental arc compositions, because both crustal thickness and magma chemistry vary coherently along strike. However, the scarcity of primitive magmas in the thick-crusted northern SVZ has hindered previous regional studies. To better address the origin of enriched continental compositions, we investigate the geochemistry (major and trace element abundances, 87Sr/86Sr and 143Nd/144Nd ratios) of new mafic samples from Don Casimiro and Maipo volcanoes in Diamante-Maipo Caldera Complex of the northern SVZ. While evolved Diamante-Maipo samples show evidence for crustal assimilation, the trace element and isotopic enrichment of the most mafic samples cannot result from crustal processing, as no known regional or global basement lithologies are enriched in all of the necessary incompatible trace elements. Subduction erosion models similarly fail to account for the enriched isotopic and trace element signature of these samples. Instead, we suggest that the enrichment of northern SVZ magmas is derived from an enriched ambient mantle component (similar to EM1-type ocean island basalts), superimposed on a northward decline in melt extent. A substantial, but nearly uniform contribution of melts from subducting sediment and altered oceanic crust are required at all latitudes. The EM1-like enrichment may arise from recycling of metasomatized subcontinental lithospheric mantle (M-SCLM), as the isotopic trajectory of primitive rear-arc monogenetic cones trend towards the compositions of SCLM melts sampled across South America. Isotopic data from spatially distributed rear-arc centres demonstrate that the arc-parallel variations in the degree of EM1-type enrichment observed in arc-front samples are also present up to 600 km behind the trench in the rear-arc. Rear-arc trace element systematics require significant but variable quantities of slab melts to be transported to the mantle wedge at these large trench distances. Overall, we show that a unified model incorporating variable mantle enrichment, slab additions, and melt extents can account for along and acrossarc trends within the SVZ. The recognition that mantle enrichment plays a key role in the production of enriched continental compositions in the SVZ has important implications for our understanding of the chemical evolution of the Earth. If ambient mantle enrichment is not taken into account, petrogenetic models of evolved lavas may overestimate the role of crustal assimilation, which, in turn, may lead models of continental crust growth to overestimate the amount of continental material that has been recycled back into the mantle

Fluvial archives of NW African climate and tectonic evolution, Atlas Mountains, central Morocco

The Atlas mountains in Morocco are a natural laboratory at the junction between the Atlantic Ocean (passive margin), the Mediterranean (subduction) and the African Craton. Here, interactions between the mantle and lithosphere, crustal compression and uplift have been recorded in river terraces, alluvial fans, drainage patterns, river long profiles, and in wedge-top & foreland sediments. Limited work on terraces in one of the catchments crossing the south Atlas thrust front has shown rates of incision are low and have been sustained since the Pleistocene. Dating of terraces using Optically Stimulated Luminescence, together with field sedimentology, links the formation of terraces in the Dades River to 100 ka climate cycles. Studies of tributary fans and fan sediments in terraces suggest coupling of hillslopes, tributaries and trunk streams vary across glacial-interglacial cycles and is geologically controlled. River long profiles extracted across the southern Atlas Mountains contain knickzones (areas of increased steepness), resulting from tectonically driven uplift. We will use newly acquired high resolution DEM data together with field mapping and Optically Stimulated Luminescence dating to constrain river terrace formation in High Atlas catchments draining into the Ouarzazate foreland basin. These data will be used to constrain further, the regional tectonic and climatic controls on river terrace formation. Integrating the terrace records with the other fluvial archives will enable challenging questions on tectonic surface processes, source-to-sink sedimentology and intra-plate tectonics to be tackled

Structural Guided Scaffold Phage Display Libraries as a Source of Bio-Therapeutics

We have developed a structurally-guided scaffold phage display strategy for identification of ligand mimetic bio-therapeutics. As a proof of concept we used the ligand of integrin avb6, a tumour cell surface receptor and a major new target for imaging and therapy of many types of solid cancer. NMR structure analysis showed that RGD-helix structures are optimal for avb6 ligand-interaction, so we designed novel algorithms to generate human single chain fragment variable (scFv) libraries with synthetic VH-CDR3 encoding RGD-helix hairpins with helices of differing pitch, length and amino acid composition. Study of the lead scFv clones D25scFv and D34scFv and their corresponding VH-CDR3 derived peptides, D25p and D34p, demonstrated: specific binding to recombinant and cellular avb6; inhibition of avb6-dependent cell and ligand adhesion, avb6-dependent cell internalisation; and selective retention by avb6-expressing, but not avb6-negative, human xenografts. NMR analysis established that both the D25p and D34p retained RGD-helix structures confirming the success of the algorithm. In conclusion, scFv libraries can be engineered based on ligand structural motifs to increase the likelihood of developing powerful bio-therapeutics

Catchment changes in response to tectonics and climate: using river terraces and DEM data in the southern High Atlas Mountains (Morocco)

Tectonics and climate drive the generation and transport of sediment in mountain rivers as these evolve over time. On a glacial-interglacial scale, in particular catchment reorganisation and catchment incision dynamics control these processes, and affect fan deposition in sedimentary basins. The Atlas Mountains in Morocco exhibit ongoing catchment reorganisation and an abundance of river terraces recording glacial fluvial aggradation and interglacial-glacial incisional periods, opening up insight into the processes behind catchment evolution over geological timescales. Topography and river profiles across drainage divides are similar in a stable divide, and if they are unequal they indicate active catchment reorganisation. When reorganisation occurs, it results in irregularities in river long profiles and changes in river valley erosion. River strath terraces are formed by transition between valley widening and downcutting of terraces in response to local divergence of sediment-transport capacity 3. Consequently, they record changes in catchments due to river capture, climate and tectonics. The presence of river terraces enables catchment processes over time to be investigated. A combination of remote sensing and field mapping and logging was completed in May 2018. River terraces have been mapped with newly released high resolution DEM data in the southern High Atlas in Morocco, and additional surveying was done in the field. Geomorphological indices suggest river catchment capture is a key control on the development of drainage networks. River long profiles suggest tectonic controls have also influenced landscape development over the last few million years. Logging of terrace sediments together with high-resolution sampling for OSL dating enables these catchment-wide effects to be compared with paleo-hydrological and sediment transport characteristics of the fluvial system. The combination of geomorphological DEM and sedimentological field data enables us to explore drivers of catchment change, and will contribute to the wider understanding of fluvial system response to climate and tectonic controls, and to its transport into the sedimentary record