Igneous Layering, Fractional Crystallization and Growth of Granitic Plutons: the Dolbel Batholith in SW Niger

This study reassesses the development of compositional layering during the growth of granitic plutons, with emphasis on fractional crystallization and its interaction with both injection and inflation-related deformation. The Dolbel batholith (SW Niger) consists of 14, kilometre-sized plutons emplaced by pulsed magma inputs. Each pluton has a coarse-grained core and a peripheral layered series. Rocks consist of albite (An≤11), K-feldspar (Or96-99, Ab1-4), quartz, edenite (XMg = 0·37-0·55), augite (XMg = 0·65-0·72) and accessories (apatite, titanite and Fe-Ti-oxides). Whole-rock compositions are metaluminous, sodic (K2O/Na2O = 0·49-0·62) and iron-rich [FeOtot/(FeOtot + MgO) = 0·65-0·82]. The layering is present as size-graded and modally graded, sub-vertical, rhythmic units. Each unit is composed of three layers, which are, towards the interior: edenite ± plagioclase (Ca/p), edenite + plagioclase + augite + quartz (Cq), and edenite + plagioclase + augite + quartz + K-feldspar (Ck). All phases except quartz show zoned microstructures consisting of external intercumulus overgrowths, a central section showing oscillatory zoning and, in the case of amphibole and titanite, complexly zoned cores. Ba and Sr contents of feldspars decrease towards the rims. Plagioclase crystal size distributions are similar in all units, suggesting that each unit experienced a similar thermal history. Edenite, characteristic of the basal Ca/p layer, is the earliest phase to crystallize. Microtextures and phase diagrams suggest that edenite cores may have been brought up with magma batches at the site of emplacement and mechanically segregated along the crystallized wall, whereas outer zones of the same crystals formed in situ. The subsequent Cq layers correspond to cotectic compositions in the Qz-Ab-Or phase diagram at PH2O = 5 kbar. Each rhythmic unit may therefore correspond to a magma batch and their repetition to crystallization of recurrent magma recharges. Microtextures and chemical variations in major phases allow four main crystallization stages to be distinguished: (1) open-system crystallization in a stirred magma during magma emplacement, involving dissolution and overgrowth (core of edenite and titanite crystals); (2) in situ fractional crystallization in boundary layers (Ca/p and Cq layers); (3) equilibrium ‘en masse' eutectic crystallization (Ck layers); (4) compaction and crystallization of the interstitial liquid in a highly crystallized mush (e.g. feldspar intercumulus overgrowths). It is concluded that the formation of the layered series in the Dolbel plutons corresponds principally to in situ differentiation of successive magma batches. The variable thickness of the Ck layers and the microtextures show that crystallization of a rhythmic unit stops and it is compacted when a new magma batch is injected into the chamber. Therefore, assembly of pulsed magma injections and fractional crystallization are independent, but complementary, processes during pluton constructio

Igneous Layering, Fractional Crystallization and Growth of Granitic Plutons: the Dolbel Batholith in SW Niger

This study reassesses the development of compositional layering during the growth of granitic plutons, with emphasis on fractional crystallization and its interaction with both injection and inflation-related deformation. The Dolbel batholith (SW Niger) consists of 14, kilometre-sized plutons emplaced by pulsed magma inputs. Each pluton has a coarse-grained core and a peripheral layered series. Rocks consist of albite (An≤11), K-feldspar (Or96-99, Ab1-4), quartz, edenite (XMg = 0·37-0·55), augite (XMg = 0·65-0·72) and accessories (apatite, titanite and Fe-Ti-oxides). Whole-rock compositions are metaluminous, sodic (K2O/Na2O = 0·49-0·62) and iron-rich [FeOtot/(FeOtot + MgO) = 0·65-0·82]. The layering is present as size-graded and modally graded, sub-vertical, rhythmic units. Each unit is composed of three layers, which are, towards the interior: edenite ± plagioclase (Ca/p), edenite + plagioclase + augite + quartz (Cq), and edenite + plagioclase + augite + quartz + K-feldspar (Ck). All phases except quartz show zoned microstructures consisting of external intercumulus overgrowths, a central section showing oscillatory zoning and, in the case of amphibole and titanite, complexly zoned cores. Ba and Sr contents of feldspars decrease towards the rims. Plagioclase crystal size distributions are similar in all units, suggesting that each unit experienced a similar thermal history. Edenite, characteristic of the basal Ca/p layer, is the earliest phase to crystallize. Microtextures and phase diagrams suggest that edenite cores may have been brought up with magma batches at the site of emplacement and mechanically segregated along the crystallized wall, whereas outer zones of the same crystals formed in situ. The subsequent Cq layers correspond to cotectic compositions in the Qz-Ab-Or phase diagram at PH2O = 5 kbar. Each rhythmic unit may therefore correspond to a magma batch and their repetition to crystallization of recurrent magma recharges. Microtextures and chemical variations in major phases allow four main crystallization stages to be distinguished: (1) open-system crystallization in a stirred magma during magma emplacement, involving dissolution and overgrowth (core of edenite and titanite crystals); (2) in situ fractional crystallization in boundary layers (Ca/p and Cq layers); (3) equilibrium ‘en masse' eutectic crystallization (Ck layers); (4) compaction and crystallization of the interstitial liquid in a highly crystallized mush (e.g. feldspar intercumulus overgrowths). It is concluded that the formation of the layered series in the Dolbel plutons corresponds principally to in situ differentiation of successive magma batches. The variable thickness of the Ck layers and the microtextures show that crystallization of a rhythmic unit stops and it is compacted when a new magma batch is injected into the chamber. Therefore, assembly of pulsed magma injections and fractional crystallization are independent, but complementary, processes during pluton constructio

Geophysical Exploration of Vesta

Dawn’s year-long stay at Vesta allows comprehensive mapping of the shape, topography, geology, mineralogy, elemental abundances, and gravity field using it’s three instruments and highprecision spacecraft navigation. In the current Low Altitude Mapping Orbit (LAMO), tracking data is being acquired to develop a gravity field expected to be accurate to degree and order ~20 [1, 2]. Multi-angle imaging in the Survey and High Altitude Mapping Orbit (HAMO) has provided adequate stereo coverage to develop a shape model accurate to ~10 m at 100 m horizontal spatial resolution. Accurate mass determination combined with the shape yields a more precise value of bulk density, albeit with some uncertainty resulting from the unmeasured seasonally-dark north polar region. The shape and gravity of Vesta can be used to infer the interior density structure and investigate the nature of the crust, informing models for Vesta’s formation and evolution

Science lives: School choices and ‘natural tendencies’

An analysis of 12 semi-structured interviews with university-based scientists and non-scientists illustrates their life journeys towards, or away from, science and the strengths and impact of life occurrences leading them to choose science or non-science professions. We have adopted narrative approaches and used Mezirow's transformative learning theory framework. The areas of discussion from the result have stressed on three main categories that include ‘smooth transition’, ‘incremental wavering transition' and ‘transformative transition’. The article concludes by discussing the key influences that shaped initial attitudes and direction in these people through natural inclination, environmental inspirations and perceptions of science

First mineralogical maps of 4 Vesta

Before Dawn arrived at 4 Vesta only very low spatial resolution (~50 km) albedo and color maps were available from HST data. Also ground-based color and spectroscopic data were utilized as a first attempt to map Vesta’s mineralogical diversity [1-4]. The VIR spectrometer [5] onboard Dawn has ac-quired hyperspectral data while the FC camera [6] ob-tained multi-color data of the Vestan surface at very high spatial resolutions, allowing us to map complex geologic, morphologic units and features. We here re-port about the results obtained from a preliminary global mineralogical map of Vesta, based on data from the Survey orbit. This map is part of an iterative map-ping effort; the map is refined with each improvement in resolution

The Aqueous Alteration of CR Chondrites: Experiments and Geochemical Modeling

CR carbonaceous chondrites are of major interest since they contain some of the most primitive organic matter known. However, aqueous alteration has more or less overprinted their original features in a way that needs to be assessed. This study was initiated by comparing the mineralogy and modal abundances of the most altered CR1 chondrite, GRO 95577, to a less altered CR2. Calculated element distributions imply that GRO 95577 may result from aqueous alteration of Renazzo by an isochemical process on their parent asteroid, whose mineralogical composition was estimated ( Unaltered CR shown included table)

Chondritic Models of 4 Vesta: Comparison of Predicted Internal Structure and Surface Composition/Mineralogy with Data from the Dawn Mission

Understanding the physical and chemical processes which led to the formation of the terrestrial planets remains one of the principal challenges of the Earth and planetary science communities. However, direct traces of the earliest stages of planet building have generally been wiped out on larger bodies such as the Earth or Mars, obscuring our view of how that process occurred. On the other hand, the planet building process would appear to have been arrested prematurely in the region between Mars and Jupiter, now populated by several hundred thousand compositionally diverse objects that escaped accretion into larger planets. Of these, the asteroid 4 Vesta is of particular interest as it is large (520 km diameter), and known to have a basaltic surface dominated by pyroxenes [1, 2]. Furthermore, visible-IR spectra of Vesta obtained by ground and space-based telescopes are remarkably similar to laboratory spectra measured on meteorites of the Howardite-Eucrite-Diogenite clan (HED), leading to the paradigm that the HEDs came from Vesta [2]. Geochemical and petrological studies of the HEDs confirm the differentiated nature of the near-surface region of their parent body, and imply that crust extraction occurred well within the first 10Ma of solar system history [3]. Vesta is therefore a prime target for studies that aim to constrain the earliest stages of planet building, and for that reason it is currently the subject of the Dawn mission [4]

Bulk Composition of Vesta as Constrained by the Dawn Mission and the HED Meteorites

Of the objects in the main asteroid belt, Vesta is of particular interest as it is large enough to have experienced internal differentiation (520 km diameter), and it is known to have a basaltic surface dominated by FeO-bearing pyroxenes. Furthermore, visible-IR spectra of Vesta and associated Vestoids are remarkably similar to laboratory spectra of Howardite-Eucrite-Diogenite (HED) meteorites, leading to the paradigm that the HEDs ultimately came from Vesta. Geochemical and petrological studies of the HEDs confirm the differentiated nature of the near-surface region of their parent body, and imply that crust extraction occurred well within the first 10 Ma of solar system history Vesta is therefore a prime target for studies that aim to constrain the earliest stages of planet building, and it is within this context that the NASA Dawn spacecraft orbited Vesta from July 2011 to September 2012. The results of the Dawn mission so far have significantly reinforced the HED-Vesta connection, confirming a significant degree of internal differentiation, a surface mineralogy compatible with that of the HEDs, and near-surface ratios of Fe/O and Fe/Si consistent with HED lithologies. The combination of data from the HED meteorites and the Dawn mission thus presents an unprecedented opportunity to use Vesta as a natural laboratory of early differentiation processes in the early solar system. However, the bulk composition of Vesta remains a significant unknown parameter, but one that plays a key role on the physical and chemical properties of the internal and surface reservoirs (core, mantle, crust). Several attempts have been made to constrain the bulk composition of the eucrite parent body, early endeavours relying on petrological or cosmochemical constraints. More recently, individual chondrite class compositions, or mixtures thereof, have been considered, constrained by considerations such as O-isotopes, trace-element ratios and siderophile element concentrations of the eucrites. The work presented here builds upon these latter studies, with the primary aims of: i) illustrating the potential diversity of the geochemical and geophysical properties of a fully differentiated Vesta-sized parent body, and ii) assessing which, if any, of the known chondritic bulk compositions are plausible analogues for proto-Vesta

Do Mesosiderites Reside on 4 VESTA? an Assessment Based on Dawn Grand Data

Almost a century ago, simple petrographic observations were used to suggest a close genetic link between eucrites and the silicates in mesosiderites [1]. Mesosiderites are composed of roughly equal proportions of silicates that are very similar in mineralogy and texture to howardites, and Fe, Ni metal (Fig. 1) [2]. This similarity has led some to conclude that mesosiderites come from the howardite, eucrite and diogenite (HED) parent asteroid [3, 4]. Subsequent petrologic study demonstrated a number of differences between mesosiderite silicates and HEDs that are more plausibly explained as requiring separate parent asteroids [5]. However, HEDs and mesosiderites are identical in oxygen isotopic composition, and this has been used to argue for a common parent 4 Vesta [6]

Analysis of Temperature Maps of Selected Dawn Data Over the Surface of Vesta

The thermal behavior of areas of unusual albedo at the surface of Vesta can be related to physical properties that may provide some information about the origin of those materials. Dawn s Visible and Infrared Mapping Spectrometer (VIR) [1] hyperspectral cubes can be used to retrieve surface temperatures. Due to instrumental constraints, high accuracy is obtained only if temperatures are greater than 180 K. Bright and dark surface materials on Vesta are currently investigated by the Dawn team [e.g., 2 and 3 respectively]. Here we present temperature maps of several local-scale features that were observed by Dawn under different illumination conditions and different local solar times