Post-fragmentation modifications of recycled juvenile fragments have been described for a phreatomagmatic event and some criteria for recognition were proposed based on vesicularity, morphology and occurrence of mud coatings 8. The two main types of mechanically induced variations that are effective during recycling are clast fracture and abrasion. Laboratory investigations have also shown that the main mechanically-induced modifications of coarse pumice clasts are rounding and comminution 12 , While no experimental data are available for these effects on ash material, they are probably reduced by the lower inertia.
Chemical alteration acts mainly on the surface of pyroclasts and it is largely effective also on fine-grained fragments, due to their high specific area. Alteration affects the few nanometres of the most external portion of the clasts 14 and may range from ash surface acid dissolution, to salt deposition or encrustation, to exfoliation 11 , Exposure of glassy material to volcanic gases results also in the modification of the bulk composition of the external glass and becomes significant after a few-days-long residence even at ambient temperatures While chemical alteration of the clasts induced by the magmatic fluids present in the eruptive plume can occur at largely variable temperatures, mainly depending on the position within the column 14 , 15 , chemical alteration related to syn-eruptive intra-crater residence always occurs at high temperature conditions.
Therefore, clasts undergoing syn-eruptive recycling are possibly subject to both chemically and thermally-induced modifications. Thermal alteration is particularly important when clasts are recycled within the crater area.
The observed large variety of modifications encompasses groundmass crystallisation and change in matrix glass composition, softening, sintering and welding of clasts, vapour-phase precipitation on external surfaces, colour and shape surface changes, up to subsolidus transformation of some mineral phases. Experimental results 16 , 17 also show that thermal effects are poorly significant at temperatures lower than T g , because the glassy structure does not relax and the kinetic of crystallisation is very slow.
Similarly, at temperature close to T e , melting of groundmass microlites starts, together with homogenization of glass composition, often resulting in the reset of the previous thermal history 16 , In particular, we concentrated on ash grains selected from: a falling-ash collected during an intense phase of ash forming explosive activity at Mt. Samples at Stromboli, Etna and Gaua were collected during the activity or a few days later. The products of the two well known historic eruptions of Vesuvius were chosen in order to extend the study to the tephra of past eruptions, where the uncertainties in the identification of the juvenile material primary and recycled can be also biased by the longer exposure to atmospheric agents and pedogenic weathering.
Collected samples were sieved and few grams in the size range 0. Total grain size distribution of mid-intensity basaltic explosive eruptions is generally centred on this class size 21 , This size class also represents the best compromise between the representativeness of the groundmass textural features of erupted magma clast size is at least one order of magnitude larger than average size of groundmass microlites and the possibility to capture the internal heterogeneity of the magma by studying an adequate number of clasts.
Component analysis was performed by separating under the binocular microscope the juvenile particles from lithics and crystals. A number of 30 ash fragments for each component was then characterized at SEM, basing on overall shape, vesicularity, surface morphology and groundmass texture, following For the definition of terms used in the description of the clast surface luster, colour, aspect we refer to terms defined in the Mineral Identification Key section of the Mineralogical Society of America website.
The luster corresponds to the overall sheen of ash surface, not to be confused with its colour. As typical of mid-intensity basaltic eruptions, the lithological features of the studied juvenile material are largely variable 2 , 3 , 7 , For convenience, we grouped the juvenile components of each studied sample into two different classes. Fragments with clearly juvenile, glassy features were named Type A and the fragments with variably crystallised external surfaces and groundmasses were named Type B Fig. Type A clasts are similar for the different volcanoes, being characterized by glassy external surfaces, generally free of vapour-phase condensates, secondary minerals or adhering dust.
The outlines of Type A clasts are largely variable from spiny due to the intersection of bubbles with the external surfaces to fluidal, with smooth external surfaces Fig.
Irrespective of the eruption or volcano, Type A clasts are present both as light brown, highly vesicular clasts and as dark brown, scoriaceous clasts, with larger bubbles and septa. The matrix glass is optically homogeneous and shows a uniform grey level in back-scattered electronic BSE images Supplementary Fig. S1 online. Groundmass is generally hypohyaline, with mainly rounded bubbles and variable amounts of euhedral microlites, possibly related to degassing-induced crystallisation of magma before fragmentation. Type A clasts closely match the distinguishing features given for sideromelane fragments 3 , 7 and then we interpret these clasts as primary juvenile material, not modified successively.
External surfaces range from irregular Fig. An important distinguishing feature from Type A clasts is the wide range of colour gradation of the external surface, from yellow, honey, light-brown to grey, black, dark-brown to orange to red Figs. Surface colour is sometimes inhomogeneous and its variations can be limited to the bubble rims or is pervasive over the entire surface of the clast.
Surfaces can be dull, or can display a metallic luster and glittering Fig. The metallic luster can be present with different intensity on clasts of different colour and it is more intense on smooth surfaces. Tiny mounds project above the clast surface, possibly related to nuclei of groundmass crystallisation Fig. In some cases, the presence of adhering dust and vapour-phase sublimates Fig. The whole vesicularity is generally lower with respect to Type A clasts and bubbles are sometimes collapsed, deformed and lined by oxides Fig.
The groundmass texture of Type B clasts varies from glassy to holocrystalline, with a wide range of crystallinity. When present, dendritic crystals of oxides are generally limited to the most external portion Fig. Often, microphenocrysts and microlites have a continuous oxidized rim Fig. For their overall features, a large part of Type B clasts strictly resembles the tachylite clasts 7.
SEM images of clasts with different external morphology, varying from irregular a , to fluidal b, c and blocky d. Arrows in c highlight cracks and wrinkles on external surface. Stereomicroscope image e of Type B clasts with different colours; f detail of the external surface of the clast shown in a ; g close-up of the area outlined in b.
The right part of the picture was captured in secondary electrons SE mode, while the left part records backscattered electrons BSE ; h enlargement of the inset in d. Olivine and Ti-mag show subsolidus transformation and lamellae texture, respectively. Surface luster of the clasts is strictly correlated to groundmass texture. In particular, metallic luster is associated with the presence of abundant crystal nuclei and microlites along the clast surface rim.
Conversely, massively crystallised groundmass is associated with the presence of an external crust, giving a dull luster to the surface Supplementary Figs. S2, S3 online. Type B clasts show also subsolidus transformations of Ti-magnetite and olivine.
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Microphenocrysts of Ti-magnetite exhibit a symplectitic texture complex intergrowths of minerals that comprises lamellae exsolution of ilmenite and magnetite Fig. Olivine microlites with alteration textures, characterized by adjacent lamellae of magnetite and pure forsterite, are sporadic, but do occur in Etna sample Fig.
While Type A clasts represent magma clasts unmodified after fragmentation, we suggest that the large variability observed in Type B clasts can be the inheritance of magma heterogeneities existing in the magma column before fragmentation large variability in microlite and bubble contents or the results of the modification of juvenile clasts after the fragmentation. Hereafter, we will discuss about the different origin of the wide internal variability shown by Type B clasts, trying to distinguish between primary features indicating heterogeneity in the erupting magma, from features related to secondary modifications following clast recycling.
Features observed in the Type B clasts will be discussed in terms of the processes that occur during the eruption or during the intra-crater recycling.
We will pay particular attention to the crystallinity and the groundmass texture, to the relationships between groundmass crystallisation and surface properties of the clasts luster, colour, texture and to the meaning of subsolidus transformations in some mineral phases. As stated before, thermal processes are responsible of the largest and most pervasive post-fragmentation modifications. For this reason, the comparison of the natural variability of Type B clasts with the results of experimental reheating of clasts of comparable composition performed at variable T and redox conditions 16 , 17 appears to be extremely useful.
First of all, we totally exclude the notion that crystal nucleation and growth is related to quenching during the ejection both for type A and B clasts. Therefore, microlites in the groundmass of studied clasts are assumed to reflect the response to cooling, or syn-eruptive degassing during magma ascent along the conduit 26 , In this context tachylites, which contain a large amount of microlites, have been quite often associated with the slowly rising and crystallising magma along the margins of the conduit 7.
As a general rule, temperature, redox conditions and increasing exposure time control the groundmass crystallisation. During reheating, groundmass crystallisation, revesiculation and cracking of external surfaces occur to restore a new equilibrium state. Thermal treatment of previously formed glass is in fact a general method for inducing massive crystallisation in the glass in order to produce glass-ceramic. Also in this case, the temperature and duration of the heating process are the most important parameters that control crystal nucleation and growth.
The reciprocal role of these parameters can be synthesized in a time-temperature transformation diagram known as TTT diagram Time-temperature transformation diagram known as TTT diagram , based on experimental results 16 , Diamonds correspond to the experiments reported in The black line separates the field of crystallisation at its right from that of unmodified glasses.
The quenching rate of 1. T n is the temperature corresponding to the minimum dwelling time, for massive groundmass crystallisation at constant temperature. The fine black arrows indicate the groundmass texture of clasts reheated at corresponding temperature and time conditions. The paths 2 and 3 represent possible trajectories of ash recycled at high and low temperature, respectively. In Fig. Thus we can infer that ash fragments quenched with comparable cooling rates preserve their original features and can be interpreted as primary juvenile clasts.
Accordingly, in case of reheating, the amount of groundmass crystals could be used as a proxy for defining the heating temperature interval and duration of recycling processes. Therefore we cannot exclude that oscillations in the feeding rate can favour the complete degassing of the magma, or that thermal fluctuations can permit the cooling of portions of magma close to the conduit walls. All these processes will induce an increase in crystallinity and heterogeneities in magma columns as those described in type B clasts.
As a result, groundmass crystallisation is a process that may occur under different conditions, which are effective both on the magma before fragmentation and on the clasts during reheating related to intra-crater recycling.
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For these reasons we cannot consider the crystal content alone as diagnostic criterion for post-fragmentation ash recycling process. The large variability in textures of microlites and groundmass crystallinity observed in Type B clasts Fig. The occurrence of microlites mainly oxides along pre-existing interfaces, such as clast surface, contacts between clasts, or the external surfaces of pre-existing crystals, is observed in both natural samples Fig.
Caudate nucleus 4. Cerebellum 5. Dura mater 6. Ear 7. Falx cerebri 8. First cervical nerve root 9. Glossopharyngeal and vagus nerve roots Hippocampus a cerebral gyrus Internal capsule Internal jugular vein Lateral geniculate body Lateral ventricle Medial geniculate body MIddle cerebellar peduncle Posterior neck muscles Parietal lobe Petrous part of temporal bone Posterior cranial fossa Sigmoid sinus Accessory nerve root Neck muscle Superior sagittal sinus Temporal lobe Tentorium cerebelli Thalamus Third cervical vertebrae Third ventricle Vertebral artery Reprinted with permission from England, M.
Frontal sinus 2. Falx cerebri 3.
Cerebral cortex 4. Temporalis muscle 5. Corpus callosum 6. Septum pellucidum 7. Internal capsule 8. Lateral ventricle 9. Parietal bone Hypodermis Skin Lentiform nucleus Caudate nucleus Systemic Descriptions Cardiovascular System The superior sagittal sinus 12 drains blood from the brain. Integumentary System The skin 11 of the scalp covers the loose connective tissue of the hypodermis 10 , which contains adipose tissue.
Muscular System The temporalis muscle 4 is a major muscle of mastication that attaches inferiorly to the mandible. Nervous System The cerebrum consists of gray matter and white matter. The gray matter forms the cortex 3 and nuclei. The caudate nucleus 15 and the lentiform nucleus 14 are basal ganglia. White matter nerve tracts are located between the gray matter. The internal capsule 7 consists of projection fibers that connect the cerebral cortex to the thalamus and brainstem. The corpus callosum consists of commisural fibers that connect the cerebral hemispheres to each other.
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