RICERCA ITALIANA     

Research program

Crustal Anatexis: Natural evidence, Experiments and Modelling (C.A.N.E.M)

University Co-ordinator

Università degli Studi di PERUGIA - SCIENZE DELLA TERRA - ()

Research Unit Leader

Diego Perugini

Description

The aim of this research project is the investigation of the behaviour of crustal anatectic melts during their separation from the source region, their migration, and their emplacement either as plutonic or volcanic bodies. The research will be carried out by detailed analyses of magma interaction processes occurring during these stages to construct a conceptual model which can explain the origin of compositional heterogeneity in magmatic masses. Such a model is fundamental to understand the influence of magma interaction among different melts, generated either by melting of different source rocks or by different degrees of partial melting of the same source region, on the original geochemical features of the single melts and, hence, to provide information about the characteristics of source regions and their geologic significance. The detailed knowledge of these processes is of paramount importance to reconstruct the geological history of crustal anatectic magma bodies, from the onset of the partial melting process to their emplacement.
Starting from the knowledge about magma interaction processes gained by combining classic petrologic techniques and the new methods of Fractal Geometry and Chaos Theory [e.g. 29, 39], and about the mechanisms of segregation and ascent of anatectic melts [e.g. 25], and also using Experimental Petrology in collaboration with the research group of Prof. D. Dingwell (Ludwig-Maximilians-Universität), the main targets of the project are:
i) to study magma interaction processes among melts generated either by melting of different source rocks or by different degrees of partial melting of the same source region, and to analyse how these processes may mask the original geochemical features of single melts by studying the compositional heterogeneity of inter-granular glasses in crustal xenoliths (or enclaves) and leucosomes in migmatitic complexes;
ii) to study magma interaction processes among anatectic melts during their migration from the source region and ascent toward the emplacement level by developing new Experimental Petrology experiments and numerical models based on 'Small World' fracture networks, and to understand the influence of fracture network topology in transferring such melts toward shallower crustal levels as a function of chemical exchanges among them;
iii) to understand in details, by studying natural outcrops, the mechanisms acting during accumulation of melts in magma chambers and in volcanic conduits, and the influence of the development of chaotic mixing processes on their geochemical compositions and on the production of geochemical heterogeneities at different length scales;
iv) to understand, by detailed studies of chemical diffusion processes, the kinetics associated to the development of compositional heterogeneities within magmatic systems and to define their time-scales as a function of the dynamics occurring during melt segregation, migration, and accumulation in plutonic and volcanic magmatic systems;
Researches will be carried out on key outcrops of plutonic and volcanic rocks where the coexistence in space and time of anatectic melts with different geochemical compositions has been documented, and on crustal xenoliths, representing fragments of source region, hosted in volcanic rocks. Part of the research will be also concentrated on a pilot study of the compositional variability of leucosomes in migmatites. In particular, regarding plutonic rocks the attention will be focused on the leucocratic anatectic facies of Pulchiana [Sardinia, Italy; 42] and Sithonia and Arnea [Greece; e.g. 3, 43, 44] plutons. Regarding volcanic rocks, lava flows and domes (with dacitic and rhyolitic compositions) cropping out on the Tuscan Magmatic Province [San Vincenzo and Roccatederighi, Italy; 45, 46], on the islands of Lipari, Vulcano, and Panarea [Aeolian Archipelago, Italy; 47, 48, 49], and on the areas of El Hoyazo and Mazarròn [Spain; e.g. 50, 51] will be studied. In these latter areas, crustal xenoliths hosted in the lavas, representing fragments of source region [e.g. 48, 50, 51, 52], will be also studied. Regarding migmatitic complexes, the attention will be focused on outcrops of the Ronda area [Spain; e.g. 53, 54].
The project will be divided into two main Activities (Activity 1 and 2), which are describe in detail as follows.

Activity 1
With the aim of understanding the dynamics associated with the segregation of anatectic melts and their interaction within the source region, xenoliths representing fragments of source rocks carried to the Earth surface by anatectic magmas will be studied in collaboration with Unit of Padova. Crustal xenoliths will be studied by detailed microanalysis of inter-granular glasses at short length scale and microanalysis of melt inclusions in different mineral phases. A pilot study will be also carried out on migmatitic rocks, in close collaboration with the Unit of Padova, by analysing the spatial distribution and the compositional heterogeneity of leucosomes at different length scales.
Inter-granular glasses in the xenoliths will be analysed by measuring their major and trace element contents (by EMPA and LA-ICP-MS) along transects and regular grids with cell dimensions varying from hundreds to tens of microns (Fig. 1), in order to map their geochemical variability. Compositional series will be extracted from grids and transects (Fig. 1) and, by applying Chaos Theory techniques, the degree of compositional disorder of the different chemical elements will be measured (Fig. 1). This approach will allow us to study the degree and the spatial distribution of the heterogeneity in the xenoliths and to make inferences on the chemical exchanges among anatectic melts during the initial stages of segregation from the source region. <br />


In conjunction with the studies on the xenoliths, a pilot study of the compositional heterogeneity of leucosomes in migmatitic complexes will be also carried out. It is well known that the geochemical composition of leucosomes cannot be strictly considered as the original composition of the partial melts because of the possible presence of restitic phases and/or cumulus phenomena in the leucosomes [e.g. 55, 56]. Nevertheless, there are geological contexts, such as the Ronda area, in which these phenomena are minimized [e.g. 53, 54]; these represent key outcrops to study in situ the initial stages of the partial melting process and mobilization of anatectic melts. The 'scale-invariance' of leucosome morphological distribution is still a subject of debate [e.g. 57, 58]. Morphological analysis will indicate if and in which conditions of the migmatite the distribution of leucosomes is fractal. This aspect is of fundamental importance because the genesis of fractal structures is strictly associated with the occurrence of chaotic dynamics. This means that mobilization and interaction of melts originates complex structures whose spatial distribution may strongly influence chemical exchanges between the different anatectic melts and, thus, can potentially obliterate their original compositions. Migmatitic outcrops will be covered by regular grids with different dimensions (from the metre to tens of centimetres) and, depending on the grid dimension, whole rock samples of leucosomes will be collected. These will be analysed by measuring their major and trace element, and isotope contents. The compositional variability at the different length scales will be studied by applying the same Chaos Theory techniques used in the study of inter-granular glasses in xenoliths, in order to attempt to quantify the efficiency of chemical exchanges among the different melts. Measurements of the degree of compositional disorder of the different chemical elements will be compared with values of fractal dimension of flow structures. This approach will allow us to define the degree and the spatial distribution of the compositional heterogeneity as a function of flow fields in the migmatite.
Together with the above studies, melt inclusions in minerals constituting both migmatites and granulites, and xenoliths will be analysed, in close collaboration with the Unit of Padova. Their geochemical variability will be compared to that observed at the micro-scale, in the inter-granular glasses, and at the macro-scale, in the migmatitic complexes. The study of compositional variations of melt inclusions will allow us to establish the possible occurrence of chaotic mixing dynamics at the scale of the single outcrops whereas, at the scale of the single melt inclusion, it will allow us to apply the conceptual model of 'diffusive fractionation' of trace elements [Fig. 2; 39]. The application of this methodology, together with the studies on the large length scale, will allow us to constrain the kinetics of mixing at the micro-scale, to understand the time-scales of melt mobilization within the source region, and to study in details how the development of fractal patterns and chaotic mixing processes among different anatectic melts may influence their original compositional features during their segregation from the source region.



Activity 2
The study of interaction among different anatectic melts during their separation from the source region and their migration toward shallower crustal levels will be carried out by using both Experimental Petrology techniques and numerical simulations.
Recent works highlighted that the topology of fracture networks (i.e. the spatial distribution of fractures at both short and large length scale and the structure of their connections) can play a very important role for an efficient delivery of magmas, preventing them to reach geochemical equilibrium with the host rocks [25]. Natural fracture networks behave as 'Small World' networks (Fig. 3) having a very high local (i.e. on short length scale) and global (i.e. on large length scale) efficiency in delivering melts from the source region toward shallow crustal levels of emplacement or toward the Earth surface. However, if compositionally different melts migrate through these fracture networks, they do not necessarily preserve their original compositions. Indeed, it is highly probable that the different melts coalesce at the intersection points of fracture and experience mixing processes.



With the aim to study the dynamics and the extent of chemical exchanges occurring when different melts interact within fracture networks having a 'Small World' topology, new experiments will be carried out by considering melt percolation within artificial fracture networks. This work will be carried out in collaboration with the research group of Prof. D. Dingwell. In detail, structures (with dimensions of the order of 10-15 cm) of interconnected channels having a 'Small World' topology will be built in refractory ceramics (Fig. 4). Synthetic melts with compositional features analogous to those of natural anatectic melts will be allowed to percolate under gravity within these networks in high temperature furnaces (1200-1300°C) (Fig. 4). The samples will be studied by detailed microanalysis of both major (EMPA) and trace (LA-ICP-MS) elements to characterise the compositional variability of the system as a function of mixing dynamics among the different melts and to understand the influence of melt migration and coalescence on the original compositions of melts.



These experimental studies represent an analogue of the mechanisms associated to the migration and coalescence of melts during their delivery from the source region and will provide the basis for the development of numerical models. In particular, by using the geochemical constraints obtained from the experiments, numerical codes will be developed to simulate the process of migration and interaction of melts on a large scale (of the order of hundreds of metres-kilometres). These studies will allow us to evaluate possible changes in the style and efficiency of chemical exchanges among melts during their migration on a large length scale.

The study of interaction processes among the different anatectic melts during their emplacement will be carried out on natural outcrops in which the occurrence of crustal anatectic melts has been documented in both the plutonic and volcanic environment. In particular, the research will proceed with increasing detail from studies at the outcrop length scale to investigations of the single mineral phases. This approach will allow us to evaluate in detail the style and the distribution of the geochemical heterogeneity within anatectic magmatic masses at different length scales. The research on this topic will begin by studying the compositional heterogeneity at the length scale of single outcrops by performing detailed major and trace element, and isotope whole rock analysis on samples collected on regular grids with variable dimensions from tens of metres to centimetres. The geochemical variability will be studied by applying the same Fractal Geometry and Chaos Theory techniques utilized in the study of inter-granular glasses and migmatitic complexes. This approach, applied to all plutonic and volcanic outcrops, will allow us to define the degree of heterogeneity of magmatic masses and to recognise the presence of different dynamic regions characterized by different mixing efficiencies [e.g. 35-36].
The different dynamic regions will be studied by detailed microanalyses. In particular, zoning patterns of plagioclase crystals, which record the propagation of the geochemical heterogeneity during mixing processes, occurring in the different regions, will be studied. Compositional series will be extracted from the zoning patterns [e.g. 34] and will be analysed by using Chaos Theory techniques (e.g. Fig. 1C-E). The application of these techniques will allow us to recognise compositional fluctuations at the crystal length scale. In addition, the style of the propagation of the compositional heterogeneity and its time-scales will be defined by correlating the analyses of different zoning patterns occurring in the same magmatic system. The results of this short length scale study will be correlated with the geochemical variability on the large length scale in order to fully define the causes and the mechanisms responsible for the observed compositional heterogeneity.

The Gantt diagram in Fig. 5 reports the time schedule of this research project.






Data diffusion
Publications on national and international journals and organization of thematic sessions and presentations at national and international meetings (e.g. FIST, EGS-AGU, EGU), in collaboration with the Unit of Padova. Dissemination of data and results at an international workshop that will be organized during the second semester of the second year and on the web site of the project.