Mixing Experiments with Shoshonitic and Trachytic Melts using a High-Temperature Centrifuge and a Viscometer: a comparative study

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Evidence of cyclic replenishment of the shallow magmatic reservoir with deeper alkali basaltic (shoshonitic) magma (Campi Flegrei, in Italy; see Arienzo et al., 2008, Bull. Volcanol.) motivated this study. Results from different mixing experiments
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  Stephan Kolzenburg 17/08/2009 Bachelor Thesis Towards timescales of magma mixing in Campi Flegrei, Italy Supervisor: Prof. Dr. Donald Bruce Dingwell Fig. 1: Tempio di Serapide, witness of volcanic motion in Campi Flegrei, from www.partecipiamo.it Ludwig-Maximilians-University Munich Department of Earth and Environmental Sciences Section for Mineralogy, Petrology and Geochemistry  Stephan Kolzenburg 17/08/2009 1 Abstract ...................................................................................................................................2   Introduction..............................................................................................................................3   Choice of end-members and end member characterisation....................................................5   Choice of end members by strontium/ neodymium isotopy..................................................5   End member A: Agnano Monte Spina..................................................................................6   End member B: Minopoli:.....................................................................................................8   Further characterisation of both end members ..................................................................10   Sample preparation and experimental conditions..................................................................12   Sample preparation............................................................................................................12   Experimental conditions.....................................................................................................14    Analytical conditions..............................................................................................................16   Results...................................................................................................................................18   Theoretical background......................................................................................................18   Comparative microprobe results........................................................................................19   Test of Bush / Langmuir equation ......................................................................................28   Discussion.............................................................................................................................30   Concluding remarks...............................................................................................................31    Acknowledgements................................................................................................................32   References............................................................................................................................33   Table of figures......................................................................................................................35   Declaration ............................................................................................................................37    Stephan Kolzenburg 17/08/2009 2  Abstract Volcanic eruptions and are among the most spectacular and destructive forces on the planet. In Europe alone, 4-5 million people live within sight of an active volcano and ten percent of the population of Europe are considered economically vulnerable to volcanic eruption. The eruptions that occurred in the Campi Flegrei caldera since the Neapolitan yellow tuff (12,000 years BP) are thought to be triggered by short-term pre-eruptive mixing of a trachytic to trachydacitic resident (AMST) and a new basaltic, trachyandesitic (=shoshonitic) magma (Minopoli) in the shallow magma chamber. The experiments of this study were motivated by this hypothesis. This work presents electron microprobe results from mixing experiments using natural volcanic samples from the Campi Flegrei caldera. Two experimental runs of 25 and 168 hours were carried out under Taylor-Couette flow, simulating forced convection under very low Reynolds numbers, hence laminar flow. The end-member melts derive from the Agnano-Monte Spina (AMST) and Minopoli eruptions. End-members are stirred together under constant angular velocity (0.5 rotations per minute) and constant temperature (1300° Celsius) using a concentric cylinder viscometer. Based on microprobe analysis of quenched melts from the two experiments, the mixing efficiency for the different diffusion speeds are revealed for the main and trace elements analysed. The results also question the linearity of mixing processes as they are proposed in Petrology.  Stephan Kolzenburg 17/08/2009 Introduction The Campi Flegrei caldera (CFc) is a densely inhabited, volcanic area that has been active for over 60.000 years (P  APPALARDO  et al., 2002 cit. in C.   C  ANNATELLI  et al., 2006). This volcanic system lies in the west of Naples, Italy. Still being active, which is proven by the Monte Nuovo eruption, on 29 Sep 1538 A.D. (D I V ITO  et al., 1987 cit. in S.  DE V ITA  et al. 1999), frequent earthquakes in the 1960´s and 1980´s and today’s high hydrothermal activity. Hence it is one of the most dangerous volcanic settings on earth. The CFc is a resurgent structure that is thought to be derived from a former stratovolcano (R ITTMAN  1950 cit. in A.P ERROTTA  et al. 2006). The caldera is a nested structure, the result of two main collapses (1) the Campanian Ignimbrite (CI;37,000 years BP) and (2) the Neapolitan Yellow Tuff (NYT;12,000 years BP) (O RSI  et al. 1996). Fig. 2: Campi flegrei caldera, faults, vents and history of activity, Orsi et al., 1996 (www.ov.ingv.it) Volcanic activity in the last 12,000 years was concentrated in the NYT caldera and can be grouped into three main phases of activity. During the first (12,000 – 9500 years BP) and second (8600 – 8200 years BP) phases, the vents were located at the structural boundary of the NYT caldera, during the third phase (4800 – 3800 years BP) activity was focused in the north-eastern sector of the resurgent block (O RSI et al. 1996, S.  DE V ITA et al. 1999). 3  Stephan Kolzenburg 17/08/2009 4Magma mixing in the shallow magma chamber has newly been to be the main factor triggering eruptions in the CFc (DE CAMPOS et al. (2008); A RIENZO  et al. (2008)). Therefore understanding the general dynamic conditions of magma mixing, especially for the Campi Flegrei volcanic province, can give insight to processes that determine the dynamics of hazardous volcanic areas Studies on analogue materials (T URNER and C  AMPBELL , 1986; S PARKS and M  ARSHALL , 1986; O TTINO , 1989; J ELLINEK et al., 1999; cit. in D E C  AMPOS et al., 2008) show, that diffusion and convection are the main factors controlling mixing. In a magma chamber free convection can derive from internal variations in temperature and/or composition (F URBISH , 1997 cit. in D E C  AMPOS  et al. 2008). Forced convection may be induced by an outside force such as wall rock assimilation or replenishment of the chamber by another magma. Forced convention, the result of magma chamber replenishment, is simulated in the experiments of this study by the stirring action of a spindle In experiments with silicate melts, the degree and the rate of mixing are directly proportional to the forced convection/advection or to the applied shear stress (Z IMANOWSKI et al., 2004, cit. In D E C  AMPOS et al., 2008). Silicate melts of different viscosities and compositions can mix if: i) the appropriate pressure and temperature conditions exist, ii) if they remain fluid after reaching thermal equilibrium and iii) if there is sufficient time for fluid motion and diffusion to occur (S PARKS et al., 1984; P ERUGINI et al., 2003 cit. in D E C  AMPOS et al., 2008). The work presented in this study shows results from two mixing experiments during which all experimental conditions were kept equal, only stirring time was changed (25h and 168h). They give insight to the differential mobility of major and minor elements which is determined by convection and diffusion. The experiments follow the same conditions as described in D E C  AMPOS  et al. (2008). Two end members were stirred at 1300°C and 0.5 rpm using a concentric cylinder viscometer simulating mixing of silicate melts under laminar flow conditions by diffusive and convective processes. Based on microprobe analysis differential mobility of major and minor elements (e.g. major network formers and network modifiers) during the mixing process is revealed and will be discussed in more detail below. The differential mobility leads to step like mixing trends that oppose the general idea of linearity of magma mixing.
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