Bachelor Thesis. Towards timescales of magma mixing in Campi Flegrei, Italy. Supervisor: Prof. Dr. Donald Bruce Dingwell

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1 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 Ludwig-Maximilians-University Munich Department of Earth and Environmental Sciences Section for Mineralogy, Petrology and Geochemistry

2 Abstract...2 Introduction...3 Choice of end-members and end member characterisation... Choice of end members by strontium/ neodymium isotopy... End member A: Agnano Monte Spina...6 End member B: :...8 Further characterisation of both end members...1 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...3 Concluding remarks...31 Acknowledgements...32 References...33 Table of figures...3 Declaration

3 Abstract Volcanic eruptions and are among the most spectacular and destructive forces on the planet. In Europe alone, 4- 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, years BP) are thought to be triggered by short-term pre-eruptive mixing of a trachytic to trachydacitic resident () and a new basaltic, trachyandesitic (=shoshonitic) magma () 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 2 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 () and eruptions. End-members are stirred together under constant angular velocity (. rotations per minute) and constant temperature (13 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. 2

4 Introduction The Campi Flegrei caldera (CFc) is a densely inhabited, volcanic area that has been active for over 6. years (PAPPALARDO et al., 22 cit. in C. CANNATELLI et al., 26). This volcanic system lies in the west of Naples, Italy. Still being active, which is proven by the Monte Nuovo eruption, on 29 Sep 138 A.D. (DI VITO et al., 1987 cit. in S. DE VITA et al. 1999), frequent earthquakes in the 196 s and 198 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 (RITTMAN 19 cit. in A.PERROTTA et al. 26). The caldera is a nested structure, the result of two main collapses (1) the Campanian Ignimbrite (CI;37, years BP) and (2) the Neapolitan Yellow Tuff (NYT;12, years BP) (ORSI et al. 1996). Fig. 2: Campi flegrei caldera, faults, vents and history of activity, Orsi et al., 1996 ( Volcanic activity in the last 12, years was concentrated in the NYT caldera and can be grouped into three main phases of activity. During the first (12, 9 years BP) and second (86 82 years BP) phases, the vents were located at the structural boundary of the NYT caldera, during the third phase (48 38 years BP) activity was focused in the north-eastern sector of the resurgent block (ORSI et al. 1996, S. DE VITA et al. 1999). 3

5 Magma mixing in the shallow magma chamber has newly been to be the main factor triggering eruptions in the CFc (DE CAMPOS et al. (28); ARIENZO et al. (28)). 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 (TURNER and CAMPBELL, 1986; SPARKS and MARSHALL, 1986; OTTINO, 1989; JELLINEK et al., 1999; cit. in DE CAMPOS et al., 28) 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 (FURBISH, 1997 cit. in DE CAMPOS et al. 28). 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 (ZIMANOWSKI et al., 24, cit. In DE CAMPOS et al., 28). 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 (SPARKS et al., 1984; PERUGINI et al., 23 cit. in DE CAMPOS et al., 28). 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 (2h 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 DE CAMPOS et al. (28). Two end members were stirred at 13 C and. 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. 4

6 Choice of end-members and end member characterisation In this section the choice of the end members will be explained by strontium/neodymium isotopy and the eruptive history of the CFc. Further, the end members will be characterised by their isotopy, rheology, petrology and chemistry. Choice of end members by strontium/ neodymium isotopy As shown below (Fig. 3 Sr/Nd isotopy of the Phlegraean volcanics) isotopic analysis of the Phlegraean volcanic rocks show a distinct trend of evolution with time. The most recent eruptions that occurred in the CFc plot between the isotopy of NYT / Agnano-Monte Spina Tephra () and (Min2). The composition, being the more evolved one, is thought to represent the resident magma in the shallow magma chamber. The composition is thought to be the magma replenishing the chamber and therefore changing its chemistry and also the physical properties of the magma in the chamber (L. CIVETTA, C. P. DE CAMPOS, personal communication). Due to such constrasting properties, as outlined above, rock samples from both and were chosen to produce the starting glasses for these experiments, and are described below. Fig. 3: Sr/Nd isotopy of the Phlegraean volcanics (changed), from L. CIVETTA 29, data from L. CIVETA, personal communication

7 End member A: Agnano Monte Spina The Agnano-Monte Spina eruption was the highest magnitude eruption in the last époque of activity in the Campi Flegrei caldera and occurred at 41 years BP (S. DE VITA et al. 1999). It erupted 1.2 km³ dense rock equivalent of trachytic magma as pyroclastic-fallout, -flow and surge beds and bedsets (S. DE VITA et al. 1999). Pumice samples from the pyroclastics were selected and thin sections were prepared at the University of Rio de Janeiro, Brazil. Five thin sections were analysed using transmitted- and reflected-light microscopy to characterize the whole rock properties of the eruptive products. The important and interesting features found during optical microscopy are explained below. Vesicle content in the pumice ranges from ~ 6% to 8%. The texture of the samples is porphyritic, with phenocrysts of plagioclase and alkali-feldspar (with sanidine crystals up to mm), biotite, pyroxene (mostly cpx), olivine (sometimes with zoning), opaques and apatite in order of decreasing abundance. Phenocrysts occur as both single crystals and crystal aggregates. No idiomorphic crystals are found; they are either fragments or have resorbed edges and are rounded. There are also tuff fragments incorporated in the pumice, they show a high content of heavy minerals, mostly rutile with some zircons. Microlites are plagioclase and apatite, which is found to be grown in most of the phenocrysts. Olivine Apatite Fig. 4: Zoned olivine crystals with apatite intergrowth (photo: KOLZENBURG) 6

8 The heavy mineral tuff is a density segregation phenomenon and may have been incorporated before or during the pumice fragmentation. Tuff fragments (Fig. ) are often elongated and shaped like the vesicles of the pumice. Therefore, at the point of incorporation temperature must have been high enough to deform the tuff according to the same stress regime experienced by the pumice. Rutile fragments Fig. : Heavy mineral tuff with rutile fragments in reflected light (photo: DE CAMPOS, KOLZENBURG) Opaque Minerals are mostly magnetite that frequently shows ilmenite exsolution lamellae and some pyrite. Oxidising fluids that moved in the samples changed the mineral chemistry along the migration surfaces. Fig. 6 shows magnetite and pyrite that have these effects. They are clustered with pyroxenes and bound by sideromelan. Oxidation surface Pyrite Pyroxene Magnetite Fig. 6: Magnetite, pyrite and pyroxene intergrowth (photo: DE CAMPOS, KOLZENBURG) The matrix is sideromelan, one sample showed small glass shards incorporated but the quantity is marginal. Vesicles in all samples are elongated. Their size ranges from tens of micrometers up to one centimeter. Needle- and plate-like-shaped crystals, such as the feldspars, and elongated vesicles are aligned in extension direction. Pumice samples of the Tephra were melted to produce the glass for the experiments. The samples were taken from a stratigraphically low section, to assure that they represent the chemical composition closest to the magma that resided in the chamber before the eruption. 7

9 End member B: : The eruption was a magmatic to phreatomagmatic eruption that occurred between 11.1 and1.3 ka BP along the regional fault system in the northern portion of the CFc (C. CANNATELLI et al., 26). The eruptive products are composed of pumice, scoria and ash airfall deposits with some ash surge beds (C. CANNATELLI et al., 26). Several scoria samples were selected and thin sections were produced at the University of Rio de Janeiro, Brazil. Two thin sections were analysed using transmitted- and reflected-light microscopy to characterize the whole rock properties of the eruptive products. The important and interesting features found during optical microscopy are explained below. Vesicle content in the Pumice ranges from ~ 4% to %, they are generally smaller and more angular than the ones in and therefore indicate a lower solidifying temperature. The texture of the samples is porphyritic, with phenocrysts of pyroxene (generally clinopyroxene, some orthopyroxene), olivine, opaques, plagioclase and zircon in order of decreasing abundance. Crystals are generally subhedral and clustered in aggregates. Pyroxenes (Fig. 7) and olivines show intensive zoning. Clinopyroxene within the clusters are often twinned, whereas single crystals are usually not. Fig. 7: Zoned pyroxene crystal in scoria in transmitted light; crossed Nichols left (photo: KOLZENBURG) 8

10 The extinction angle of two clinopyroxenes was determined to be ~ 43 which indicates that they have augite composition. Two orthopyroxenes were identified as enstatite by their 2v angle of 7. The scoria shows heavy mineral tuff fragments similar, in internal structure and mineral content, to the ones of (Fig. ). These tuff components are often found within vesicles and tend to be concentrated on one end of the vesicles throughout the sample which might be a sign of depositional orientation. Opaques in the samples, though less in quantity, are of more variable composition. Pyrite, magnetite, hematite and chalcopyrite, in order of decreasing abundance, are found. The Matrix is build up of ~ % sideromelan, % microlites (plagioclase and olivine) and 4% glass shards. Pyrite Glass shards Fig. 8: Pyrite and matrix of scoria (photo: DE CAMPOS, KOLZENBURG) The intense zoning of olivine and pyroxene in the samples shows that the crystals are not in equilibrium with the matrix glass. In order to avoid contamination of the experimental melt by the melting of these crystals, the samples were crushed and the pyroxene and olivine crystals were picked out by hand. In doing so the composition was slightly changed towards higher silicium content. 9

11 Further characterisation of both end members Microprobe analysis of the glasses that were used in the experiments gave the results shown below (Table 1; for analytical conditions see the referring paragraph): SiO 2 TiO 2 Al2O 3 FeOt MnO MgO CaO Na 2 O K 2 O P 2 O Cl Total Std. dev Std. dev Table 1: Composition of starting melts; mean values of 1 measurements per end-member in weight% of oxides These values were normalized and then used, without further correction, for the following representations and calculations. The - glass plots in the Trachyte / Trachydacite field, the glass in the Basaltic Trachyandesite field in the K 2 O vs. SiO 2 diagram after LE BAS et al. (Fig. 9). The analytical results are in good correlation with other studies on materials of the same eruptions (C. CANNATELLI et al., 26 for ; S. DE VITA et al., 1999 for ). Fig. 9: Na 2 O+K 2 O vs. SiO 2 diagram after LE BAS et al.,

12 From the Values given in Table 1 the viscosities of the melts, at the experimental temperature of 13 C, were calculated using the model of GIORDANO D, RUSSELL J. K., & DINGWELL D. B., (28) assuming dry conditions and not considering the chlorine content. : : log η = 1.69 ~ 49 Pa s log η = 3.2 ~ 18 Pa s This model also gives the value of viscosity contrast A. (η 1 being the viscosity of and η 2 the viscosity of, at 13 C) A = η 2 η 1 η 2+η1 The Viscosity contrast has great influence on the ability of two silicate melts to mix. The system gives value of.94, for which efficient Mixing of silicate melts is thought to be unlikely. The viscosity ratio U is calculated after JELLINEK et al. (1999). U = η η a i η a being the higher and η i the lower viscosity of the mixed fluids. It gives a value of 32.4 for which a mixing efficiency below 3% is suggested. Nevertheless the performed experiments show efficient mixing for several elements between these end members. This work started from ready made glasses, choice of the end members, handpicking of crystals from the samples and preparation of the thin sections had been done beforehand by other colleagues of the Magma Mixing project at LMU. 11

13 Sample preparation and experimental conditions The following paragraph describes how the end member glasses were prepared before the experiments and gives details about the experimental conditions such as geometry, theoretical background and sample processing after the experiments. Sample preparation Sample preparation was done as described in DE CAMPOS et al., 24. Rock powders from both end members were melted and homogenized at 13 C, then quenched and drilled as cylinders. By this time the glasses were completely crystal free. A 22mm diamond drill was used for the glass. However, due to deformations in one of the crucibles a 22mm and an 18mm diamond drill had to be used for the glass samples. The glasses were then cut to fit the intended ratio of 7%vol. and 3%vol.. This ratio is considered to be the most suitable for the experiments and derives from studies of the chemical composition of the younger eruptive products of the CFc (L. CIVETTA and C. P. DE CAMPOS pers. comm.). The sample geometry and mass of the glass slices used in the two experiments (2h and 168h) are given below (size was measured using a digital calliper (precision ±.1mm) and mass was measured with a high-precision digital balance (precision ± 1 x 1-g)): 2h experiment: : 21.96mm diameter and 1.74mm height 3.8 %vol.; weight: g : 21.9mm diameter and 2mm height %vol.; weight: h experiment: : 21.96mm diameter and 1.6mm height 29.9 %vol.; weight: 1,7947g : 21.92mm, 17.87mm in diameter and 8.79mm, 24.22mm height respectively together 7. %vol.; weight (total): g 12

14 With these values the density at room temperature was calculated to be roughly 2.66 g/cm³ for - and 2.47 g/cm³ for -glass. The Pt8Rh2 crucible and spindle were cleaned in hydrofluoric acid for a minimum of 24h prior to the experiments. The crucible was first tested for leaks with acetone; the low viscosity of acetone will source even very small cracks that could lead to molten material leaking dangerously in the experimental furnace. Contact surfaces between the glasses were carefully polished to (1) yield accurate sample size measurements and (2) to avoid bubble formation during the melt process by providing the best contact possible. Fig. 1: Experiment setup (photo: KOLZENBURG) 13

15 Experimental conditions In this study a disc of denser glass of was mounted at the bottom of the crucible (Fig. 11) and the less dense glass of was mounted on top (Fig. 11), using the methodology outlined by DE CAMPOS et al. 28, as to avoid mingling forced by buoyancy. Setup of the experiments and experimental conditions were kept the same for both the 2h and the 168h experiment. Fig. 11: Experimental geometry and conditions: a bottom layer of end-member A () and a top layer of end-member B () have been mounted in a Pt 8 Rh 2 crucible. The two starting materials were then melted and stirred at 13 C, using a concentric cylinder viscometer, described in detail in DINGWELL (1986). A low rotation speed of. rpm was applied by the rotation of the spindle (~3 mm in diameter), which is centrally located within the double-cylinder (fig. and description from DE CAMPOS et al 28) Rotation during flow may be an effective mean of mixing two silicate melts (DE CAMPOS et al 24; ZIMANOWSKI et al.24 cit. in DE CAMPOS et al. 28). Rotation, in these experiments, was applied by rotation of the spindle. The angular velocity was kept constant (. rpm) but not the relative vorticity. Vorticity is the amount of rotation in a flow (CHORIN, 1994 cit. in DE CAMPOS et al. 28). It increases in these experiments over time and contributes to stretching and folding of the melts. Therefore it greatly increases the contact surface between the melts which has strong influence on the effectiveness of diffusion. 14

16 To simulate and enhance the combined process of convection and diffusion a Taylor-Couette stirring geometry provided by a concentric cylinder viscometer was used, as suggested in DE CAMPOS et al., 28. Forced convection was simulated for two different periods of time (2h and 168h). As in DE CAMPOS et al., (28) both temperature (13 C) and shear rate (~1s -1 ) have been maintained constant during the whole experiment. Only vorticity and therefore the surface area and therefore diffusion between the different end members increased with time. Flow conditions in these experiments are assumed to be laminar. The samples were heated, remelted and then stirred at 13 C using a concentric cylinder viscometer, described in detail in DINGWELL (1986). After the samples started melting (meniscus forming at the wall of the Pt 8 Rh 2 crucible) the spindle (~3mm in diameter), which is centrally located within the cylinder, was lowered in the melt until touching the bottom of the crucible and raised back up 2mm. Then rotation, with a constant angular velocity of. rpm, was started. The distance of 2mm between the spindle and the bottom of the crucible was necessary to avoid the tilting of the spindle due to thermal extension within the experimental setup. This would have led to non laminar flow conditions. The experiments were terminated by stopping the spindle rotation. After the spindle had completely stopped, it was carefully extracted from the melt, as to avoid disturbance of the sample. The sample was then allowed to cool slowly to room temperature in a relaxation furnace, to allow stress relaxation. The cooling process lasted for approximately four hours. Fig. 12: Concentric cylinder viscometer used for melting and stirring (photo: KOLZENBURG) 1

17 Analytical conditions This paragraph reports the Processing of the samples for microprobe analysis and the analytical conditions that were preset on the electron microprobe as well as the standards that were used for comparison and details of the measuring procedure. The samples were drilled out of the crucible using a 22mm diamond drill. Due to inhomogeneities of the crucible bottom the samples could not be drilled out completely. The 2h sample lost about 2mm of its bottom whereas the 168h sample lost about.3mm. Height differences in the samples are also due to slightly different starting volumes, as only the volume ratio of 7/3 was considered. The samples were sliced in half and prepared for microprobe analysis by polishing and coating them with carbon (to make them conductible). In order to fit the sample holder (shown in Fig. 13) the sample of the 2h experiment was cut at the edges that were not needed for analysis, the sample of the 168h experiment was mounted in two pieces that broke apart (loss-free) on an internal discontinuity. The beginning and end of the profile lines were marked with conductive silver as to find them in the microprobe. This silver was also used to conductively join the samples with the holder. Fig. 13: Profile lines on samples and standards mounted for analysis (photo: DE CAMPOS, KOLZENBURG) The measurements were carried out on a Cameca SX1 electron microprobe in the Department of Earth and Environmental Sciences of the LMU Munich. The electron microprobe was operated at 1 kev acceleration voltage and 2nA beam current. A defocused 1 µm was used for all glass analysis in order to avoid loss of volatile elements (such as alkalines and chlorine). For the crystal standards a normally focused beam was used. Albite (Na), corundum (Al), orthoclase (K), synthetic wollastonite (Ca, Si), vanadinite (Cl), Ilmenite; BY21 (Mn, Ti), hematite (Fe), periclase (Mg), apatite (P) were used as standards and a Basalt-Olivine-Melilitite-glass (BY 1) as reference. 16

18 Matrix correction was performed by PAP procedure (POUCHOU and PICHOIR, 1984 cit. in DE CAMPOS et al., 28). The reproducibility of standard analyses was < 1% for each element routinely analyzed. For analytical deviations from standard in the range ± 1% < St < ± 3%, data was corrected by means of a simple correction factor: measured amount of element in the standard/expected amount of element in the standard. Data presented here have standard deviations for all elements < ± 2.%. Vertical profiles (Fig. 13) were measured along the midpoint between crucible wall and spindle to keep the influence of either at minimum. The step length was set to be.6 ± 2μm for the 2h sample and ± 1μm for the 168h sample. Due to a crack running through the 168h sample the profile line for data points was moved 68μm towards the spindle in order to avoid inaccurate measurements. To be able to precisely continue the profile line from the top to the bottom part of the 168h sample photo correlation was done and the error was assessed to be < μm. All the profile lines were checked to be in sound condition (e.g. no large bubbles etc.) prior to measurement and the beam was auto focused during the measurements after every five measuring points. The differential concentrations, or mobility, of major and minor elements will be presented as normalized weight percents of oxides (SiO2, TiO2, Al2O3, K2O, Na2O, MgO, CaO, P2O, FeO total, MnO and Cl) in a comparative way, as it has been done in DE CAMPOS et al., 28 showing how this mobility may change with time. 17

19 Results The following part covers a short introduction to the theoretical background of fluid mixing and its nomenclature. Subsequently the comparative microprobe results of the profiles measured along the 2h and 168h samples are presented and elucidated. Finally different mixing trends that occurred and changed with time during the experiments are shown and explained. Theoretical background The experiments started with two multi-component silicate melts. The composition of the end members is expected to converge towards a theoretical hybrid of this system (7%vol. 3% vol. ). The can be calculated via the classical two-end-member-mixing equation of G. W. BUSH (1761) which has been applied to petrology by LANGMUIR et al.1978: CH = CAx+ C B(1 x) C C C Where H, A and B are the concentrations of the hybrid, most evolved () and least evolved () magma, respectively, and x is the volumetric proportion of (7%vol. x =.7) in the mixture. The concentration of the composition for each analyzed major and minor oxide has been calculated by considering the end member compositions reported in Table 1 and the original proportions of the two magmas (i.e. 7%vol. of and 3%vol. of ) 18

20 The main physical phenomenon driving element distribution within the experimental system is known as double diffusive convection (DDC). It has been studied by TURNER (1973) who describes it as a series of phenomena occurring in fluids where there are gradients of two (or more) properties with different molecular diffusivities. His studies were looking at the effect of temperature and salinity gradients in aqueous solutions. According to the study a temperature gradient is the most effective way to generate density profiles consisting of well-mixed-, diffusivelayers and spreading horizons that lead to patterns as shown in Fig. 14. In his study TURNER (1973) shows that the same patterns may arise without a temperature gradient, only by the interaction of two (or more) concentration gradients with different molecular diffusivities. Fig. 14: Density variation in aqueous solutions under DDC, when heated from below, from TURNER (1973) Different molecular diffusivities in the experiments is obvious and arises through the general chemical properties of the studied elements especially in relationship of their contribution to the melt structure (e.g. Si and Al generally act as network formers and Na and K generally act as network modifiers). As patterns like the ones TURNER described arise in the results of these mixing experiments the Terms given in Fig. 14 will be used to describe the following diagrams Comparative microprobe results Errors for the diagrams are.6 ± 2μm for the 2h and ± 1μm for the 168h experiment in sample height; errors in concentration for the experimental glasses are the same as for the end members. Error bars for the analysis of the experimental glasses are not shown in the diagrams as to keep them clear. The differences in sample depth are due to different starting volumes and loss after drilling as described earlier. 19

21 The change in slope at the / contact and the general smoothing of patterns from the 2h to the 168h run are due to the change towards a more diffusion dominated system through time. The diffusion efficiency is highly time dependent, especially because vorticity greatly increases. All elements show two distinct convection cells on the top and bottom that approximately correlate in size with the height of the end members at the start of the experiments. Most of them also show a third convection cell in between the two that changes with time. Plots of elements showing similar patterns are discussed together and general observations are given at the end of this section. Although the two experiments were carried out with completely new setups used for either of the two the results show good reproducibility. Statements on qualitative changes over time are therefore possible within analytical error Mean 2h experiment 2 Mean 168h experiment SiO 2 wt % SiO 2 wt % h experiment 2 168h experiment Al 2 O 3 wt % Al 2 O 3 wt % Fig. 1: Comparison of normalized major and minor oxide distributions of the 2h and the 168h run (SiO 2 norm, Al 2 O 3 norm) 2

22 Si and Al generally behave similar in the mixing experiments, which is due to their role as network-formers in silicate melts. Both show well mixed layers in the bottom cell with a spreading horizon that separates the bottom- from the top-cell in the 2h run. Within the topcell of the 2h run both elements show a zigzag pattern that originates from internal up- and downhill-diffusion. These patterns are also seen in the 168h run but they are smoothened due to increasing diffusion. Si shows hints of the separation of a third well mixed zone which will be obvious in plots discussed later. The slight increase in Al content in the bottom cell from 2-to 168h and the stronger change in slope of the spreading horizon of Al show that Al is more mobile within the melts that Si. Other than a general smoothening, Si does not show obvious signs of mixing h experiment 2 168h experiment FeO wt % FeO wt % Fig. 16: Comparison of normalized major and minor oxide distributions of the 2h and the 168h run (Fe 2 O 3 norm) The bottom cell of the 2h run is a relatively homogenous well mixed layer with a small diffusive layer of decreasing concentration at its base and slight uphill diffusion before the onset of the spreading horizon. A zigzag pattern of up-and downhill diffusion has developed in the upper convection cell which could, on a bigger scale, be described as a well mixed layer. This zigzag pattern is still existent after 168h but is smoothened by diffusion and transfers at its lower end into a diffusive layer that grades into the spreading horizon. The resemblance of Fe to Si and Al behaviours that are shown below will be discussed later. 21

23 h experiment 2 168h experiment CaO wt % CaO wt % h experiment 2 168h experiment MgO wt % MgO wt % Fig. 17: Comparison of normalized major and minor oxide distributions of the 2h and the 168h run (CaOnorm, MgOnorm) Ca and Mg show three convection cells that are separated by two spreading horizons of different intensity in the 2h run. Their chemical affinity in terms of ion- size and charge leads to the analogical behaviour. Both show a change from a small diffusive to a larger well mixed layer in the bottom cell and a fairly sable well mixed top layer. The convection cell in between behaves interestingly different. Ca shows a well mixed layer that is linked to two diffusive layers on top and bottom whereas the well mixed layer of Mg is more sharply delimited. Both, the top and bottom of this cell, exhibit a flip in slope, the top even shows slight uphill diffusion before changing to the spreading horizon. In the 168h run the top and bottom convection cells stay stable and evolve into undisturbed well mixed layers. Ca and Mg concentrations decreased homogenously in the bottom- and stayed constant in the top-cell. The convection cell in the middle vanished and turned into a diffusive layer that affiliates the spreading horizon. 22

24 h experiment 2 168h experiment K 2 O wt % K 2 O wt % h experiment 2 168h experiment Na 2 O wt % Na 2 O wt % Fig. 18: Comparison of normalized major and minor oxide distributions of the 2h and the 168h run (K 2 Onorm, Na 2 Onorm) K and Na, due to their high ion- size and charge, are the most mobile of the analysed elements. When K still follows the geometry and distribution as most of the other elements do Na almost decouples from the system speeding through the melts. K sows almost the same development as Ca does with 3 well mixed layers separated by spreading horizons developing after 2h. K is a little more stable in terms of element concentration in the convection cells than Ca and Mg, the development of the well mixed layer in the middle into a diffusive layer in the 168h run is nevertheless visible. Also the homogenization of the bottom cell with time is present for K. 23

25 Na, as stated above, behaves exceptionally different. Its diffusion speed is the highest observed in the analysed elements. A third well mixed layer after 2h, as seen for most other elements, is not present. The pattern of Na after 2h resembles the patterns that most other elements show after 168h, with a diffusive layer as the remain of a former well mixed layer between the top- and bottom-cell. The chemistry strongly tends towards the of the system after 168h. The top cell is developed as a diffusive layer with a gradual change in slope. Homogenisation rate in the bottom cell is a lot faster than in the top cell. This phenomenon is, although not as strong as for Na, also seen for other elements and will further be discussed below h experiment 2 168h experiment P 2 O wt % P 2 O wt % h experiment 2 168h experiment TiO 2 wt % TiO 2 wt % Fig. 19: Comparison of normalized major and minor oxide distributions of the 2h and the 168h run (P 2 O norm, TiO 2 norm) The larger analytical error leads to more blurry patterns for P and Ti. Nevertheless three well mixed layers are obvious. Both P and Ti show a well mixed layer in the lower convection cell that has an uphill diffusion tendency after 2h. 24

26 This tendency changes with time, for both elements, to a homogenous well mixed layer that grades into a diffusive layer and transfers into the spreading horizon. The.1 wt% offset between the two well mixed layers at the top of the sample after 2h vanishes with time for P but is still visible for Ti after 168h. Up- and downhill diffusion tendencies in the lower convection cells for P and Ti after 2h seem to be there but proper evidence is not possible at this resolution h experiment h experiment Cl wt % Cl wt % Fig. 2: Comparison of normalized major and minor oxide distributions of the 2h and the 168h run (Clnorm) Cl shows three layers after 2h that are of almost equal height. Two well mixed layers on top and bottom that are linked by a diffusive layer. After 168h one single diffusive layer has developed from top to bottom of the sample. Due to the small difference in concentration together with the relatively high analytical error convergence towards the can not be proven but, as Cl is a highly mobile element, is likely. The volatile behaviour of Cl is seen in the 168h results, after 1 week at 13 C the uppermost.mm show a massive decrease in Cl content due to degassing. 2

27 2 2 2h experiment h experiment MnO wt % MnO wt % Fig. 21: Comparison of normalized major and minor oxide distributions of the 2h and the 168h run (MnOnorm) Mn, although end member compositions are very close to the, does not show homogenization. Even though resolution in these diagrams is poor due to scattering of the analysis points two diffusive-, a downgrading and an upgrading-, and a well mixed layer (from bottom up) are obvious. This positioning is seen for both experimental runs and increases from the 2h to the 168h run. The in- and decrease of concentration is significantly bigger than the analytical error, a tendency towards more heterogeneous conditions has set in instead of the expected convergence towards the. This effect is only seen for Mn in these experiments but has been described for other elements in experiments with silicate melts of similar chemistry to the ones used here, but with a smaller viscosity contrast (DE CAMPOS et al. 28). 26

28 Since Na has the highest diffusion speed of the analysed elements some general effects of the experimental system are more obvious in its behaviour than in the one of other elements. The diagram displaying the combined results for the 2h and168h run shows these in a good h experiment 2h experiment Na 2 O wt % way. The bottom cell is shifted by,3wt% after only 2h and by 1,wt% after 168h. This change is homogenous throughout the bottom cell. The top cell only changes by.2 wt% at its top but displays a general trend converging towards the. Nonetheless the top cell does not develop as a well mixedbut as a diffusive layer. Homogenization speed in the bottom-cell cell is obviously faster than in the top-cell. Fig. 22: Comparison of normalized major and minor oxide distributions of the 2h and the 168h run (Na 2 Onorm) h experiment 168h experiment The same effect as described before is also visible for Ca but with a different magnitude. The bottom cell shifts slightly from 2h to 168h and the pattern within the cell becomes more homogenous. The upper cell homogenizes slower than the bottom one CaO wt % Fig. 23: Comparison of normalized major and minor oxide distributions of the 2h and the 168h run (CaOnorm) The difference in homogenization rate between the top and bottom layer is inferred to be viscosity dependent, since this is the only known parameter that massively varies among the end members. 27

29 Test of Bush / Langmuir equation Petrologists assume that the chemistry of mixed magmatic systems plots as a straight line on an inter elemental diagram (LANGMUIR et al. 1978; FOURCADE and ALLÉGRE, 1981; ROLLINSON, 1993 cit. in DE CAMPOS et al. 28). If the statement given above truly describes the dynamics of magma mixing the ratio of two fractions involved (elements analysed as oxides) should plot on a straight line between the end member ratios. Ratios of elements that were found to behave similarly during mixing were chosen to test the former statement. Diagrams given below clearly contrast with the assumption of linearity for some elements and agree with it for others h experiment linear mixing trend Col 2 vs Col h experiment linear mixing trend Col 2 vs Col Al 2 O3 wt% Al 2 O3 wt% SiO 2 wt% SiO 2 wt% h experiment linear mixing trend 8 168h experiment linear mixing trend 7 7 K 2 O K 2 O Na 2 O Na 2 O Fig. 24: Element ratios for SiO 2 /Al 2 O 3 and Na 2 O/K 2 O (normalized), after 2h and 168h displaying nonlinearity of magma mixing 28

30 After 2h just about 2 to 3 analysis points plot on a mixing trend in the SiO 2 /Al 2 O 3 ratio diagram, this number is boosted after 168h testifying that mixing efficiency, especially for elements behaving as network-formers, is highly time dependent. The non-linear characteristics of the mixing trend manifested in the 2h experiment become more detailed after 168h as resolution increases due to stronger mixing of the system. The results plot along a curve that shows a step-like shape on two scales. On a big scale one step is visible; the high slope of the curve flattens out roughly half way and increases again, as it approaches the second end member. On a smaller scale at least eight steps appear with sharp changes in slope between almost flat and almost vertical. This step like behaviour is well known for double diffusive convection regimes (TURNER 1973). Only detailed analysis of the experimental data allows seeing the described phenomena, a simple regression would have let to a fairly good correlation with the linear mixing assumption. The Na 2 O/K 2 O ratios after 2h and 168h show a similar behaviour as the SiO 2 /Al 2 O 3 ratios in terms of the development of a step like mixing trend. The intensity of mixing for Na and K is a lot stronger than for Si and Al as more analysis points plot along the mixing trend. Interestingly the observed ratios and therefore also the trend of mixing change with time. In the upper cell (former glass) the ratios seem to be fairly constant whereas a distinct change towards higher Na concentrations is seen in the lower cell (former glass). This is due to the big difference in diffusion speed between Na and K, a linear and/ore stable mixing trend can only develop if the diffusion coefficients for the compared elements are very similar h experiment linear mixing trend 168h experiment linear mixing trend 4 4 MgO wt% 3 MgO wt% CaO wt% CaO wt% Fig. 2: Element ratios for CaO/MgO (normalized), after 2h and 168h displaying apparent linearity of magma mixing 29

31 Fig. 2 shows an almost linear trend of mixing for the CaO/MgO ratios that is stable in slope over time; it also develops towards higher mixing efficiency. With time the CaO/MgO ratio decreases linearly towards the ratio of end member. Discussion In some of the comparative diagrams the analysis of the experimental glass show a general shift of concentration of the same magnitude away from the end members throughout the whole sample. For example Al shifts to higher, and Fe shifts to lower values. The same shifts are observed in the ratio comparing diagrams. These offsets may be generated by up- and downhill diffusion but this can not be testified here. More analysis along other profile lines would be needed to test if there are areas that show the correlating enrichment/depletion to the ones analysed. Offsets like the ones observed could also derive from analytical errors or wrong calibration, a reprocessing of the microprobe data with different calibration files could reveal this. Bordering the spreading horizons apparent up- and downhill diffusion can be seen. Ti, for example, shows uphill diffusion in the bottom cell of the 2h experiment. Due to the low resolution of the analysed profiles (roughly μm) these effects can not be described in detail, Areas that show these effects could be analysed with a smaller step length to verify this. Temperature in these experiments was chosen to be super liquidus in order to assure that crystallization does not influence the mixing process. To test if the chosen parameters are realistic for natural processes geothermometers of melt-inclusions in crystals should be analysed to get the temperatures that determine the regime of mixing. Also the rate of forced convection should be tested to resemble natural processes. The third convection cell, that developed in the 2h experiments and changes its properties after 168h can obviously be reproduced, since the experiments were performed under the same conditions. If the geometry and the development of this effect is driven chemistry or the experimental geometry could not be discovered in this work, comparison to data of former experiments or new experiments that only change either of the components would have to be performed in order to substantiate the origin of this phenomenon. 3

32 As seen in the section Test of Bush / Langmuir equation the linear approach of magma mixing fits well for elements with very similar diffusion coefficients (Ca/Mg) but does not fit for ones with different diffusion coefficients (Na/K). In addition to this mixing It is further complicated by the roles of the element in the melt structure; network-formers for example have a smaller degree of freedom as their diffusion is hindered by structure effects. Non linear mixing trends as observed in these experiments on a micron scale may be a matter of scaling. If the experiments represent mixing in natural magma chambers chemical cross sections through magma chambers with evidence of magma mixing should show this on a larger scale. The notable similarity in mixing behaviour of Fe to Si and Al that has been described earlier may be explained by the property of Fe to build up tetrahedral structures. Therefore it can act, like Si and Al that are well known for their tetrahedral configuration which is the reason for the network forming properties, as a network former. The acquired data clearly shows that the two end members mix, and that mixing efficiency is highly time dependent. Quantifying of the mixing efficiency may be performed by a method described in De Campos et al. 29 using statistical composition distribution. Concluding remarks Under the conditions chosen for these experiments mixing of the two end members is possible. This is testified by the convergence of concentrations towards the of the system shown in the comparative diagrams and the propagation of oxide/oxide ratios with time along certain mixing trends. The mixing trends observed contrast with the general idea of linearity of magma mixing, step like and incontinuous trends are found instead. This is ascribed to differences in diffusion speeds of elements in the melt that are controlled by diffusion coefficients and the melt structure. Mixing between the end members is not homogenous. Due to differences in diffusion speed (that derives from diffusion coefficients, melt structure and viscosity) as well as double diffusive convection effects some elements homogenize faster than others. Mixing trends even evolve with time, as seen in the oxide/oxide ratio diagrams. 31

33 Acknowledgements I especially want to thank Cristina De Campos for her encouragement, assistance, discussions and suggestions during this work. Further I want to thank Donald Bruce Dingwell and Werner Ertl-Ingrisch for their assistance in Laboratory work, Lucia Civetta for the choice of the end-members, Michael J. Heap for suggestions in phrasing, Saskia Bernstein for assistance in microprobe analysis and everyone that helped in the sample preparation. This work was funded by INGV project unrest and DFG Project IDCP towards timescales of assimilation and magma mixing in the large igneous province of snake river plain Yellowstone, northwest united states 32

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