Integrated petrologic, geochemical and experimental investigation of intraplate alkaline magmas at Dunedin Volcano: constraints on mantle sources, differentiation trends and magmatic processes

Intraplate volcanic settings have long been recognized for the compelling diversity of magma compositions if compared to plate boundary magmatism. Alkaline volcanoes are typically characterized by an impressive variety of crystalline cargoes carried by their magmas, witnessing a complex source-to-surface path. Understanding intraplate alkaline volcanoes is a task of fundamental scientific value, as it relates to the very processes of magma genesis and differentiation. Moreover, it is crucial for urban development, as they are often the sites of human activity, where they provide natural resources as well as constituting a natural hazard. The Dunedin Volcano (South Island of New Zealand) represents a classic case study in which some of the early concepts on the differentiation of intraplate alkaline suites were originally proposed. To investigate processes that lead to magma generation, polybaric storage and magma emplacement, in this thesis whole-rock major elements, trace elements and isotopic compositional data have been collected by means of X-Ray Fluorescence (XRF), Inductively Coupled Plasma Mass Spectrometer (ICPMS) and Multicollector ICPMS, respectively. These have been integrated with mineral-scale studies of intrusive and eruptive products, performed with Electron Probe Micro Analyzer (EPMA) and Laser Ablation ICPMS. The composite volcano erupted magmas with silica-saturated to undersaturated affinity, the more abundant eruptive products being represented by primitive (alkali basalts) and highly differentiated (trachyte-phonolite) magmas. Basalts are characterized by a range of alkali contents, incompatible element abundances, isotopic compositions, as well as mineral populations. High-alkali basalts and mid-alkali basalts have complexly zoned clinopyroxene crystals (Mg#65-82), rich in incompatible elements and characterized by FOZO-HIMU isotopic signatures (87Sr/86Sri = 0.70277-0.70315, 143Nd/144Ndi = 0.51286-0.51294, and 206Pb/204Pb = 19.348-20.265). Low-alkali basalts are characterized by clinopyroxenes (Mg#69-84) with resorbed mafic cores (Mg#78-88), and plagioclase crystals (An43-84), and have lower incompatible element abundances and isotopic compositions that trends toward EMII (87Sr/86Sr = 0.70327-70397, 143Nd/144Ndi = 0.51282-0.51286, and 206Pb/204Pb = 19.278-19.793). Partial melting models indicate that the variable alkalinity and isotopic composition of basaltic rocks result from interaction of low-alkali melts derived from fertile asthenospheric mantle domains with melts derived from the metasomatized lithospheric mantle. Basaltic magmas and intermediate magma compositions (phonotephrite, mugearite, tephriphonolite and benmoreite) crystallized at depths mostly comprised between the lower and the mid-crust (13-27 km). Basaltic rocks crystallized olivine + clinopyroxene + oxides, while intermediate magmas are produced after fractionation of clinopyroxene + amphibole + plagioclase + titanomagnetite. Accumulation of crystals segregated from the intermediate magmas is witnessed by a suite of crystal-rich mafic enclaves (gabbros, amphibolites and clinopyroxenites), entrained by the ascending magmas. At shallow crustal levels (5-11 km) phonolites and trachytes are produced by different processes. Trachytes differentiated from benmoreite magmas, following significant feldspar fractionation (~50 wt.%) and small amount of country rock assimilation (≤10 wt.%). Phonolites instead differentiated from tephriphonolites, and represent the interstitial liquids of crystal mush systems, constituted by crystalline networks of alkali feldspars and minor amphibole, represented by syenitic and feldspar-rich enclaves. Trachyte and phonolite may represent two stages in the development of Dunedin Volcano plumbing system, where crustal assimilation becomes more difficult as a crystal mush forms and shields later injections in the upper crust. Differentiation trends have been constrained by high pressure–high temperature piston cylinder experiment, geochemical models and thermodynamic modelling, and results indicate that two distinct differentiation trends coexist, as supported by previous studies. The silica–saturated magmatic lineage is related to mid- and high-alkali basalts parental compositions crystallizing at 15-25 km, 1050-1200°C with H2O>3 wt.% and evolves as mugearitic to benmoreitic magmas crystallize clinopyroxene + amphibole + plagioclase + titanomagnetite at mid- to upper crustal levels. The highest silica-enrichment observed in trachytes (SiO2>60 wt.%) relates to the lowest crystallization temperatures (850-950°C) and to high water contents (H2O>6 wt.%). Silica-undersaturated magmas differentiate after high-alkali basalts at high pressure (20-30 km) and high temperatures (1100-1250°C) in relatively water-poor magmas (H2O ≤ 2.5 wt.%) and fractionate clinopyroxene + olivine + titanomagnetite. Further fractionation of clinopyroxene + amphibole + titanomagnetite from phonotephrite produced tephriphonolite magmas, whose higher volatile budget (H2O ≥ 4.5 wt.%) and lower temperatures (900-1000°C) caused abundant crystallization of alkali feldspars ± biotite ± amphibole in the shallow crust, resulting in the formation of highly crystalline regions (≥60 wt.% crystals) with alkali enriched interstitial melts of phonolitic composition. The variety of alkaline rock compositions and coexisting crystalline populations are not only related to fractionation along differentiation trends, but are strongly influenced by a range of complex processes as magma transfers across diverse storage regions. Different magmas mixed and entrained crystalline material segregated from previous magma batches. The multidisciplinary approach used in this thesis allowed the reconstruction of an integrated source-to-surface history of Dunedin Volcano magmas, with the aim of contributing to our understanding of the processes that control the origin and differentiation of intraplate alkaline systems on a global scale

Integrated petrologic, geochemical and experimental investigation of intraplate alkaline magmas at Dunedin Volcano: constraints on mantle sources, differentiation trends and magmatic processes

Intraplate volcanic settings have long been recognized for the compelling diversity of magma compositions if compared to plate boundary magmatism. Alkaline volcanoes are typically characterized by an impressive variety of crystalline cargoes carried by their magmas, witnessing a complex source-to-surface path. Understanding intraplate alkaline volcanoes is a task of fundamental scientific value, as it relates to the very processes of magma genesis and differentiation. Moreover, it is crucial for urban development, as they are often the sites of human activity, where they provide natural resources as well as constituting a natural hazard. The Dunedin Volcano (South Island of New Zealand) represents a classic case study in which some of the early concepts on the differentiation of intraplate alkaline suites were originally proposed. To investigate processes that lead to magma generation, polybaric storage and magma emplacement, in this thesis whole-rock major elements, trace elements and isotopic compositional data have been collected by means of X-Ray Fluorescence (XRF), Inductively Coupled Plasma Mass Spectrometer (ICPMS) and Multicollector ICPMS, respectively. These have been integrated with mineral-scale studies of intrusive and eruptive products, performed with Electron Probe Micro Analyzer (EPMA) and Laser Ablation ICPMS. The composite volcano erupted magmas with silica-saturated to undersaturated affinity, the more abundant eruptive products being represented by primitive (alkali basalts) and highly differentiated (trachyte-phonolite) magmas. Basalts are characterized by a range of alkali contents, incompatible element abundances, isotopic compositions, as well as mineral populations. High-alkali basalts and mid-alkali basalts have complexly zoned clinopyroxene crystals (Mg#65-82), rich in incompatible elements and characterized by FOZO-HIMU isotopic signatures (87Sr/86Sri = 0.70277-0.70315, 143Nd/144Ndi = 0.51286-0.51294, and 206Pb/204Pb = 19.348-20.265). Low-alkali basalts are characterized by clinopyroxenes (Mg#69-84) with resorbed mafic cores (Mg#78-88), and plagioclase crystals (An43-84), and have lower incompatible element abundances and isotopic compositions that trends toward EMII (87Sr/86Sr = 0.70327-70397, 143Nd/144Ndi = 0.51282-0.51286, and 206Pb/204Pb = 19.278-19.793). Partial melting models indicate that the variable alkalinity and isotopic composition of basaltic rocks result from interaction of low-alkali melts derived from fertile asthenospheric mantle domains with melts derived from the metasomatized lithospheric mantle. Basaltic magmas and intermediate magma compositions (phonotephrite, mugearite, tephriphonolite and benmoreite) crystallized at depths mostly comprised between the lower and the mid-crust (13-27 km). Basaltic rocks crystallized olivine + clinopyroxene + oxides, while intermediate magmas are produced after fractionation of clinopyroxene + amphibole + plagioclase + titanomagnetite. Accumulation of crystals segregated from the intermediate magmas is witnessed by a suite of crystal-rich mafic enclaves (gabbros, amphibolites and clinopyroxenites), entrained by the ascending magmas. At shallow crustal levels (5-11 km) phonolites and trachytes are produced by different processes. Trachytes differentiated from benmoreite magmas, following significant feldspar fractionation (~50 wt.%) and small amount of country rock assimilation (≤10 wt.%). Phonolites instead differentiated from tephriphonolites, and represent the interstitial liquids of crystal mush systems, constituted by crystalline networks of alkali feldspars and minor amphibole, represented by syenitic and feldspar-rich enclaves. Trachyte and phonolite may represent two stages in the development of Dunedin Volcano plumbing system, where crustal assimilation becomes more difficult as a crystal mush forms and shields later injections in the upper crust. Differentiation trends have been constrained by high pressure–high temperature piston cylinder experiment, geochemical models and thermodynamic modelling, and results indicate that two distinct differentiation trends coexist, as supported by previous studies. The silica–saturated magmatic lineage is related to mid- and high-alkali basalts parental compositions crystallizing at 15-25 km, 1050-1200°C with H2O>3 wt.% and evolves as mugearitic to benmoreitic magmas crystallize clinopyroxene + amphibole + plagioclase + titanomagnetite at mid- to upper crustal levels. The highest silica-enrichment observed in trachytes (SiO2>60 wt.%) relates to the lowest crystallization temperatures (850-950°C) and to high water contents (H2O>6 wt.%). Silica-undersaturated magmas differentiate after high-alkali basalts at high pressure (20-30 km) and high temperatures (1100-1250°C) in relatively water-poor magmas (H2O ≤ 2.5 wt.%) and fractionate clinopyroxene + olivine + titanomagnetite. Further fractionation of clinopyroxene + amphibole + titanomagnetite from phonotephrite produced tephriphonolite magmas, whose higher volatile budget (H2O ≥ 4.5 wt.%) and lower temperatures (900-1000°C) caused abundant crystallization of alkali feldspars ± biotite ± amphibole in the shallow crust, resulting in the formation of highly crystalline regions (≥60 wt.% crystals) with alkali enriched interstitial melts of phonolitic composition. The variety of alkaline rock compositions and coexisting crystalline populations are not only related to fractionation along differentiation trends, but are strongly influenced by a range of complex processes as magma transfers across diverse storage regions. Different magmas mixed and entrained crystalline material segregated from previous magma batches. The multidisciplinary approach used in this thesis allowed the reconstruction of an integrated source-to-surface history of Dunedin Volcano magmas, with the aim of contributing to our understanding of the processes that control the origin and differentiation of intraplate alkaline systems on a global scale

The effect of melt water content and isothermal annealing time on the formation and evolution of clinopyroxene-titanomagnetite clusters

Crystal clustering influences the formation of crystal mushes and the rheology and differentiation of magmas. Heterogeneous nucleation is known to be an important cluster-forming mechanism, but there has been little systematic experimental study of cluster formation and evolution.In this study, we analysed dynamic crystallization experiments from Pontesilli et al. (2019), focusing on clusters of clinopyroxene (cpx) and titanomagnetite (tmt). These experiments aimed to reproduce the crystallisation behaviour of dry (nominally 0 wt.% H2O) and hydrous (2 wt.% H2O added) Etnean trachybasalt at mid-crustal storage conditions (400 MPa, 1100°C, NNO+1 oxygen buffer, corresponding to undercooling of 120°C and 80°C respectively). After superheating at 1300°C for 30 minutes, samples were cooled at 80°C/min to 1100°C and annealed for dwell times ranging from 0.5h to 8h.Electron backscatter diffraction (EBSD) maps and image analysis were used to quantify clustering parameters such as tmt number density, "shared perimeter fraction" ("SPF", the fraction of total tmt boundary length shared between cpx and tmt), "fraction of touching tmt" ("FTT", the fraction of all tmt grains that are touching cpx), and the crystallographic orientation relationships (CORs) between cpx and tmt. Dry samples generally show a higher number density of tmt crystals than wet samples. SPF and FTT are highest (≥ 0.40 and ≥ 0.93 respectively) in the 0.5h duration dry experiments. Both parameters fall to ≤ 0.25 and ≤ 0.75 respectively after 4h of annealing. In wet experiments, SPF and FFT are lower (≤ 0.33 and ≤ 0.79 respectively) at 0.5h annealing time and do not decrease strongly with annealing.EBSD maps reveal that > 70 % of tmt grains are in contact with cpx in all analysed samples. Tmt exhibits two closely related CORs to cpx. More than 60% of total tmt-cpx boundary length in all samples follows COR 1 ([-110]tmt[010]cpx, [111]tmt(100)*cpx, [-1-12]tmt[001]cpx) or COR 2 ([-110]tmt[010]cpx, [-1-11]tmt(-101)*cpx, [112]tmt[101]cpx). COR frequencies suggest a strong influence of water content and annealing time on their formation. In the 0.5h duration dry experiment, tmt-cpx boundaries following COR 1 are twice as frequent by length as those following COR 2, whereas in the 0.5h duration wet experiment, COR 2 boundaries are 5 times more frequent by length than COR 1 boundaries. In both wet and dry experiments the length ratio of COR 1 : COR 2 boundaries approaches 1 with longer annealing times.The degree of undercooling (as imposed by the different water contents) is the most important influence on the microstructural clustering parameters, leading to lower overall number densities of tmt as well as affecting the SPF and FTT values at short durations and the subsequent evolution of these parameters with increasing annealing time. The high frequency of tmt-cpx CORs is consistent with heterogeneous nucleation. However, the mechanisms controlling which CORs develop are unclear. Annealing does not fully erase CORs or microstructural signatures of clustering, suggesting that crystal clusters erupted in volcanic products could still preserve signs of their formation

The origin of clinopyroxene-titanomagnetite clustering during crystallisation of synthetic trachybasalt

Crystal clustering impacts rheology and differentiation in magmatic systems, and also offers insights into nucleation processes. Electron backscatter diffraction (EBSD) is ideal for studying interactions between crystals at interfaces. Clinopyroxene (Cpx) – titanomagnetite (Timt) clusters formed in time series experiments on synthetic trachybasaltic melt were studied using EBSD to understand the cause of clustering. Experiments were performed at 400 MPa and the NNO +2 buffer, at both anhydrous and hydrous (2 wt.% H2O) conditions, by cooling from 1300 °C (superliquidus) to 1100 °C with a rate of 80°C/min and holding at the target temperature for 4 – 8 hours before isobaric quenching. All experiments crystallize dendritic Cpx (Lmax = 50 – 60 µm) and isometric euhedral to hopper-shaped Timt (Lmax = 5 – 6 µm). Infrequent (~ 10 mm-2) unmelted Cr-oxide crystals are surrounded by polycrystalline Cr-bearing Timt rims (Lmax Cr-oxide + rim = 20 µm). Cpx dendrite “rosettes” radiate from the polycrystalline rims, but many dendrites do not belong to rosettes, at least in 2D. Individual Timt crystals (Cr-free) are strongly associated with the sides and tips of Cpx dendrites. About 75% of Timt grains are in contact with Cpx in 2D. Cpx-Timt interfaces are irregular, and Timt is often attached only by thin necks. Timt grain centers are weakly clustered (R = 0.87 – 0.95, 1 = random). Timt shows a strong crystallographic orientation relationship (COR) with Cpx, with 75 – 89% of Timt grains in contact with Cpx lying within 6° of a single fixed (“specific”) COR, OR1 = Cpx [010] // Timt <110>; Cpx (100) // Timt <111>; Cpx [001] // Timt <112>. The axes Cpx [010] // Timt <110> show the least dispersion (< 3°) from the ideal alignment. Relative to Cpx, individual Timt may be rotated up to 6° away from OR1, around an axis close to Cpx [010]. There are two peaks in this continuous distribution, corresponding to OR1 (above) and OR2 = Cpx [010] // Timt <110>; Cpx (-101) // Timt <111>; Cpx [101] // Timt <112>. The misorientation between OR1 and OR2 is 5.3°. OR1 and OR2 together represent 68 – 77% of Timt grains in contact with Cpx (tolerance angle 2.6°). Cpx dendrite branches bend around Cpx [010]. The anhydrous sample with dwell time 4 hours shows continuous bending of up to ~15°, whereas the hydrous sample with dwell time 8 hours shows bending of up to only ~7° and subgrain boundaries (1 - 2°) separating undistorted domains, suggesting recovery of bent crystals during annealing. Initial Cpx nucleation likely occurred heterogeneously as rosettes on Cr-bearing Timt rims around Cr-oxide crystals. Multiple Timt grains touching different branches of the same bent Cpx crystal all maintain a close COR with the Cpx orientation immediately adjacent to the Cpx-Timt interface, indicating that Timt nucleated on (or attached to) dendrite branches during or after their growth. In conclusion, EBSD is a powerful method for understanding crystallization and cluster formation. Future work will study the effect of annealing time, water content, and undercooling on Cpx – Timt cluster development

Alpine subduction zone metamorphism in the Paleozoic successions of the Monti Romani (Northern Apennines, Italy)

The hinterland of the Cenozoic Northern Apennines fold-and-thrust belt exposes the metamorphic roots of the chain, vestiges of the subduction-related tectono-metamorphic evolution that led to the buildup of the Alpine orogeny in the Mediterranean region. Like in other peri-Mediterranean belts, the tectono-metamorphic evolution of the Palaeozoic continental basement in the Apennines is still poorly constrained, hampering the full understanding of their Alpine orogenic evolution. We report the first comprehensive tectono-metamorphic study of the low-grade metasedimentary (metapsammite/metapelite) succession of the Monti Romani Complex (MRC) that formed after Palaeozoic protoliths and constitutes the southernmost exposure of the metamorphic domain of the Northern Apennines. By integrating fieldwork with microstructural studies, Raman spectroscopy on carbonaceous material and thermodynamic modelling, we show that the MRC preserves a D1/M1 Alpine tectono-metamorphic evolution developed under HP–LT conditions (~1.0–1.1 GPa at T ~ 400°C) during a non-coaxial, top-to-the-NE, crustal shortening regime. Evidence for HP–LT metamorphism is generally cryptic within the MRC, dominated by graphite-bearing assemblages with the infrequent blastesis of muscovite ± chlorite ± chloritoid ± paragonite parageneses, equilibrated under cold palaeo-geothermal conditions (~10°C/km). Results of this study allow extending to the MRC the signature of subduction zone metamorphism already documented in the hinterland of the Apennine orogen, providing further evidence of the syn-orogenic ductile exhumation of the HP units in the Apennine belt. Finally, we discuss the possible role of fluid-mediated changes in the reactive bulk rock composition on mineral blastesis during progress of regional deformation and metamorphism at low-grade conditions

Anatomy of intraplate monogenetic alkaline basaltic magmatism. Clues from magma, crystals, and glass

Intraplate basaltic systems, often occurring as fields of small monogenetic volcanoes, are dominated by eruption of alkaline basaltic rocks, ranging from nephelinite/basanite to transitional/subalkaline. Their primitive erupted compositions imply limited crustal modification, thus providing an important probe into deep, lithospheric mantle processes. The whole-rock chemical variability within single eruptions is controlled by the characteristics of the primary melting source, as well as near-source percolative/reaction processes. Complex crystal textures and compositions have so far demonstrated that basaltic magmas are principally processed and modified within the lithospheric mantle with minor modification en route through the crust. Fractional crystallization and magma mixing modify melts throughout ascent, and can imprint secondary chemical intra-eruptive variability. Quantifiable temperature and pressure parameters constrain the depth of formation, and hence provide information about the role of different mineral phases in deep versus shallow chemical evolution. Volatile components in the melt can be quantified on glass and melt inclusions. These analyses may help to reconstruct initial dissolved volatile content to further constrain the source characteristics and magmatic ascent dynamics. Integrated studies of crystals and melt paint a picture of extended lithospheric mantle to minor crustal processing resulting from the complex deep plumbing of monogenetic basaltic systems. This highlights the need for improved resolution to characterize true primary signatures and hence elucidate the formation of intraplate alkaline basalt

Tracking Thermal Pathways of Magma Ascent and Eruption Using Trace Element Partitioning in Sector Zoned Clinopyroxene

Igneous minerals provide valuable records of magmatic processes in active volcanoes, witnessing the conditions which lead up to eruptions. The solidification pathway of a magma plays a key role in eruption style and thus potential hazards, however discerning ascent processes from mineral records is not always straightforward. Different solidification pathways in a plumbing system will experience different cooling and degassing rates, which result in different degrees of magma undercooling (ΔT = Tliquidus – Tcrystallisation) and crystallisation conditions. Understanding how changing ΔT is reflected by mineral chemistry and zoning is integral in our interpretation of magmatic histories. Sector zoning in clinopyroxene, characterised by the presence of distinct crystal domains (low Al-Ti hourglass sectors and high Al-Ti prism sectors) which grow simultaneously, yet differ in chemistry, is strongly related to ΔT. Investigating these zonations in active volcano settings provides an opportunity to explore pre-eruptive processes associated with ΔT changes for a range of eruption styles[1]. We apply a new REE+Y ΔT calibration, developed specifically for sector zoned clinopyroxenes using experimentally-derived trace element data, to a range of augitic clinopyroxene megacrysts (>5mm), phenocrysts (0.5 – 5mm) and microcrysts (<0.5mm) from central conduit and eccentric eruptions at Mt Etna, Italy, to investigate differences in pre-eruptive processes and solidification pathways in the volcano plumbing system. We also apply the same calibration to clinopyroxene megacrysts erupted at Stromboli, Italy, for comparison. Results show that hourglass sectors typically record low ΔT conditions (<45°C), which aligns with previous experimental results based on major element partitioning [2]. Prism sectors, on the other hand, return higher ΔT results, suggesting that these domains may be unreliable recorders of ΔT due to enhanced uptake of highly charged cations. Additionally, clinopyroxene microcrysts return higher ΔT than phenocrysts from the same eruption, highlighting the ability of clinopyroxene to record a range of magma crystallisation regimes. Our data will be used to explore differences in polythermal and polybaric processes between eruption styles, and aim to better constrain magma ascent pathways prior to eruption