An investigation of crustal contamination through petrology and geochemisty

The contamination of mantle-derived magmas by the continental crust is an important process during petrogenesis of volcanic rocks at active continental margins e.g. The Andes. Investigating the evolution of continental arc magmas is, however, hampered by our limited knowledge of, and poor constraints on, the nature of the underlying crustal basement and the mechanisms of crustal anatexis. This thesis reports results from: 1) a whole rock geochemical and in-situ geochronological investigation of a suite of crustal xenoliths from the Bolvian Altiplano, Central Andes; 2) a whole rock geochemical study of the xenoiths’ host lavas and; 3) detailed in-situ geochemical studies of crustal partial melts (quenched to glasses) trapped within their crustal progenitors from Bolivia, NE China and SE Spain. Sampled crustal xenoliths from the Bolivan Altiplano provide a rare insight into the nature of the Central Andean continental basement and reveal lithological and geochemical heterogeneity exists at depth with 87 Sr/86 Sr values extending to 0.7368 which is more radiogenic than any Srisotopic signature exhibited by the recent (< 60 Ma) volcanic record. In-situ U-Pb dating of zircon separates reveal predominant age peaks at 1.7-1.9 Ga, 1.0-1.2 Ga and 495-380 Ma which correspond to periods of supercontinent formation and break-up e.g. construction of Rodinia. Lavas erupted from monogenetic centres on the eastern Bolivian Altiplano show petrographic and geochemical evidence for crustal contamination. The geochemical heterogeneity exhibited by the lavas is, however, difficult to reconcile through simple two component crust-magma interaction models (bulk mixing, AFC and EC-AFC). Instead, contamination is inferred to have involved numerous crustal components. The geochemical signatures observed in lavas from monogenetic centres towards the active Andean arc (between ~18-21o S) are distinct (e.g. lower 87 Sr/86 Sr, higher Sr/Y, higher Ba/Nb at higher Zr/Nb) and may indicate a lower degree of crust-magma interaction, an increase in the contribution from slab-derived fluids and thinner crust arc-wards, the latter which has previously been inferred from geophysical studies. In-situ analysis of anatectic melts reveals that Sr-isotopic disequilibrium between a crustal melt and its source can exist on the sub-millimetre scale. This is understood to reflect the melting of aged minerals with different Rb/Sr (and therefore 87 Sr/86 Sr) more quickly than the isotopic composition can diffusively equilibrate between melt and minerals. Results suggest therefore that crustal anatexis can produce melts which are geochemically heterogeneous both spatially and temporally. This highlights the need for detailed microscopic investigations coupled with petrogenetic modelling in order to develop a more robust characterisation and well-constrained quantification of crustal contamination in open magmatic systems

Critical Eruptive Controls of an Intra-plate Volcano: Ascension Island, South Atlantic

Understanding what drives transitions in eruptive style is an important challenge in volcanology. Existing descriptions of small-volume trachytic eruptions often record transitions in eruptive behaviour, including and outwith explosive-effusive transitions, but detailed ascent and eruption dynamics reconstructions are rare. Historically, poor deposit preservation and exposure and unconstrained trachytic melt physical properties have contributed to a dearth of relevant literature. Here, I first make use of recent advances in understanding trachytic melt properties to reconstruct ascent and eruption dynamics of a particularly well-preserved and -exposed small-volume eruption on Ascension Island, South Atlantic - the Echo Canyon eruption (EC). Forensic stratigraphy, petrographic analysis, reconstruction of bulk magma properties and quantitative textural analysis reveal: the EC eruption underwent several transitions in eruptive behaviour, driven by rapid ascent, decompression and vesiculation. Further, peak explosion intensity equivalent to volcanic explosivity index (VEI) 6 eruptions was transiently achieved before transition to effusive activity by rapid evolution of permeable vesicle networks in conduit margin shear zones. Next, I use 2D and 3D textural analyses of a stratigraphically and compositionally well-constrained basalt-rhyolite ‘Mingled Fall’ deposit to assess how mingling impacts vesiculation during ascent. I find vesiculation in mingled clasts’ basaltic and rhyolitic regions progressed independently with little–no shear. In the basaltic regions, connectivity development was only slightly inhibited relative to scoriaceous clasts. I show 2D and 3D studies in texturally complex samples return different vesiculation histories. The 3D vesicle size distribution studies are more reliable for complex clasts, whereas 2D shape descriptors are better constrained and more useful for inter-eruption comparisons. Finally, I demonstrate how frameworks developed throughout offer insights into the evolution of the volumetrically comparable La Soufrière St Vincent, 2020-21 eruption. This thesis makes a useful contribution to understanding ascent and eruption dynamics of small-volume, but potentially high-impact events, common to isolated ocean island settings

Exposed ocean crust on Masirah Island, SE Oman: Crustal accretion and melt evolution at an ancient slow-spreading mid-ocean ridge

The accretion of oceanic crust at mid-oceanic ridges (MOR) accounts for the most voluminous magmatism on Earth and occurs across a range of spreading-rates. Slow-spreading ridges represent 50% of the present-day global MOR-system and produce heterogeneous oceanic crust that deviates fundamentally from the conventional 6 – 7 km layer-cake Penrose-crust formed at fast-spreading ridges. Instead, the sparse and intermittent magmatism at slow-spreading ridges requires plate separation to be partly accommodated by faulting, producing ocean lithosphere with a discontinuous igneous crust of variable thickness, commonly disrupted by large 'detachment' faults that exhume the deep crust and shallow mantle directly onto the seafloor. Due to the difficulty in accessing oceanic crust in situ, our knowledge of the processes occurring at MORs is still relatively limited and geologists often turn to ophiolites as their analogues. There is however an increasing awareness of a preservation bias towards ocean lithosphere formed at atypical spreading ridges, such as marginal ocean basins near subduction zones, and that potentially key differences between many ophiolites and ‘true’ MOR ocean lithosphere (especially in their chemical composition) complicate direct comparisons. The Masirah Ophiolite, exposed over ~650 km2 on an isolated island off the southeast coast of the Sultanate of Oman, is near-unique in that it is believed to have formed at a ‘true’ MOR, unaffected by the influence of subduction. It can therefore provide valuable geological insights into crustal accretion process along modern MORs. Although its basic structure was mapped the 1990s, previous studies did not examine Masirah from the perspective of modern MOR processes in any detail. Previous work determined that Masirah formed at a slow-spreading ridge in the young Indian Ocean at ~150 Ma, followed by an episode of alkaline magmatism during intraplate rifting some 20 Ma later, before being emplaced onto the Arabian continental margin in the late Cretaceous. The work presented in this thesis updates the geochronological model for the evolution of Masirah, describes a previously unrecognised type of ocean lithospheric architecture (‘Penrose on a diet’) that accreted at the paleo-spreading ridge, and investigates what mantle processes might be responsible for this style of accretion. In a key finding, new radiometric dates show that the ophiolite formed at 135 – 130 Ma, and that the alkaline magmatic activity overlapped with crustal accretion as an episode of ‘near-axis’ magmatism. Trace element compositions of the two magmatic suites show a large degree of overlap and define a continuum, supporting a transitional event where magmas, derived by variable degrees of melting of a heterogeneous mantle source, were initially delivered to the ridge axis, whereupon subsequent melts, derived from continued lower-degree melting of a more enriched mantle component, were delivered near off-axis. The ‘Penrose on a diet architecture’ of the Masirah lithosphere is noteworthy for having a thin igneous crust (2 km) and a thin lower crust with respect to the upper crust (lower crust : upper crust = ~0.4 : 0.6). Despite these characteristics suggesting a low melt supply, field relations nevertheless indicate the crust formed by magmatic spreading. This contradicts current crustal accretion models, which predict the formation of 6 – 7 km thick crust for conditions of high melt supply and spreading dominated by detachment faulting when melt supply is low. The Masirah Ophiolite shows that the variability of permissible slow-spreading crustal geometries is greater than previously thought and that a basaltic seafloor should not automatically be interpreted to correlate with a thick magmatic crust. The mantle section exposed on Masirah is characterised by highly refractory peridotites with a mineral chemistry indicating high degrees of melting, despite the overlying igneous crust being thin. By considering the effect of mantle fertility on the overall melt production, a model is advanced where upwelling of an ancient domain of depleted mantle led to an overall reduced melt supply and a high proportion of melts from fertile mantle components relative to those from the depleted peridotites. Consequently, observations from Masirah suggest that besides rates of upwelling and seafloor spreading, the time-integrated melting history of the mantle underneath modern MORs exercises an important control on the degree of mantle melting, as well as the resulting lithospheric architecture and compositions of the erupted mid-ocean ridge basalts

EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERE DURING MESOZOIC TETHYAN RIFTING: CONSTRAINTS FROM THE EXTERNAL LIGURIAN MANTLE SECTION (NORTHERN APENNINE, ITALY)

Our study is focussed on mantle bodies from the External Ligurian ophiolites, within the Monte Gavi and Monte Sant'Agostino areas. Here, two distinct pyroxenite-bearing mantle sections were recognized, mainly based on their plagioclase-facies evolution. The Monte Gavi mantle section is nearly undeformed and records reactive melt infiltration under plagioclase-facies conditions. This process involved both peridotites (clinopyroxene-poor lherzolites) and enclosed spinel pyroxenite layers, and occurred at 0.7–0.8 GPa. In the Monte Gavi peridotites and pyroxenites, the spinel-facies clinopyroxene was replaced by Ca-rich plagioclase and new orthopyroxene, typically associated with secondary clinopyroxene. The reactive melt migration caused increase of TiO2 contents in relict clinopyroxene and spinel, with the latter also recording a Cr2O3 increase. In the Monte Gavi peridotites and pyroxenites, geothermometers based on slowly diffusing elements (REE and Y) record high temperature conditions (1200-1250 °C) related to the melt infiltration event, followed by subsolidus cooling until ca. 900°C. The Monte Sant'Agostino mantle section is characterized by widespread ductile shearing with no evidence of melt infiltration. The deformation recorded by the Monte Sant'Agostino peridotites (clinopyroxene-rich lherzolites) occurred at 750–800 °C and 0.3–0.6 GPa, leading to protomylonitic to ultramylonitic textures with extreme grain size reduction (10–50 μm). Compared to the peridotites, the enclosed pyroxenite layers gave higher temperature-pressure estimates for the plagioclase-facies re-equilibration (870–930 °C and 0.8–0.9 GPa). We propose that the earlier plagioclase crystallization in the pyroxenites enhanced strain localization and formation of mylonite shear zones in the entire mantle section. We subdivide the subcontinental mantle section from the External Ligurian ophiolites into three distinct domains, developed in response to the rifting evolution that ultimately formed a Middle Jurassic ocean-continent transition: (1) a spinel tectonite domain, characterized by subsolidus static formation of plagioclase, i.e. the Suvero mantle section (Hidas et al., 2020), (2) a plagioclase mylonite domain experiencing melt-absent deformation and (3) a nearly undeformed domain that underwent reactive melt infiltration under plagioclase-facies conditions, exemplified by the the Monte Sant'Agostino and the Monte Gavi mantle sections, respectively. We relate mantle domains (1) and (2) to a rifting-driven uplift in the late Triassic accommodated by large-scale shear zones consisting of anhydrous plagioclase mylonites. Hidas K., Borghini G., Tommasi A., Zanetti A. &amp; Rampone E. 2021. Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy). Lithos 380-381, 105855