Plagioclase archives of depleted melts in the oceanic crust

Mid-ocean ridge and ocean island basalts provide vital but incomplete insights into the chemical structure of Earth’s mantle. For example, high-anorthite plagioclase carried by these basalts is generally too primitive and incompatible-element depleted to have crystallized from them. Moreover, erupted basalts rarely preserve the strong isotopic and incompatible-element depletions found in some melt inclusions and mantle residua represented by abyssal peridotites. By integrating experimental observations with published analyses of natural crystals and glasses, we demonstrate that high-anorthite plagioclase is in equilibrium with melts generated by high-degree melting of depleted mantle sources. Although such melts seldom erupt, their imprints on crystal and melt inclusion records nonetheless suggest that high-anorthite plagioclase grows from endmember but essentially unexotic magmas. The widespread occurrence of high-anorthite plagioclase in both oceanic basalts and the oceanic crust hence indicates that depleted melts are pervasive in the upper mantle and lower crust despite rarely reaching the surface. Plagioclase archives therefore imply that depleted melts play much a greater role in lower crustal accretion than typically recognized and that the upper mantle may also be more depleted than previously thought.

Post-Oruanui supereruption recovery, reconstruction and evolution of Taupo volcano, New Zealand

<p>This thesis research presents geochemical perspectives on the magmatic recovery of Taupo volcano (New Zealand) in the aftermath of the 25.4 ka Oruanui supereruption. Following the Oruanui, and after only ~5 kyr of quiescence, Taupo erupted three small volume (~0.1 km3) dacitic units, followed by another ~5 kyr break, and then the modern sequence from ~12 ka onwards of 25 rhyolitic units organised into 3 geochemically distinct subgroups (SG1-SG3). The eruptive units are stratigraphically constrained over exceptionally short time intervals, providing fine-scale temporal snapshots of the magma system. In this thesis I compare and contrast whole-rock, mineral and glass compositions of Oruanui and post-Oruanui magmas through time to investigate the post-supereruption reconstruction and evolution of Taupo through to the latest eruption. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U/Th disequilibrium model-ages in zircons from Taupo SG1 rhyolites (erupted 12 ka-10 ka) and SG2 rhyolites (erupted 7 ka-2.6 ka) imply the presence of only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains. U-Pb dating of these equiline grains shows that both 300-450 ka plutonic-derived and pre-100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units which were vented within the published Whakamaru caldera outline. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolites have notably high concentrations of U, Th, P, Y+ (REE)3+ and Nb but with only minor changes in Hf and Ti. SG2 zircons typically have higher Sc, reflecting large-scale changes in melt chemistry and crystallising mineral phases with time. The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U-Th ages from individual post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallisation, acting within a crustal protolith chemically independent of that which built the Oruanui. Cooling periods alternate with times of rejuvenation and eruption, in some cases demonstrably accompanying syn-eruptive regional rifting and mafic injection. Not only were the processes that developed the supersized Oruanui magma body unusually rapid, but this huge magma system was effectively reset and rebuilt on a comparably short timescale. Major and trace element whole rock, glass and mineral chemistry of post-Oruanui eruptive products indicate how the host magma system re-established and evolved. The dacite units show wide variations in melt inclusion compositions and strongly zoned minerals consistent with interaction of less-evolved mafic magmas at a depths of >8 km, overlapping with the inferred base of the old Oruanui mush system. The dacites reflect the first products of the rebuilding silicic magma system, as most of the Oruanui mush was reconfigured or significantly modified in composition following thermal fluxing accompanying post-caldera collapse readjustment. The first (SG1) rhyolites erupted from 12 ka formed through shallow fractionation (4-5 km depth) and cooling of a parental melt similar in composition to the earlier dacite melts, with overlapping melt inclusion and crystal core compositions between the two magma types. For the younger rhyolite units, fine-scale temporal changes in melt chemistry and mineral phase stability occur over time, which are closely linked to the development, stabilisation and maturation of a new and likely unitary rhyolite mush system at Taupo. The new mush system is closely linked to and sometimes physically interacts with the underlying mafic melts, which are similar in composition to those involved in the Oruanui eruption and provide the long-term thermal and chemical driving force for magmatism. We consider that the new mush body has expanded to >250 km3 (and possibly up to 1000 km3) but has not yet been located by geophysical investigations. For the most recent SG3 eruptions, the system once again underwent widespread destabilisation, resulting in increased levels of melt extraction from the silicic mush. Trends in whole-rock chemistry and close links between melt inclusions and mineral zoning with earlier units indicates that the 35 km3 Unit Y (Taupo eruption) melt dominant body formed in response to mafic disruption of the silicic mush pile. Associated Fe-Mg diffusion timescales in orthopyroxene suggest that Taupo is capable of changing behaviour and generating large eruptible melt bodies on timescales as short as decades to centuries. The 232 AD Unit Y eruption culminated from a critical combination of high differential tectonic stress build up, and increased potency in the silicic magma system resulting from elevated levels of mafic magma input, resulting in one of the largest and most violent worldwide Holocene eruptions. The post-Y magma system then responded to further disruption with the eruption of sub-lacustrine dome(s). Taupo is considered to be capable of rapidly recovering in its modern form to continue its hyperactive eruptive behaviour on timescales that are of human interest and concern.</p>

Post-Oruanui supereruption recovery, reconstruction and evolution of Taupo volcano, New Zealand

<p>This thesis research presents geochemical perspectives on the magmatic recovery of Taupo volcano (New Zealand) in the aftermath of the 25.4 ka Oruanui supereruption. Following the Oruanui, and after only ~5 kyr of quiescence, Taupo erupted three small volume (~0.1 km3) dacitic units, followed by another ~5 kyr break, and then the modern sequence from ~12 ka onwards of 25 rhyolitic units organised into 3 geochemically distinct subgroups (SG1-SG3). The eruptive units are stratigraphically constrained over exceptionally short time intervals, providing fine-scale temporal snapshots of the magma system. In this thesis I compare and contrast whole-rock, mineral and glass compositions of Oruanui and post-Oruanui magmas through time to investigate the post-supereruption reconstruction and evolution of Taupo through to the latest eruption. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U/Th disequilibrium model-ages in zircons from Taupo SG1 rhyolites (erupted 12 ka-10 ka) and SG2 rhyolites (erupted 7 ka-2.6 ka) imply the presence of only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains. U-Pb dating of these equiline grains shows that both 300-450 ka plutonic-derived and pre-100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units which were vented within the published Whakamaru caldera outline. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolites have notably high concentrations of U, Th, P, Y+ (REE)3+ and Nb but with only minor changes in Hf and Ti. SG2 zircons typically have higher Sc, reflecting large-scale changes in melt chemistry and crystallising mineral phases with time. The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U-Th ages from individual post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallisation, acting within a crustal protolith chemically independent of that which built the Oruanui. Cooling periods alternate with times of rejuvenation and eruption, in some cases demonstrably accompanying syn-eruptive regional rifting and mafic injection. Not only were the processes that developed the supersized Oruanui magma body unusually rapid, but this huge magma system was effectively reset and rebuilt on a comparably short timescale. Major and trace element whole rock, glass and mineral chemistry of post-Oruanui eruptive products indicate how the host magma system re-established and evolved. The dacite units show wide variations in melt inclusion compositions and strongly zoned minerals consistent with interaction of less-evolved mafic magmas at a depths of >8 km, overlapping with the inferred base of the old Oruanui mush system. The dacites reflect the first products of the rebuilding silicic magma system, as most of the Oruanui mush was reconfigured or significantly modified in composition following thermal fluxing accompanying post-caldera collapse readjustment. The first (SG1) rhyolites erupted from 12 ka formed through shallow fractionation (4-5 km depth) and cooling of a parental melt similar in composition to the earlier dacite melts, with overlapping melt inclusion and crystal core compositions between the two magma types. For the younger rhyolite units, fine-scale temporal changes in melt chemistry and mineral phase stability occur over time, which are closely linked to the development, stabilisation and maturation of a new and likely unitary rhyolite mush system at Taupo. The new mush system is closely linked to and sometimes physically interacts with the underlying mafic melts, which are similar in composition to those involved in the Oruanui eruption and provide the long-term thermal and chemical driving force for magmatism. We consider that the new mush body has expanded to >250 km3 (and possibly up to 1000 km3) but has not yet been located by geophysical investigations. For the most recent SG3 eruptions, the system once again underwent widespread destabilisation, resulting in increased levels of melt extraction from the silicic mush. Trends in whole-rock chemistry and close links between melt inclusions and mineral zoning with earlier units indicates that the 35 km3 Unit Y (Taupo eruption) melt dominant body formed in response to mafic disruption of the silicic mush pile. Associated Fe-Mg diffusion timescales in orthopyroxene suggest that Taupo is capable of changing behaviour and generating large eruptible melt bodies on timescales as short as decades to centuries. The 232 AD Unit Y eruption culminated from a critical combination of high differential tectonic stress build up, and increased potency in the silicic magma system resulting from elevated levels of mafic magma input, resulting in one of the largest and most violent worldwide Holocene eruptions. The post-Y magma system then responded to further disruption with the eruption of sub-lacustrine dome(s). Taupo is considered to be capable of rapidly recovering in its modern form to continue its hyperactive eruptive behaviour on timescales that are of human interest and concern.</p>

Constraints on the behaviour and content of volatiles in Galápagos magmas from melt inclusions and nominally anhydrous minerals

Despite their relatively low concentration in most oceanic basalts, volatile species (e.g. H2O, CO2 and S) have a disproportionately large influence on a wide range of mantle and magmatic processes. However, constraining the concentration of H2O (and other volatiles) in basaltic magmas is not straightforward as submarine glass analyses are influenced by assimilation of hydrothermal brines, and the melt inclusion record is often reset by post-entrapment processes. Nevertheless, in this study we show that it is possible to reconstruct a detailed history of the volatile content of basaltic magmas through integration of multiple discreet volatile records and careful consideration of secondary processes. We present new analyses of volatiles in olivine-hosted melt inclusions, melt embayments and nominally anhydrous minerals (NAMS, clinopyroxene and orthopyroxene) found in basalts erupted on Floreana Island in the south-eastern Galápagos Archipelago. Our results indicate that the Floreana magmas, which are characterised by the most radiogenic Pb and Sr isotope signatures in the Galápagos Archipelago, contain H2O concentrations between 0.4 and 0.8 wt% (at a melt Mg# of 0.65, where Mg# = Mg/(Mg + Fe) molar). These are marginally greater than the H2O contents of magmas beneath Fernandina in the western Galápagos Archipelago (cf. 0.2–0.7 wt% H2O at Mg# = 0.65). While the volatile content of magmas from the western archipelago follow trends defined by concurrent mixing and crystallisation, NAMs from Floreana reveal the presence of rare, volatile-rich magmas (~2 wt% H2O) that form as a consequence of reactive porous flow in mush-dominated magmatic systems beneath the south-eastern Galápagos. Furthermore, the Floreana magmas have similar H2O/light Rare Earth Element ratios to basalts from the western Galápagos but contain F/Nd and Cl/K ratios that are ~2 – 3 times greater, indicating that the mantle source of the Floreana lavas might represent an important halogen reservoir in the Galápagos mantle plume.

Boron isotope evidence for devolatilized and rehydrated recycled materials in the Icelandic mantle source

Enriched mantle heterogeneities are widely considered to be generated through subduction, but the connections between specific subducted materials and the chemical signatures of mantle heterogeneities are not clearly defined. Boron is strongly isotopically fractionated at the surface and traces slab devolatilization, making it a potent tracer of previously subducted and recycled materials. Here, we present high-precision SIMS boron concentrations and isotope ratios on a comprehensive suite of quenched basaltic glasses from all neovolcanic zones in Iceland, two rhyolite glasses, and a set of primitive melt inclusions from central Iceland. Boron isotope ratios (δ11B) in Icelandic basalts and melt inclusions range from -11.6‰ to -1.0‰, averaging -4.9‰, which is higher than mid-ocean ridge basalt (MORB; δ11B = -7.1‰). Because the δ11B value of the Icelandic crust is low, the high δ11B compositions of the Icelandic lavas are not easily explained through crustal assimilation processes. Icelandic basalt glass and melt inclusion B/Ce and δ11B values correlate with trace element ratio indicators of the degree of mantle partial melting and mantle heterogeneity (e.g. Nb/Zr, La/Yb, Sm/Yb), which indicate that the boron systematics of basalts are controlled by mantle heterogeneity. Additionally, basalts with low B/Ce have high 206Pb/204Pb, further indicating mantle source control. These correlations can be used to deduce the boron systematics of the individual Icelandic mantle components. The enriched endmember within the Iceland mantle source has a high δ11B value and low B/Ce, consistent with the composition of “rehydrated” recycled oceanic crust. The depleted endmember comprises multiple distinct components with variable B/Ce, likely consisting of depleted MORB mantle and/or high 3He/4He mantle and two more minor depleted components that are consistent with recycled metasomatized mantle wedge and recycled slab gabbro.The compositions of these components place constraints on the devolatilization history of recycled oceanic crust. The high δ11B value and low B/Ce composition of the enriched component within the Iceland mantle source is inconsistent with a simple devolatilization process and suggests that the recycled oceanic crust component may have been isotopically overprinted by B-rich fluids derived from the underlying hydrated slab lithospheric mantle (i.e. “rehydration”). Further, the B/Ce and δ11B systematics of other OIBs can be used to constrain the devolatilization histories of recycled components on a global scale. Globally, most OIB B/Ce compositions suggest that recycled components have lost >99% of their boron, and their δ11B values suggest that rehydration may be a sporadic process, and not ubiquitous.

Periodically Released Magmatic Fluids Create a Texture of Unidirectional Solidification (UST) in Ore-Forming Granite: A Fluid and Melt Inclusion Study of W-Mo Forming Sannae-Eonyang Granite, Korea

The Upper Cretaceous Sannae-Eonyang granite crystallized approximately 73 Ma and hosted the Sannae W-Mo deposit in the west and the Eonyang amethyst deposit in the east. The granite contained textural zones of miarolitic cavities and unidirectional solidification texture (UST) quartz. The UST rock sampled in the Eonyang amethyst mine consisted of (1) early cavity-bearing aplitic granite, (2) co-crystallization of feldspars and quartz in a granophyric granite, and (3) the latest unidirectional growth of larger quartz crystals with clear zonation patterns. After the UST quartz was deposited, aplite or porphyritic granite was formed, repeating the prior sequence. Fluid and melt inclusions occurring in the UST quartz and quartz phenocrysts were sampled and studied to understand the magmatic-hydrothermal processes controlling UST formation and W-Mo mineralization in the granite. The composition of melt inclusions in the quartz phenocrysts suggested that the UST was formed by fractionated late-stage granite. Some of the melt inclusions occurring in the early-stage UST quartz were associated with aqueous inclusions, indicating fluid exsolution from a granitic melt. Hypersaline brine inclusions allowed the calculation of the minimum trapping pressure of 80–2300 bars. Such a highly fluctuating fluid pressure might be potentially due to a lithostatic-hydrostatic transition of pressure-attending fluid loss during UST formation. Highly fluctuating lithostatic-hydrostatic pressures created by fluid exsolution allowed shifting of the stability field from a quartz-feldspar cotectic to a single-phase quartz. The compositions of brine fluid assemblages hosted in the quartz phenocrysts deviated from the fluids trapped in the UST quartz, especially regarding the Rb/Sr and Fe/Mn ratios and W and Mo concentrations. The study of melt and fluid inclusions in the Eonyang UST sample showed that the exsolution of magmatic fluid was highly periodic. A single pulse of magmatic fluids of variable salinities/densities might have created a single UST sequence, and a new batch of magmatic fluid exsolution would be required to create the next UST sequence.

VESIcal Part II: A critical approach to volatile solubility modelling using an open-source Python3 engine

Accurate models of H2O and CO2 solubility in silicate melts are vital for understanding volcanic plumbing systems. These models are used to estimate the depths of magma storage regions from melt inclusion volatile contents, investigate the role of volatile exsolution as a driver of volcanic eruptions, and track the degassing paths followed by magma ascending to the surface. However, despite the large increase in the number of experimental constraints over the last two decades, many recent studies still utilize the earlier generation of models, which were calibrated on experimental datasets with restricted compositional ranges. This may be because many of the available tools for more recent models require large numbers of input parameters to be hand-typed (e.g., temperature, concentrations of H2O, CO2, and 8--14 oxides), making them difficult to implement on large datasets. Here, we use a new open-source Python3 tool, VESIcal, to critically evaluate the behaviours and sensitivities of different solubility models for a range of melt compositions. Using literature datasets of andesitic-dacitic experimental products and melt inclusions as case studies, we illustrate the importance of evaluating the calibration dataset of each model. Finally, we highlight the limitations of particular data presentation methods such as isobar diagrams, and provide suggestions for alternatives, and best practices regarding the presentation and archiving of data. This review will aid the selection of the most applicable solubility model for different melt compositions, and identifies areas where additional experimental constraints are required