fractional crystallisation
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Polybaric fractional crystallisation of arc magmas: an experimental study simulating trans-crustal magmatic systems
AbstractCrystallisation-driven differentiation is one fundamental mechanism proposed to control the compositional evolution of magmas. In this experimental study, we simulated polybaric fractional crystallisation of mantle-derived arc magmas. Various pressure–temperature trajectories were explored to cover a range of potential magma ascent paths and to investigate the role of decompression on phase equilibria and liquid lines of descent (LLD). Fractional crystallisation was approached in a step-wise manner by repetitively synthesising new starting materials chemically corresponding to liquids formed in previous runs. Experiments were performed at temperatures ranging from 1140 to 870 °C with 30 °C steps, and pressure was varied between 0.8 and 0.2 GPa with 0.2 GPa steps. For most fractionation paths, oxygen fugacity (fO2) was buffered close to the Ni-NiO equilibrium (NNO). An additional fractionation series was conducted at fO2 corresponding to the Re-ReO2 buffer (RRO ≈ NNO+2). High-pressure experiments (0.4–0.8 GPa) were run in piston cylinder apparatus while 0.2 GPa runs were conducted in externally heated pressure vessels. Resulting liquid lines of descent follow calc-alkaline differentiation trends where the onset of pronounced silica enrichment coincides with the saturation of amphibole and/or Fe–Ti–oxide. Both pressure and fO2 exert crucial control on the stability fields of olivine, pyroxene, amphibole, plagioclase, and Fe–Ti–oxide phases and on the differentiation behaviour of arc magmas. Key observations are a shift of the olivine–clinopyroxene cotectic towards more clinopyroxene-rich liquid composition, an expansion of the plagioclase stability field and a decrease of amphibole stability with decreasing pressure. Decompression-dominated ascent trajectories result in liquid lines of descent approaching the metaluminous compositional range observed for typical arc volcanic rocks, while differentiation trends obtained for cooling-dominated trajectories evolve to peraluminous compositions, similar to isobaric liquid lines of descent at elevated pressures. Experiments buffered at RRO provide a closer match with natural calc-alkaline differentiation trends compared to fO2 conditions close to NNO. We conclude that decompression-dominated fractionation at oxidising conditions represents one possible scenario for arc magma differentiation.
<p>The Kermadec Arc-Havre Trough (KAHT) is widely regarded as a classical example of an intra-oceanic arc-back-arc system, where subduction-driven arc magmatism is focused at the Kermadec volcanic arc-front, and magmatism within the Havre Trough back-arc system results from decompression-related melting. In detail, however, the Havre Trough has not been well-studied, and data for very few lavas have been reported. Recent mapping undertaken in the southern Havre Trough has resulted in the discovery of several prominent submarine stratovolcanoes, Gill Seamount, Rapuhia Seamount and the related Rapuhia Ridge, Yokosuka Seamount, and Giljanes Seamount, situated in the middle of deep rifts and on elevated crustal plateaux. The origin and evolution of these stratovolcanoes is unknown. The first detailed dataset of whole rock major and trace element geochemistry, mineral chemistry, and ⁴⁰Ar/³⁹Ar isotope data, for lavas erupted from these volcanoes is presented here, and used to investigate the processes that drive volcanism in the Havre Trough back-arc. ⁴⁰Ar/³⁹Ar ages obtained from back-arc stratovolcanoes range from ca. 1167 - 953 ka for Gill Seamount, and ca. 107 - 50 ka for Rapuhia Ridge. These ages overlap with known ages for arc-front lavas, indicating that both back-arc and arc-front volcanism are coeval. These ages are all significantly younger than the inferred initation of Havre Trough rifting ca. 5 - 6 Ma. Lavas analysed from Gill Seamount and Rapuhia Ridge are basaltic to basaltic-andesitic in whole rock composition and contain a phenocryst assemblage of olivine ± orthopyroxene + clinopyroxene ± plagioclase. Lavas from Rapuhia Seamount, Yokosuka Seamount and Giljanes Seamount range from andesitic to dacitic in composition, and have a phenocryst assemblage consisting primarily of plagioclase ± clinopyroxene ± amphibole ± Fe-Ti oxides ± apatite. Variations in mineral assemblages and whole rock compositions of the lavas are consistent with crystal fractionation of their respective phenocryst phases. The more evolved compositions of Rapuhia Seamount, Yokosuka Seamount and Giljanes Seamount, all sited on an elevated crustal plateau, are inferred to result from prolonged assimilation + fractional crystallisation (AFC) in the mid- to upper- crust. Mineral compositions provide additional evidence for fractional crystallisation, and most crystals are inferred to have crystallised in equilibrium with their host melt. However, compositions of some olivine phenocrysts in Gill Seamount and Rapuhia Ridge indicate multiple populations of olivine, suggesting their magmatic systems were open to contributions from secondary processes. Variations in Or content in plagioclase crystals for a given lava suite suggests the sample suites crystallised from melts with different starting K₂O compositions. Elevated ratios of Nb/Yb in the mafic Gill Seamount and Rapuhia Ridge lavas indicate the back-arc volcanoes and ridges originated from a less depleted mantle than that present underneath the Kermadec volcanic arc-front, likely a consequence of trenchward advection of mantle within a suprasubduction wedge and/or partial melting of a fusible enriched mantle component. All whole rock samples from these back-arc volcanoes have trace element characteristics that resemble those of typical volcanic arc magmas, indicating that they are variably modified by subducting plate-derived components despite their rear-arc setting. However, the extent of fluid enrichment is less than that at the Kermadec volcanic arc-front. Elevated REE patterns and (La/Sm)N ratios suggest the subduction-component modifying back-arc volcano magmas is dominated by subducting sediment. This sediment component is not consistent with aqueous fluid transfer or bulk mixing, but by the addition of a sediment-derived partial melt with residual accessory phases monazite + zircon + rutile. HFSE/REE fractionated trace element patterns overlap for unmodified basalts from Gill Seamount and Rapuhia Ridge, and Rumble V Ridge back-arc constructional volcanism to the south. This suggests that a similar mechanism triggers constructional back-arc volcanism at both locations in the southern Havre Trough, likely a consequence of thermal anomalies inferred to be present in the mantle wedge (Todd et al. (2011)).</p>
<p>The Kermadec Arc-Havre Trough (KAHT) is widely regarded as a classical example of an intra-oceanic arc-back-arc system, where subduction-driven arc magmatism is focused at the Kermadec volcanic arc-front, and magmatism within the Havre Trough back-arc system results from decompression-related melting. In detail, however, the Havre Trough has not been well-studied, and data for very few lavas have been reported. Recent mapping undertaken in the southern Havre Trough has resulted in the discovery of several prominent submarine stratovolcanoes, Gill Seamount, Rapuhia Seamount and the related Rapuhia Ridge, Yokosuka Seamount, and Giljanes Seamount, situated in the middle of deep rifts and on elevated crustal plateaux. The origin and evolution of these stratovolcanoes is unknown. The first detailed dataset of whole rock major and trace element geochemistry, mineral chemistry, and ⁴⁰Ar/³⁹Ar isotope data, for lavas erupted from these volcanoes is presented here, and used to investigate the processes that drive volcanism in the Havre Trough back-arc. ⁴⁰Ar/³⁹Ar ages obtained from back-arc stratovolcanoes range from ca. 1167 - 953 ka for Gill Seamount, and ca. 107 - 50 ka for Rapuhia Ridge. These ages overlap with known ages for arc-front lavas, indicating that both back-arc and arc-front volcanism are coeval. These ages are all significantly younger than the inferred initation of Havre Trough rifting ca. 5 - 6 Ma. Lavas analysed from Gill Seamount and Rapuhia Ridge are basaltic to basaltic-andesitic in whole rock composition and contain a phenocryst assemblage of olivine ± orthopyroxene + clinopyroxene ± plagioclase. Lavas from Rapuhia Seamount, Yokosuka Seamount and Giljanes Seamount range from andesitic to dacitic in composition, and have a phenocryst assemblage consisting primarily of plagioclase ± clinopyroxene ± amphibole ± Fe-Ti oxides ± apatite. Variations in mineral assemblages and whole rock compositions of the lavas are consistent with crystal fractionation of their respective phenocryst phases. The more evolved compositions of Rapuhia Seamount, Yokosuka Seamount and Giljanes Seamount, all sited on an elevated crustal plateau, are inferred to result from prolonged assimilation + fractional crystallisation (AFC) in the mid- to upper- crust. Mineral compositions provide additional evidence for fractional crystallisation, and most crystals are inferred to have crystallised in equilibrium with their host melt. However, compositions of some olivine phenocrysts in Gill Seamount and Rapuhia Ridge indicate multiple populations of olivine, suggesting their magmatic systems were open to contributions from secondary processes. Variations in Or content in plagioclase crystals for a given lava suite suggests the sample suites crystallised from melts with different starting K₂O compositions. Elevated ratios of Nb/Yb in the mafic Gill Seamount and Rapuhia Ridge lavas indicate the back-arc volcanoes and ridges originated from a less depleted mantle than that present underneath the Kermadec volcanic arc-front, likely a consequence of trenchward advection of mantle within a suprasubduction wedge and/or partial melting of a fusible enriched mantle component. All whole rock samples from these back-arc volcanoes have trace element characteristics that resemble those of typical volcanic arc magmas, indicating that they are variably modified by subducting plate-derived components despite their rear-arc setting. However, the extent of fluid enrichment is less than that at the Kermadec volcanic arc-front. Elevated REE patterns and (La/Sm)N ratios suggest the subduction-component modifying back-arc volcano magmas is dominated by subducting sediment. This sediment component is not consistent with aqueous fluid transfer or bulk mixing, but by the addition of a sediment-derived partial melt with residual accessory phases monazite + zircon + rutile. HFSE/REE fractionated trace element patterns overlap for unmodified basalts from Gill Seamount and Rapuhia Ridge, and Rumble V Ridge back-arc constructional volcanism to the south. This suggests that a similar mechanism triggers constructional back-arc volcanism at both locations in the southern Havre Trough, likely a consequence of thermal anomalies inferred to be present in the mantle wedge (Todd et al. (2011)).</p>
<p>The Coromandel Volcanic Zone (CVZ) was the longest-lived area of volcanism in New Zealand hosting the commencement of large explosive rhyolitic and ignimbrite forming eruptions. The NW trending Coromandel Peninsula is the subaerial remnant of the Miocene-Pliocene CVZ, which is regarded as a tectonic precursor to the Taupo Volcanic Zone (TVZ), currently the most dynamic and voluminous rhyolitic volcanic centre on Earth. This study presents new single glass shard major and trace element geochemical analyses for 72 high-silica volcanic tephra layers recovered from well-dated deep-sea sediments of the SW Pacific Ocean by the Ocean Drilling Program (ODP) Leg 181. ODP Site 1124, ~720 km south and east from the CVZ, penetrated sediments of the Rekohu Drift yielding an unprecedented record of major explosive volcanic eruptions owing to the favourable location and preservation characteristics at this site. This record extends onshore eruptive sequences of CVZ explosive volcanism that are obscured by poor exposure, alteration, and erosion and burial by younger volcanic deposits. Tephra layers recovered from Site 1124 are well-dated through a combination of biostratigraphic and palaeomagnetic methods allowing the temporal geochemical evolution of the CVZ to be reconstructed in relation to changes in the petrogenesis of CVZ arc magmas from ~ 10 to 2 Ma. This thesis establishes major and trace element geochemical "fingerprints" for all Site 1124-C tephras using well-established (wavelength dispersive electron probe microanalysis) and new (laser ablation inductively coupled plasma mass spectrometry) in situ single glass shard microanalytical techniques. Trace element analysis of Site 1124-C glass shards (as small as 20 um) demonstrate that trace element signatures offer a more specific, unequivocal characterisation for distinguishing (and potentially correlating) between tephras with nearly identical major element compositions. The Site 1124-C core contains 72 unaltered Miocene-Pliocene volcanic glass-shard-bearing laminae > 1 cm thick that correspond to 83 or 84 geochemical eruptive units. Revised eruptive frequencies based on the number of geochemical eruptive units identified represent at least one eruption every 99 kyr for the late Miocene and one per 74 kyr for the Pliocene. The frequency of tephra deposition throughout the history of the CVZ has not been constant, rather reflecting pulses of major explosive eruptions resulting in closely clustered groups of tephra separated by periods of reduced activity, relative volcanic quiescence or non-tephra deposition. As more regular activity became prevalent in the Pliocene, it was accompanied by more silicic magma compositions. Rhyolitic volcanic glass shards are characterised by predominantly calc-alkaline and minor high-K enriched major element compositions. Major element compositional variability of the tephras deposited between 10 Ma and 2 Ma reveals magma batches with pre-eruptive compositional gradients implying a broad control by fractional crystallisation. Trace element characterisation of glass shards reveals the role of magmatic processes that are not readily apparent in the relatively homogeneous major element compositions. Multi-element diagrams show prominent negative Sr and Ti anomalies against primitive mantle likely caused by various degrees of plagioclase and titanomagnetite fractional crystallisation in shallow magma chambers. Relative Nb depletion, characteristic of arc volcanism, is moderate in CVZ tephras. HFSEs (e.g. Nb, Zr, Ti) and HREEs (e.g. Yb, Lu) remain immobile during slab fluid flux suggesting they are derived from the mantle wedge. LILE (e.g. Rb, Cs, Ba, Sr) and LREE (e.g. La, Ce) enrichments are consistent with slab fluid contribution. B/La and Li/Y ratios can be used as a proxy for the flux of subducting material to the mantle wedge, they suggest there is a strong influence from this component in the generation of CVZ arc magmas, potentially inducing melting. CVZ tephra show long-term coherent variability in trace element geochemistry. Post ~ 4 Ma tephras display a more consistent, less variable, chemical fingerprint that persists up to and across the CVZ/TVZ transition at ~ 2 Ma. Initiation of TVZ volcanism may have occurred earlier than is presently considered, or CVZ to TVZ volcanism may have occurred without significant changes in magma generation processes.</p>
<p>The Coromandel Volcanic Zone (CVZ) was the longest-lived area of volcanism in New Zealand hosting the commencement of large explosive rhyolitic and ignimbrite forming eruptions. The NW trending Coromandel Peninsula is the subaerial remnant of the Miocene-Pliocene CVZ, which is regarded as a tectonic precursor to the Taupo Volcanic Zone (TVZ), currently the most dynamic and voluminous rhyolitic volcanic centre on Earth. This study presents new single glass shard major and trace element geochemical analyses for 72 high-silica volcanic tephra layers recovered from well-dated deep-sea sediments of the SW Pacific Ocean by the Ocean Drilling Program (ODP) Leg 181. ODP Site 1124, ~720 km south and east from the CVZ, penetrated sediments of the Rekohu Drift yielding an unprecedented record of major explosive volcanic eruptions owing to the favourable location and preservation characteristics at this site. This record extends onshore eruptive sequences of CVZ explosive volcanism that are obscured by poor exposure, alteration, and erosion and burial by younger volcanic deposits. Tephra layers recovered from Site 1124 are well-dated through a combination of biostratigraphic and palaeomagnetic methods allowing the temporal geochemical evolution of the CVZ to be reconstructed in relation to changes in the petrogenesis of CVZ arc magmas from ~ 10 to 2 Ma. This thesis establishes major and trace element geochemical "fingerprints" for all Site 1124-C tephras using well-established (wavelength dispersive electron probe microanalysis) and new (laser ablation inductively coupled plasma mass spectrometry) in situ single glass shard microanalytical techniques. Trace element analysis of Site 1124-C glass shards (as small as 20 um) demonstrate that trace element signatures offer a more specific, unequivocal characterisation for distinguishing (and potentially correlating) between tephras with nearly identical major element compositions. The Site 1124-C core contains 72 unaltered Miocene-Pliocene volcanic glass-shard-bearing laminae > 1 cm thick that correspond to 83 or 84 geochemical eruptive units. Revised eruptive frequencies based on the number of geochemical eruptive units identified represent at least one eruption every 99 kyr for the late Miocene and one per 74 kyr for the Pliocene. The frequency of tephra deposition throughout the history of the CVZ has not been constant, rather reflecting pulses of major explosive eruptions resulting in closely clustered groups of tephra separated by periods of reduced activity, relative volcanic quiescence or non-tephra deposition. As more regular activity became prevalent in the Pliocene, it was accompanied by more silicic magma compositions. Rhyolitic volcanic glass shards are characterised by predominantly calc-alkaline and minor high-K enriched major element compositions. Major element compositional variability of the tephras deposited between 10 Ma and 2 Ma reveals magma batches with pre-eruptive compositional gradients implying a broad control by fractional crystallisation. Trace element characterisation of glass shards reveals the role of magmatic processes that are not readily apparent in the relatively homogeneous major element compositions. Multi-element diagrams show prominent negative Sr and Ti anomalies against primitive mantle likely caused by various degrees of plagioclase and titanomagnetite fractional crystallisation in shallow magma chambers. Relative Nb depletion, characteristic of arc volcanism, is moderate in CVZ tephras. HFSEs (e.g. Nb, Zr, Ti) and HREEs (e.g. Yb, Lu) remain immobile during slab fluid flux suggesting they are derived from the mantle wedge. LILE (e.g. Rb, Cs, Ba, Sr) and LREE (e.g. La, Ce) enrichments are consistent with slab fluid contribution. B/La and Li/Y ratios can be used as a proxy for the flux of subducting material to the mantle wedge, they suggest there is a strong influence from this component in the generation of CVZ arc magmas, potentially inducing melting. CVZ tephra show long-term coherent variability in trace element geochemistry. Post ~ 4 Ma tephras display a more consistent, less variable, chemical fingerprint that persists up to and across the CVZ/TVZ transition at ~ 2 Ma. Initiation of TVZ volcanism may have occurred earlier than is presently considered, or CVZ to TVZ volcanism may have occurred without significant changes in magma generation processes.</p>
<p>This thesis presents a chemical and isotopic investigation of well-dated silicic tephra layers sourced from the Taupo Volcanic Zone (TVZ), central North Island, New Zealand, that were recovered from deep ocean sediment cores at Ocean Drilling Program Site 1123 (41 degrees 47.16' S, 171 degrees 29.94' W; 3290 m water depth), located approximately 1000 km east of the TVZ. The relative quiescence of the deep ocean sedimentary setting, the continuous supply of biogenic and terrigenous sediment and the favourable location of Site 1123 close to the main TVZ ash dispersal path have resulted in an extensive TVZ tephra record (70 Quaternary tephra layers preserved in 3 sediment cores) at Site 1123. This record extends and compliments the onshore record of silicic TVZ volcanism which has been obscured by erosion of non-consolidated volcanic material and burial of older units by younger volcanic deposits. The Site 1123 cores comprise an important paleo-oceanographic record for the Southwest Pacific Ocean and as a result of previous paleo-environmental studies, the Site 1123 tephras have been assigned orbitally tuned stable isotope ages that are more precise than is currently possible by any radiometric dating techniques. These features of the Site 1123 tephra record highlight its potential to be established as a type section for Quaternary tephrochronological studies in the New Zealand region. In addition, the continuous stratigraphy and precise age control of these tephras enables the Site 1123 record to be used as a petrogenetic archive to investigate changes in chemical and isotopic composition of these tephras that may be related to changes in the petrogenesis of TVZ silicic magmas during the last ~ 1.65 Ma. This thesis establishes major and trace element chemical 'fingerprints' for the Site 1123 tephras using traditional (electron probe microanalysis) and novel (laser ablation inductively coupled plasma mass spectrometry) in situ geochemical techniques. Trace element fingerprints are demonstrated to provide a more precise means of correlating and distinguishing between tephras with essentially identical major element chemistries. These fingerprints are used to refine the original Site 1123 composite stratigraphy and age model and identify a section of repeated sediments in the Site 1123 cores that have introduced a significant error into the original composite stratigraphy and age model for the interval ~1.1 to 1.4 Ma. Correlation of the tephra layers between the 3 sediment cores (1123A, B and C) establishes that ~37-38 individual tephra units are recorded with ages ranging from 1.655 Ma to 27.1 ka. Approximately 50% of the eruptive units and cumulative tephra thickness at the site were recorded during the first ~ 150 ka of silicic TVZ volcanism (1.65 to 1.50 Ma). The fragmentary onshore record does not preserve clear evidence for this early period of hyperactivity. Four broad silicic melt types are identified on the basis of chemistry and eruptive age. Trace element indices of fractional crystallisation suggests the origin of the four melt types is primarily due to differential degrees of fractional crystallisation of accessory zircon, hydrous mineral phases and Fe-Ti oxides. Sr-Nd-Pb isotopic compositions of 13 representative Site 1123 tephras cannot be generated using traditional models in which Torlesse meta-sedimentary rocks are the sole contaminant of mafic magmas. Instead the data support a model in which ascending TVZ basalts assimilate crustal rocks of both meta-greywacke terranes: firstly up to 15% of Waipapa crust is assimilated at depth, followed by assimilation of between 20 and 45% Torlesse crust at shallower levels. In this model the majority of Site 1123 tephras indicate a remarkably uniform amount of crust (~ 35%) with the most evolved sample requiring 45% crustal contribution. However, extensive fractional crystallisation (55-85%) is required to have accompanied crustal assimilation in order to drive the relatively low SiO2 compositions of these contaminated mafic magmas (SiO2 = 53-58 wt% after crustal contamination) to the high SiO2 rhyolite (74-78 wt%) compositions of the Site 1123 tephras. The large crustal contributions to TVZ silicic magmas (35-45%) implied by these data are high compared to large volume silicic magmas from different settings (e.g. Yemen-Ethiopia; Long Valley, USA), a feature that likely reflects the thin crust and high thermal flux into the continental crust beneath the TVZ.</p>
<p>The petrogenesis of silicic arc magmas is controversial with end-member models of fractional crystallisation and crustal anatexis having been invoked. A prime example of this is the archetypical continental Taupo Volcanic Zone and the adjacent oceanic Kermadec Arc. Insights into the genesis and timescales of magmatic processes of four continental rhyolitic magmas (Whakamaru, Oruanui, Taupo and Rotorua eruptives) and an oceanic (Healy seamount) rhyodacitic magma are documented through micro-analytical chemical studies of melt inclusions and crystal zonation of plagioclase and quartz. Electron probe microanalysis, laser ablation inductively coupled plasma mass spectrometry and Fourier transform infrared spectroscopy have been used to measure major, trace and volatile element concentrations, respectively, of melt inclusions and crystals. Melt inclusions are high silica (e.g. 74 - 79 wt%) irrespective of arc setting and display a wide range of trace element compositions (e.g. Sr = 17 - 180 ppm). Taupo Volcanic Zone melt inclusions exhibit higher K2O and Ce/Yb relative to Healy melt inclusions reflecting the assimilation of continental lithosphere. Quantitative trace element modelling of melt inclusion compositions: (a) demonstrates that magma genesis occurred through 62 - 76% fractional crystallisation at Healy whereas assimilation of continental lithosphere (greywacke) in addition to 60 - 80% fractional crystallisation is required for the Taupo Volcanic Zone magmas; and (b) suggests the presence of crystal mush bodies beneath silicic magma chambers in both continental and oceanic arc environments. Water concentrations of melt inclusions ranged between 1.4 - 5.1 wt% for the Whakamaru, Taupo and Healy samples. However, the inconsistency in the measured molecular water to hydroxyl concentrations of melt inclusions relative to those determined experimentally for groundmass rhyolitic glasses provide evidence for the degassing of inclusions prior to quenching, by diffusion of hydroxyl groups through the crystal host. Thus, partial pressures of water estimated from the inclusions and inferred depths of the crystallising magma bodies are underestimated. Chemical profiles of mineral zonation, however, indicate a more complex origin of silicic melts than simple fractionation and assimilation. For example, trace element modelling of Whakamaru plagioclase suggests that the three distinct textural plagioclase populations present in Whakamaru samples crystallised from four physiochemically discrete silicic melts. This modelling indicates a strong petrogenetic link between andesitic and silicic magmas from the chemical variation of selected Whakamaru plagioclase crystals possessing high anorthite (45-60 mol %) cores and low anorthite (~ 30 mol %) rim compositions and the interaction of greywacke partial melts. Furthermore, Sr diffusion modelling of core-rim interfaces of the same plagioclase crystals indicate the amalgamation of the magma chamber occurred continuously over the 15,000 years preceding the climactic eruption. Conversely, the major element zonation of Taupo plagioclases implies magma genesis occurred solely through assimilation and fractional crystallisation without the incorporation of evolved crystal mush magmas, indicating a spectrum of magmatic processes are occurring beneath the Taupo Volcanic Zone with each eruption providing only a snapshot of the petrogenesis of the Taupo Volcanic Zone.</p>
<p>This thesis presents a chemical and isotopic investigation of well-dated silicic tephra layers sourced from the Taupo Volcanic Zone (TVZ), central North Island, New Zealand, that were recovered from deep ocean sediment cores at Ocean Drilling Program Site 1123 (41 degrees 47.16' S, 171 degrees 29.94' W; 3290 m water depth), located approximately 1000 km east of the TVZ. The relative quiescence of the deep ocean sedimentary setting, the continuous supply of biogenic and terrigenous sediment and the favourable location of Site 1123 close to the main TVZ ash dispersal path have resulted in an extensive TVZ tephra record (70 Quaternary tephra layers preserved in 3 sediment cores) at Site 1123. This record extends and compliments the onshore record of silicic TVZ volcanism which has been obscured by erosion of non-consolidated volcanic material and burial of older units by younger volcanic deposits. The Site 1123 cores comprise an important paleo-oceanographic record for the Southwest Pacific Ocean and as a result of previous paleo-environmental studies, the Site 1123 tephras have been assigned orbitally tuned stable isotope ages that are more precise than is currently possible by any radiometric dating techniques. These features of the Site 1123 tephra record highlight its potential to be established as a type section for Quaternary tephrochronological studies in the New Zealand region. In addition, the continuous stratigraphy and precise age control of these tephras enables the Site 1123 record to be used as a petrogenetic archive to investigate changes in chemical and isotopic composition of these tephras that may be related to changes in the petrogenesis of TVZ silicic magmas during the last ~ 1.65 Ma. This thesis establishes major and trace element chemical 'fingerprints' for the Site 1123 tephras using traditional (electron probe microanalysis) and novel (laser ablation inductively coupled plasma mass spectrometry) in situ geochemical techniques. Trace element fingerprints are demonstrated to provide a more precise means of correlating and distinguishing between tephras with essentially identical major element chemistries. These fingerprints are used to refine the original Site 1123 composite stratigraphy and age model and identify a section of repeated sediments in the Site 1123 cores that have introduced a significant error into the original composite stratigraphy and age model for the interval ~1.1 to 1.4 Ma. Correlation of the tephra layers between the 3 sediment cores (1123A, B and C) establishes that ~37-38 individual tephra units are recorded with ages ranging from 1.655 Ma to 27.1 ka. Approximately 50% of the eruptive units and cumulative tephra thickness at the site were recorded during the first ~ 150 ka of silicic TVZ volcanism (1.65 to 1.50 Ma). The fragmentary onshore record does not preserve clear evidence for this early period of hyperactivity. Four broad silicic melt types are identified on the basis of chemistry and eruptive age. Trace element indices of fractional crystallisation suggests the origin of the four melt types is primarily due to differential degrees of fractional crystallisation of accessory zircon, hydrous mineral phases and Fe-Ti oxides. Sr-Nd-Pb isotopic compositions of 13 representative Site 1123 tephras cannot be generated using traditional models in which Torlesse meta-sedimentary rocks are the sole contaminant of mafic magmas. Instead the data support a model in which ascending TVZ basalts assimilate crustal rocks of both meta-greywacke terranes: firstly up to 15% of Waipapa crust is assimilated at depth, followed by assimilation of between 20 and 45% Torlesse crust at shallower levels. In this model the majority of Site 1123 tephras indicate a remarkably uniform amount of crust (~ 35%) with the most evolved sample requiring 45% crustal contribution. However, extensive fractional crystallisation (55-85%) is required to have accompanied crustal assimilation in order to drive the relatively low SiO2 compositions of these contaminated mafic magmas (SiO2 = 53-58 wt% after crustal contamination) to the high SiO2 rhyolite (74-78 wt%) compositions of the Site 1123 tephras. The large crustal contributions to TVZ silicic magmas (35-45%) implied by these data are high compared to large volume silicic magmas from different settings (e.g. Yemen-Ethiopia; Long Valley, USA), a feature that likely reflects the thin crust and high thermal flux into the continental crust beneath the TVZ.</p>
<p>The petrogenesis of silicic arc magmas is controversial with end-member models of fractional crystallisation and crustal anatexis having been invoked. A prime example of this is the archetypical continental Taupo Volcanic Zone and the adjacent oceanic Kermadec Arc. Insights into the genesis and timescales of magmatic processes of four continental rhyolitic magmas (Whakamaru, Oruanui, Taupo and Rotorua eruptives) and an oceanic (Healy seamount) rhyodacitic magma are documented through micro-analytical chemical studies of melt inclusions and crystal zonation of plagioclase and quartz. Electron probe microanalysis, laser ablation inductively coupled plasma mass spectrometry and Fourier transform infrared spectroscopy have been used to measure major, trace and volatile element concentrations, respectively, of melt inclusions and crystals. Melt inclusions are high silica (e.g. 74 - 79 wt%) irrespective of arc setting and display a wide range of trace element compositions (e.g. Sr = 17 - 180 ppm). Taupo Volcanic Zone melt inclusions exhibit higher K2O and Ce/Yb relative to Healy melt inclusions reflecting the assimilation of continental lithosphere. Quantitative trace element modelling of melt inclusion compositions: (a) demonstrates that magma genesis occurred through 62 - 76% fractional crystallisation at Healy whereas assimilation of continental lithosphere (greywacke) in addition to 60 - 80% fractional crystallisation is required for the Taupo Volcanic Zone magmas; and (b) suggests the presence of crystal mush bodies beneath silicic magma chambers in both continental and oceanic arc environments. Water concentrations of melt inclusions ranged between 1.4 - 5.1 wt% for the Whakamaru, Taupo and Healy samples. However, the inconsistency in the measured molecular water to hydroxyl concentrations of melt inclusions relative to those determined experimentally for groundmass rhyolitic glasses provide evidence for the degassing of inclusions prior to quenching, by diffusion of hydroxyl groups through the crystal host. Thus, partial pressures of water estimated from the inclusions and inferred depths of the crystallising magma bodies are underestimated. Chemical profiles of mineral zonation, however, indicate a more complex origin of silicic melts than simple fractionation and assimilation. For example, trace element modelling of Whakamaru plagioclase suggests that the three distinct textural plagioclase populations present in Whakamaru samples crystallised from four physiochemically discrete silicic melts. This modelling indicates a strong petrogenetic link between andesitic and silicic magmas from the chemical variation of selected Whakamaru plagioclase crystals possessing high anorthite (45-60 mol %) cores and low anorthite (~ 30 mol %) rim compositions and the interaction of greywacke partial melts. Furthermore, Sr diffusion modelling of core-rim interfaces of the same plagioclase crystals indicate the amalgamation of the magma chamber occurred continuously over the 15,000 years preceding the climactic eruption. Conversely, the major element zonation of Taupo plagioclases implies magma genesis occurred solely through assimilation and fractional crystallisation without the incorporation of evolved crystal mush magmas, indicating a spectrum of magmatic processes are occurring beneath the Taupo Volcanic Zone with each eruption providing only a snapshot of the petrogenesis of the Taupo Volcanic Zone.</p>
Impurities are transferred out at the boundary of the liquid. Velocities normal to the boundary are small. Therefore, for efficient removal contact areas and times should be large. Transfer depends on the chemical and physical properties of the liquid and the phase that captures the impurities at the boundary. This phase may be a liquid, gas (vacuum) or solid. Properties can be described in terms of equilibrium and empirical mass transfer coefficients. Vacuum may be applied to remove volatile elements. Refining can be carried out by partial solidification or fractional crystallisation, using the segregation that occurs during freezing of an alloy. Finally, an element can be added to form a reactive compound followed by removal of the compound by sedimentation or filtration.
