Real-time tracking of the 2019 pumice raft in the southwest Pacific

Jutzeler, Dr Martin1, Van Sebille, Dr Erik2, Marsh, Prof Robert3

1Centre for Ore Deposit and Earth Sciences, University of Tasmania, Hobart, Australia, 2Institute for Marine and Atmospheric research, Utrecht University, Utrecht, Netherlands, 3School of Ocean and Earth Science, University of Southampton, Southampton, United Kingdom

Pumice rafts can be hazardous to maritime traffic due to their ability to clog engine water intakes, block harbours and divert maritime traffic for weeks. On 7 August 2019, a 195 km2 pumice raft was produced at an unnamed submarine volcano in the Tonga Islands (Southwest Pacific Ocean). The raft quickly expanded in size and got segmented into multiple smaller rafts that reached the Lau and Fiji Islands over the following weeks. Yachts that crossed the raft as early as two days post-eruption sent an alert to the Rescue and Co-Operation Centre New Zealand (RCCNZ), the Maritime Safety Authority of Fiji, who relayed us the information. The coupling of real-time satellite observations with weather reports and oceanographic Lagrangian simulations allowed near-real time forecasting of raft dispersal. The abundance of satellite images allowed us to contrast virtual particle tracking methods with ocean model currents to explore the relative influence of surface currents, wind, and wave action on pumice flotsam dispersal. We produced bi-weekly hazard maps to RCCNZ and key local individuals for dissemination to the yachting, shipping and fishing communities via social media and word of mouth. This strategy successfully prevented further vessels from encountering the pumice raft, and facilitated contact with sailing crews for information on the raft and samples. The dispersal models built for this pumice raft can be used for global maritime hazard mitigation.


Biography

Martin Jutzeler is a Senior Research Fellow at the University of Tasmania. Martin uses trans-disciplinary strategies to tackle frontier questions in ancient and modern marine volcanism. His research includes volcanic architecture, submarine volcanism, and dispersal of volcanic material in the oceans.

Solution Pipes and Focussed Vertical Water Flow in Carbonates with High Matrix Porosity

White, Susan1, Lipar, Matej 2, Szymczak, Piotr 3,  Webb, John 1

1Environmental Geoscience, Department of Ecology, Environment and Evolution, La Trobe University, Melbourne, Bundoora, Victoria, 3086, Australia, 2Anton Melik Geographical Institute, Research Centre of the Slovenian Academy of Sciences and Arts, Gosposka ulica 13, SI-1000 Ljubljana, Slovenia, 3 Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warszawa, Poland

Focused vertical flow is one of the distinctive features of karst processes within porous calcareous media with matrix porosity, especially in calcarenites, and forms distinctive features termed solution pipes. These are vertical cylindrical voids of variable diameter and depth predominantly in the vadose (epikarstic) zone. Although dissolutional in origin, the initial trigger that focuses the water flow in the host rock matrix is poorly understood, although they appear to be related to vegetation and rock heterogeneities.

Measuring of the morphology of pipes e.g. pipe diameter, depth, density and shape,  is common but has not resolved the problem and neither has a 2D statistical data based on 5 x 5 m area in Australia which included inside and outside diameter of the pipes, their elongation, the thickness of cemented rim, and distance and direction to nearest neighbour. The stratigraphy, mineralogy, fabric and geochemistry of the surrounding rock and pipe rims and fills provide insights into pipe formation but do not address the issue of focussed flow. Thin section and X-ray diffraction has been used to distinguish between rim and host rock fabric. Stable isotope analysis partly correlated with host rock age indicates that the dissolutional process is under vadose conditions. Dating of the rim cement has not been successful due to the minute size of the carbonate cement samples. And the rock clasts of the host rock are obviously older.

Triggers to flow include lithological irregularities e.g. cracks in calcrete, surface geomorphology e.g. potholes and vegetation e.g. stem flow, buried tree trunks. If these are not present the instability of the wetting front infiltrating into the unsaturated zone breaks advances into unsaturated host rock as self organised fingers. This fingered flow, a non-uniform accelerated transport of sinking water, through the porous material due to an unstable wetting front, is particularly favoured if the media is water repellent, which causes uneven infiltration.

The roles of dynamic instabilities and self-organisation in the emergence of focusing patterns and the various challenges, especially in the vadose zone, can probably be only resolved with modelling, conjunction with field data. From the numerical point of view, modelling of solution pipe formation requires solving the coupled equations for the groundwater flow, chemical transport and porosity evolution. The ground-water system may be either saturated, or partly or completely unsaturated. Morphologically similar pipes seem to develop under both conditions. Pipes formed in deglaciated Miocene sediments in Poland  can be modelled using Darcian flow equations. However the pipes found in unglaciated areas e.g. Cape Bridgewater, Victoria, were clearly formed by dissolution in the unsaturated (vadose) so this model is inappropriate. However the Richards equation applies a  continuity requirement resulting in a general partial differential equation describing water movement in unsaturated non-swelling media. This appears to be the best option for modelling  water flow in the vadose zone

As most solution pipes form under vadose conditions and show potential as palaeoclimatic indicators, further understanding of their formation and behaviour is important.


Biography

A karst geomorphologist  who has worked on features and processes in high matrix porsity  Neogene and  Pleistocene carbonates for many years.

Variations in mid- to late Holocene nitrogen supply to northern Great Barrier Reef Halimeda macroalgal bioherms

McNeil, Mardi1; Nothdurft, Luke1; Erler, Dirk2; Hua, Quan3; Webster, Jody M.4

1Queensland University of Technology, Brisbane, Australia, 2Southern Cross University, Lismore, Australia, 3ANSTO, Lucas Heights, Australia, 4The University of Sydney, Australia

The northern Great Barrier Reef (GBR) Halimeda bioherms have accumulated on the outer continental shelf from calcium carbonate algal sediments over the past ~10,000 years and cover >6000 km2 of shelf area. As such, Halimeda bioherms play a key role in the shallow marine carbon cycle over millennial timescales. The main source of nitrogen (N) to these bioherms is thought to be westward transport of upwelled NO3-rich water from the Coral Sea. However, the primary N source has not been traced geochemically, and we have no understanding of any temporal variation. Here, we reconstruct patterns of N supply to Halimeda bioherms in the GBR since the mid-Holocene using the 15N/14N ratio of skeletal-bound organic N (δ15N-SOM) in modern and fossil Halimeda sediment cores.

Average Halimeda skeletal δ15N-SOM was 6.28 ± 0.26‰, consistent with δ15N-NO3– from western tropical South Pacific (WTSP) thermocline waters. Thus geochemically validating shelf-break upwelling of an oceanic N source that appears to regulate the Halimeda bioherm spatial distribution. Halimeda δ15N-SOM decreased by 1-2 ‰ from 5000 to 2000 cal. yr BP, reaching a minima of 5.5‰ that persisted for almost 1000 years. The Halimeda δ15N-SOM variation reflects mid- to late Holocene changes in regional climate and intensified El Niño activity that likely facilitated elevated N2 fixation in the WTSP, thereby lowering thermocline δ15N-NO3. Thus, Halimeda skeletal material provides a valuable high-resolution geochemical archive of past oceanographic and climatic processes over centennial to millennial timescales, complementing existing paleoclimate proxy records.


Biography

Mardi’s interest is in cross-disciplinary research in Marine Geoscience, specifically biogenic carbonate sediments and structures at various scales from calcareous epiphytes on seagrass, to carbonate bioherms and reefs; their physical, chemical, and biological processes and their application in understanding past environments and current environmental change.

Insights into palaeo-hydrology of the Coorong Lagoon, South Australia, based on Strontium Isotope Tracers (87Sr/86Sr and _88/86Sr) in fossil carbonates

Yuexiao Shao1,2, Zara Woolston1, Juraj Farkaš1,2, Briony Chamberlayne1, John Tibby3 Deborah Haynes1, Jonathan Tyler1

1Department of Earth Sciences, School of Physical Sciences, University of Adelaide, Australia; 2Metal Isotope Group (MIG), University of Adelaide, Australia; 3Department of Geography, Environment and Population, School of Social Sciences, University of Adelaide, Australia

The Coorong lagoon, as part of the wetland system at the terminus of the River Murray, is recognised not only for its ecological importance but also for its unique geomorphology and salinity gradient that ranges from fresh/brackish (< 35 PSU) in the North Lagoon to hypersaline (> 70 PSU) in the South Lagoon. The lagoon hydrology is controlled by seawater-continental water mixing processes that are traceable via the radiogenic Strontium (Sr) isotopes (87Sr/86Sr ratios). The hypersaline South Lagoon, being more geomorphologically restricted, is known for high degree of evaporation, which leads to ongoing calcium carbonate precipitation, which also acts as a sink for dissolved inorganic carbon (DIC) [1]. These processes involving carbonate formation and a local inorganic carbon cycling are traceable via the novel stable Sr isotope (δ88/86Sr), which is particularly sensitive to mass-dependent isotope fractionation linked to carbonate precipitation / dissolution. Importantly, the South Lagoon has seen dramatic hydrological and ecological changes over the last ~200 years (since the European settlement), which is evident from geochemical and diatom records of the Coorong sediment cores [2], and has implications for water resource management and future strategies for the recovery of local ecosystem to more natural conditions[3]. In order to reconstruct paleo-hydrology of the Coorong before and after the European settlement, this study calibrated the δ88/86Sr fractionation between recent aragonitic bivalve shells (Arthritica helmsi species) and local water in the modern Coorong lagoon, and a constant difference of δ88/86Sr between the shells and the local water Δ88/86Sr (δ88/86Srsolid – δ88/86Srwater) = -0.92‰ was discovered. Such calibrations, coupled with 87Sr/86Sr and δ88/86Sr analyses of fossil A. helmsi shells from a sediment core in the South Lagoon, are complemented by radiocarbon dating and elemental concentration data, to better constrain (i) variability in the mixing of water sources in modern Coorong and over the last ~2500 years; and (ii) to reconstruct palaeo-salinity changes and associated carbonate precipitation/dissolution processes. Primary results based on 87Sr/86Sr of shells indicated the source of water in the South Lagoon were never purely marine; however, according to δ88/86Sr of the shells, the the South Lagoon in the past ~2500 years was probably less evaporated than it has been in recent times (i.e., post European settlement).

[1] Shao et al (2018), Geochimica et Cosmochimica Acta 239, 90-108.

[2] McKirdy et al. (2010) Organic Geochemistry 41, 96-110.

[3] Brookes et al. (2018). Goyder Institute for Water Research Technical Report Series No. 18/04, Adelaide, South Australia. ISSN: 1839-2725


Biography

3rd year PhD student in University of Adelaide, mainly interested in alkaline metal isotopes in coastal systems as tracers of water source mixing, carbonate dynamics, and redox states, and further application of these tracers in reconstructing the paleo-climate of the local coastal system.

Holocene microbialite records of terrigenous influence on water quality for the offshore southern Great Barrier Reef

Salas-Saavedra, Dr Marcos1, Webb, Professor Gregory1, Sanborn, Dr Kelsey2, Zhao, Professor Jian-xin1, Webster, Professor Jody 2, Nothdurft, Dr Luke3, Nguyen, Dr Ai1

1School of Earth and Environmental Sciences, The University of Queensland, Brisbane, Australia, 2Geocoastal Research Group, School of Geoscience, University of Sydney, Sydney, Australia, 3School of Earth and Atmospheric Sciences, Queensland University of Technology, Brisbane, Australia

Anthropocene climate change and water quality degradation represent unprecedented challenges to modern coral reef ecology. Projected trends for the Great Barrier Reef (GBR) suggest continuing declines in reef health. Although declining reef health after European colonization is well documented around the world and increased terrigenous sediment flux is known to have terminated deglacial reefs in the GBR, longer-term patterns of water quality are poorly understood. Without historical data, it is difficult to disentangle natural and anthropogenic reef behaviour to better model anthropogenic effects. Here we present the first proxy-based long-term Holocene water quality reconstruction for any reef. The geochronological framework provided by rotary coring on Heron and One Tree reefs (offshore, southern GBR) allowed reconstruction of offshore water quality from 8,200 to 1,800 years before present (BP) using centennial resolution microbialite-based geochemical proxies. Microbialites, which form part of growing reef framework, contain a robust proxy record of water quality through incorporation of trace elements (e.g., rare earth elements-REEs, Zr, Th, etc.) from ambient seawater. Trace elements associated with terrigenous flux were measured in dated microbialites as well as poorly consolidated Pleistocene limestone and palaeosol formed at the Pleistocene-Holocene unconformity at Heron and One Tree reefs. Paleosol samples have REE patterns similar to the Queensland based shale proxy Mud of Queensland (MuQ) consistent with local soil formed on exposed reefal limestone. Framboidal pyrite within the palaeosol suggests anoxic soil conditions during initial inundation. Younger microbialite-hosted REE and yttrium (REY) normalised to MuQ (subscript SN) have seawater-like patterns (e.g., light REE – LREE – depletion and high Y/Ho ratios) but with a well-defined, non-linear trend of changing water quality through the ensuing Holocene.

Immediately following reef initiation (>8,300 yrs BP) data suggest increasing terrigenous influence to 8,000 yrs BP on the basis of coordinated, more mud-like microbialite proxies, including reduced LREE depletion (e.g., (Nd/Yb)SN > 0.4) and lower Y/Ho ratios (< 53) with higher concentrations of lithofile elements. Proximal seawater became ‘cleaner’ from ~7,000 years ago, with opposing REY trends reflecting seawater with less terrigenous influence but showed marked mid-Holocene variability related to changing regional climatic factors. The strong fluctuation between intervals of high and low relative terrigenous sediment influence correlates well with particular regional and more global climate records, such as, the Indian-Australian Summer Monsoon (IASM) strength, high turbidity periods at 7.0, 5.4, and 2.7 ka BP with dampened El Niño Southern Oscillation (ENSO) frequency and fluctuations in local relative sea level. Water quality improved significantly after 3,200 yrs BP.

The new microbialite geochemical record provides a fully independent new water quality proxy and a means to interpret reef growth dynamics in relation to changing water quality associated with climate evolution at centennial to millennial scales. Such records provide valuable context for predictions of modern reef behavior in a changing world where coastal water quality is more likely to decline than to improve.


Biography

Dr. Marcos Salas Saavedra obtained his BSC in Biological Sciences at the Austral University of Chile in 2010 and he finished his PhD in Geochemistry and Geochronology at the University of Queensland in 2019. His research interest are in Biogeochemistry, Isotope Geochemistry, Carbonate Reefs Geology, U-series dating and Palaeoenvironment.

Rafted benthic microfossils as proxies of Neogene ocean current history in the Bass Strait seaway, south-eastern Australia

Warne, Mark1, McDonald, Abbey1

1School of Life and Environmental Sciences, Deakin University, Melbourne, Australia

Cenozoic marine strata along the southeast Australian coastline contain a record of Southern Ocean evolution over geological time.  A significant aspect of this oceanic evolution, is the development and interplay of surface currents such as the East Australian Current, Leeuwin Current and Antarctic Circumpolar Current. These, and other ocean currents, have substantially shaped marine biological diversity along the southern Australian continental margin, and have left distinctive stratigraphic markers within the marine sedimentary rock record.

Traditionally, proxy records of past surface ocean currents have been derived from fossils of marine microplankton preserved in deep ocean and continental shelf sediments.  However, in life, zooplankton often exhibit variation in depth distribution, which complicates relationships with surface ocean currents.  In contrast, and paradoxically, shallow marine benthic microfossils associated with floating macroalgae, such as epiphytal ostracods and foraminifera, can provide more direct evidence of changes in global patterns of surface ocean circulation, because dispersal only occurs at the sea surface. The dispersal mechanism for these benthic micro-organisms is via attachment to seaweed, ripped up from shallow marine environments by coastal storms, and sent drifting vast distances across oceans on surface currents, until colonization occurs in new, distant shallow marine realms.

Key Neogene rafting-related ostracod migration and extinction events apparent in carbonate and siliciclastic marine strata along the southern Victorian coastline, and which defined broad phases in the oceanographic history of Bass Strait, are as follows:

Strong warm plumes of East Australian Current waters entering eastern Bass Strait (around 16.4, 5.8 and 3.2 million years ago), as evidenced by fossil occurrences of warm water, western Pacific Neohornibrookella species.  Increases in the influence of East Australian Current waters in the Bass Strait region around 3 Ma, also likely facilitated the expansion of Pacific Ambostracon spp (pumila group species) into SE Australian coastal shallow and marginal marine realms.

Sporadic incursions of Antarctic Circumpolar Current waters entering western Bass Strait between approximately 9 and 5 million years ago, as evidenced by the influx of mid to high latitude Tasmanocypris (dartnalli group) species.

The inception of warm Leeuwin Current (aka Zeehan Current) waters entering western Bass Strait (4.4 million years ago), which created a confluence with East Australian Current waters.  This is evidenced by an east-west biogeographic differentiation of shallow marine ostracod faunas across this seaway. Notable is the widespread disappearance of Neohornibrookella and Tasmanocypris (dartnalli group) species from warm shallow marine ostracod faunas west of Cape Otway, during the early Pliocene.

Widespread extinction of warm SW Pacific derived marine ostracod taxa across the entirety of Bass Strait (e.g. Neohornibrookella species) due to the inception of the cold winter Bass Cascade current (1.8 to 2.2 million years ago), which led to winter water temperature minima too cold for these taxa.  The inception of the Bass Cascade was associated with an early Quaternary northward shift in the position of the southern hemisphere, mid latitude westerly wind belt.


Biography

Mark Warne’s main work is university teaching in the disciplines of geology, physical geography, environmental science and palaeobiology. He is currently the course director for the Bachelor of Science at Deakin University. He has research interests in the fields of micropalaeontology (principally fossil Ostracoda), marine stratigraphy, palaeoecology and palaeo-oceanography.

Transitions in Eruptive Style During the 2012 Deep Submarine Silicic Eruption of Havre Volcano, Kermadec Arc, New Zealand

Clark, Acacia1; Carey, Rebecca1; Jutzeler, Martin1; Mitchell, Samuel2

1University of Tasmania, Hobart, Australia, 2University of Bristol, Bristol, United Kingdom

Submarine eruptions are poorly understood compared to their subaerial counterparts due to challenges accessing and observing them. The 2012 silicic submarine eruption of Havre Volcano in the Kermadec Arc was the largest deep ocean eruption (~900 – 1220 meters below sea level) ever recorded.

The main vent transitioned in eruption style during the event. The current eruption framework describes the onset of magma disruption on the seafloor at high (107 kgs-1) eruption rates, which produced a large pumice raft (~1 km3) accompanied with a giant pumice seafloor deposit. This phase transitioned to an intermediate phase of unknown intensity that produced an ash-lapilli-block (ALB) deposit proximal to the vent. The final eruptive phase was low intensity (104 kgs-1) effusive magma emplacement that produced a 250 m-high dome complex (Dome OP) over the vent. Previous studies have focused on microtextures of these main phases to understand shallow conduit processes.

We have identified lobe deposits around Dome OP which stratigraphically sit above the ALB deposit but were emplaced prior to the end of the effusive phase. These deposits represent a transitional phase between high to low eruption rates. Detailed microtextural studies were conducted on four representative clasts from in-situ Dome OP together with three clasts from surrounding lobe deposits, and two dense end-member ALB clasts not previously studied.

Microlites of the same crystal types and habits are present in lobe deposits and in-situ Dome OP clasts, where they are most abundant. ALB clasts are microlite free with almost spherical vesicles. Clasts from lobe deposits have elongated vesicles with round edges and in-situ Dome clasts have elongated and flattened vesicles. ALB clasts have the highest vesicle number density, followed by lobe deposits and then in-situ Dome clasts. Rounded vapor-phase cristobalite is present in lobe deposits and in-situ Dome O clasts, whereas in-situ Dome P clasts contain an abundance of oblong cristobalite crystals that exist entirely within the groundmass. No discernible correlation could be made between vesicle size and cristobalite crystal size. Silicic submarine domes are morphologically and texturally similar to subaerial domes, indicating hydrostatic pressure has a minor role in outgassing and emplacement processes of lava domes.


Biography

Acacia graduated with a Bachelor of Science from the University of Sydney in 2015. After taking some time to explore Canada she returned to undertake her Honours at the University of Tasmania (UTAS) and graduated with First Class. She intends to begin her PhD at UTAS in late 2020.

Volcanic stratigraphy and eruption mechanisms from the last remaining outcrops at Wiri Mountain, Auckland Volcanic Field, New Zealand

Foote, April1, Németh, Károly2, Handley, Heather1

1Department of Earth and Planetary Sciences, Macquarie University, Sydney, Australia, 2Institute of Agriculture and Environment, Massey University, Palmerston North, New Zealand

Dispersed volcanic fields (commonly labelled as monogenetic volcanic fields) are of great interest in volcanology as they provide a relatively simple volcanic architecture to study regarding magma source to surface processes, edifice growth and subsequent destructions. The Auckland Volcanic Field (AVF) in New Zealand is among the few hundred documented dispersed volcanic fields worldwide that were active through the Holocene and can broadly be defined as a mafic intraplate monogenetic volcanic field. Recent research has highlighted that the transition between monogenetic and polygenetic volcanism is far more continuous than was originally thought, where volcanoes traditionally viewed as monogenetic are commonly found to have had multiple eruptions and complex magmatic plumbing systems.

This study focuses on the last remaining outcrops of Matukutūruru, or Wiri Mountain, one of the southernmost volcanic centres in the AVF. Wiri Mountain presents a unique situation where despite the large extent to which deposits have been removed, the remaining spectacular outcrops allow a clear picture to be formed, including analysis of stratigraphy and facies, vent location and extent of deposits, fragmentation depth, eruptive styles and their transitions and the eruption history of the volcanic centre.

Wiri Mountain has had a complex eruption history, beginning with a pre-existing tuff ring/maar landscape. An initial basal tuff ring was deposited by predominantly pyroclastic density currents with ballistic curtain deposits and some pyroclastic fall, through a debris filled vent that widened mostly at depth through the course of the eruptions. At least two smaller, satellite tuff rings were then deposited on the outer flanks of the first by a combination of pyroclastic density currents and pyroclastic fall, with a transition from phreatomagmatic to Strombolian eruptive style. A central scoria cone was then deposited within the initial tuff ring, followed by lava spatter and lava flows that covered the tuff rings, the scoria cone, and the surrounding area. This complex eruption history highlights the range and transition of eruptive styles leading to the production of multiple types of eruptive products and deposits that can be typical for the AVF.

The small magma volumes typical of monogenetic volcanism allow for significant influence of fragmentation and eruptive products by external water, resulting in a wide variety of volcanic landforms. However, based on the results of this study, volcanic activity at Wiri Mountain and the surrounding area of the southern end of the AVF was potentially more complex than would typically be expected from the textbook definition of monogenetic activity, with a large enough eruptive volume to allow a complex eruptive evolution over time, multiple satellite cones, and potential connections to nearby centres; highlighting the grey area on the concept boundary of monogenetic volcanism.


Biography

April Foote is a PhD candidate at Macquarie University. Her studies are based on the volcanology of the southern end of the Auckland Volcanic Field.

Geophysical and geochemical constraints on the formation of Holocene intraplate volcanism in East Asia

Ward, Jack F.1; Rosenbaum, Gideon1; Ubide, Teresa1; Wu, Jonny2; Caulfield, John T. 1,3; Sandiford, Mike4; Gürer, Derya1

1School of Earth and Environmental Sciences, The University of Queensland, Brisbane, Australia, 2Department of Earth and Atmospheric Sciences, University of Houston, Houston, USA, 3Central Analytical Research Facility, Institute for Future Environments, Queensland University of Technology, Brisbane, Australia, 4School of Earth Sciences, University of Melbourne, Melbourne, Australia

East Asia contains many Holocene volcanic centres, several of which are located far (between 600 and 1500 km) from the Pacific and Philippine Sea plate subduction zones. The origin of these intraplate volcanoes, which include Jeju, Ulleungdo, Tianchi, Jingbohu, Erkeshan and Wudalianchi, remains enigmatic. Geodynamic processes proposed to explain the occurrence of the East Asian Holocene intraplate volcanoes include mantle plume activity, subduction processes with slab fluid involvement, and subduction processes without slab fluid involvement. Here, we evaluate a variety of geophysical datasets and a compilation of geochemical data to assess the feasibility of these mechanisms. High-resolution tomography data provide no evidence for the rise of deep-seated mantle plumes. Instead, the tomographic and seismic data highlight the stagnation of the Pacific slab at the 660 km discontinuity below Tianchi, Longgang, Jingbohu, Erkeshan and Wudalianchi. The geophysical data also provide evidence for the stagnation of the Philippine Sea slab at the 410 km discontinuity below Jeju and Ulleungdo. Although the intraplate volcanoes appear to be located above subducted slabs, the geochemical data do not provide evidence for melt generation due to slab metasomatism. Instead, the intraplate volcanoes are alkaline in composition and display primitive mantle normalised trace element characteristics comparable to those shown by ocean island basalts. In light of the absence of evidence for plume activity or slab metasomatism, we suggest that convective upwellings occurring at the edges of the Pacific and Philippine Sea slabs may be responsible for Holocene intraplate volcanism in East Asia. Because it is likely that the Pacific and Philippine Sea slabs have been stagnant in the mantle transition zone for millions of years, we speculate that slab-edge convection and volcanism may be driven by regional-scale tectonic events. We conclude by discussing possible Neogene–Quaternary tectonic events that may have contributed to the occurrence of East Asian Holocene intraplate volcanism. 


Biography

Jack Ward is a PhD student at the School of Earth and Environmental Sciences, The University of Queensland. He uses geophysical and geochemical data to better understand the processes that cause anomalous subduction-related magmatism.

Timescales of magma ascent recorded by olivine zoning patterns from Mount Leura and Mount Noorat, Newer Volcanics Province, Australia

Didonna, Rosa1, Handley, Heather1, Cas, Ray2, Fidel, Costa3, Murphy, Timothy1

1Department of Earth and Environmental Sciences, Macquarie University, Sydney, NSW, Australia 2School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, Australia 3Earth Observatory of Singapore, Nanyang Technological University, Singapore

Intraplate continental basaltic volcanic provinces (ICBVPs) occur on all continents, but the timescales of magmatic processes that lead to eruption in such settings are poorly understood due to the temporal infrequency and lack of spatial pattern in eruptions. Therefore, unravelling the timescale of magma ascent is a critical aspect to advance our understanding of volcanic hazard and risk. Here we focus on the Newer Volcanics Province (NVP) of SE Australia is an active intraplate basaltic province that contains over 400 volcanic centres. Volcanic landforms include maars, tuff rings, scoria cones, lava fields along with more complex eruption centres and the rate of activity has varied in space and time. Despite a large number of studies on the bulk-rock geochemistry and physical volcanology of the deposits (e.g. stratigraphy, eruption styles), few constraints are available on the timescales of magma ascent in the province.

We have investigated the olivine crystals within entrained mantle xenoliths and as individual crystals within the groundmass of basaltic volcanic rocks using compositional X-ray maps, backscattered electron (BSE) images and electron microprobe analyses (EMPA). We focus on NVP samples from Mount Leura (Lehurra kang) and Mount Noorat (Knorart) to shed a light on the dynamic processes that lead to the eruption and the relative timescales of magma ascent. Olivine crystals in mantle xenoliths are mainly unzoned with Mg#90 (Mg# = 100×Mg/[Mg+Fe]) with few crystals normally zoned (<Mg#75 rim). Olivine grains in the groundmass are commonly up to 1 mm in size and are mainly skeletal, with significant variation in Mg#, CaO, MnO and NiO content from core to rim. Olivine grains in the Mount Noorat samples are largely normally zoned with crystal interiors characterised by >Mg#90 and rims by <Mg#75. Olivine crystals from Mount Leura contain complexly zoned olivines suggesting a more complex crystallisation and transport history (Mg#77-79 cores and rims up to Mg#85). We model the chemical zonation patterns in olivine crystals that reveal a short time of magmatic processes before the eruption. The insight of magma storage, ascent and the pathway to the surface at NVP is a crucial information in understanding the volcanic hazard and mitigation risk in the region for which too little consideration is still given at the present.


Biography

Research Fellow in the Dept of Earth and Environmental Sciences at Macquarie University. PhD from Macquarie University in April 2020.

Research interests in igneous petrology and volcanology. Focus on crystal records of magmatic processes and timescales in the lead-up to volcanic eruptions combining field observations and detailed microscopy analytical techniques.

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About the GSA

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