Magmatic mass transfer through deep crust: Field relationships, chemistry and rheology
Dr Sandra Piazolo (SEE), Dr. Thomas Mueller (SEE)Project partner(s): Centre of Core to Crust Fluid Systems, Macquarie University (CASE)Contact email: firstname.lastname@example.org
This exciting project aims to shed light on the long standing problem of how melt is transferred through the crust by a combination of field studies in Greenland and Ireland, combined with lab-based microstructural and geochemical analyses. Depending on student’s interests investigations will be augmented by a choice of high temperature or analogue experiments and/or numerical modelling. The student will be part of an international research group involving partners and students based in Greenland, Australia and Italy. Throughout the studentship, the student will have the opportunity to visit partners.
Fluids are instrumental in the evolution of Earth’s crust and mantle; they facilitate chemical exchanges that change basic rock properties and are important for crustal differentiation at the large scale. Fluid advection of heat and mass is central to nuclear waste storage, CO2 sequestration, geothermal systems, and the formation of ore deposits.
The motivation for examining transfer of melt is rooted in a fundamental gap in our knowledge. It is poorly understood how magmatic mass transfer occurs through the deep crust. This project builds on observations that significant migration of melt and mass transfer at the kilometre-scale can occur in localized areas resulting in significant changes to both the melt and the host through which melt migrates (Daczko, Piazolo et al. 2016) (Fig. 1).
This project aims to achieve a new level of understanding and quantification of the underlying principles governing magmatic mass transfer through deep crust. Three main questions will be addressed:
- Processes: What physiochemical processes are involved in magmatic mass transfer through deep crust?
- Recognition: How can geologists recognize prior magmatic mass transfer in natural rocks? What is the physical and chemical fingerprint at micro- to meso-scales?
- Effect: How does magmatic mass transfer affect the chemistry, geochronology, melt fertility and rheology (strength) of the crust it transfers through as well as the crust it forms at higher levels?
Figure 1. Field example of melt transfer zone in the lower crust, Fiordland, New Zealand. It shows a gabbroic gneiss (light grey) which has changed due to fluxing of a hydrous melt to an ultrabasic granofels rock (hornblendite, dark) in a channel of melt-rock interaction (~40m wide). The melt channel is inferred to be a zone of significant mass transfer on the basis of the change in rock chemistry and mineral assemblage. For scale see geologists in the foreground (modified after Daczko, Piazolo et al. 2016).
In this project, you will work with leading scientists at Leeds, UK, and the Centre of Excellence, (Macquarie University, Australia), together with experts on the geology of the field areas to develop an in-depth understanding of how melt moves through the crust and how such melt flux influences the chemical make-up of both the transgressing melt and the material that the melt passes through. Special emphasis will be given to the feedback between deformation and melt migration.
The studentship will involve
- Field work in remote areas of W. Greenland, and Connemara, Ireland (Fig. 2).
- In-depth analysis of samples from the two field areas. This will include chemical analysis including major and minor elements, bulk rock geochemical analysis, quantitative microstructural analysis (e.g. Smith et al. 2015) and high resolution trace element analysis using synchrotron analysis (Fig. 3).
Figure 2 Field relationships of melt related structures. (left) Melt flux zone in a high strain zone in the lower crust of W. Greenland. (right) Melt rock interaction at Conomarra; note the reaction between feldspar rich intrusive layers ( “melt”) and gabbro where a hornblende- rich reaction rim is formed.
In order to develop an in-depth understanding of the processes involved, the student will be able to utilize additional tools, the choice made depending on the student’s individual background and interests:
- Numerical modelling of reactive flow
- High temperature- high pressure experiments
- Analogue modelling with real-time analysis (see for example Bons et al. 2001, 2008)
- Trace element analysis using laser ablation and synchrotron techniques (Stuart et al. 2016)
Potential for high impact outcome
Geochemical signatures in upper crustal magmatic and volcanic rocks suggest that they are sourced from lower crustal or even mantle environments (e.g. Bourdon et al. 2002; Gray and Kemp 2009; Vigneresse 2006). However, we do not fully understand the mechanisms responsible for magmatic mass transfer through the lower and middle crust; these are widely debated [Brown, 2004; Petford, 1996; Weinberg, 1996] and the recognition of these processes in the rock record is poor.
We are in a unique position at Leeds in collaboration with the Australian Research Council Centre of Excellence “Core to Crust Fluid Systems” (CCFS, http://www.ccfs.mq.edu.au/, Macquarie University, Australia) to answer important unresolved questions about how magma moves through the crust. This knowledge is fundamental to our understanding of Earths’ evolution. At the same time, the understanding of reactive flow and tools to recognize, predict and model reactive flow is fundamental to many problems facing society (e.g. security of nuclear waste deposits, ore mineral formation). Hence, beside the advances in the fundamental understanding of crustal formation and evolution, the tools developed throughout this project have immediate policy-relevant implications. Consequently, we anticipate the project generating several papers being suitable for submission to high impact journals.
|Figure 3: Trace element fingerprinting of melt flux using high end synchrotron analysis (Stuart et al. 2016). a: Overview photomicrograph of a fluxed gabbroic gneiss in plain polarized light showing pyroxenes with incipient coronas of hornblende and quartz intergrowths. FOV = 18 mm. c: Plagioclase Sr concentration (black low, white high concentrations) of same area as shown in (a); pyroxenes are marked in black. Sr is enriched next to replacement microstructures and in bands connecting individual coronas across samples. e: Schematic diagram illustrating the interpreted flow of melt as highlighted by the high Sr concentrations; grey areas are plagioclase rich domains. Melt is interpreted to have moved along pyroxene/corona boundaries parallel to foliation (red), accumulated and reacted in embayments (green), and moved along plagioclase-plagioclase boundaries forming ‘bridges’ (blue). Note that bands connect individual coronas across samples (modified after Stuart et al. 2016).|
Training & Framework of the project
The student will work under the supervision of Assoc. Prof. Sandra Piazolo and Dr. Thomas Mueller within the IGT metamorphic and structural geology group. This project provides a high level of specialist scientific training in: (i) Field work and targeted sampling in lower to mid crustal sections, (ii) state-of-the-art analytical techniques with special emphasis on both chemical and structural analysis of geomaterials; along with a selection of other skills including numerical modelling of reactive flow, high temperature and pressure experiments and analogue modelling. Co-supervision will involve regular meetings between partners and extended visits for the student to the Centre of Excellence “Core to Crust Fluid Systems” (CCFS, Macquarie University, Australia), where the student will work under the supervision of Assoc. Prof. Nathan Daczko.
The successful PhD student will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of training workshops in numerical modelling, managing your degree, and preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/). The student will not only benefit from the experts and facilities at Leeds University but also the ARC Centre of Excellence “Core to Crust Fluid Systems” located at Macquarie University.
The student will be part of a larger joint research effort currently under way both at Leeds University and CCFS (Macquarie University) involving at least 3 PhD students with collaborators within the School at Leeds and partners in Sydney and Padua. As such, the student will be part of an active group excited to unravel the processes and signatures involved in reactive fluid flow and mass transfer.
The student should have a strong interest in structural geology, metamorphic geology and igneous processes, love for field work and thinking out of the box and a strong background in a quantitative science (maths, physics, chemistry). Willingness to work within a research team is essential. A Masters of Research in a relevant area will increase the chances to get a scholarship.
The proposal has been agreed as a “Partnership Project” (a potential CASE project) with CCFS, Macquarie University providing extra funding additional to the NERC student stipend.
Bons, P. D., Elburg, M. A. and Dougherty-Page, J. (2001). Analogue modelling of segregation and ascent of magma. In: Ailleres, L. and Rawling, T. 2001. Animations in Geology. Journal of the Virtual Explorer, 4.
Bons, Paul D., et al. (2008) Finding what is now not there anymore: Recognizing missing fluid and magma volumes. Geology 36, 851-854.
Bourdon, E., J.-P. Eissen, M. Monzier, C. Robin, H. Martin, J. Cotten, and M. L. Hall (2002), Adakite-like lavas from Antisana Volcano (Ecuador): Evidence for slab melt metasomatism beneath Andean Northern Volcanic Zone, Journal of Petrology, 43(2), 199-217.
Brown, M. (2004), The mechanism of melt extraction from lower continental crust of orogens, Geological Society of America Special Papers, 389, 35-48.
Daczko, N.R., Piazolo, S., Meek, U., Stuart, C.A. and Elliott, V. (2016), Hornblendite delineates zones of mass transfer through the lower crust, Scientific Reports, 6, 31369, doi:10.1038/srep31369.
Gray, C. M., and A. I. S. Kemp (2009), The two-component model for the genesis of granitic rocks in southeastern Australia — Nature of the metasedimentary-derived and basaltic end members, Lithos, 111(3–4), 113-124.
Klepeis, K. A., Schwartz, J., Stowell, H., & Tulloch, A. (2016), Gneiss domes, vertical and horizontal mass transfer, and the initiation of extension in the hot lower-crustal root of a continental arc, Fiordland, New Zealand, Lithosphere, 8(2), 116-140.
Petford, N. (1996), Dykes or diapirs?, Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 87(1-2), 105-114.
Stuart, C.A., Piazolo, S. and Daczko, N.R. (2016), Mass transfer in the lower crust: evidence for incipient melt assisted flow along grain boundaries in the deep arc granultes of Fiordland, New Zealand, Geochemistry, Geophysics, Geosystems (G3), doi: 10.1002/2015GC006236.
Smith, J. R., Piazolo, S., Daczko, N. R., & Evans, L. (2015). The effect of pre‐tectonic reaction and annealing extent on behaviour during subsequent deformation: insights from paired shear zones in the lower crust of Fiordland, New Zealand. Journal of Metamorphic Geology, 33(6), 557-577.
Vigneresse, J. L. (2006), Granitic batholiths: from pervasive and continuous melting in the lower crust to discontinuous and spaced plutonism in the upper crust, Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 97(04), 311-324.
Weinberg, R. F. (1996), Ascent mechanism of felsic magmas: news and views, Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 87(1-2), 95-103.
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