Heterogeneous deep mantle sources or shallow melt-rock interaction? Has the mantle zoo paradigm run its course?

This Project has been filled

Project background: The Earth has evolved through large scale differentiation events such as core segregation and by staggered production of continental and oceanic lithosphere. Since the onset of plate tectonics at ca. 3 Ga, differentiated material has been mixed back into the mantle. This is widely accepted as a pre-requisite for any explanation of the observed composition of many mantle plume-related ocean island basalts (OIB) erupted at the Earth’s surface, giving rise to the so-called “Mantle Zoo” paradigm of deep magma sources (Figure 1). The distribution of compositional heterogeneity remains poorly constrained due to the inaccessibility of actual deep mantle samples. Combined with the historical reliance upon bulk-rock measurements of primitive basalts to infer the complex composition of the mantle source(s) of magmas, our understanding of mantle physical and chemical evolution has reached an impasse. A new perspective based on understanding the physical mechanisms of isotope inheritance is needed to contextualise patterns in bulk-rock measurements. Recent technological advances now make this possible. Similar to how zoos of old have evolved from animals in individual pens with simple labels, to open-plains and rich descriptions of ecosystems, the mantle zoo as we know it may transform to an altogether more elegant concept. DTP 2019 Figure 1

It is a fundamental tenet in geochemistry that during mantle melting the isotopic fingerprint of the mantle source is faithfully recorded in the melt it produces. This information is complemented by the major and trace element signature of the magma that is generated. Determining the unequivocal composition of a mantle source can, however, be problematic because the effect of mixed pyroxenite-peridotite melting, and interaction with the overlying lithosphere prior to eruption, introduces further layers of uncertainty to petrogenetic models. Scores of studies have explained the correlations between isotopic signatures and trace element systematics of basalts as arising naturally from varying degrees of melting of a compositionally heterogeneous source. This interpretation has constituted the state of the art for decades and is firmly rooted in the bulk-rock analysis of primitive basalts.

What is not clear though, is to what degree compositional heterogeneity of magma can be caused by a) melting of a heterogeneous source versus b) contamination through shallow interaction with host rocks (combinations of lithospheric mantle, gabbroic crust, terrestrial or marine sediments). In particular, the nature of the EM-1 (enriched mantle-1) magmatic signature has been attributed to both deeply mixed subducted lithosphere (Zindler and Hart, 1986; Hofmann, 1997; Boyet et al., 2019) and possible shallow interaction with foundered slabs of continental passive margins that became detached during the early stages of oceanic rifting (e.g. Class and le Roex, 2006, Millet et al., 2008).

DTP 2019 Figure 2 In this study we will use the Canary Islands archipelago, which traverses a continental margin, as a natural laboratory. Here we can examine the effects on melt composition as a function of shallow interaction with different lithosphere, transitioning from one that is dominantly oceanic (La Palma, El Hierro) to increasingly continental in its nature (through Tenerife, La Gomera and Gran Canaria, to Lanzarote and Fuerteventura). In particular, this study will use melt inclusions, trapped in early crystallising phases such as olivine, in primitive lavas to compare melt compositions prior to interaction with the overlying lithosphere with the final erupted products. Melt inclusions provide a snapshot of primary mantle melt composition therefore preserving evidence of the compositional heterogeneity in the mantle source. Melt inclusions are, however, small (sub-mm diameter) and, until recently, analysis of more than one potential tracer of mantle source in any individual melt inclusion had been challenging.

This project, using a powerful combination of major, trace and volatile elements with Sr, Nd and Pb isotope measurements, and using methods recently developed by the supervisors and their collaborators (see Harvey et al., 2009; Reinhard et al., 2017, 2018, Pankhurst et al., 2018, 2019) will fingerprint the nature of the chemical and isotopic heterogeneities in individual melt inclusions that are representative of the ingredients that formed the melts that ultimately built the Canary Islands. This combination of tracers will not only provide a unique perspective on the scale, magnitude and distribution of mantle heterogeneities within the mantle plume that has underlying the Canaries for >30 Myr, but also provide valuable insights into whether interaction of a melt with a passive margin can replicate the EM-1 signature attributed to deep-mantle heterogeneity. Differences between melt inclusion compositions and final erupted magmas will reveal any changes in the nature of shallow contamination approaching a passive continental margin.

Specific objectives of the project and project milestones: The specific objective of this study is to determine the effects on primitive basalt composition with proximity to a passive continental margin. During the project you can expect to collect a comprehensive set of primitive basalts and coexisting mantle xenoliths from across the Canary Island archipelago during fieldwork early in the project. Sample characterisation and preparation will be key aspects of the project and it is envisaged that this, along with the fieldwork, will encompass the majority of year 1. Year 2 will be focussed on gathering isotopic (potentially Sr-Nd-Pb-Hf-Os) data on key primitive basalts and peridotite xenoliths from across the archipelago, moving towards a study of melt inclusions during the remainder of the project.

Potential for high impact outcome: Several recent studies (Koornneef et al., 2015; Reinhard et al., 2017, 2018) have demonstrated the potential for these methods to generate high-impact publications. The methods to be employed in this project take advantage of the most recent advances in mass spectrometer technology and draw upon the supervisors’ extensive chemical and petrological experience. The resultant datasets will set a new benchmark for the nature of the information that can be extracted from individual melt inclusions. This is a field in its infancy and any high-quality dataset resulting from this study would likely generate a great deal of interest across several fields in the geological sciences.

Training: In addition to the programme-wide training delivered by the Doctoral Training Programme, the successful candidate can expect to receive training in sample preparation, chemical and isotopic separation and mass spectrometric analysis of melt inclusions. The use of MELTS modelling software and diffusion modelling could also play a role in the project and appropriate training can be given in-house for these methods.

Student profile: Candidates should have a good degree in an Earth Science discipline, an interest in geochemistry and volcanology, and be willing to assist with method development in a world-class geochemistry facility. The nature of this project means that a high degree of competency in sample preparation and mass spectrometry will be necessary. Given the cutting-edge nature of both the mass spectrometry and sample preparation methods that will be employed, it is unlikely that a potential candidate would have necessarily already acquired these skills, although some geochemical experience would be an advantage. This is a technical geochemical project that will require a keen eye for detail, patience and a willingness to work for extended periods in a clean laboratory. An interest in developing mass spectrometry skills would also be an advantage. The project will also require fieldwork in the Canary Islands.

 

References:

Boyet, M, Doucelance, R., Israel, C., Bonnand, P., Auclair, D., Suchorski, K., Bosq, C. 2019. New Constraints on the Origin of the EM‐1 Component Revealed by the Measurement of the La‐Ce Isotope Systematics in Gough Island Lavas. Geochemistry, Geophysics, Geosystems 20 (5), 2484-2498. https://doi.org/10.1029/2019GC008228

Class, C., & le Roex, A. P. 2006. Continental material in the shallow oceanic mantle—How does it get there? Geology, 34(3), 129–132. https://doi.org/10.1130/G21943.1

Harvey, J. & Baxter, E.F. 2009. An improved method for TIMS high precision neodymium isotope analysis of very small aliquots (1–10 ng). Chemical Geology 258, 251-257. doi:10.1016/j.chemgeo.2008.10.024

Hofmann, A.W. 1997. Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219-229

Koornneef, J. M., Nikogosian, I., van Bergen, M. J., Smeets, R., Bouman, C. & Davies, G. R., 2015. TIMS analysis of Sr and Nd isotopes in melt inclusions from Italian potassium-rich lavas using prototype 1013 Ohm amplifiers. Chemical Geology. 397,14-23

Millet, M., Doucelance, R., Schiano, S., David, K., Bosq, C. 2008. Mantle plume heterogeneity versus shallow-level interactions: A case study, the São Nicolau Island, Cape Verde archipelago. Journal of Volcanology and Geothermal Research, 176, 265-276 https://doi.org/10.1016/j.jvolgeores.2008.04.003

Pankhurst, M.J., Vo, N.T, Butcher, A.R., Long, H., Wang, H, Nonni, S., Harvey, J., Guðfinnsson, G, Fowler, R, Atwood, R., Walshaw, R, Lee, P.D. 2018. Quantitative measurement of olivine composition in three dimensions using X-ray micro-computed tomography. American Mineralogist 103(11), 1800-1811 https://doi.org/10.2138/am-2018-6419 

Pankhurst, M.J., Gueninchault, N., Andrews, M, Hill, E. 2019. Non-destructive three-dimensional crystallographic orientation analysis of olivine using Laboratory Diffraction Contrast Tomography. Mineralogical Magazine, in press https://doi.org/10.1180/mgm.2019.51

Reinhard, A.A, Jackson, M.G., Harvey, J., Brown, C.R., Koornneef, J.M. 2017.Extreme differences in 87Sr/86Sr between Samoan lavas and the magmatic olivines they host: Evidence for highly heterogeneous 87Sr/86Sr in the magmatic plumbing system sourcing a single lava. Chemical Geology 439, 120-131. https://doi.org/10.1016/j.chemgeo.2016.05.017

Reinhard, A.A., Jackson, M.G., Koornneef, J.M., Rose-Koga, E.F., Blusztajn, J., Konter, J.G., Koga, K.T., Wallace, P.J, .Harvey, J. (2018) Sr and Nd isotopic compositions of individual olivine-hosted melt inclusions from Hawai’i and Samoa: Implications for the origin of isotopic heterogeneity in melt inclusions from OIB lavas. Chemical Geology 495, 36-49. https://doi.org/10.1016/j.chemgeo.2018.07.034