IAP2-22-355

Is the onset of plate tectonics linked to enhanced recycling of bioessential nutrient?

Early life on Earth was likely less proliferous than it is today, due to severe limitations in the supply of key nutrients, such as phosphorus and nitrogen. The major source of nitrogen to the biosphere is biological N2 fixation; however, this pathway requires molybdenum in its most efficient form, which was poorly soluble in the anoxic Archean Ocean (Johnson et al. 2021, Stüeken et al. 2016). Further, the solubility of phosphorus may have been suppressed by ferruginous (Fe2+-rich) conditions, causing rapid scavenging of phosphorus by iron oxides and carbonates (Reinhard et al. 2017). However, so far, these nutrient constraints are focused on the open marine realm and neglect terrestrial or near-shore settings, where nutrient fluxes may potentially have been elevated, due to weathering of rocks enriched in N and P.

There was a step change in the composition of Earth’s continental crust in the mid- to late-Archean, when early sodic TTGs were intruded by voluminous potassic granites (Laurent 2014). We hypothesize that these rocks mark the point when the Archean continental crust became enriched in N and P after the onset of widespread plate tectonics, when subduction and melting of sedimentary deposits created a pathway of N and P into newly formed granitic rocks. Weathering of these granitoids over time could have created a bioavailable flux of N and P to the biosphere, stimulating productivity and possibly facilitating biological innovation in the lead-up to the Great Oxidation Event around 2.4 Ga.

Both N and P behave as incompatible elements in the ferromagnesian minerals comprising the mantle. Nitrogen follows potassium and can be enriched in micas and feldspars (Hall 1999). Phosphorus may reach apatite saturation in felsic melts. The secular evolution of N in granitic rocks is largely unknown while P data from whole-rocks suggest a progressive increase in the global P abundances at the Archean-Proterozoic boundary and in the Neoproterozoic (Bucholz 2022, Cox et al. 2018). These rises in P are linked to recycling of increasingly P-rich sediments. However, the linkage between P recycling and the onset of plate tectonics in the early Archean has not been explored.

This project therefore has three main aims:
(1) Measure concentrations and isotopic compositions of N in early Archean granitoids to determine if and when a sedimentary source signature becomes visible. Mafic seafloor volcanic rocks will also be analysed to constrain this non-sedimentary source of N to the crust.
(2) Measure concentrations of P in zircon growth rings from both early and late granitoids, to obtain a time-resolved record of P abundances in felsic melts. Assess if this can be linked to geochemical signatures of sedimentary input, using additional tracers and MELTS modelling.
(3) Experimentally explore the solubility of N-bearing (and possibly P-bearing) mineral phases under Archean conditions. These data will provide constraints on the bioavailability of mineral-bound N to the Archean biosphere.

The results from this work will shed new light on linkages between the solid Earth and the habitability of surface environments at a time when life was becoming increasingly established.

Methodology

Archean granitoids will be obtained from existing collections and collaborators of the project PIs. This will include some of the oldest preserved granitoids in Greenland, as well as younger Neoarchean specimens from southern Africa. In addition, there is scope to collect samples in the field in southern Africa in Year 1 or 2. The samples will be analysed in three work packages (WP), each of which has the potential to yield a stand-alone publishable manuscript.

WP1: Whole-rock powder will be prepared and analysed by XRF to obtain information about major element composition and potential alteration. Alteration will also be assessed with thin section petrography. The whole-rock powders, as well as separates of micas and feldspars, will be analysed for nitrogen isotopes and abundances, using an offline-combustion setup and a tube-cracker coupled to a gas-source mass spectrometer. The mineral separates will provide insights into the major ammonium host phases.

WP2: Zircon separates from the samples will be studied for U-Pb age and phosphorus content using laser-ablation triple quadruplole ICP-MS. Analyses of individual zircon growth rings will provide information about temporal trends in P content of the source melt. These will be compared to abundances of elements that are indicative of magma source (Al, K, etc.). Thermodynamic models in MELTS may be used to derive quantitative linkages between source material and apatite saturation in felsic melts.

WP3: Solubility experiments will be performed under anoxic conditions at different temperatures and fluid compositions to explore the mobility of ammonium in Archean surface environments compared to the modern. Input minerals will be taken from WP1. Analyses of dissolved nitrogen species in solution (including ammonium, as well as potential nitrite or nitrate resulting from ammonium oxidation) will be conducted by UV-VIS spectrophotometry and ion chromatography.

All analytical techniques are available at the University of St Andrews.

Project Timeline

Year 1

Literature review, review historic data from case studies. Gather samples for analysis from collaborators and field sites; lab training and initial data collection

Year 2

Collect geochemical data for whole-rock composition (XRF), P abundances (LA-ICP-MS) and N abundances & isotopes (IRMS); petrographic analysis; present results at domestic conference; compile first manuscript for publication.

Year 3

Complete data collection on Archean granites and perform solubility experiments; begin to compile second manuscript

Year 3.5

Complete the thesis and final manuscripts; present data at an international conference

Training
& Skills

Laboratory training in advanced analytical techniques, including laser-ablation inductively-coupled plasma mass spectrometry, gas-source mass spectrometry, UV-VIS spectrophotometry, and ion chromatography.
• Training in sample collection, preparation and curation, as well as data and time management
• Opportunity to attend CEED courses (Centre for Educational Enhancement and Development) at the University of St Andrews to develop computational skills in areas such as statistical data analysis using R, or crafting publication-quality figures in Adobe Illustrator
• Participation in workshops on MELTS modelling
• Training in scientific writing through crafting of publishable manuscript and training in public speaking via oral presentation at conferences
• Transferrable skills training through the IAPETUS2 DTP network

References & further reading

• Bucholz, C.E., 2022. Coevolution of sedimentary and strongly peraluminous granite phosphorus records. Earth and Planetary Science Letters, 596, p.117795.

• Cox, G.M., Lyons, T.W., Mitchell, R.N., Hasterok, D. and Gard, M., 2018. Linking the rise of atmospheric oxygen to growth in the continental phosphorus inventory. Earth and Planetary Science Letters, 489, pp.28-36.

• Hall, A., 1999. Ammonium in granites and its petrogenetic significance. Earth-Science Reviews, 45(3-4), pp.145-165.

• Johnson, A.C., Ostrander, C.M., Romaniello, S.J., Reinhard, C.T., Greaney, A.T., Lyons, T.W. and Anbar, A.D., 2021. Reconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth. Science advances, 7(40), p.eabj0108.

• Laurent, O., Martin, H., Moyen, J. F. & Doucelance, R. 2014. The diversity and evolution of late-Archean granitoids: Evidence for the onset of “modern-style” plate tectonics between 3.0 and 2.5 Ga. Lithos 205, 208-235.

• Reinhard, C.T., Planavsky, N.J., Gill, B.C., Ozaki, K., Robbins, L.J., Lyons, T.W., Fischer, W.W., Wang, C., Cole, D.B. and Konhauser, K.O., 2017. Evolution of the global phosphorus cycle. Nature, 541(7637), pp.386-389.

• Stüeken, E.E., Kipp, M.A., Koehler, M.C. and Buick, R., 2016. The evolution of Earth’s biogeochemical nitrogen cycle. Earth-Science Reviews, 160, pp.220-239.

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