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Production and recycling of Archean continental crust

Production and recycling of Archean continental crust

Continents are essential for the development and survival of life on Earth. However, as surprising as it may sound, there did not exist a planetary scale numerical model to show the formation of the oldest continents until the recent study ‘Growing primordial continental crust self-consistently in global mantle convection models‘ in Gondwana Research by Jain et al., 2019. Hot off the press, the first author of this study himself, Charitra Jain, Post Doctoral Research Associate in the Department of Earth Sciences at Durham University, shares the scoop in our News & Views!

Why do we care?

Uniquely positioned within the habitable zone [1], Earth is the sole planet within our solar system that sustains life. Understanding the factors that make a planet habitable [2,3] are becoming increasingly relevant with the rapidly expanding catalogue of extrasolar planets over the last decade [4,5]. The operation of plate tectonics and the formation and stability of continental landmasses have played a crucial role in the atmospheric evolution and development of life on Earth [6]. Plate tectonics is a theory where the outer surface of the Earth (lithosphere) is fragmented into a number of mobile plates that drift at a speed of few centimetres per year relative to each other, atop a convecting mantle. Mountains, volcanoes, and earthquakes are found at the boundaries of these plates. Continents cover about a third of the planet’s surface area and are made of thin crust overlying a thick and undeformable cratonic lithosphere [7,8]. Even after having assembled a reasonable model for its origins and internal workings, many fundamental questions pertaining to Earth sciences remain unresolved, for example:

  • How did the first continents form and what accounts for their stability over billions of years? Was their growth an episodic process [9] or a continuous process with a significant drop in crustal growth rate around 3 Ga (billion years ago) [10]? How much of the continental crust has recycled into the mantle [11]?
  • What was the global geodynamic regime exhibited by early Earth? When and how did the subduction-driven plate tectonics commence [12]? Did the presence of continents play a role in a regime transition from vertical tectonics to horizontal tectonics around 3Ga [13]?

Owing to the increasing paucity of natural observational data as we go back further in time [14], numerical modelling constrained by experimental and field data has thus become indispensable to uncover the secrets of Earth’s evolutionary history. Our recent study [15] is a step in the right direction where we have developed a new two-stage melting algorithm to create oceanic (basaltic/mafic) and continental (felsic) crust in self-consistent global mantle convection models for the first time.

What’s new in these models?

Generally, two stages of mantle differentiation are inferred to generate continental crust as shown in the schematic in Fig. 1A. First, basaltic magma is extracted from the mantle. Second, it is buried and partially melts to form felsic continental crust. During the much hotter Archean conditions [16,17], majority of continental crust was made of Tonalite-Trondhjemite-Granodiorite (TTG) [18,19] rocks. Experimental data suggests that TTGs are formed when hydrated basalt melts at garnet-amphibolite, granulite or eclogite facies conditions [20,21] and specific P-T conditions [22] have been employed as a criterion for generating TTGs by the authors in their models.

Interested in the long-term planetary evolution, we parametrised the processes of melt generation and melt extraction. If the melt is generated within the top 300 km of the mantle (Fig.1B1), it is instantaneously removed from the depth (Fig.1B2) [23,24] and transported both to the bottom of the crust (plutonism/intrusion) and to the top of the model domain (volcanism/eruption) (Fig. 1B3). The intruded melt stays molten while a temperature adjustment to account for adiabatic decompression is applied, and tends to result in a warm, weak lithosphere. The erupted melt is rapidly solidified by setting its temperature to the surface temperature (300K), resulting in a strong and cold lithosphere [25]. The mass ratio of erupted to intruded melt is controlled by a parameter called eruption efficiency, which is tested extensively in our simulations. Geological data suggests that the majority of mantle-derived melts intrude at a depth, corresponding to an eruption efficiency between 9% and 20% [26].


Figure 1: A, One-dimensional compositional variation with basalt-harzburgite continuum consisting of a mixture of olivine (ol) and pyroxene-garnet (px-gt) mineralogies in different proportions. Upon initialisa- tion, the whole mantle has a pyrolytic composition. B, Cartoon depicting a section of a mesh column (not to scale) in a stage: B1, where crustal production has already happened; B2, after melt removal but before compaction or opening gaps in lithosphere for magmatism; B3, with the eruption and intrusion of the melt with the white downgoing arrows representing compaction of tracers/markers.

How do these results stack up against data?

By varying initial core temperature, eruption efficiency, and limiting the mass of TTG that can be generated in our simulations to 10% and 50% of basalt mass, we present results from two sets of simulations. The crustal volumes have the same order of magnitude and the crustal composition follows similar trends as reported from geological and geochemical data [10,27,28]. Interestingly, we report two stages of TTG production without needing a significant change in convection regime: a period of continuous linear growth with time and intense recycling fuelled by strong plume activity and lasting for around 1 billion years, followed by a stage with reduced TTG growth and moderate recycling. The production of TTGs happen at the tip of deformation fronts driven by the lateral spreading of plumes (mantle upwellings) that rise to the surface (Fig. 2). These results indicate that the present-day slab- driven subduction was not required for the genesis of Archean TTGs [29,30] and early Earth exhibited a “plutonic squishy lid” or vertical-tectonics geodynamic regime [31–33].


Figure 2: Thermal (top) and compositional (bottom and zoom-in) evolution with time for a simulation with initial core temperature of 6000 K, mantle potential temperature of 1900 K, eruption efficiency of 40%, and TTG mass limited to 10% of basalt mass. The lighter shades of teal in the composition field represent progressive mantle depletion (higher harzburgite content) with time.

What’s coming next?

These results are parameter-dependent to some extent and they would change in a 3D domain as that would limit the impact of plumes on lithosphere dynamics. Future models will aim for forming the viscous cratonic roots and incorporating the effects of water [34,35] and grain-size [36–38] on mantle rheology. Going forward, modelling coupled core-mantle-atmosphere systems will shed more light on the role of different tectonic modes towards planetary habitability [39,40] and help solve these contentious aspects.

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The Sassy Scientist – Research Relevance

The Sassy Scientist – Research Relevance

Every week, The Sassy Scientist answers a question on geodynamics, related topics, academic life, the universe or anything in between with a healthy dose of sarcasm. Do you have a question for The Sassy Scientist? Submit your question here.

Meghan asks:

Why is your research relevant?

Dear Meghan,

Because I like it. My supervisor is in my office every day to talk about my results. I talk to people outside my department and they say it all looks very promising. Who cares I did not produce a Nature or Science paper? I’m having fun… *cough*

But seriously: this is a frequently asked question and a particularly difficult question to answer when you’re a young scientist. It also depends on the goal: should your research be applicable to society (most funding agencies seem to head in that direction), or is fundamental research in trying to understand the present-day state and history of our (and other) planet(s) also relevant? Oftentimes geodynamics research is only indirectly related in terms of societal impact. Does this mean that our research is irrelevant? I doubt it.

To be clear, when you design a research project or scroll through research job postings, the only thing you should be thinking of is if this is interesting enough to work on for a couple of years. Then, when you are doing this research, the answer to your question is an easy one: because it’s interesting enough for you to work on it.

Yours truly,

The Sassy Scientist

PS: This post was written after seeing the umpteenth ‘exciting observation’ for some space thingy a gazillion miles away on a national news program.

How the EGU works: Experiences as GD Division President

How the EGU works: Experiences as GD Division President

In a new regular feature, Paul Tackley,  president of the EGU geodynamics division, writes about his role as a president, and gives us an insider’s view on how EGU works and is preparing for the future. 

Paul Tackley. Professor at ETH Zürich and EGU geodynamics division president. Pictured here giving an important scientific talk, or maybe at karaoke. Your pick.

Stepping into the role of GD Division President has given me a big learning experience about how the European Geosciences Union is run and about how members are represented and can participate. Here I convey some impressions, give a quick overview of how EGU functions and the role of division presidents, and mention a few other activities you may not be aware of.

Firstly, I was impressed just how much a bottom-up organisation the EGU is – how it is run by members for the benefit of members. EGU employs only 7 full-time staff – very few compared to the 140+ employed by the American Geophysical Union! Thus, most of the organisation is run my volunteers, including the big jobs of President, Vice-President, Treasurer and General Secretary, and also the presidents of the 22 scientific divisions and members of eight committees. Of course, the fact that so few staff are needed is helped by the fact that Copernicus (the company) deals with publishing all the journals and organising the General Assembly (GA), and Copernicus has 54 employees.

Secondly, I now appreciate that EGU does a lot more beyond organising the General Assembly and publishing 18 open access journals. In particular, EGU is active in the areas of Education and Outreach, and supports various Topical Events, with each area coordinated by a committee. Additionally, a Diversity and Equality working group was recently set up. I encourage you to read more about these various activities on EGU’s web site.

What must a division president do?  The main tasks are to organise the division’s scientific programme at the General Assembly, and to attend three EGU Council + Programme Committee meetings per year: short ones at the General Assembly, and longer (2-3 days) ones in October near Munich, and in January somewhere warm (such as Nice or Cascais). Practically, this involves sitting in a darkened room for 2-3 days with a lot of other people (there are many other members in addition to the division presidents, including early career scientists) listening to information of variable interest level and discussing and making decisions (voting) when necessary. The EGU Council discusses the full range of EGU activities, so meetings consist of a series of reports: from the president, the treasurer, the various committees, the ECS representative, etc., often with much time spent discussing and voting on new points and developments that arise. Programme Committee meetings are focussed on the General Assembly, both discussing general issues and accomplishing the specific tasks of finalising the list of sessions (October meeting) and the session schedule (January meeting). Throughout all these meetings, I have found the council members to be very collegial and constructive in trying to do what is best for improving EGU activities and making optimal arrangements for the General Assembly (although of course, opinions about what is best can vary). Additionally, Copernicus is continually improving their online tools to make scheduling easier.

The President Alberto Montanari, Programme Committee Chair Susanne Buiter and Copernicus Managing Director Martin Rasmussen, celebrating the EGU General Assembly.

I am happy that there are several other people actively taking care of various tasks in the GD Division. Division officers stimulate sessions in their respective areas of the GA programme and judge the Outstanding Early Career Scientist Award nominations, while judging of the OSPP (Outstanding Student Poster and Pico) awards is organised by an Early Career Scientist (now Maelis Arnould). Our Early Career Scientists are incredibly active, maintaining this blog and the Facebook page, and organising social events at the GA. Finally, the Medal committee decides the winner of the Augustus Love Medal.

Changes are ongoing at EGU! In a multi-year process the finances are being moved from France to Germany, a complicated process as described by our Treasurer at the GA Plenary session. Moving the EGU office (where the 7 people work) from a confined space on the campus of Ludwig Maximilian University of Munich to a much larger modern office premises is happening around now and will allow some expansion of the staff and a suitable space to greet visitors. In the longer term, it may be necessary to move the location of the General Assembly from Vienna due to the ever-increasing number of attendees!

To conclude, EGU is our organisation and we can contribute to the running of it and the decision-making process, so I encourage you to get involved and to make your views about possible future improvements or other issues known to your representative (i.e. me, or our Early Career Scientist representative Nicholas Schliffke). And if anyone wants to take over as the next GD Division President, (self-)nominations can be submitted starting in September with the vote coming in November!

Programme Committee of EGU, which includes its chair, all the division presidents, the executive board, key people from Copernicus and Programme Committee Officers including the ECS representative and OSPP coordinator.

The Sassy Scientist – PhD angst

The Sassy Scientist – PhD angst

Every week, The Sassy Scientist answers a question on geodynamics, related topics, academic life, the universe or anything in between with a healthy dose of sarcasm. Do you have a question for The Sassy Scientist? Submit your question here.

Iris asks:

Will I ever finish my PhD?

Dear Iris,

Most researchers won’t admit to it publicly, but they all had doubts when trying to complete their PhD research. Sometimes the daunting task may seem impossible: why did I ever think I was smart enough and could graduate to become a doctor in philosophy? There are too many reasons to throw yourself into a depression: whether it is the ferocious comments on first versions of paper manuscripts, a stumbling and embarrassing presentation at a large conference in front of a room of expert strangers, deleting your work halfway through your project without a back-up, waiting for months for lab time only to find out that the one piece of equipment you needed to process your field study samples just broke down and it will take months and a new grant proposal to replace it: the list goes on and on and for some reason always keeps expanding. Before you find yourself googling the nearest psychiatrist or — even worse — decide to pack up and go home to live in your parents’ basement while working as a barista like every up-and-coming movie star ever, take comfort in this: everybody around you feels, or has felt, the same as you. Talk to your colleagues, your supervisor, your professor or (I dare you) a stranger at a conference: you’ll get positive feedback on your research and encouragement that you’ll make it. Sure, it will take effort and you will see some nights through ‘till daylight, but eventually you’ll be there. And then you’re one of the few…

Waiting for you at the other side…

Yours truly,

The Sassy Scientist

PS: This post was written after struggling to finish a PhD myself, just as every single scientist has in the past.