TOURMALINE: A TRACER OF METAMORPHISM IN THE EOARCHEAN

The Eoarchean was an epoch of change for the early Earth. The crust, atmosphere, and hydrosphere stabilized, and life developed and interacted with this early surface. Understanding the crust and hydrosphere can provide important insights into how the early Earth operated and how it may have affected life’s development. B is of particular interest as it is regarded as key to stabilize organic molecules, including ribose, critical for RNA [4]. Here we evaluate what the borosilicate mineral tourmaline can tell us about B in the Eoarchean Isua basin.

Tourmaline is the most widespread B-bearing mineral in the Earth’s crust and is present in all major rock types [1]. Due to its refractory nature and ability to incorporate a wide variety of elements it can be used as a resilient tracer of processes and composition of its geologic growth environments.

We have studied tourmalines from the Isua Belt, SW Greenland using SEM, EMPA, microscopy, and LA-ICPMS, and determined their growth history from nucleation to peak metamorphic conditions. This growth history was then linked to the rest of the paragenesis to determine the relationships among phases and growth zones. Also, experiments were conducted to determine experimental partition coefficients between tourmaline and fluid at both nucleation and peak conditions.

Optical zoning shows prolonged and complex growth histories with tourmaline intersector zoning thermometry indicating nucleation at ca. 300C and growth to peak conditions around 600C ([2], [3]) in agreement with garnet-biotite thermometry. Pseudosections provide pressure estimates and the partition coefficients combined with trace element analysis of growth zones an estimate of the composition of the formation fluid.

This holistic study of the tourmalines and their associated fluids from the Isua Belt indicates that these tourmalines are multigenerational and have complex growth histories. This will bring important new interpretations of the Isua Belt and its metamorphic history. However, it also complicates efforts to use these tourmalines as direct probes of the B content of the Eoarchean surface.

References:

1. Henry, D. J., and B. L. Dutrow. Boron: Min, Pet, and Geochem, (2018).
2. Hinsberg, V. J. van. Min. (2006).
3. Hinsberg, Vincent J. van, and J. Schumacher (2007).
4. Mishima, S, Y. Ohtomo, and T. Kakegawa. (June 2016).