Key Questions
How have Earth's magnetic field and ocean/atmosphere evolved since the Archean?
Background
Banded irons formations (BIFs) are enigmatic deposits that reflect the conditions in Earth’s early oceans1. Many aspects of their formation are still debated, in particular, their primary mineralogy during deposition and their temporal distribution2–5. Today, BIF mineralogy is dominated by magnetic minerals such as magnetite and hematite. Their mineralogy, and continuous deposition through much of geologic time, make BIFs a compelling target for filling in gaps in the paleomagnetic record.
Aims of the Project
The aims of this project are twofold: first, to characterize the paleomagnetic signals hosted in BIFs from under-sampled time periods e.g. 700 Ma and 3000 Ma. These time periods are of particular interest for looking for trends in magnetic field intensity and morphology which may indicate the mode of convection driving the core dynamo. For example, it is currently unknown when the inner core nucleated, and debated how the dynamo was driven prior to this. The second aim is to investigate the present day mineralogy of BIFs using microscopy and rock magnetism to establish the mechanism by which the primary BIF mineralogy was replaced. Both diagenetic and metamorphic processes have been suggested, and the two should be distinguishable through variations in the grain size and shape of the secondary mineral assemblage. The timing and mechanism of mineral replacement is essential in order to constrain how much the present day mineralogy can inform us about the chemistry of the ancient oceans.
Project Description
Banded irons formations (BIFs) are enigmatic deposits that reflect the conditions in Earth’s early oceans1. Many aspects of their formation are still debated, in particular, their primary mineralogy during deposition and their temporal distribution2–5. Today, BIF mineralogy is dominated by magnetic minerals such as magnetite and hematite. Their mineralogy, and continuous deposition through much of geologic time, make BIFs a compelling target for filling in gaps in the paleomagnetic record. The aims of this project are twofold: first, to characterize the paleomagnetic signals hosted in BIFs from under-sampled time periods e.g. 700 Ma and 3000 Ma. These time periods are of particular interest for looking for trends in magnetic field intensity and morphology which may indicate the mode of convection driving the core dynamo. For example, it is currently unknown when the inner core nucleated, and debated how the dynamo was driven prior to this. The second aim is to investigate the present day mineralogy of BIFs using microscopy and rock magnetism to establish the mechanism by which the primary BIF mineralogy was replaced. Both diagenetic and metamorphic processes have been suggested, and the two should be distinguishable through variations in the grain size and shape of the secondary mineral assemblage. The timing and mechanism of mineral replacement is essential in order to constrain how much the present day mineralogy can inform us about the chemistry of the ancient oceans. The implications of this research are extensive. By simultaneously improving the paleomagnetic record and our understanding of the environmental conditions in the oceans on early Earth, the relationship between planetary magnetic fields and surface environments can be further constrained. It is currently debated whether the shielding generated by Earth’s magnetosphere is critical for maintaining a stable atmosphere and liquid water on the surface. This is critical for understanding the role of magnetic fields in planetary habitability.
Methods to be used
Fieldwork will be conducted at the Rapitan Formation, Canada and the West Rand Formation, South Africa to collect block samples and oriented drill cores for paleomagnetic analysis. The paleomagnetism of samples will be measured using a superconducting rock magnetometer. The rock magnetic properties of samples will be characterized using the quantum diamond microscope and vibrating sample magnetometer, and supplemented with scanning electron microscopy. U-Pb chronology will also be conducted on magnetite grains to establish their timing of formation.
Specialised skills required
Background in Earth Sciences
Please contact Claire Nichols on claire.nichols@earth.ox.ac.uk and James Bryson james.bryson@earth.ox.ac.uk if you are interested in this project
