New Paper – Signatures of Reductive Magnetic Mineral Diagenesis From Unmixing of First‐Order Reversal Curves

One of the key themes in my early research career was trying to understand the magnetic signature of rocks where the primary remanence was not carried by iron oxides like magnetite, but instead iron sulphides, particularly greigite. Studies of sediment cores made it clear that it could form relatively early during diagenesis. But my PhD research in New Zealand showed it could also form very late in diagenesis – sometimes during tectonic events millions of years after the host rock had formed. Frustratingly, the combined the rock magnetic and electron microscope observations we used at the time to infer the presence of greigite did not give us any way of identifying if that greigite formed early or late. More generally, we could only semi-confidently identify the dominant magnetic phase in sediments that clearly contained several different phases.

Ten years later, I am a coauthor on a new paper available at JGR – Solid Earth that uses a new analytical technique – FORC unmixing – that is offering new insights into the diagenesis of magnetic minerals. We used FORCs – a form of partial hysteresis curve that allow you to estimate the distribution of coercivities and interaction fields of magnetic particles in a rock sample – in the studies linked to above, but this refinement uses Principal Component Analysis to identify and characterise different magnetic grain populations present in the same sample.

This gives us a much more detailed view of diagenetic processes, and where and how different magnetic minerals form or are altered during diagenesis. Although the problem of early and late forming greigite is not solved by this paper, there are hints that the two processes involved do indeed generate magnetic grain distributions with slightly different properties.

I can’t claim any credit for developing the FORC unmixing technique: my previous work on some of the samples just gives me some expertise in interpreting the results. But I am very happy to see new techniques being used to revisit and refine our previous conclusions – especially when it shows that, for the most part, we were on the right track!

ABSTRACT

Diagenetic alteration of magnetic minerals occurs in all sedimentary environments and tends to be severe in reducing environments. Magnetic minerals provide useful information about sedimentary diagenetic processes, which makes it valuable to use magnetic properties to identify the diagenetic environment in which the magnetic minerals occur and to inform interpretations of paleomagnetic recording or environmental processes. We use a newly developed first‐order reversal curve unmixing method on well‐studied samples to illustrate how magnetic properties can be used to assess diagenetic processes in reducing sedimentary environments. From our analysis of multiple data sets, consistent magnetic components are identified for each stage of reductive diagenesis. Relatively unaltered detrital and biogenic magnetic mineral assemblages in surficial oxic to manganous diagenetic environments undergo progressive dissolution with burial into ferruginous and sulfidic environments and largely disappear at the sulfate‐methane transition. Below the sulfate‐methane transition, a weak superparamagnetic to largely noninteracting stable single domain (SD) greigite component is observed in all studied data sets. Moderately interacting stable SD authigenic pyrrhotite and strongly interacting stable SD greigite are observed commonly in methanic environments. Recognition of these characteristic magnetic components enables identification of diagenetic processes and should help to constrain interpretation of magnetic mineral assemblages in future studies. A key question for future studies concerns whether stable SD greigite forms in the sulfidic or methanic zones, where formation in deeper methanic sediments will cause greater delays in paleomagnetic signal recording. Authigenic pyrrhotite forms in methanic environments, so it will usually record a delayed paleomagnetic signal.

Roberts et al., Journal of Geophysical Research, 123, 2018.