Classification of a Complexly Mixed Magnetic Mineral Assemblage in Pacific Ocean Surface Sediment by Electron Microscopy and Supervised Magnetic Unmixing

Hongmiao Pan,Shuangchi Liu,Yan Liu,Jinhua Li,Yongxin Pan,Andrew Philip Roberts,Tian Xiao

doi:10.3389/feart.2020.609058

Abstract

Unambiguous magnetic mineral identification in sediments is a prerequisite for reconstructing paleomagnetic and paleoenvironmental information from environmental magnetic parameters. We studied a deep-sea surface sediment sample from the Clarion Fracture Zone region, central Pacific Ocean, by combining magnetic measurements and scanning and transmission electron microscopic analyses. Eight titanomagnetite and magnetite particle types are recognized based on comprehensive documentation of crystal morphology, size, spatial arrangements, and compositions, which are indicative of their corresponding origins. Type-1 particles are detrital titanomagnetites with micron- and submicron sizes and irregular and angular shapes. Type-2 and -3 particles are well-defined octahedral titanomagnetites with submicron and nanometer sizes, respectively, which are likely related to local hydrothermal and volcanic activity. Type-4 particles are nanometer-sized titanomagnetites hosted within silicates, while type-5 particles are typical dendrite-like titanomagnetites that likely resulted from exsolution within host silicates. Type-6 particles are single domain magnetite magnetofossils related to local magnetotactic bacterial activity. Type-7 particles are superparamagnetic magnetite aggregates, while Type-8 particles are defect-rich single crystals composed of many small regions. Electron microscopy and supervised magnetic unmixing reveal that type-1 to -5 titanomagnetite and magnetite particles are the dominant magnetic minerals. In contrast, the magnetic contribution of magnetite magnetofossils appears to be small. Our work demonstrates that incorporating electron microscopic data removes much of the ambiguity associated with magnetic mineralogical interpretations in traditional rock magnetic measurements.

Highlights

We show that by incorporating detailed scanning electron microscope (SEM) and Transmission electron microscope (TEM) characterizations, much of the ambiguity that is inherent to magnetic mineralogy interpretations when using only rock magnetic measurements is eliminated
Stepwise saturation IRM (SIRM) acquisition and demagnetization indicate that the sample is saturated and demagnetized largely below ∼300 mT, and completely at
Normalized isothermal remanent magnetization (IRM) acquisition and direct current demagnetization (DCD) curves are roughly symmetric with a calculated R-value of 0.45 for the Wohlfarth-Cisowski test (Cisowski, 1981)

Summary

Introduction

Sedimentary sequences provide important geological records for understanding long-term variations of Earth’s magnetic field and paleoclimate (e.g., Valet and Meynadier, 1993; Guyodo and Valet, 1999; Kissel et al, 1999; Evans and Heller, 2001; Evans and Heller, 2003; Yamazaki, 2009; Hao et al, 2012; Liu et al, 2012; Roberts et al, 2013; Kissel et al, 2020; Valet et al, 2020). Magnetic mineral identification in sediments is fundamentally important for both paleomagnetic and environmental magnetic studies because the type, concentration, size and shape of magnetic minerals control their magnetic properties, including magnetic recording quality (e.g., Dunlop and Özdemir, 1997; Dekkers, 2003; Liu et al, 2012; Chang et al, 2014a; Larrasoaña et al, 2014; Roberts et al, 2019). They generally involve fitting of functions to derivatives of isothermal remanent magnetization (IRM) acquisition or direct current demagnetization (DCD) curves (e.g., Robertson and France, 1994; Kruiver et al, 2001; Heslop et al, 2002; Egli, 2003, Egli, 2004a; Egli, 2004c; Heslop and Dillon, 2007; Maxbauer et al, 2016), alternating field demagnetization curves of an anhysteretic remanent magnetization or IRM (Egli and Lowrie, 2002; Egli, 2004a, Egli, 2004b, Egli, 2004c), or analysis of hysteresis loops (e.g., Roberts et al, 1995; Dunlop, 2002a, Dunlop, 2002b; Tauxe et al, 2002; Heslop and Roberts, 2012a; Heslop and Roberts, 2012b), ferromagnetic resonance (FMR) spectra (e.g., Weiss et al, 2004; Kopp et al, 2006a; Kopp et al, 2006b; Gehring et al, 2011; Kind et al, 2011; Gehring et al, 2013; Chang et al, 2014b), or first-order reversal curve (FORC) diagrams (e.g., Roberts et al, 2014; Lascu et al, 2015; Channell et al, 2016; Harrison et al, 2018; Roberts et al, 2018)

Methods

Results

Discussion

Conclusion