Apparent magnetic polarity reversals due to remagnetization resulting from late diagenetic growth of greigite from siderite

Abstract

S U M M A R YA mixed-polarity zone, representing alternations between remagnetized and non-remagnetized strata, has been documented within the lower few metres of the CRP-1 core (Ross Sea, Antarctica). Detailed rock magnetic investigation of this interval indicates that the normal polarity remagnetization is carried by magnetostatically interacting single-domain particles of a ferrimagnetic iron sulphide mineral, while the reversed-polarity magnetization of non-remagnetized strata is carried by magnetite with a broad range of grain sizes and negligible magnetostatic interactions. Scanning electron microscope observations of polished sections indicate that the ferrimagnetic iron sulphide mineral is greigite (Fe 3 S 4 ). Based on microtextural relationships, it is not possible to determine the relative timing of formation for much of the greigite. However, a significant proportion of the greigite has grown on the surface of authigenic siderite (FeCO 3 ) grains that occur as microconcretions and as cement surrounding detrital matrix grains. In such cases, microtextural relationships indicate that siderite post-dates early diagenetic pyrite and that greigite post-dates the siderite. Siderite usually forms in environments with abundant dissolved iron and carbonate, but without dissolved pore water H 2 S. This set of geochemical conditions occurs in methanic settings below the sulphate reduction zone (in which early diagenetic pyrite forms). We interpret the observed remagnetization of the lower part of the CRP-1 core as due to a late diagenetic pore water migration event where abundant iron on the surface of siderite grains reacted with fluids containing limited dissolved sulphide, thereby causing precipitation of greigite. The distribution of siderite (and associated greigite) in the lower part of the CRP-1 core is patchy, which accounts for the apparent alternation of polarities. This study is part of a growing catalogue of remagnetizations involving greigite, which suggests that occurrences of greigite should be treated with caution in palaeomagnetic and environmental magnetic studies.The reliability of a magnetic polarity stratigraphy depends on the validity of the assumption that the magnetic polarity recorded by a sedimentary sequence was acquired at or near the time of deposition. This assumption does not always hold, and it must be tested by carefully investigating the nature, origin and age of the magnetic minerals carrying the observed characteristic remanent magnetization (ChRM). There have recently been an increasing number of cases where the ferrimagnetic iron sulphide minerals greigite (Fe 3 S 4 ) and pyrrhotite (Fe 7 S 8 ) carry ChRMs with magnetic polarities opposite to those carried by coexisting magnetite or where the ChRM is contradictory to the expected polarity for the depositional age of the sedimentary sequence. These observations suggest a late, and sometimes complex, diagenetic origin for the iron sulphides (e.g.

A Greigite-Based Magnetostratigraphic Time Frame for the Late Miocene to Recent DSDP Leg 42B Cores from the Black Sea

“…The amount of magnetic interaction and the domain state in these samples, emerges from FORC diagrams that have contours centering around a single domain (SD) peak at B c ∼70 mT ( Figure 2J). The FORC diagrams are similar to those previously reported for greigite (Roberts et al, 2000;Sagnotti et al, 2004). The central peak has considerable vertical spread and is centered slightly below B u = 0, which indicates relatively strong magnetic interaction among particles (Pike et al, 1999;Newell, 2005).…”

Environmental control on the occurrence of high-coercivity magnetic minerals and formation of iron sulfides in a 640 ka sediment sequence from Lake Ohrid (Balkans)

“…It is important to note that these parameters can sometimes provide ambiguous results; high SIRM / χ is not always accompanied by high GRM / NRM; however, MDFs are high if any of the former proxies are elevated. Missing GRM acquisition of greigite bearing samples has been reported in other studies (Roberts et al, 1998;Sagnotti et al, 2005), and could be related to grain-size effects, because only fine-grained (SD) greigite acquires GRM. As observed in the images of the magnetic extracts, coarse-grained iron sulfides are abundant in samples with low GRM and high SIRM / χ and we therefore hypothesize that greigite either grew to too coarse grain size for GRM acquisition or was transformed into other iron sulfides, e.g., pyrrhotite, which is also characterized by high SIRM / χ (Maher and Thompson, 1999).…”

“…Newly formed iron sulfides have been observed to overgrow earlier diagenetic iron minerals, such as siderite Sagnotti et al, 2005).…”

“…In addition to the "early" diagenetic processes, concurring with organic matter degradation, authigenic Fe minerals can form at a late diagenetic phase. Such precipitation and dissolution processes occur mainly as a result of a variety of different mechanisms associated with the migration of mineralized fluids and with changes in the pore water chemistry, which disrupt the steady-state diagenetic progression Sagnotti et al, 2005).…”

Magnetic mineralogy and its implication of contemporary coastal sediments from South China

“…A drop between 300 and 320°C is also occurring during ThD of a triaxial SIRM (in particular, in the direction of highest acquisition field) and susceptibility measured after each step of ThD (Fig. 4a, b) indicating that iron sulfides are existing in the sediments (Roberts 1995;Sagnotti et al 2005). A gyroremanent magnetization (GRM) is obvious above about 60 mT during AFD (Fig.…”