Contradictory magnetic polarities in sediments and variable timing of neoformation of authigenic greigite

Jiang, WT Horng, CS Roberts, AP Peacor, DR

Wt. Jiang et al., Contradictory magnetic polarities in sediments and variable timing of neoformation of authigenic greigite, EARTH PLAN, 193(1-2), 2001, pp. 1-12

34

INGLESE

Article

Earth Sciences

EARTH AND PLANETARY SCIENCE LETTERS

0012-821X → ACNP

193

1-2

2001

1 - 12

ISI

0012-821X(20011130)193:1-2<1:CMPISA>2.0.ZU;2-E

In several recent published Studies. paleomagnetic results from greigite-be aring sediments reveal characteristic remanences that are anti-parallel to those carried by coexisting detrital magnetic minerals and polarities that are opposite to those expected for the age of the rock unit. These observat ions have important implications for the reliability of paleomagnetic data from greigite-bearing sediments. We have investigated the origin of such co ntradictory magnetic polarities by studying the formation mechanisms of gre igite in mudstones from the Lower Gutingkeng Formation. southwestern Taiwan . Scanning electron microscope observations indicate that the Gutingkeng gr eigite has three modes of occurrence, including nodular, framboidal and mat rix greigite. Microtextural observations, including transection of bedding by iron-sulfide nodules with no deviation of sediment textures, the presenc e of partially dissolved edges around detrital and early diagenetic phases, and neoformation of greigite and Fe-rich clays around detrital phyllosilic ates. indicate that all three types of greigite have a diagenetic origin th at post-dates early diagenetic pyrite. In addition. paleomagnetic data yiel d contradictory polarities even for greigite-bearing sister samples from th e same stratigraphic horizon. The data are collectively interpreted to indi cate that neoformation of the Gutingkeng greigite occurred after partial di ssolution of syngenetic or early diagenetic pyrite. The timing of greigite formation can apparently vary enough to give contradictory polarities for d ifferent greigite components even within a single stratigraphic horizon. Di rect petrographic observation of authigenic magnetic iron-sulfide phases, a s carried out in this study, can provide important constraints on formation mechanisms and timing of remanence acquisition for these minerals and sugg ests that care should be taken when interpreting magnetostratigraphic data from greigite-bearing sediments. (C) 2001 Elsevier Science B.V. All rights reserved.