Contradictory magnetic polarities in sediments and variabletiming of neoformation of authigenic greigite

Wei-Teh Jiang a;*, Chorng-Shern Horng b, Andrew P. Roberts c,Donald R. Peacor d

a Department of Earth Sciences, National Cheng Kung University, Tainan 70101, Taiwanb Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, Taiwan

c School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre,European Way, Southampton SO14 3ZH, UK

d Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA

Received 29 May 2001; received in revised form 25 August 2001; accepted 5 September 2001

Abstract

In several recent published studies, paleomagnetic results from greigite-bearing sediments reveal characteristicremanences that are anti-parallel to those carried by coexisting detrital magnetic minerals and polarities that areopposite to those expected for the age of the rock unit. These observations have important implications for thereliability of paleomagnetic data from greigite-bearing sediments. We have investigated the origin of such contradictorymagnetic polarities by studying the formation mechanisms of greigite in mudstones from the Lower GutingkengFormation, southwestern Taiwan. Scanning electron microscope observations indicate that the Gutingkeng greigite hasthree modes of occurrence, including nodular, framboidal and matrix greigite. Microtextural observations, includingtransection of bedding by iron-sulfide nodules with no deviation of sediment textures, the presence of partially dissolvededges around detrital and early diagenetic phases, and neoformation of greigite and Fe-rich clays around detritalphyllosilicates, indicate that all three types of greigite have a diagenetic origin that post-dates early diagenetic pyrite. Inaddition, paleomagnetic data yield contradictory polarities even for greigite-bearing sister samples from the samestratigraphic horizon. The data are collectively interpreted to indicate that neoformation of the Gutingkeng greigiteoccurred after partial dissolution of syngenetic or early diagenetic pyrite. The timing of greigite formation canapparently vary enough to give contradictory polarities for different greigite components even within a singlestratigraphic horizon. Direct petrographic observation of authigenic magnetic iron-sulfide phases, as carried out in thisstudy, can provide important constraints on formation mechanisms and timing of remanence acquisition for theseminerals and suggests that care should be taken when interpreting magnetostratigraphic data from greigite-bearingsediments. ß 2001 Elsevier Science B.V. All rights reserved.

Keywords: greigite; pyrite; magnetite; mudstone; framboidal texture; nodules; magnetization; Taiwan

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 4 9 7 - 6

* Corresponding author. Tel. : +886-6-2757575 ext. 65437; Fax: +886-6-2740285.E-mail addresses: atwtj@sparc3.cc.ncku.edu.tw (W.-T. Jiang), cshorng@earth.sinica.edu.tw (C.-S. Horng),

arob@mail.soc.soton.ac.uk (A.P. Roberts), drpeacor@umich.edu (D.R. Peacor).

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1. Introduction

Greigite (Fe3S4) is a minor but potentially im-portant magnetic constituent of many marinesediments. Along with other sul¢des, such as py-rite and pyrrhotite, it plays a major role in thegeochemical cycling of O, C, N and S. Greigiteformation is usually related to early diagenetic,reducing conditions [1^7], although it can formduring later diagenesis depending on the diage-netic environment [5]. Greigite is ferrimagneticand its natural remanent magnetization (NRM)is potentially important for paleomagnetic studies.In several reported instances, paleomagnetic re-sults from greigite-bearing sediments have yieldedpolarities opposite either to that expected for theage of the rock unit or opposite to that indicatedby coexisting detrital magnetic minerals [7^9].More than one generation of greigite has beenreported within some sedimentary rocks, includ-ing early diagenetic and epigenetic greigite, whichwill have important implications for the NRMrecorded by such rocks [3^5]. The interpretationsof the above studies have generally been based onpaleomagnetic measurements, with independentidenti¢cation of greigite, and occasionally withassistance from sulfur isotopic data. In this study,we address the problem of NRM acquisition aris-ing from variable timing of greigite formation bydescribing detailed electron microscopic observa-tions from sediments for which the polarities ofdetrital magnetite and authigenic greigite havebeen demonstrated to be contradictory [9].

2. Geological setting and methods

The studied samples were collected from thelower portion of the Lower Gutingkeng Forma-tion in the eastern segment of the Erhjen-chi(EJE) river section in southwestern Taiwan (Fig.1). The Lower Gutingkeng Formation is a Plio^Pleistocene marine mudstone sequence and isabout 1500 m thick in the EJE section [10]. Pre-vious rock magnetic studies of three clastic se-quences in the region have shown that detritalmagnetite and authigenic greigite are widely dis-tributed [9,11,12]. Thermal demagnetization re-

sults (25^400³C) from one sequence indicate thatthe polarities of greigite below 320^340³C are op-posite to those revealed by detrital magnetiteabove 340³C [9]. The greigite is generally very¢ne-grained (6 1 Wm) and is dispersed throughoutthe matrix of the mudstones. It also occurs asaggregates of grains forming concretionary nod-ules of 6 1 to 5 mm in diameter (Fig. 2). Suchnodules are sporadically distributed through thestudied mudstone sequences and an early Plioceneportion of the nodule-bearing and surroundingsediments was investigated in more detail in thepresent study (Fig. 3).

X-ray di¡raction (XRD) analysis (CuKK radi-ation operated at 40 kV and 30 mA) was per-formed to identify minerals from magnetic ex-tracts and from powdered greigite nodules.Samples of about 100^200 g were crushed, stirredinto a slurry, and pumped through the poles of anelectromagnet with a peristaltic pump to obtainmagnetic extracts (ca. 1^2x of the sampleweight). Low-temperature (5^300 K) measure-ments of an isothermal remanent magnetization(IRM), imparted with a 1 T ¢eld at 5 K, werecarried out in zero ¢eld on small bulk subsamples(V100 mg) as they were warmed to room temper-ature using a Quantum Design Magnetic PropertyMeasurement System (MPMS-2). These measure-ments were conducted to enable detection of mag-netic phase transitions for identi¢cation of mag-netite or pyrrhotite [13^16]. The NRM wasinvestigated to study the magnetic polarity ofthe sediments. Paleomagnetic measurements weremade with a 2G Enterprises cryogenic magnetom-eter. Thermal demagnetization was conductedwith an ASC thermal demagnetizer that has alow ¢eld (6 10 nT) cooling chamber. The magne-tometer and thermal demagnetizer are both lo-cated in a magnetically shielded room. Sampleswere heated from room temperature to 400³C inthe following steps: 25, 120, 180, 240, 280, 300,320, 340, 360, 380 and 400³C. Magnetic suscepti-bility was measured for selected samples aftereach heating step to monitor for thermal altera-tion.

A polished section of one of the nodule-bearingsamples (EJEB380) was prepared for microtextur-al observations utilizing scanning electron micro-

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scopes (SEMs) equipped with X-ray energy-dis-persive spectrometers (EDSs) for elemental iden-ti¢cation. The SEMs were operated at 15 or 20kV, and back-scattered electron (BSE) imageswere digitally recorded with contrast relating tothe densities and average atomic weights of min-eral phases. Greigite normally has brighter con-trast than pyrite in BSE images. This normal con-trast was observed in the present study, but, insome cases, very ¢ne-grained greigite showsslightly darker contrast than coarser-grained py-rite because of the relatively irregular scatteringsurface and possible intergrowth with or coatingby silicates or other phases. Mineral identi¢cationwas primarily based on EDS spectra in conjunc-tion with XRD data.

3. Results

3.1. XRD and low-temperature IRM analyses

XRD analysis on magnetic extracts and on the

powdered nodules indicates that greigite is thedominant magnetic carrier of the studied samples(Fig. 4). Pyrite coexists with greigite in variableratios in the nodule-bearing samples, and quartzis common. Characteristic XRD peaks of pyrrho-tite were not detected. In addition, a Verwey tran-sition was detected at ca. 120 K in the low-tem-perature IRM measurements of all studiedsamples (Fig. 5). This suggests that magnetite isubiquitous in these sediments, although it occursonly in low concentrations. The magnetic transi-tion characteristic of pyrrhotite at ca. 30^35 K[13,14,16] was not detected in this study.

3.2. NRM

Representative thermal demagnetization dataare shown in Fig. 6. They display either simpleone-component or multiple-component NRMs.For samples with a simple one-componentNRM, including samples from sites EJEB374,375, 377 and 379, the remanence directions main-tained normal polarity during the entire demag-

Fig. 1. (a) Location map of the eastern part of the EJE river section in southwestern Taiwan. (b) Map of the EJE section, show-ing bedding attitudes of the Lower Gutingkeng Formation and nannofossil datum ages based on Horng and Shea [10]. The starsymbol indicates the locality of the studied early Pliocene greigite nodule-bearing sediments.

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netization sequence (Fig. 6a). Samples from site381 have a greater secondary remanence compo-nent but, nevertheless, normal polarity is also evi-dent throughout the demagnetization sequence forthis site (Fig. 6f). On the other hand, samplesfrom sites EJEB376, 378 and 380 (except forEJEB380.GA) have more than one componentof magnetization. For samples EJEB378.CB andEJEB380.GB, a reversed polarity component isisolated throughout a signi¢cant proportion ofthe unblocking temperature spectrum; an essen-tially anti-parallel normal polarity component isevident for the last one or two steps (Fig. 6c,e).Even for the two sister samples EJEB380.GA andEJEB380.GB, which were collected from the samecore at the same stratigraphic level, the NRMsexhibit opposite polarities between 25 and 380³C

(Fig. 6d,e). In summary, the NRM behavior ofthe samples in the temperature range of 25 to380³C can be quite di¡erent between neighboringsites, or even in a single site. Regardless of thepolarity below 380³C, any magnetization that per-sists to 380^400³C in these samples has normalpolarity. Greigite does not persist to such hightemperatures and readily transforms into pyrrho-tite and pyrite [9,16^19]. The newly formed pyr-rhotite may acquire a thermal remanent magnet-ization (TRM) and may a¡ect the NRMdirection, but the TRM acquisition is insigni¢cantif the ambient ¢eld is well shielded during thermaldemagnetization [9]. Thus, we infer that the nor-mal polarity component is mainly contributed bydetrital magnetite that was deposited during oneof the early Pliocene normal polarity subchrons ofthe Gilbert Chron.

3.3. Microtextural observations

SEM observations indicate that the greigite has

Fig. 2. Black iron-sul¢de nodules, ranging from submillime-ter to 5 mm in size, embedded in the gray mudstone matrix(samples from sites EJEB376 and EJEB378).

Fig. 3. Photograph of paleomagnetic sites from the EJE riversection, where greigite-bearing nodules were found. Samplingsites are spaced at stratigraphic intervals of 30 cm or less.Detailed sampling was carried out at sites EJEB376, 378 and380. The bedding plane is N40³E/62³SE, with southeast ori-ented to the left of the photograph.

Fig. 4. XRD patterns of (a) magnetic extracts from mud-stone matrix, and (b) powdered nodules, both from sampleEJEB380. Characteristic peaks occur for greigite (G), pyrite(P) and quartz (Q).

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three modes of occurrence, including: (1) ¢ne-grained crystal aggregates in iron-sul¢de nodules,(2) ¢ne-grained crystal aggregates within pyriteaggregates surrounding pyrite framboids, and (3)small grains within phyllosilicate {001} cleavagesor grains that are dispersed in the sediment ma-trix. These three textural types of greigite arehereafter referred to as nodular, framboidal andmatrix greigite, respectively.

Nodular greigite occurs in iron-sul¢de-dominat-ed concretionary nodules surrounded by silicate-dominated matrix phases (Fig. 7a). The noduleshave circular, elliptical, or irregularly elongatedshapes in cross-section and contain abundant sil-icate grains with variable sizes. The silicate grainsare embayed by pyrite, which is in turn enclosedby greigite aggregates having crystal sizes that are¢ner than the resolution limit of BSE images (Fig.7b,c). Both embayed silicate and pyrite grainshave partially dissolved edges. Intergrowths ofgreigite with ¢ne-grained iron-rich clays fre-

quently occur in the peripheral regions of nodules(Fig. 7d). The iron-rich clays (EDS spectra imply-ing a mixture of Fe-rich chlorite and minor illite/smectite) occur only in the vicinity of greigite andthey ¢ll space between the greigite and silicategrains with corroded edges, consistent with anorigin via diagenetic neoformation. The surround-ing mudstone matrix exhibits no evidence of de-formation or bending with respect to each nodule.EDS spectra of greigite have a high ratio of ironto sulfur compared to pyrite (Fig. 7e,f). Standard-less calculations of the greigite and pyrite compo-sitions suggest ca. 44% Fe (at%) and 56% S forgreigite and ca. 34% Fe and 66% S for pyrite,consistent with the chemical formulae Fe3S4 andFeS2, respectively. Nodular greigite is the mostabundant of the three types of greigite in nod-ule-bearing samples.

Framboidal greigite occurs within irregularlyelongated patches or aggregates of pyrite crystalsthat locally enclose spherical pyrite framboids(Fig. 8a). The pyrite aggregates consist of ¢ne-grained crystals with submicron to ca. 1-Wm diam-eter sizes. They ¢ll space between silicate grainsand have irregular boundaries in contact with thesurrounding silicate-dominated matrix (Fig. 8b).Greigite crystals with sizes ranging from 0.1 to0.5 Wm form aggregates that occupy intersticesbetween and around pyrite crystals within pyriteaggregates, and locally occur in the peripheral re-gions of pyrite framboids (Fig. 8c,d). Crystaledges of most of the framboidal pyrite crystalsare not sharp, in contrast to those with well-de-¢ned crystal edges precipitated in other marinesediments (e.g. [20]).

Matrix greigite commonly occurs as separatesubmicron to ca. 1-Wm grains dispersed in thematrix and locally forms arrays within detritalchlorite {001} cleavages (Fig. 9a). In some cases,¢ne-grained greigite forms aggregates that ¢llspaces between distorted packets of large detritalchlorite grains (Fig. 9b). Coarse-grained detritalmuscovite is also present, but greigite occursmostly in detrital chlorite grains. Magnetite wasnot observed by BSE imaging probably becauseits concentration is rather low, as suggested by thelow-temperature IRM measurements and XRDdata.

Fig. 5. Low-temperature analysis of IRM for small bulksamples, with measurements made during zero-¢eld warming.The ¢rst derivative of the IRM with respect to temperatureis expressed as 3log[3d(IRM)/d(K)] with scales on the right-hand axis and data curves in the upper portion to illustratethe Verwey transition of magnetite at about 120 K.

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4. Discussion

4.1. Diagenetic formation of greigite in iron-sul¢denodules

Three lines of evidence suggest that the iron-sul¢de nodules in these sediments formed post-depositionally, including: (1) transection of bed-ding by nodules, with no deviation of sedimenttexture, (2) the presence of partially dissolvededges around numerous detrital silicate grainsthat are embayed by pyrite within or in the periph-eral areas of the iron-sul¢de nodules, (3) replace-ment of detrital phyllosilicates by greigite and neo-formed Fe-rich clays in the vicinity of nodules.These features are consistent with diagenetic for-mation of sul¢de nodules [21]. Pyrite grains withinthe nodules have partially dissolved edges and areenclosed by greigite aggregates, therefore the nod-ular greigite must also be diagenetic in origin,having formed subsequent to pyrite.

4.2. Post-depositional precipitation of greigite inpyrite framboids

Framboidal aggregates and euhedral crystalsare common growth textures of pyrite in clasticsediments. The formation of framboidal pyrite isdirectly related to the production of hydrogensul¢de via bacterial reduction of sulfate in anoxicbottom waters and porewaters and to the avail-ability of reactive iron-bearing minerals or aque-ous species. Framboidal pyrite forms either diage-netically at shallow depths in normal marinesediments or syngenetically in anoxic bottomwaters or at the sediment^water interface in eu-xinic environments, with or without the presenceof precursor iron monosul¢des. Pyrite tends tooccur as small framboids or euhedral crystals atthe surface of iron-bearing minerals during laterstages of diagenesis when the availability of reac-

tive iron-bearing species and/or sulfate in theporewaters is limited [1,2,20,22^28]. In the studiedmudstones, spheroidal pyrite framboids couldhave formed syngenetically or diagenetically.The core^rim growth relationships imply thatthe surrounding ¢ner grained pyrite aggregatesor framboids of irregular shape formed after thespherical framboids. The occurrence of greigiteframboids or aggregates in peripheral regions ofspherical pyrite framboids and in interstices be-tween pyrite crystals within pyrite aggregates sug-gests that the framboidal greigite formed afterframboidal pyrite or contemporaneously with ir-regularly shaped pyrite aggregates.

4.3. Neoformation of matrix greigite

The common observation of neoformed greigiteon or within layered phyllosilicate minerals im-plies that the layer surfaces of phyllosilicates arereactive sites for greigite formation. Such greigiteoccurrences may result from a build-up of sul¢dicsulfur in pore waters at depth during diagenesis[25,26]. The reaction rates of iron-bearing phyllo-silicate minerals toward dissolved sul¢dic sulfurare much slower than for the highly reactiveiron oxyhydroxides or iron oxides [25]. Early-formed sul¢dic sulfur from bacterial sulfate reduc-tion quickly reacts with iron oxyhydroxides oroxides to form iron sul¢des and its reaction withiron-bearing phyllosilicate minerals occurs muchlater during diagenesis. The greigite associatedwith phyllosilicates in our sample (mostly chlo-rite) probably formed through later diagenetic re-action of iron in the phyllosilicates with sul¢dicsulfur. The greigite that is sporadically dispersedin the matrix shows no evidence for the timing offormation, but its random spatial distributionmight be consistent with formation at the sametime as the greigite that is intervened with thephyllosilicate layers.

Fig. 6. Representative vector-component diagrams for stepwise thermal demagnetization of samples from sites EJEB374 toEJEB381. The diagrams are plotted after application of a bedding correction (N40³E/62³SE). Solid circles, projection onto thehorizontal plane; open circles, projection onto the vertical plane; J0 = initial remanence intensity; Jt/J0 = normalized remanenceintensity. Thermal demagnetization paths are also shown on equal-area stereographic projections. Solid circles and lines, lowerhemisphere projection; open circles and dashed lines, upper hemisphere projection.6

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The microtextural data described above suggestthat all three observed types of greigite formeddiagenetically after pyrite formation. Post-deposi-tional formation of greigite has been reported fora variety of marine sediments [3^6,8,9,19,29]. Inthe case of marine sediments from South Island,New Zealand, greigite was inferred to be a prod-uct of early diagenesis, with pyrite formation

being inhibited by low permeability and consump-tion of available H2S [6]. However, Roberts andTurner [6] reported that chlorite was a commonconstituent, in addition to greigite, in their mag-netic extracts. It is therefore possible that thisgreigite formed during later diagenesis alongwith neoformed Fe-rich clays, as we have sug-gested for the Gutingkeng greigite studied here.

Fig. 7. Back scattered electron images of iron-sul¢de nodules from the Gutingkeng mudstone. (a) A spherical iron-sul¢de nodulein a silicate-dominated matrix (dark gray contrast) with the nodular core and rim dominated by pyrite (gray) and greigite (lightgray), respectively. (b) Pyrite (gray; labeled P) and silicate (black) grains embayed by aggregates of small greigite crystals (lightgray; G) within a nodule. (c) Pyrite grains (light gray; P) directly adjacent to partially dissolved silicate grains (gray and darkgray; Q = quartz; K = potassium feldspar), with dissolved edges in contact with greigite (bright; G) within a nodule. (d) Inter-growth of greigite (bright; G) and diagenetic clays (light gray; C) which partially replace a detrital muscovite grain (gray; M) inthe vicinity of an iron-sul¢de nodule. (e, f) X-ray energy-dispersive spectra of pyrite and greigite, respectively, obtained from aniron-sul¢de nodule.

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In the Simpson Oil ¢eld, Alaska, epigenetic grei-gite formation was related to sulfate supplied byhydrocarbon seepage [3^5]. Observations of grei-gite that formed substantially later than early bur-

ial (i.e. after establishment of a detrital magne-tization) will have signi¢cant implications for thetiming of remanence acquisition in greigite-bear-ing sediments.

Fig. 9. BSE photomicrographs of the Gutingkeng mudstone. (a) Arrays of greigite grains (bright contrast; labeled G) occur alongchlorite {001} cleavages (labeled C) and are surrounded by other silicate minerals (dark gray; M = muscovite; Q = quartz) in thematrix. (b) A chlorite grain (C) with spaces between distorted {001} cleavages ¢lled by greigite aggregates (bright; G). Smallbright spots in the matrix are also greigite. Q = quartz.

Fig. 8. BSE images of iron-sul¢de aggregates and framboids from the Gutingkeng mudstone. (a) Irregular aggregates of iron sul-¢des (bright contrast) surrounding pyrite framboids (brightest). The surrounding dark areas are mostly silicates. (b) Intergrowthof aggregates of pyrite crystals of variable sizes (bright; labeled P) with coarser-grained framboidal pyrite (brightest ; P). G = grei-gite. (c) Ultra-¢ne-grained greigite (gray; G) within the pyrite aggregate (bright; P) shown in (b). (d) Irregular patches of ¢ne-grained greigite (gray; G) display irregular boundaries within pyrite aggregates (bright; P) and occur around the peripheral re-gions of pyrite framboids.

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4.4. Paleomagnetic implications of greigiteformation

A widely recognized formation sequence forsedimentary iron-sul¢de minerals is as follows:cubic/noncrystalline monosul¢desCmackinawiteCgreigiteCpyrite/marcasite [2,30]. A high levelof supersaturation with respect to iron sul¢desfavors formation of monosul¢des and greigite,which subsequently transform into pyrite. The re-action sequence is not only suggested by geolog-ical evidence but is also directly supported by low-temperature laboratory synthesis [31,32]. Theseprocesses occur during early diagenesis at shallowburial depths. The greigite formed in this mannershould therefore record a near-depositional polar-ity.

The NRMs carried by greigite in some of theGutingkeng samples reported here exhibit paleo-magnetic polarities that are sometimes opposite tothose recorded by sister samples at the samestratigraphic level (Fig. 6), as was also shownfor the Tsailiao-chi section where detrital magne-tite exhibits dissolution pits and invariably showsnormal polarity consistent with depositional age[9]. The contradictory magnetic polarity signalsimply that the Gutingkeng greigite could haveformed during diagenesis at depth after syngeneticor early diagenetic formation of pyrite. The mi-crotextural evidence shown in this study stronglysupports this interpretation. The contradictorypolarities recorded either by di¡erent greigitecomponents or by greigite and detrital compo-nents, respectively, are apparently dependent onthe time lag between greigite formation and de-positional age. The studied sediments have anearly Pliocene depositional age that correspondsto the Gilbert Chron. Geomagnetic polarity rever-sals were relatively frequent in the Gilbert Chron.In sediment samples such as the Gutingkeng mud-stone, where parts of the NRM were acquired atdi¡erent times, frequent polarity reversals couldproduce composite NRMs that are occasionallyconsistent (i.e. all components have the same po-larity) or occasionally inconsistent. That is, a laterdiagenetic magnetization carried by greigite couldhave either a normal or reversed polarity, as couldthe depositional magnetization (carried by detrital

minerals or by early diagenetic greigite). Thus,documentation of consistent polarities at strati-graphic horizons containing neoformed greigitedoes not guarantee that a near-depositional earlydiagenetic polarity has been recorded.

4.5. Iron sources for diagenetic greigite

The neoformation of greigite after pyrite for-mation is apparently related to a change in phys-icochemical conditions. Formation of pyrite, espe-cially framboidal pyrite, is favored by highconcentrations of dissolved sul¢de species withFe available from reactive iron oxyhydroxides oroxides [2,23,24,30]. The Fe source for pyrite for-mation is most likely provided primarily by dis-solution of detrital magnetite in the Gutingkengmudstone. The concentration of magnetite musthave therefore decreased with time and the con-centration of sul¢dic sulfur must have lowered asit was taken up by crystallization of pyrite. Thepyrite subsequently experienced partial dissolu-tion and greigite formed concurrently or laterwhen the diagenetic processes involving P, Tand redox changes and reactions between Fe-and S-bearing species reached a physicochemicalstate that was suitable for greigite formation. Thefact that only little or no magnetite was detectedin greigite-rich mudstones, and that magnetite isrelatively abundant in greigite-poor or -barrensamples of the Lower Gutingkeng formation sup-ports this interpretation [9,11,12]. Although thereare no published results on hydrocarbon migra-tion in the region, the common occurrence of mudvolcanoes associated with hydrocarbon-bearing£uids and gas suggests the possible existence ofhydrocarbon activity, which may have causedchanges in redox and chemical conditions thatresulted in greigite formation within the Guting-keng mudstone, as was the case in the SimpsonOil ¢eld, Alaska [3^5]. Detrital chlorite apparentlyprovided sites for nucleation of matrix greigiteand part of the available iron was derived fromchlorite, as implied by common intergrowths ofgreigite with detrital chlorite.

Local overgrowth of pyrite by greigite is im-plied because the concretionary pyrite has parti-ally dissolved edges and is embayed by greigite

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aggregates. Also, the iron-sul¢de nodules invari-ably have pyrite-dominated cores and greigite-en-riched rims, implying replacement from rim to-ward core by interaction with pore £uids. Pyriteis generally a relatively stable phase in marinesediments, but it can be oxidized to form magne-tite during oxic diagenesis [33,34]. It has been sug-gested that pyrite could react with oxidizing acidsand other oxidants such as Fe3� and molecularhalogens [26]. The nodular greigite could haveformed as a result of partial oxidation and disso-lution of pyrite. The occurrence of neoformediron-rich clays only in the vicinity of iron-sul¢denodules is consistent with the release of iron andsulfur from such a process. The released sulfurand iron may have provided material for the for-mation of other textural types of greigite else-where in the sediment. The general absence ofsharp crystal edges in the framboidal pyrite alsoimplies some degree of pyrite dissolution. Thesedata collectively suggest that the neoformation ofgreigite was associated with diagenetic oxidationand dissolution of pyrite.

5. Conclusions

In this study, we have shown that greigite oc-curs in nodular, framboidal and matrix forms inmudstones from the Lower Gutingkeng Forma-tion, southwestern Taiwan. The nodular greigiteis intergrown with partially dissolved pyrite grainswithin iron-sul¢de nodules that have greigite-dominated rims. The framboidal greigite ¢lls in-tergranular space within pyrite aggregates, where-as the matrix greigite occurs between phyllosili-cate layers or is dispersed in the sedimentmatrix. We infer that these three types of greigiteformed during late diagenesis after syngenetic orearly diagenetic formation of pyrite. Framboidaland matrix greigite probably formed via dissolu-tion and neoformation processes, but nodulargreigite was a replacement product of early-formed nodular pyrite involving oxidation/disso-lution and neocrystallization. These di¡erentmodes of diagenetic greigite formation give riseto NRMs that are often anti-parallel to theNRMs carried by detrital magnetic minerals or

even by greigite within the same stratigraphic ho-rizon. It is likely that similar mechanisms are re-sponsible for other contradictory polarity recordsgiven by greigite and magnetic detrital minerals,as reported in the literature from several otherlocations. Direct petrographic observation of au-thigenic magnetic iron sul¢des, as performed inthis study, provides important constraints on thetiming of magnetizations carried by such miner-als. These results suggest that care should be tak-en to constrain the formation mechanisms of grei-gite in a given situation before attempting tointerpret the paleomagnetic direction in terms ofa near-depositional early diagenetic signal.

Acknowledgements

We thank Dr. Masayuki Torii for making thelow-temperature IRM measurements at theOkayama University of Science, Japan, and forhis comments on the manuscript. We are gratefulto Leonardo Sagnotti and Martin Schoonen fortheir constructive reviews of the manuscript. Thisstudy was supported by the National ScienceCouncil of the Republic of China under GrantsNSC89-2116-M-006-019 to W.T.J., NSC89-2911-I-001-082-2 and NSC91-2911-I-001-001 to C.S.H.,by a grant from the Royal Society of London toA.P.R., and by National Science FoundationGrant EAR 9814391 to D.R.P.[RV]

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