Diagenetic controls on multiphase pyritization of graptolites

Abstract

Observations on pyritized graptolites have revealed a variety of pyrite morphotypes, several of which often coexist within the same fossil steinkern. The commonest forms of pyrite are those consisting of size-sorted microcrysts, which show degrees of ordering ranging from well-ordered framboids to a more homogeneous, unordered groundmass. Larger, euhedral pyrite crystals may be scattered throughout the microcrystalline groundmass, or be the dominant pyrite form in themselves. ' Floriform' pyrite frequently overgrows earlier framboids and may act to mould the inner periderm microstructure of the graptolite. Overpyrite occurs either as a primary 'overspill' of internal pyrite, or as a separate, later phase.All of the diagenetic pyrite has light sulphur isotope compositions, indicating formation during early diagenesis; isotopic evidence of the relative timing of pyrite generations matches the morphological paragenesis. Under most conditions, available iron appears not to be a limiting factor in pyrite formation, with reactive organic matter only being limiting at the lowest concentrations encountered in these sediments. The availability of organic matter does, however, exert a control on the timing of fossil pyritization. At one locality, a change in diagenetic conditions has allowed for the formation of a morphologically late generation of pyrite with isotopically light and therefore apparently early diagenetic signatures.

The hydroid fossil record and analytical techniques for assessing the affinities of putative hydrozoans and possible hemichordates

“…Like hydroids, graptolites preserved through kerogenization (Gupta et al . ), pyritization (Underwood and Bottrell ) and aluminosilicification (Page et al . ) are common in the lower Palaeozoic but relatively rare in the upper Palaeozoic, perhaps because of the extinctions of graptoloids and dendroids in the Devonian and Carboniferous, respectively (Ruedemann ).…”

“…Graptolites can be preserved through kerogenization (Gupta et al . ), pyritization (Underwood and Bottrell ) and/or aluminosilicification (Page et al . ).…”

Porewater sulphur geochemistry and fossil preservation during phosphate diagenesis in a Lower Cretaceous shelf mudstone

“…Relatively few studies have combined detailed sediment C–S–Fe systematics with sulphur isotope measurements in order to examine the conditions necessary for the pyritization of either skeletal material ( Fisher, 1986; Underwood & Bottrell, 1994) or soft tissues ( Briggs et al , 1991 ; Briggs et al , 1996 ). However, a common feature of the aforementioned studies and the present study is the occurrence of high levels of porewater dissolved iron, in association with low concentrations of organic matter.…”

“…Pyrite in particular may exhibit a range of morphologies, which can be related to the specific diagenetic conditions encountered within the sediment ( Hudson, 1982; Beier & Feldman, 1991; Canfield & Raiswell, 1991). Recent studies of fossil pyritization have combined detailed sediment C–S–Fe systematics with host sediment and fossil sulphide‐S isotope compositions, in order to elucidate the conditions necessary for the preservation of both skeletal material ( Underwood & Bottrell, 1994) and soft tissues ( Briggs et al , 1991 ; Briggs et al , 1996 ).…”

Darriwilian (Middle Ordovician) graptolite faunas of the Sandia Region, southern Peru

“…Thus, internal details and rhabdosome development structures have not been recognized. The graptolites generally consist of fractured periderm (organic material) surrounded by a variable amount of pressure shadow minerals (see Underwood, 1992;Underwood and Bottrell, 1994;Mitchell et al, 2008). The material is not well enough preserved for a detailed taxonomic analysis.…”