Anita S. Andrew

Middle Palaeozoic extinction events: Faunal and isotopic data

Abstract At least 9 and possibly as many as 12 extinction events of global or near-global impact can be discriminated in the mid-Palaeozoic (earliest Silurian to Early Carboniferous) on the basis of brachiopod, coral, conodont and ammonoid data, and the history of carbonate build-ups. Isotopic data from whole-rock samples are presented for three of these events, based on Australian carbonate sequences constrained by conodont data. These data represent the initial phase of a more extensive investigation using C, O, S, Sr and Nd isotopic signatures derived from conodonts, articulate and inarticulate brachiopod shell, and fish remains from numerous Australian and European sequences. The aim of the project is to identify isotopic responses to extinction events, and address causes for these changes. In limestone sections analysed so far, variations in carbon isotope compositions on a whole-rock scale are most marked at horizons that can be correlated with times of significant reduction in biomass and diversity. This is despite the fact that the whole rocks are in fact multicomponent systems with respect to carbon, in part arising from diagenetic rearrangements of carbon distribution within the scale of the whole rock. Thus, the carbon isotope data do provide evidence on a whole-rock scale for fundamental changes in the global carbon cycle that are correlative with extinction events. It is, however, unlikely that the magnitude of the isotopic shift will be precisely documented from such whole-rock analyses. The oxygen isotope results are less obviously related to the times of carbon isotope excursion; this contrast in apparent resilience to post-depositional modification of pristine isotope compositions exemplifies the extreme caution needed in evaluating isotopic data from carbonates.

Unusual mineral inclusions and carbon isotopes of alluvial diamonds from Bingara, eastern Australia

Abstract A suite of diamonds from eastern Australia, “Group B”, are essentially eclogitic. However, mineral inclusion compositions are unlike eclogitic diamonds from other occurrences. Garnets are very Ca-rich (Gr 61–84 ) and clinopyroxenes are mostly diopsidic (16±10 mol% Jd). Other inclusions in these diamonds are SiO 2 , titanite, olivine, molybdenite and melilite—a truly unique suite! Garnet and clinopyroxene are not accompanied by indications of fertile mantle conditions. Rather, we find depletions in Fe, Mn, Na, K, coupled with high Ni and Cr in some clinopyroxenes. The chemistry of the garnets and clinopyroxenes suggests affinities with rodingites—metasomatised mafic rocks that occur in ophiolites. “Group B” diamonds are 13 C-enriched ( δ 13 C=+1.1±1.3‰ (1 S.D.)). Internal structures include evidence of brittle failure and of rapid growth. The diamonds have high nitrogen contents and positive nitrogen isotopes ( δ 15 N=+10.1±4.7‰). This may indicate that the diamonds formed from 13 C-enriched carbon (+3‰), such as that derived from marine carbonates within the mafic rocks. The many unusual features of these diamonds can be interpreted to indicate that the diamonds formed in a subduction environment. However, resorption on the surfaces of the diamonds indicates that they were brought to the upper crust by magmatic activity.

Timing of mineralization and source of fluids in a slate-belt auriferous vein system, Hill End goldfield, NSW, Australia: Evidence from 40Ar39Ar dating and O- and H-isotopes

Abstract 40 Ar 39 Ar dating of metamorphic biotite and alteration muscovite from the auriferous veins and host rocks at the Hill End goldfield, N.S.W., Australia, has distinguished four major geological events, including the timing of gold mineralization. The earliest hydrothermal event occurred during the Middle Devonian Tabberabberan Orogeny (370–380 Ma) and resulted in the formation of quartz veins barren of Au. A second and major episode of vein emplacement occurred in the Early Carboniferous during the principal phase of metamorphism and deformation at 359–363 Ma. This was followed by Au accumulation in two stages: (1) after the major phases of quartz deposition, and (2) during and after the development of conspicuous internal vein laminations (~ 357 Ma and ~ 343 Ma, respectively). Two sources of fluid are proposed for vein and ore formation. The first is a local metamorphic fluid characterized by δ 18 O H 2 O values of 8.9 to 12.5 per mil and δD H 2 O values of −87 to −90 per mil. The second is a mixed ore fluid with δ 18 O and δD values in the range of δ 18 O H 2 O 8.4 to 11 per mil and δD H 2 O of −49 to −36 per mil. Progressive entry of this second fluid, sourced from trough-fill or deeper crustal rocks, is linked closely to cycles of gold precipitation at Hill End.

Late Ludfordian Correlations and the Lau Event

Changes in whole conodont faunas and δ13C values are combined to achieve high-resolution correlations of Upper Silurian successions in many areas (primarily Gotland, Skane, Lithuania, Bohemia, Austria, Sardinia and Queensland); other areas are correlated with lower precision. Four of the widely recognised subzones average considerably less than 0.1 Ma and a fifth interval less than 0.01 Ma. The main constraints on resolution and precision now achievable are the amount of, and the precision in, new or previously published data from each local section; centimetric scale collecting would be worthwhile in many sections. Some stratal characters are more widespread during certain intervals than might have been expected, for example, the presence of oncolites and algal coatings in the Icriodontid Zone and the lower part of the O. snajdri Zone. Similarly, the Dayia navicula bloom in the Upper P. siluricus Subzone was widespread. So also were muddy-sandy sediments followed by oolite low in the O. snajdri Zone; crinoids flourished widely when the lower part of the Lower Icriodontid Subzone was formed. Closely spaced samples show that, in some intervals, the now well-known δ13C spike was modified considerably by fluctuations; that is, it is not a smooth plateau. The best-documented fluctuations (19 analyses) are 2.5 smooth cycles with up to 2.7‰ in amplitude during ca 12,000 years or less in the base of the O. snajdri Zone. An enigma is the depletion of the spike in some sections, especially in the best Bohemian section.