 |
Azmy, K.1,2; Kaufman, A.J.3, Misi, A. 4; Kimura, H.5; Oliveira, T.F.6
1. Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NL A1B 3X5, Canada - kazmy@mun.ca 2. Mineralogisch-Petrographisches Institut, Universität Basel, CH-4056 Basel, Switzerland 3. Department of Geology, University of Maryland, College Park, MD 20742, USA - kaufman@geol.umd.edu 4. Universiade Federal da Bahia, Centro de Pesquisa em Geofísica e Geologia, Campus da Federação, 40210-340 Salvador - BA, Brazil - misi@ufba.br 5. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA - hkimura@gps.caltech.edu 6. Votorantim Metais, P.O. Box 03, 38780-000 Vazante - MG, Brazil - flavio@vz.cmm.com.br
ABSTRACT
Keywords: Neoproterozoic carbonates, chemostratigraphy Introduction The successful use of primary stable isotope signatures, encrypted in marine carbonates (Veizer et al., 1999 and references therein), to understand the Earth’s evolution during the Phanerozoic (≤0.54 Ga) encouraged the geological community to apply the same technique to investigate the evolution of earlier environments and life on Earth during the Proterozoic (2.5-0.54 Ga). The lack of biostratigraphic controls and suitable minerals for radiometric age measurements made chemostratigraphy a potential alternative (e.g. Jacobson and Kaufman, 1999; Azmy et al., 2001, more references therein). The establishment of a reliable isotope stratigraphic profile for the Neoproterozoic sequence in South American is necessary to advance the issue of its correlation with other Neoproterozoic sequences in the area and beyond, and to develop an improved understanding of the major conditions that dominated during early Earth’s history. Geologic Setting and Sequence Stratigraphy The Neoproterozoic carbonate-dominated deposits of the Vazante Group (Dardenne, 2001) extends along more than 300 km N-S in the external zone of the Brasilia Fold Belt in São Francisco Basin (Fig. 1). The sequence stratigraphy of the Vazante Group has been studied in detail and refined by several authors (e.g. Azmy et al., 2001; Dardenne, 2001). The Vazante Group (Fig. 2) is a carbonate dominated sequence, which rests on a major lower glaciogenic unit (DI) recognized by Dardenne (2001) and also contains another distinguishable upper (younger) diamictite unit (DII) near its top at the base of the Serra de Lapa Formation (Fig. 3)
Figure 1. Location map of the study area
Methodology Several cores, provided by the Brazilian mining company Votorantim Metais, representing different depositional settings were investigated. Thin sections were examined petrographically with a standard polarizing microscope and luminoscope. A mirror-image slab of each thin section was also prepared for microsampling. Powder samples were extracted by a low-speed microdrill. For O- and C-isotope analyses, powder samples (~ 500 mg) were analyzed using a continuous-flow mass spectrometer system (Thermo-Finnigan Gasbench II linked to a ThermoFinnigan DELTA plus XP isotope ratio mass spectrometer). Powder samples were also digested in 5 % (v/v) acetic acid for 70-80 min. and analysed for Ca, Mg, Sr, Fe and Mn using an ICP. Selected samples were run for Sr-isotopes analysis and S-isotope ratios were measured on trace sulfate trapped in whole-rock carbonate powders (e.g. Azmy et al., 2001).
Evaluation of Sample Preservation
Several petrographic and geochemical techniques have been utilized to ascertain the degree of preservation of individual samples (e.g. Kaufman et al., 1995; Azmy et al., 2001). Thin sections were examined petrographically and cathodoluminescence was also employed to study the different rock components and to refine the selection of best preserved samples (e.g. Azmy et al., 2001). Microsamples were extracted from carbonates of preserved textures and minimal recrystallization (near-micritic size). Samples with
Figure 2. Vazante Group stratigraphy (after Dardenne, 2001). DI and DII are glaciogenic intervals.
Figure 3. Photographs of (A) lower diamictite unit (DI) and (B) upper diamictie unit (DII).
Mn/Sr ratios of up to 10 (Fig. 4) are considered to yield near primary carbon isotope signals (e.g. Kaufman and Knoll, 1995; Azmy et al., 2001, and references therein).
Figure 4. Scatter diagram of Mn vs. d13C of the Poço Verde, Morro do Calcário and Lapa carbonates.
Isotope Stratigraphy The Neoproterozoic is believed to have had
two major global glacial episodes, referred to as the Sturtian in
early Neoproterozoic and the Vendian in late Neoproterozoic (Jacobsen
and Kaufman, 1999; Bleeker, 2004). The Global Neoproterozoic d13C
pattern (Jacobsen and Kaufman, 1999) reveals clear plunges; each consists
of two shifts, associated with the immediate warming that followed
the global major glacial events. The
d13C profiles
of the investigated cores (Fig. 5) show major negative shifts in dolomicrites
at the top of the Serra Poço Verde Formation (~4 ‰) and in two intervals
of the carbonate rhythmites of the Lapa Formation (up to 8 ‰), immediately
above the younger (upper) glacial unit (DII). The plunge at the top
of the Serra Poço Verde Formation has
been suggested to correlate most likely with a Sturtian event (Azmy
et al., 2001), a proposition favored by the relative abundance of
Conophyton stromatolites and by the retained primary Sr- and
S-isotope signatures (see also discussion below). The Lapa d13C excursions (Fig. 5), above diamictites DII, are persistent
and consistent in parallel sections across the formation, suggesting
that the signal is primary and regional. The Lapa pattern shows two
major negative d13C shifts of up to 8 ‰, a lower shift immediately above
the basal diamictites (DII) and an upper shift near the top of sequence.
The highest d13C values obtained from carbonate clasts (~1.7 ‰ VPDB)
embedded in DII (Figs. 3 and 5) are comparable to the enriched values
of the
Figure 5. The d13C profiles of the studied Poço Verde, Calcário and Lapa carbonates.
underlying upper most Pamplona carbonates (cf. Azmy et al., 2001), suggesting that these clasts were broken down by weathering from the exposed Pamplona carbonates during the glacial episode. If so, it seems that the Lapa Formation is a cap carbonate and its two major d13C shifts (Fig. 5) are suggested to likely correlate with those of the global younger Vendian events (cf. Jacobsen and Kaufman, 1999), a proposition favored by the scarcity of Conophyton stromatolites in the Lapa carbonates relative to their older Vazante counterparts (cf. Azmy et al., 2001). In Brazil, comparable negative d13C shifts have been documented in the isotope profiles of the neighboring Bambuí carbonates (e.g. Misi and Veizer, 1998; Powis et al., 2001; Misi et al., in press), immediately above diamictites, at the neighboring Irecê Basin. The occurrence of two diamictite units (DI and DII) in the entire Vazante Group (Fig. 2), separated by 2.4 km thick sediments, implies that they are not phases of a single glacial episode but likely two different major glaciations. The occurrence of a major d13C plunge at the top of the Serra Poço Verde Formation (Pamplona carbonates) without a close basal glaciogenic units, such as in case of the Lapa Formation, might still suggest a possible Sturtian pre-glacial shift similar to that of the Marinoan Ghaub diamictite in Otavi Group of Namibia (Hoffman et al., 1998; Hoffmann et al., 2004). However, the Pamplona plunge consists of two distinctive shifts whereas the pre-Ghaub plunge is only a single smooth one that starts before the glacial unit and continues through the overlying carbonate sequence without inflexions. If so, the lowermost diamictite (DI) at the base of the Vazante Group (~4 km deep) is possibly a very early Sturtian glacial event (cf. Jacobsen and Kaufman, 1999, their Fig. 5). Yet, more investigation is still suggested for the sequences below Lapa rocks to better understand the stratigraphic framework in the area.
Strontium Isotopes In many cases, least altered carbonate samples retain their near-primary Sr-isootpe signatures because the Sr-isotope signal in diagenetic solutions is often buffered by the dissolving precursor phase (e.g. Jacobsen and Kaufman, 1999, Misi et al., in press) and therefore the least radiogenic signatures can be used as the best approximation for ambient seawater (e.g. Jacobsen and Kaufman, 1999). The Neoproterozoic carbonates of the Vazante Group have generally 87Sr/86Sr values that vary from 0.706144 to 0.752196. Despite the overprint of radiogenic signatures of the argillaceous inclusions, few samples seem to have retained their near-primary 87Sr/86Sr values (cf. Azmy et al., 2001; Misi et al., in press) that were as low as 0.706144 (from upper Pamplona carbonates) and 0.706841 (from Serra do Lapa carbonates). These lowest values are correlated with negative d13C shifts on the C-isotope profile. Although the difference in values is not big, the least radiogenic Pamplona value is slightly lower than that of Lapa which is consistent with the suggested least radiogenic global signatures for the Sturtian (0.70650) and Vendian (0.70679) seawaters, respectively (cf. Jacobsen and Kaufman, 1999).
Sulfur Isotopes The global sulfur cycle comprises several reservoirs (e.g. seawater, evaporites, reduced sulfates, igneous and metamorphic rocks, volcanoes, the atmosphere and fresh water) and fluxes and recycling processes between these reservoirs are highly complex (cf. Strauss, 1997; Schröder et al., 2004). Structurally substituted sulfates in carbonates may provide a more representative d34S signal of open ocean water conditions (Strauss, 1997; Azmy et al., 2001). The evolution of sulfur isotope composition of Precambrian seawaters, based on the evaporites record, shows a scatter of d34S values around a mean value of ~20 ‰ CDT that increases drastically to ~30 ‰ CDT around the Precambrian-Cambrian boundary (Strauss, 1997; Azmy et al., 2001; Schröder et al., 2004). The d34S values, obtained from trace sulfates trapped in the Vazante carbonates, show a considerable pattern of increase from the Serra Poço Verde and Morro do Calcário carbonates (~11 to 17 ‰) to the overlying Lapa carbonates (~12 to 24 ‰), which is expected with the proximity to the Precambrian-Cambrian boundary.
Conclusions Despite dolomitization, the Vazante Group carbonates may still retain their near-primary d13C signatures. Their C-isotope profile shows distinctive negative plunges at the top of the Serra Poço Verde carbonates (~4 ‰) and in the overlying Lapa sequence (~8 ‰), immediately above a glaciogenic unit. The association of these plunges with glacial deposits suggests a possible global correlation with either of the Sturtian or the Vendian events, although fossil evidence favors a Vendian age for the Lapa rocks. Alternatively, DII glacial event could be related to a late Sturtian glacial phase. Because of the siliciclastic inclusions, few samples retained their primary 87Sr/86Sr signatures, correlating with the negative d13C shifts. These signatures, along with the d34S signals and fossil evidence favor a Sturtian and Vendian ages for the Pamplona and Lapa carbonates, respectively.
References Azmy, K., Veizer, J., Misi, A.,
de Oliveira, T.F., Sanches,
A.L., Dardenne, M. 2001. Isotope Stratigraphy of the Neoproterozoic Carbonate
of Vazante Formation São Francisco Basin, Brazil. Precambrian Res.,
v. 112, 303-329. Bleeker, W. 2004. Towards a
‘nature’ time scale for the Precambrian - A proposal. Lethaia, v.
37:219-222. Dardenne, M. A. 2001. Lithostratigraphic sedimentary sequences of
the Vazante Group. IGCP 450 Proterozoic sediment-hosted base metal
deposits of western Gondwana (Abs.), Belo
Horizonte, Brazil:48-50. Hoffmann, K.H., Condon, D.J.,
Bowring, S.A., Crowley, J.L. 2004. U-Pb zircon date from the Neoproterozoic
Ghaub Formation, Namibia: Constraints on Marinoan glaciation. Geological
Society of America, v. 32:817–820. Hoffman, P.F., Kaufman, A.J.,
Halverson, G.P., Schrag, D.P. 1998. A Neoproterozoic snowball Earth. Science, v. 281, 1342–1346. Jacobsen, S. B., Kaufman, A.
J. 1999. The Sr, C and O isotope evolution of Neoproterozoic seawater.
Chem. Geol., v. 161:37-57. Kaufman, A. J., Knoll, A. H.
1995. Neoproterozic variations in the C-isotopic composition of seawater:
stratigraphic and biogeochemical implications. Precam. Res., v. 73, 27-49. Misi, A.,
Kaufman, A. J., Veizer, J., Powis, K., Azmy, K., Boggiani, P. C.,
Gaucher, C., Teixeira, J.B.G., Sanches, A. L.,
Iyer, S. S. (in press). Chemostratigraphic correlation of Neoproterozoic
successions in South America. Chemical Geology. Misi, A., Veizer,
J. 1998. Neoproterozoic carbonate sequences of the Una Group, Irecê
Basin, Brasil: chemostratigraphy, age and correlations. Precambrian
Research, v. 89, 87–100. Powis, K., Misi,
A., Veizer, J. 2001. Chemostratigraphy of the Neoproterozoic Bambuí
Group at Serra do Ramalho, Bahia, Brazil. Preliminary report (unpublished),
15 p. Schröder,
S., Schreiber, B.C., Amthor, J., Matter, A. 2004. Stratigraphy and environmental conditions of the terminal
Neoproterozoic–Cambrian Period in Oman: evidence from sulphur isotopes.
Journal of the Geological Society, London, v. 161, 489–499. Strauss, H. 1997. The isotopic
composition of sedimentary sulfur through time. Paleogeogr., Paleoclim.,
Paleoecol., v. 132, 97-118. Veizer, J., Ala, D., Azmy, K.;
Bruckschen, P., Bruhn, F, Buhl, D. Carden, G., Diener, A., Ebneth,
S., Goddris, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O.,
Strauss, H. 1999. 87Sr/86Sr, δ18O and δ13C
evolution of Phanerozoic seawater. Chemical Geology, v. 161, 59-88.
|
||||||||||||||||||||||||
