Other
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.126..315t&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 126, no. 4, p. 315-331
Other
13
Brazil, Carbon Isotopes, Earth Mantle, Geothermal Anomalies, Heat Sources, Magma, Oxygen Isotopes, Carbonates, Geochemistry
Scientific paper
Geochemical and Sr, Pb, O and C isotopic data are reported for carbonatite samples from five locations in southeast Brazil. Elemental abundances and delta ((C-13)(sub PDB)) data (between -5.8 and -7.2/mil prove that all samples are derived from the mantle. Dupal isotopic characteristics in the all carbonatite samples from five locations in southeast Brazil are found in this study, characteristics that have not previously been recognized in carbonatites. Brazilian carbonatites possess average delta(8)/4 values of between 101 and 145, average delta(7)/4 values of between 5.2 and 10.3, and initial Sr isotopic compositions of between 0.7046 and 0.7062. The Brazilian carbonatites comprise two groups: The northern group is coincident with the passage of the Trindade non-Dupal hotspot at ca. 80 Ma, while the southern group mainly corresponds to the passage of the Tristan de Cunha Dupal hotspot at ca. 130 Ma. Although we expected a geochemical correlation between the Brazilian carbonatites and the South Atlantic hotspots, the enriched isotopic signature (EM1) of all the carbonatite samples is similar to that of alkali basalts on Tristan de Cunha. The combined O-Sr isotopic diagram indicates that the southern group carbonatites have negligible or only slight crustal contamination. The northern group samples show significantly higher delta (O-18(sub SMOW)) values of 9 - 14/mil more radiogenic Pb isotopic ratios, and a Sr-87/Sr-86 value of 0.705. Even if these signatures are derived from the contamination of a lower crustal component with mantle sources, it is clear that the parental magma also has inherent EM1 isotopic characteristics. The interpretation of the origin of EM1 in the Brazilian carbonatites (subcontinental lithospheric mantle (SCLM) vs. asthenosphere) is dependent on the model of Parana volcanism at ca. 130 Ma, which remains controversial. One possibility is that both the northern and southern carbonatites came from enriched SCLM under a part of Gondwanaland. In this case, a hotspot would provide the thermal energy to melt the lithospheric source region for both the Parana flood basalts and the alkali carbonatitic volcanism. Another possibility is that the source of the northern carbonatites is also the Tristan plume carbonate-rich material which had once been trapped under the crust and reactivated by the Trindade hotspot, on the assumption that Parana volcanism at ca. 130 Ma was mainly triggered by the hugh Tristan plume activity. If anything, we favor the latter and believe an asthenospheric mantle plume origin for both the ultimate carbonate and the Dupal anomaly in the Brazilian carbonatites.
Horiuchi Hiroyuki
Tokonami Masayasu
Toyoda Kazuhiro
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