Tracing carbonate diagenesis and subsurface fluid activity during the opening of the Central South Atlantic

It is well acknowledged that the Central South Atlantic is a prime example of continental breakup preceded by several rift-sag basins. Many basins - developed between the Rio Grande/Walvis ridge and present-day equatorial Brazilian/African margins - experienced a complex history of rift-related mafic/felsic magmatism, lacustrine carbonate sedimentation, and deposition of a massive salt layer that today seals major oil reservoirs off the coasts of Brazil and Africa. The carbonates below the salt (or pre-salt carbonates) are home to significant volumes of offshore hydrocarbon, which are currently exploited in the Campos/Santos Basins (Brazil) and explored in the Kwanza Basin (Angola). The deposition of the salt giant seems to have occurred during the transition from the rift to the sag phase, most likely before the development of a mid-oceanic ridge and the onset of effective seafloor spreading in the Central South Atlantic. Defining the age and fluid sources of the lacustrine carbonates and the diagenetic textures they record is critical for understanding the depositional/post-depositional setting and the sediment-fluid interactions at the birth of the Central South Atlantic. In particular, the timing of the formation of the confined lake basins and the salt layer bears significant implications for the breakup of West Gondwana and the factual opening of the South Atlantic.

Previous studies have shown that the distinct water chemistry of the alkaline lakes (in which the pre-salt carbonates accumulated) produced characteristically unique rocks, such as carbonates consisting mainly of Mg-clays with spherulitic calcite and abiogenic crystal shrub limestones, locally in lateral transition to travertines. While these carbonates are essential markers of the paleoenvironment, interpretations of the depositional/diagenetic fabrics suggests intense reactivity of the carbonate phases during the burial history, including multiple and temporally variable hydrothermal pulses.

Seeking to determine the composition, fluid sources, and sequence of diagenetic/hydrothermal carbonate phases of the pre- to post-salt carbonate rocks, we carried out in situ Sr isotope tracing and trace element geochemistry on different calcite and dolomite phases occurring as framework components and diagenetic products in rocks of the Santos and Campos basins.  Sr isotope signatures and trace element data of depositional/eodiagenetic carbonate phases revealed fluid sources that are characteristically continental, with typical water chemistries of enclosed lacustrine environments. Calcites of the framework and a range of cement generations of travertine facies record significantly distinct Sr isotopes and REE signatures indicating the presence of high T fluids with strong Eu anomalies and significantly less radiogenic Sr isotopes, thereby characterizing multiple hydrothermal fluid circulation events. In addition, Sr signatures and trace element maps show extreme mobilization of U and Pb isotopes during diagenesis, while Sr isotopes remained remarkably unchanged. This observation has important implications for U-Pb dating and isotope tracing. When examined in the nascent South Atlantic Ocean scenario, the new isotope and trace element data indicate a tectonically active depositional environment, more consistent with a rift phase for the pre-salt carbonates. Small cm-scale carbonate layers deposited within the salt layer are less radiogenic than the pre-salt carbonates. Sr isotope tracing indicates a significant change in the water chemistry and fluid sources.  In summary, our results highlight the significance of high-temperature fluid circulation during burial diagenesis of the pre-salt carbonates and bear key temporal implications for: (1) the evolution of the pre-salt basins and (2) the evolving sub-surface hydrothermal systems that acted during the opening of the Central South Atlantic.