Isotopic geochemistry of Fernando de Noronha

Volcanic and hypabyssal rocks ranging in age from 12 to 3 Ma from the Fernando de Noronha archipelago in the western equatorial Atlantic Ocean can generally be divided into two age-compositional groups that have variable and distinct isotopic compositions. Predominantly older alkali basalts and trachytes are generally characterized by more radiogenic Sr-isotopic (⁸⁷Sr/⁸⁶Sr= 0.70457–0.70485) compositions and less radiogenic Nd-isotopic (¹⁴³/Nd¹⁴⁴Nd= 0.51271–0.51281) and Pb-isotopic (²⁰⁶Pb/²⁰⁴Pb= 19.132–19.282) compositions relative to the generally younger, more alkaline Si-undersaturated rocks which include nephelinites, ankaratrites, and melilitites (⁸⁷Sr/⁸⁶Sr= 0.70365–0.70418,¹⁴³Nd/¹⁴⁴Nd= 0.51277–0.51290,²⁰⁶Pb/²⁰⁴Pb= 19.317–19.565). These variations suggest the influence of at least two separate components in the source(s) of both series. One component is characterized by highRb/Sr and low μ, possibly derived from delaminated subcontinental lithosphere, whereas the other has high μ and lowRb/Sr similar to the source of St. Helena lavas. A third component is suggested by correlated compositions in the latest alkaline, Si-undersaturated lavas, and this component may be derived from depleted mantle. These isotopic variations in conjunction with the generally increasing degree of alkalinity with time are consistent with the temporal depletion of a low-μ, highRb/Sr component and increasing contributions from a high-μ component in the sources of the volanic rocks of Fernando de Noronha.     

Testing the preservation potential of early diagenetic dolomites as geochemical archives

Early marine diagenetic dolomite is a rather thermodynamically-stable carbonate phase and has potential to act as an archive of marine porewater properties. However, the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity. Here, the archive potential of early marine dolomites exposed to extreme post-depositional processes is tested using various types of analyses, including: petrography, fluid inclusion data, stable δ¹³C and δ¹⁸O isotopes, ⁸⁷Sr/⁸⁶Sr ratios, and U-Pb age dating of various dolomite phases. In this example, a Triassic carbonate platform was dissected and overprinted (diagenetic temperatures of 50 to 430°C) in a strike-slip zone in Southern Spain. Eight episodes of dolomitization, a dolostone cataclasite and late stage meteoric/vadose cementation were recognized. The following processes were found to be diagenetically relevant: (i) protolith deposition and fabric-preservation, and marine dolomitization of precursor aragonite and calcite during the Middle–Late Triassic; (ii) intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a Proto-Atlantic opening stress regime (T ca 250°C) during the Early–Middle Jurassic; (iii) dolomite cement precipitation during early Alpine tectonism, rapid burial to ca 15 km, and high-grade anchizone overprint during Alpine tectonic evolution in the Early Eocene to Early Miocene; (iv) brecciation of dolostones to cataclasite during the onset of the Carboneras Fault Zone activity during the Middle Miocene; and (v) late-stage regression and subsequent meteoric overprint. Data shown here document that, under favourable conditions, early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 10⁸ or more years. Evidence for this preservation includes preserved Late Triassic seawater δ¹³C_DIC values and primary fluid inclusion data. Data also indicate that oversimplified statements based on bulk data from other petrographically-complex dolomite archives must be considered with caution.