ABSTRACT LXXIV, DECEMBER 2005 n.3:

Abstract - The new generation Thermal Ionisation Mass Spectrometer (ThermoFinnigan Triton-Ti^®) and a clean lab for sample preparation have been established in Firenze in the last few years. Our research group has carried out the calibration of the analytical methods and the technical procedures to be applied in a number of geological research fields. In this contribution, we present the analytical procedures set up in the Radiogenic Isotope Geology Laboratory of Firenze with particular attention to the instrumental methodologies utilised in order to obtain high performances in terms of precision and accuracy on the measurement of Sr, Nd, and Pb reference samples.

MENGIST TEKLAY, YEMANE ASMEROM and THEOFILOS TOULKERIDIS - Geochemical and Sr-Nd isotope ratios in Cenozoic basalts from Eritrea: evidence for temporal evolution from low-Ti tholeiitic to high-Ti alkaline basalts in Afro-Arabian Continental Flood Basalt Province

Abstract - Mid-Tertiary continental flood basalts covering large part of Eritrea, Ethiopia and Yemen comprise an igneous province linked to a mantle plume still active beneath Afar. In Eritrea, Oligocene-Miocene basalts on the Central and Southern Highlands unconformably overlie and partially cover a lateritised Palaeozoic-Mesozoic sedimentary strata or a basement of Neoproterozoic island-arc rocks. The Oligocene-Miocene basalts, respectively from Menguda and Ona/Durko sections, have been analysed for major and trace elements and Sr-Nd isotopic compositions; such data are the first for the Eritrean flood basalts.
The older products from Menguda section are fine to medium-grained, olivine-phyric low-Ti tholeiitic basalts. They are characterised by relatively high MgO, compatible element contents and high CaO/Al₂O₃ ratios, and by low Fe, P, K and incompatible element contents. REE profiles are flat at near ten times chondritic values. No intercalated silicic volcanics are found. The younger basalts from Ona and Durko sections contain interbedded pyroclastic rocks up to 60 m total thickness. In contrast to the Menguda basalts, these younger lavas are high-Ti transitional to alkaline basalts. The temporal sequence in Eritrea is therefore from low-Ti tholeiitic basalts to high-Ti alkaline basalts. Furthermore, the younger basalts have relatively high Fe, P, K and incompatible element contents and low CaO/Al₂O₃ ratios.
⁸⁷Sr/⁸⁶Sr and 143Nd/144Nd ratios in Menguda and Ona/Durko basalts show similar and restricted ranges, respectively 0.70331-0.70423 and 0.51280-0.51288 (εNd = +3.2 to +4.7), and 0.70341-0.70434 and 0.51284-0.51288 (εNd = +3.9 to +4.8). Correlation of Sr isotope ratios with MgO used as index of fractionation indicates that the rocks underwent crustal contamination. Moreover, the earlier magmas (low-Ti tholeiitic basalts) were derived from a higher degree of partial melting of an incorporated depleted component in the Afar plume. Whereas, the later magmas (high-Ti alkaline basalts) were derived from a lower degree of melting of the plume itself.

ALAN R. WOOLLEY, KEN BAILEY, FRANCESCA CASTORINA, GIANLUIGI ROSATELLI, FRANCESCO STOPPA and FRANCIS WALL - Reply to: "Carbonate-rich pyroclastic rocks from central Apennines: carbonatites or carbonated rocks? A commentary". A. Peccerillo

Abstract - In a recent paper Peccerillo (2004) quoted chemical, including isotopic, mineralogical and field data which he claimed "..cast serious doubt on the hypothesis that the carbonate-rich pyroclastics from central Italy represent carbonatitic magmas.". He concluded that the carbonate is derived from sedimentary limestone. He also noted that he had enunciated these ideas earlier (Peccerillo, 1998) but that he had not been answered and further that "the Editor of this journal [Periodico di Mineralogia] has repeatedly asked me to set down my case again and I have, reluctantly, agreed to do so.". We, some of the principal advocates of the carbonatitic interpretation for the igneous carbonate-rich rocks of central Italy, have, therefore, taken this opportunity to answer, in detail, the various arguments advanced by Peccerillo. We remain firmly convinced that the evidence demonstrates that the carbonate-rich rocks at the five localities of San Venanzo, Cupaello, Polino, Oricola and Vulture are carbonatitic, the carbonate deriving from deep within the mantle, and that sedimentary limestones played no immediate part in their genesis.

ANGELO PECCERILLO - On the nature of carbonate-rich volcanic rocks in Central Italy. A reply to comments by Woolley et al.

Abstract - In an early paper (Peccerillo, 1998a) and in a more recent one (Peccerillo, 2004), I raised doubts about the hypothesis of Woolley and coworkers that the carbonate-rich pyroclastic rocks from the Intra-Apennine Province of central Italy (IAP) represent carbonatites. Eight years after publication of my 1998 paper, an answer finally arrives from Woolley and coworkers. Woolley et al. (2005) accept that mineralogical criteria and abundances of incompatible elements, including REE, are insufficient to establish a carbonatitic nature for IAP rocks, but they remain firmly convinced that carbonate-rich rocks from IAP are carbonatites, and that there is no significant interaction between magmas and carbonate wall rocks in the IAP. However, they do not provide an explanation for the high oxygen isotopic ratios of silicate phases, and do not account for the diluent effect of carbonates on silicate fraction observed in pairs of coexisting lavas and carbonate-rich rocks. Therefore, my key objections remain unanswered.
I believe that high oxygen isotope ratios of both carbonates (δ¹⁸O‰ = +20 to + 25) and, in particular, of silicate phases (δ¹⁸O‰ = +11 to + 14) of IAP rocks, together with the diluent effect of carbonate fraction observed in pairs of bulk carbonate-rich rocks and associated lavas at San Venanzo and Cupaello, cast doubts on a magmatic origin for carbonates. Whatever the opinion one may have on IAP carbonate-rich rocks, however, the alleged equivalence between IAP and Vulture volcano is unfounded, since the two occurrences differ for major, trace element and isotopic compositions, as well as for volcanological characteristics and geodynamic setting. Even more so, is for African carbonatite-kamafugite association whose composition and geodynamic significance have little or nothing to do with IAP. Incompatible element distribution and radiogenic isotopes of IAP rocks are very similar or identical to those of mafic rocks from the Roman province, indicating a genesis in similar metasomatic mantle sources and the same geodynamic significance.

D.K. BAILEY - Carbonate volcanics in Italy: numerical tests for the hypothesis of lava-sedimentary limestone mixing

Abstract - Simple mixing of fragmented silicate lava with high level sedimentary limestone has been advocated for the formation of the carbonate-rich volcanic rocks of Italy. This is tested on the classic San Venanzo volcanics, using major element analyses. The limiting composition that would need to be added to San Venanzo lava (containing 41% SiO₂) to produce the carbonate rich volcanic (with 39% SiO₂) is not calcite, but silicate (with 38% SiO₂). Alternatively, subtraction of calcite from the carbonate rich volcanic yields a limiting composition (with 53% SiO₂) quite unlike the lava. Simple mixing cannot be reconciled with the major element chemistry of the rocks.

ANGELO PECCERILLO - Numerical tests and qualitative approach to study of lavas and associated carbonate-rich pyroclastic rocks from the Intra-Apennine volcanoes. A reply to comments by D.K. Bailey

Abstract - Bailey (2005) reports on major element mass balance calculations for San Venanzo volcano to exclude that carbonate-rich rocks were generated by simple two end-member mixing of kamafugitic lava and sedimentary carbonates. I believe that the comments by Bailey (2005) are invalid because: 1) simple two-end-member mixing is a simplistic assumption, which has never been proposed in my previous papers (Peccerillo, 1998, 2004); 2) Quantitative comparison between average lavas and a single sample of bulk pyroclastic rock is methodologically incorrect, since the latter has much more variable composition than lava, because of diversity of juvenile fraction, post-depositional alteration and probable modification during transport of pyroclastic material. Therefore, Bailey's discussion does not help to shed light on the very nature of carbonate-rich rocks from internal zones of Apennines, and leaves the key-questions I raised in my previous papers (Peccerillo, 1998, 2004), still unanswered