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• W2044874998 abstract "[1] There are many difficulties with regard to Marra et al.'s [2004] hypothesis of a recurrence time of about 48 kyr for volcanic activity in the Roman Comagmatic Province (RCP), which developed along the Tyrrhenian margin of central Italy. Their hypothesis is based on statistical analysis of a compilation of age data for RCP volcanites that have been published since 1980. I will confine my discussion and criticism to the compilation and use of this database, which represents the analytical database used by Marra et al. [2004] to derive the cyclic nature of volcanic activity in the area and the geodynamic implications. [2] Marra et al. [2004] start from the eruptive history of the Alban Hills Volcanic District, which has been thoroughly investigated in recent years as regards its stratigraphic and geochronologic aspects [Karner and Renne, 1998; Karner et al., 2001; Marra et al., 2003], as summarized in Figure 2 of Marra et al. [2004]. The latter have obtained quiescence intervals of volcanic activity of the order of 45 (±17) kyr on the basis of data with strong stratigraphic constraints and representative of known volcanic activity. This recurrence time is characterized by a standard deviation of 38%, so it is poorly defined. Figure 1 shows all the used and rejected age data obtained on the Alban Hills volcanites: it is evident that there are periods of strong activity and periods of quiescence, but they do not seem to be regularly spaced in time. The figure also evidences a period of continuous activity from 308 to 250 ka. [3] There are two interesting points that must be addressed with regard to the Alban Hills database, and kept in mind when discussing the RCP database. In their statistical calculations Marra et al. [2004] do not use some 40Ar/39Ar data, which are apparently valid from a geochronologic point of view, but are considered inconsistent with the stratigraphy of their outcrops, due to a dominant nonjuvenile component [Marra et al., 2003, p. 234]. Marra et al. [2004] use a few K/Ar data taken from the literature for the Alban Hills, and reject a couple of K/Ar ages that are in disagreement with the 40Ar/39Ar ages (see the ages reported for the Capo di Bove and Ponte del Divino Amore lava flows in Table 1 of Marra et al. [2004]). Figure 1 also shows the age data of the Sabatini Volcanic Complex, which, like the Alban Hills, has undergone a recent detailed stratigraphic and geochronologic study [Karner and Renne, 1998; Karner et al., 2001]. It is evident that the 45 kyr recurrence time of the Alban Hills does not match the activity of the Sabatini complex. [4] Marra et al. [2004] list the geochronological data obtained on the RCP volcanoes, with the isotopic methods related to the decay of 40K to 40Ar (conventional K/Ar, unspiked K/Ar, 40Ar/39Ar) and published since 1980. The volcanic districts under consideration are, from south to north, Roccamonfina, Alban Hills, Sabatini, Vico, Vulsini, and Monte Amiata, which, although not a part of the RCP, is in the vicinity. The compilation is in chronologic order with the result that it is very difficult to insert a single datum in the activity of a volcanic district (Table 1 contains a summary of the data listed by Marra et al. [2004]). There is no stratigraphic reconstruction of the volcanic activity in the different areas, such as that given in Figure 2 of Marra et al. [2004] for the Alban Hills. The majority of the papers sourced from the literature deal with a limited part of the activity of the volcanoes, Monte Amiata being the only exception [Bigazzi et al., 1981]. The authors do not say whether the data were representative of all the known activity of the selected volcanoes and in fact, they are not. They list 23 samples from unknown deposit types (and used thirteen in their statistical calculation). [5] Some samples from the Alban Hills and Sabatini volcanic districts are listed twice because the age of the standard was revised between one set of analyses and another, leading to a small increase in the age of the samples (Table 1 of Karner and Renne [1998] and Table 2 of Karner et al. [2001]); only one datum was considered in the calculation, but there is a biased enlargement of the data set. This leads us to the problem of the standards used in the 40Ar/39Ar geochronology and their ages. There is no mention of this problem in the text: although the order of magnitude of the age variation resulting from normalization to a unique standard is on average smaller than the 15 kyr time window that Marra et al. [2004] allow for each sample, the problem nonetheless does exist. For example, Turbeville [1992] (Latera volcano samples) uses an age of 17.9 Ma for the standard B4M (muscovite), which is far lower than all other values found in the literature (see Baksi et al. [1996] for a compilation). [6] The degree of the reported error (1 or 2σ levels) is not declared in Table 1 of Marra et al. [2004], although it must be admitted that at times it is not specified in the source papers. Sometimes errors clearly stated in the original text are, however, reported at different confidence levels. As the error is one of the declared criteria for inclusion of samples in the calculation, it is important that we are consistent in this sector. [7] Marra et al. [2004] sometimes misquote the ages reported in the papers: for Monte Amiata they list and use as K/Ar isochron ages of three samples the following values: 310 ± 5 (MA77-19), 310 ± 4 (MA77-1) and 301 ± 1 ka (MA 80–89), which represent the 40Ar/36Ar initial ratios [Bigazzi et al., 1981], whereas the isochron ages are 240 ± 18, 226 ± 8 and 271 ± 4 ka, respectively. For the Sabatini District they report a value (550 ± 10 ka) for the age of the Tufo Giallo della Via Tiberina (TGVT) that, in the authors' opinion [Cioni et al., 1993], was only meant as a limit. These mistakes will attain even greater importance if the compilation is used in the future as a reference for RCP geochronology instead of the original source papers. [8] Marra et al. [2004] included 158 of the 233 age data related to the time interval 600–100 ka in their statistical calculation, which resulted in the histogram shown in their Figure 4. The only criterion of choice declared in the footnote of Table 1 is “preferentially 40Ar/39Ar data with statistical error ≤20 ka.” With this note, the authors acknowledge that the 40Ar/39Ar method is more precise than the K/Ar methods. Yet 71 K/Ar dates were included in their statistical calculation, so a consistent part of the data set is represented by model ages with an imposed atmospheric 40Ar/36Ar initial ratio. This does not necessarily mean that the K/Ar dates are wrong, but there is no constraint on them, as in many cases there is no clear stratigraphic control. Marra et al. [2004] have often disregarded their declared preference for 40Ar/39Ar ages because there are formations that, according to the current stratigraphic reconstruction, are represented in their calculation by discordant K/Ar and 40Ar/39Ar ages. [9] Discrepancies between ages obtained with the K/Ar and 40Ar/39Ar methods on the same volcanic unit are relatively common in the RCP, but the two methods have rarely been applied on the same sample, which would have allowed a true technical comparison. In other cases the observed discrepancies might also derive from uncertainty in the stratigraphic attribution or from different degrees of purity of mineral separates (for K/Ar and 40Ar/39Ar multigrain analyses). The presence of disturbed 40Ar/39Ar age spectra among samples of RCP volcanites [Cioni et al., 1993; Barberi et al., 1994; Villa, 1991], caused either by parentless 40Ar or mixed populations, and the presence of xenocrystic contamination in 40Ar/39Ar laser fusion ages [Marra et al., 2003] demonstrate that some volcanites are unsuitable for K/Ar dating. Another process that may affect K/Ar ages, but on sanidine only, is the incomplete recovery of radiogenic Ar during fusion, because a very high temperature is required to fuse the viscous feldspatic melt [McDowell, 1983]. This results in an underestimated age, as exemplified by a sample from Monte Amiata: a K/Ar date of 241 ± 7 ka (1σ) (sanidine MA77-19 [Bigazzi et al., 1981]) becomes, on the same mineral separate, a 40Ar/39Ar plateau age of 303 ± 5.5 ka (1σ) [Laurenzi and Villa, 1991]. Giannetti [2001] also provides us with the opportunity of comparing K/Ar and 40Ar/39Ar dates that were presumably performed on the same sample. Discrepancies can be observed between K/Ar dates of different mineral phases of the same sample and between K/Ar and 40Ar/39Ar total fusion datings of the same phase; in this case the sanidine K/Ar dates are not systematically younger than the 40Ar/39Ar ages. [10] The Marra et al. [2004] database contains individual volcanic formations represented by multiple analyses as well as samples on which many phases were measured. They use the notation yes/no (active/not active) for the chosen time interval to reconstruct a histogram of the volcanic activity, so the input of two or more identical ages for the same formation does not influence the final result. The problems arise when discordant dates emerge from different laboratories, and/or from many mineral phases on the same sample; this is a fairly common occurrence and entails making a choice. Marra et al. [2004] have, however, often failed to make this choice, with the result that there are formations that were considered in the calculation with more than one date. The Bagnoregio ignimbrite, Vulsini Volcanic District, for example, was computed in the calculation with three dates: a 40Ar/39Ar step-heating age on sanidine of 319 ± 4 ka (1σ) [Barberi et al., 1994], a K/Ar age on sanidine of 333 ± 4 ka (2 σ) [Nappi et al., 1995], and a 40Ar/39Ar single-crystal laser fusion age on sanidine of 296 ± 4 ka (the degree of the error is unknown, because the weighted average recalculated on the displayed data gives an error of ±25 ka at the 95% confidence level and a MSWD = 5.5) [Turbeville, 1992]. [11] Often in their calculation Marra et al. [2004] use discordant ages obtained on different phases of the same formation. At Roccamonfina, for example, sample R-247, like others, forms part of the accepted data with the following “ages”, all listed in the same paper on pages 310 and 311 [Giannetti, 2001]: 373 ± 4 ka, deposit type lava dome, K/Ar on sanidine; 393 ± 12 ka, deposit type unknown, K/Ar on sanidine (in fact, it is an Ar-Ar age); 430 ± 6 ka, deposit type lava dome, K/Ar on groundmass. At Vico volcano, on the other hand, Marra et al. [2004], surprisingly, consider for their calculation the 40Ar/39Ar leucite age of the ignimbrite C and not the 40Ar/39Ar sanidine age (they are coincident, and there is also a combined sanidine-leucite isochron age in the paper) [Laurenzi and Villa, 1987]. [12] The input of two or more different ages for the same formation in theory introduces a bias, because no more than one age will eventually be true. If there are other “true ages” of the same volcano in the wrong age interval, no bias is, in practice, introduced. However, this operation is analytically incorrect, and it is difficult to understand why only one out of two coincident ages on the same volcanic formation was not considered and three discordant ages on the same lava flow were all considered in the calculation. [13] It is evident that the critical approach used by Marra et al. [2004] for the Alban Hills and Sabatini volcanites was abandoned in their treatment of the literature data for the other volcanic districts. This is also evident in Table 1, which contains a summary of the K/Ar and 40Ar/39Ar data listed and used; the table reveals a far higher percentage of rejected data for the Sabatini and Alban Hills, which are the volcanic districts with the best age coverage of their volcanic activities. The percentage of accepted data is far higher for the Monte Amiata (all age data were accepted), Vulsini (50 accepted out of 71), Vico (8 out of 10), and Roccamonfina (37 accepted and 6 not) districts than for the Alban Hills (25 rejected and 31 accepted) and Sabatini volcanites (21 rejected and 18 accepted). The data used in the calculation for the Roccamonfina, Vulsini, Vico, and Monte Amiata volcanites are of questionable validity, as they are either unrelated to a stratigraphic reconstruction, incoherent from sample to sample, or just plain erroneous. [14] The main problem is that the chronostratigraphic reconstruction of the volcanic activity of Roccamonfina, Vico, Vulsini, and Monte Amiata products is not sufficiently detailed and complete at the moment to apply the statistical treatment of Marra et al. [2004]. Reconstruction of the entire activity of the different volcanic districts is a necessary prerequisite to any treatment of the recurrence interval in the Roman Comagmatic Province as a whole. Some sort of periodicity may eventually emerge, but at the moment it is no more than a working hypothesis. [15] This paper was supported by funding from the Italian National Research Council (CNR). The author wishes to thank Robert Fleck for the constructive review. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article." @default.
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• W2044874998 title "Comment on “Recurrence of volcanic activity along the Roman Comagmatic Province (Tyrrhenian margin of Italy) and its tectonic significance” by F. Marra, J. Taddeucci, C. Freda, W. Marzocchi, and P. Scarlato" @default.
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