Fractionation of Nb/Ta during subduction of carbonate-rich sediments
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Keywords: Nb/Ta, HFSE, sediment melt, carbonate subduction, carbon cycle, SiO2 saturation, Italian magmatism
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Abstract
Figures
 Figure 1 Variations of Nb/Ta relative to other geochemical proxies expressing intraplate metasomatism of the lithosphere (K/K*), subduction affinity (K/K* and Nb/Nb*), K-enrichment (K/K* and K2/Na2O) and degree of silica saturation (FSSI; Feldspathoid Silica Saturation Index). The K* and Nb* values are calculated from the geometrical mean of PM-normalised concentrations of Nb-U and La-U, respectively. “LD” refers to “Latium District”. |  Figure 2 Nb/Ta against other proxies sensitive to carbonate-rich fluids/melts (e.g., carbonatites). |  Figure 3 (a) Ba/Th vs. 143Nd/144Nd, showing the variable contribution of melts and (supercritical) liquids in different subduction related magmas. (b, c, d) Nb/Ta vs. Nd, Sr and Hf isotopes, respectively, in different subduction related magmas. Arc magmas dominated by sediment melts display the largest variations in radiogenic isotopes. Among them, silica-undersaturated volcanic rocks (highlighted by an open circle within the symbol) show elevated Nb/Ta due to recycling of subducting carbonate sedimentary lithologies. Only samples where Nb/Ta was determined by isotope dilution are plotted (see Supplementary Information for references). |
| Figure 1 | Figure 2 | Figure 3 |
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Introduction
The relative concentrations of Nb and Ta remain nearly constant during most magmatic processes and, thus, variations of Nb/Ta in volcanic rocks can reveal specific processes. Among geological environments, the highest Nb/Ta values are observed in carbonatites, lithosphere-derived rocks, and subduction related rocks (e.g., Green, 1995
Green, T.H. (1995) Significance of Nb/Ta as an indicator of geochemical processes in the crust-mantle system. Chemical Geology 120, 347–359. https://doi.org/10.1016/0009-2541(94)00145-X
; Münker et al., 2003Münker, C., Pfänder, J.A., Weyer, S., Büchl, A., Kleine, T., Mezger, K. (2003) Evolution of Planetary Cores and the Earth-Moon System from Nb/Ta Systematics. Science 301, 84–87. https://doi.org/10.1126/science.1084662
; Klemme et al., 2005Klemme, S., Prowatke, S., Hametner, K., Günther, D. (2005) Partitioning of trace elements between rutile and silicate melts: Implications for subduction zones. Geochimica et Cosmochimica Acta 69, 2361–2371. https://doi.org/10.1016/j.gca.2004.11.015
). The extremely high Nb/Ta of carbonatites can be used to recognise the involvement of carbonate-rich melts and fluids in different geological settings (e.g., Green, 1995Green, T.H. (1995) Significance of Nb/Ta as an indicator of geochemical processes in the crust-mantle system. Chemical Geology 120, 347–359. https://doi.org/10.1016/0009-2541(94)00145-X
). For instance, the high Nb/Ta of some intraplate magmas was shown to derive from mantle metasomatism by carbonatite-like melts (Bragagni et al., 2022Bragagni, A., Mastroianni, F., Münker, C., Conticelli, S., Avanzinelli, R. (2022) A carbon-rich lithospheric mantle as a source for the large CO2 emissions of Etna volcano (Italy). Geology 50, 486–490. https://doi.org/10.1130/G49510.1
).The high Nb/Ta of some arc magmas was attributed to residual rutile in the subducting slab. However, it remains ambiguous why only some arc magmas show high Nb/Ta whilst the occurrence of residual rutile is rather ubiquitous, as suggested by the characteristic HFSE depletions of all subduction related magmas. Fractionation of Nb/Ta was ascribed to supercritical fluids (e.g., W. Chen et al., 2018
Chen, W., Xiong, X., Wang, J., Xue, S., Li, L., Liu, X., Ding, X., Song, M. (2018) TiO2 Solubility and Nb and Ta Partitioning in Rutile-Silica-Rich Supercritical Fluid Systems: Implications for Subduction Zone Processes. Journal of Geophysical Research: Solid Earth 123, 4765–4782. https://doi.org/10.1029/2018JB015808
; T.-N. Chen et al., 2022Chen, T.-N., Chen, R.-X., Zheng, Y.-F., Zhou, K., Yin, Z.-Z., Wang, Z.-M., Gong, B., Zha, X.-P. (2022) The effect of supercritical fluids on Nb-Ta fractionation in subduction zones: Geochemical insights from a coesite-bearing eclogite-vein system. Geochimica et Cosmochimica Acta 335, 23–55. https://doi.org/10.1016/j.gca.2022.08.013
) or melts (Klemme et al., 2005Klemme, S., Prowatke, S., Hametner, K., Günther, D. (2005) Partitioning of trace elements between rutile and silicate melts: Implications for subduction zones. Geochimica et Cosmochimica Acta 69, 2361–2371. https://doi.org/10.1016/j.gca.2004.11.015
; Stolz et al., 1996Stolz, A.J., Jochum, K.P., Spettel, B., Hofmann, A.W. (1996) Fluid- and melt-related enrichment in the subarc mantle: Evidence from Nb/Ta variations in island-arc basalts. Geology 24, 587–590. https://doi.org/10.1130/0091-7613(1996)024<0587:FAMREI>2.3.CO;2
) in equilibrium with rutile, whereas aqueous fluids are not expected to significantly influence the bulk Nb/Ta due to their low HFSE abundance (e.g., Brenan et al., 1994Brenan, J.M., Shaw, H.F., Phinney, D.L., Ryerson, F.J. (1994) Rutile-aqueous fluid partitioning of Nb, Ta, Hf, Zr, U and Th: implications for high field strength element depletions in island-arc basalts. Earth and Planetary Science Letters 128, 327–339. https://doi.org/10.1016/0012-821X(94)90154-6
). To evaluate if Nb/Ta could be affected by carbon-rich fluids/melts released by subducted carbonate sediments, we investigated volcanic rocks from the Italian peninsula and Tyrrhenian seafloor. Here, chemical variations are well constrained and reflect different lithologies of the subducted sediments, being silicate-rich in the so called Tuscan magmatic province and carbonate-rich in the younger Roman magmatic province (e.g., Conticelli and Peccerillo, 1992Conticelli, S., Peccerillo, A. (1992) Petrology and geochemistry of potassic and ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources. Lithos 28, 221–240. https://doi.org/10.1016/0024-4937(92)90008-M
; Conticelli et al., 2015Conticelli, S., Avanzinelli, R., Ammannati, E., Casalini, M. (2015) The role of carbon from recycled sediments in the origin of ultrapotassic igneous rocks in the Central Mediterranean. Lithos 232, 174–196. https://doi.org/10.1016/j.lithos.2015.07.002
).top
Elevated Nb/Ta in Italian Silica-Undersaturated Magmas
HFSE concentrations measured by isotope dilution and 176Hf/177Hf data were obtained for representative samples of the Plio-Quaternary Italian volcanism (see Supplementary Information for analytical methods and the full data set). The new data from Tuscan and Roman magmatic provinces and IODP drill cores of the Tyrrhenian Sea (representative of mantle sources not affected by subduction) were integrated with published data from Etna, Stromboli, Vulture, and Pantelleria (Bragagni et al., 2022
Bragagni, A., Mastroianni, F., Münker, C., Conticelli, S., Avanzinelli, R. (2022) A carbon-rich lithospheric mantle as a source for the large CO2 emissions of Etna volcano (Italy). Geology 50, 486–490. https://doi.org/10.1130/G49510.1
).The high Nb/Ta of Etna and Vulture were previously explained by mantle-derived carbonatite-like metasomatism in the subcontinental lithospheric mantle (Bragagni et al., 2022
Bragagni, A., Mastroianni, F., Münker, C., Conticelli, S., Avanzinelli, R. (2022) A carbon-rich lithospheric mantle as a source for the large CO2 emissions of Etna volcano (Italy). Geology 50, 486–490. https://doi.org/10.1130/G49510.1
). The influence of intraplate metasomatism in the subcontinental lithospheric mantle is attested by the relative deficit of K expressed as K/K* < 1 (Fig. 1a). Conversely, both Tuscan and Roman lavas have elevated K/K*, typical of subduction zones, but with different Nb/Ta (Fig. 1). Tuscan and Tyrrhenian magmas have unfractionated Nb/Ta, similar to the BSE (14 ± 0.3; Münker et al., 2003Münker, C., Pfänder, J.A., Weyer, S., Büchl, A., Kleine, T., Mezger, K. (2003) Evolution of Planetary Cores and the Earth-Moon System from Nb/Ta Systematics. Science 301, 84–87. https://doi.org/10.1126/science.1084662
), whilst Roman lavas display higher ratios (up to 24).Figure 1 Variations of Nb/Ta relative to other geochemical proxies expressing intraplate metasomatism of the lithosphere (K/K*), subduction affinity (K/K* and Nb/Nb*), K-enrichment (K/K* and K2O/Na2O) and degree of silica saturation (FSSI; Feldspathoid Silica Saturation Index). The K* and Nb* values are calculated from the geometrical mean of PM-normalised concentrations of Nb-U and La-U, respectively. “LD” refers to “Latium District”.
Both Tuscan and Roman lavas have high K content and K2O/Na2O (Fig. 1b), ranging in composition from shoshonitic to ultrapotassic (lamproites in the Tuscan and plagioleucitites/leucitites/kamafugites in the Roman provinces). Their trace element budget is dominated by a strong subduction signature (Conticelli and Peccerillo, 1992
Conticelli, S., Peccerillo, A. (1992) Petrology and geochemistry of potassic and ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources. Lithos 28, 221–240. https://doi.org/10.1016/0024-4937(92)90008-M
; Avanzinelli et al., 2009Avanzinelli, R., Lustrino, M., Mattei, M., Melluso, L., Conticelli, S. (2009) Potassic and ultrapotassic magmatism in the circum-Tyrrhenian region: Significance of carbonated pelitic vs. pelitic sediment recycling at destructive plate margins. Lithos 113, 213–227. https://doi.org/10.1016/j.lithos.2009.03.029
; Conticelli et al., 2015Conticelli, S., Avanzinelli, R., Ammannati, E., Casalini, M. (2015) The role of carbon from recycled sediments in the origin of ultrapotassic igneous rocks in the Central Mediterranean. Lithos 232, 174–196. https://doi.org/10.1016/j.lithos.2015.07.002
; Lustrino et al., 2019Lustrino, M., Fedele, L., Agostini, S., Prelević, D., Salari, G. (2019) Leucitites within and around the Mediterranean area. Lithos 324–325, 216–233. https://doi.org/10.1016/j.lithos.2018.11.007
) as shown also by the very low Nb/Nb* (Fig. 1c). Previous studies discussed the differences between magmas from the Tuscan and Roman magmatic provinces, suggesting mantle metasomatism related to subducted Si-rich metapelites, in the former, and carbonate-rich metapelites (marls) in the latter (Avanzinelli et al., 2009Avanzinelli, R., Lustrino, M., Mattei, M., Melluso, L., Conticelli, S. (2009) Potassic and ultrapotassic magmatism in the circum-Tyrrhenian region: Significance of carbonated pelitic vs. pelitic sediment recycling at destructive plate margins. Lithos 113, 213–227. https://doi.org/10.1016/j.lithos.2009.03.029
; Frezzotti et al., 2009Frezzotti, M.L., Peccerillo, A., Panza, G. (2009) Carbonate metasomatism and CO2 lithosphere–asthenosphere degassing beneath the Western Mediterranean: An integrated model arising from petrological and geophysical data. Chemical Geology 262, 108–120. https://doi.org/10.1016/j.chemgeo.2009.02.015
; Conticelli et al., 2015Conticelli, S., Avanzinelli, R., Ammannati, E., Casalini, M. (2015) The role of carbon from recycled sediments in the origin of ultrapotassic igneous rocks in the Central Mediterranean. Lithos 232, 174–196. https://doi.org/10.1016/j.lithos.2015.07.002
). This hypothesis is supported by the contrasting silica saturation, being saturated to oversaturated in the Tuscan and saturated to strongly undersaturated in the Roman volcanic rocks (Conticelli and Peccerillo, 1992Conticelli, S., Peccerillo, A. (1992) Petrology and geochemistry of potassic and ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources. Lithos 28, 221–240. https://doi.org/10.1016/0024-4937(92)90008-M
). Other evidence for recycling of carbonates in the Roman but not in the Tuscan magma sources, includes i) 87Sr/86Sr buffered at a composition typical of carbonate-rich sediments for Roman, whilst reaching more radiogenic values for Tuscan lavas (e.g., Avanzinelli et al., 2009Avanzinelli, R., Lustrino, M., Mattei, M., Melluso, L., Conticelli, S. (2009) Potassic and ultrapotassic magmatism in the circum-Tyrrhenian region: Significance of carbonated pelitic vs. pelitic sediment recycling at destructive plate margins. Lithos 113, 213–227. https://doi.org/10.1016/j.lithos.2009.03.029
), ii) low Ni content and high Ca/Fe of high-Fo olivine within Roman lavas (Ammannati et al., 2016Ammannati, E., Jacob, D.E., Avanzinelli, R., Foley, S.F., Conticelli, S. (2016) Low Ni olivine in silica-undersaturated ultrapotassic igneous rocks as evidence for carbonate metasomatism in the mantle. Earth and Planetary Science Letters 444, 64–74. https://doi.org/10.1016/j.epsl.2016.03.039
), iii) 238U-excess in Vesuvius magmas (Avanzinelli et al., 2018Avanzinelli, R., Casalini, M., Elliott, T., Conticelli, S. (2018) Carbon fluxes from subducted carbonates revealed by uranium excess at Mount Vesuvius, Italy. Geology 46, 259–262. https://doi.org/10.1130/G39766.1
), iv) similar trace element patterns between Roman lavas and marls (Grassi et al., 2012Grassi, D., Schmidt, M.W., Günther, D. (2012) Element partitioning during carbonated pelite melting at 8, 13 and 22 GPa and the sediment signature in the EM mantle components. Earth and Planetary Science Letters 327–328, 84–96. https://doi.org/10.1016/j.epsl.2012.01.023
), v) melt inclusions in the Roman lavas with high CaO (up 22 wt. %) and CaO/Al2O3 (Nikogosian and van Bergen, 2010Nikogosian, I.K., van Bergen, M.J. (2010) Heterogeneous mantle sources of potassium-rich magmas in central-southern Italy: Melt inclusion evidence from Roccamonfina and Ernici (Mid Latina Valley). Journal of Volcanology and Geothermal Research 197, 279–302. https://doi.org/10.1016/j.jvolgeores.2010.06.014
) and vi) Ca isotopes of Roman leucitites (Ren et al., 2024Ren, H., Casalini, M., Conticelli, S., Chen, C., Foley, S.F., Feng, L., Liu, Y. (2024) Calcium isotope compositions of subduction-related leucite-bearing rocks: Implications for the calcium isotope heterogeneity of the mantle and carbonate recycling in convergent margins. Geochimica et Cosmochimica Acta 364, 100–113. https://doi.org/10.1016/j.gca.2023.11.022
).Subducting carbonate-rich sediments release minor CO2-rich melts/supercritical fluids (Chen et al., 2023
Chen, C., Förster, M.W., Foley, S.F., Shcheka, S.S. (2023) Carbonate-rich crust subduction drives the deep carbon and chlorine cycles. Nature 620, 576–581. https://doi.org/10.1038/s41586-023-06211-4
) but in sufficient amounts to induce CO2-excess and produce silica-undersaturated magmas upon mantle partial melting (Conticelli et al., 2015Conticelli, S., Avanzinelli, R., Ammannati, E., Casalini, M. (2015) The role of carbon from recycled sediments in the origin of ultrapotassic igneous rocks in the Central Mediterranean. Lithos 232, 174–196. https://doi.org/10.1016/j.lithos.2015.07.002
; Gülmez et al., 2023Gülmez, F., Prelević, D., Förster, M.W., Buhre, S., Günther, J. (2023) Experimental production of K-rich metasomes through sediment recycling at the slab-mantle interface in the fore-arc. Scientific Reports 13, 19608. https://doi.org/10.1038/s41598-023-46367-7
and references therein). In leucite-bearing lavas, the degree of silica undersaturation shows a negative correlation with Nb/Ta (Fig. 1d). The highest Nb/Ta are recorded in leucitites, which have the strongest subduction signature (Fig. 1c) and degree of silica undersaturation (Fig. 1d). Silica-rich supercritical fluids or melts in equilibrium with residual rutile, which is usually proposed to explain the high Nb/Ta, are not expected to generate such trends, especially when compared to the degree of silica undersaturation. Therefore, we propose that elevated Nb/Ta derive from melts liberated by subducting carbonate-rich marls.top
Elevated Nb/Ta from Carbonate-Rich Melts/Fluids in Subduction Zones
Since carbonates are typically HFSE poor, the silicate fraction of the marls would account for the required HFSE budget, whereas the carbonate fraction would liberate carbon-rich fluids/melts required to fractionate Nb/Ta. Recently, Gülmez et al. (2023)
Gülmez, F., Prelević, D., Förster, M.W., Buhre, S., Günther, J. (2023) Experimental production of K-rich metasomes through sediment recycling at the slab-mantle interface in the fore-arc. Scientific Reports 13, 19608. https://doi.org/10.1038/s41598-023-46367-7
showed that the reaction of carbonate-rich sediments with peridotites at 800–850 °C forms carbonatitic and K-rich silicic melts, explaining the genesis of ultrapotassic silica-undersaturated magmas, such as the Roman ones. It is yet difficult to identify the exact nature of such melts/supercritical fluids. This is because different melts/supercritical liquids interact, mix, and exsolve as function of the physical conditions of the mantle wedge (e.g., P–T–fO2) and chromatographic effects in the slab and within veined peridotite (e.g., Chen et al., 2022Chen, T.-N., Chen, R.-X., Zheng, Y.-F., Zhou, K., Yin, Z.-Z., Wang, Z.-M., Gong, B., Zha, X.-P. (2022) The effect of supercritical fluids on Nb-Ta fractionation in subduction zones: Geochemical insights from a coesite-bearing eclogite-vein system. Geochimica et Cosmochimica Acta 335, 23–55. https://doi.org/10.1016/j.gca.2022.08.013
). Moreover, the behaviour of trace elements will also depend on several unconstrained parameters describing the melting processes (i.e. degree of partial melting, mineralogy, partition coefficients) in the slab and in the metasomatised mantle. Two tentative simple models are reported in the Supplementary Information to show that Roman magmas can be quantitatively explained by melting of carbonate-rich sediments. Nevertheless, there are several lines of evidence suggesting that the high Nb/Ta of Roman magmas derives from carbonate-rich sediments. 1) The Nb/Ta ratios of the Roman volcanic rocks correlate with proxies for carbonatite-like components (Fig. 2). 2) Natural melts produced from silica- and carbonate-rich lithologies, as observed in inclusions in high P–T metamorphic rocks, show variable enrichment in HFSE and K contents (Korsakov and Hermann, 2006Korsakov, A.V., Hermann, J. (2006) Silicate and carbonate melt inclusions associated with diamonds in deeply subducted carbonate rocks. Earth and Planetary Science Letters 241, 104–118. https://doi.org/10.1016/j.epsl.2005.10.037
). Interestingly, among these inclusions, the highest HFSE and K contents are recorded in melts with high Nb/Ta (∼30). 3) Carbonatite-like melts interpreted to derive from slab melting of carbonate-rich sediments also show elevated Nb/Ta (Ravna et al., 2017Ravna, E.K., Zozulya, D., Kullerud, K., Corfu, F., Nabelek, P.I., Janák, M., Slagstad, T., Davidsen, B., Selbekk, R.S., Schertl, H.-P. (2017) Deep-seated Carbonatite Intrusion and Metasomatism in the UHP Tromsø Nappe, Northern Scandinavian Caledonides—a Natural Example of Generation of Carbonatite from Carbonated Eclogite. Journal of Petrology 58, 2403–2428. https://doi.org/10.1093/petrology/egy016
). 4) Rutile, which likely controls HFSE in the subducting slab, shows the lowest DNb/DTa (0.35) when in equilibrium with a carbonatite melt (Green, 2000Green, T.H. (2000) New partition coefficient determinations pertinent to hydrous melting processes in subduction zones. In: Smith, I.E.M., Davidson, J.P., Gamble, J.A., Price, R.C. (Eds.) Processes and Time Scales in the Genesis and Evolution of ARC Magmas. Royal Society of New Zealand, Wellington, 92–95.
).Figure 2 Nb/Ta against other proxies sensitive to carbonate-rich fluids/melts (e.g., carbonatites).
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Comparison with Other Potassic and Ultrapotassic Rocks in the Mediterranean Area
Zn-Mg isotope compositions in other circum-Mediterranean magmas suggests the recycling of marls (Shu et al., 2023
Shu, Z.-T., Liu, S.-A., Prelević, D., Wang, Y., Foley, S.F., Cvetković, V., Li, S. (2023) Recycled Carbonate-Bearing Silicate Sediments in the Sources of Circum-Mediterranean K-Rich Lavas: Evidence From Mg-Zn Isotopic Decoupling. Journal of Geophysical Research: Solid Earth 128, e2022JB025135. https://doi.org/10.1029/2022JB025135
). In the area, high precision HFSE data are available for the Rhodopes (Bulgaria) and Santorini (Kirchenbaur and Münker, 2015Kirchenbaur, M., Münker, C. (2015) The behaviour of the extended HFSE group (Nb, Ta, Zr, Hf, W, Mo) during the petrogenesis of mafic K-rich lavas: The Eastern Mediterranean case. Geochimica et Cosmochimica Acta 165, 178–199. https://doi.org/10.1016/j.gca.2015.05.030
). Among these, high Nb/Ta (19–20) was only observed in leucite-bearing absarokites from the Rhodopes. Instead, all silica-saturated volcanic rocks show lower Nb/Ta (12–16). Considering also conventional ICP-MS data, among potassic and ultrapotassic rocks of the circum-Mediterranean, the highest Nb/Ta was observed in ultrapotassic rocks from Macedonia (average of 20; Prelević et al., 2008Prelević, D., Foley, S.F., Romer, R., Conticelli, S. (2008) Mediterranean Tertiary lamproites derived from multiple source components in postcollisional geodynamics. Geochimica et Cosmochimica Acta 72, 2125–2156. https://doi.org/10.1016/j.gca.2008.01.029
). Even if these rocks were originally classified as lamproites, they are leucite-bearing and were later classified as plagioleucitites (Lustrino et al., 2019Lustrino, M., Fedele, L., Agostini, S., Prelević, D., Salari, G. (2019) Leucitites within and around the Mediterranean area. Lithos 324–325, 216–233. https://doi.org/10.1016/j.lithos.2018.11.007
), making them comparable to Roman rocks. In the Mediterranean area, leucitites and plagioleucitites occur also in the Pontides (Turkey) and the average Nb/Ta is slightly higher than the BSE (18 in Eastern Pontides; Altherr et al., 2008Altherr, R., Topuz, G., Siebel, W., Şen, C., Meyer, H.-P., Satır, M., Lahaye, Y. (2008) Geochemical and Sr–Nd–Pb isotopic characteristics of Paleocene plagioleucitites from the Eastern Pontides (NE Turkey). Lithos 105, 149–161. https://doi.org/10.1016/j.lithos.2008.03.001
; 20 in Central Pontides; Gülmez et al., 2016Gülmez, F., Genç, Ş.C., Prelević, D., Tüysüz, O., Karacik, Z., Roden, M.F., Billor, Z. (2016) Ultrapotassic Volcanism from the Waning Stage of the Neotethyan Subduction: a Key Study from the Izmir–Ankara–Erzincan Suture Belt, Central Northern Turkey. Journal of Petrology 57, 561–593. https://doi.org/10.1093/petrology/egw021
). Importantly, other silica-saturated potassic and ultrapotassic rocks from Spain, Serbia and Turkey, have lower Nb/Ta, analytically indistinguishable from the BSE value (Prelević et al., 2008Prelević, D., Foley, S.F., Romer, R., Conticelli, S. (2008) Mediterranean Tertiary lamproites derived from multiple source components in postcollisional geodynamics. Geochimica et Cosmochimica Acta 72, 2125–2156. https://doi.org/10.1016/j.gca.2008.01.029
). High Nb/Ta in subduction-related potassic and ultrapotassic circum-Mediterranean rocks are a peculiar feature of silica-undersaturated rocks, ultimately reflecting the recycling of carbonate-bearing sedimentary lithologies.top
A Common High Nb/Ta Signature in Silica-Undersaturated Magmas from Carbonate-Rich Subduction Zones
We further investigate the relationship between elevated Nb/Ta and recycled carbonate-rich sediments considering isotope dilution HFSE data from subduction-related magmas worldwide. In Figure 3a (Ba/Th vs. 143Nd/144Nd), magmas define two trends reflecting the contribution of fluids from the subducted basaltic crust (high Ba/Th) or melts dominated by sediments (low Ba/Th).
Figure 3 (a) Ba/Th vs. 143Nd/144Nd, showing the variable contribution of melts and (supercritical) liquids in different subduction related magmas. (b, c, d) Nb/Ta vs. Nd, Sr and Hf isotopes, respectively, in different subduction related magmas. Arc magmas dominated by sediment melts display the largest variations in radiogenic isotopes. Among them, silica-undersaturated volcanic rocks (highlighted by an open circle within the symbol) show elevated Nb/Ta due to recycling of subducting carbonate sedimentary lithologies. Only samples where Nb/Ta was determined by isotope dilution are plotted (see Supplementary Information for references).
Radiogenic isotope compositions (Sr-Nd-Hf), plotted against Nb/Ta (Fig. 3b–d), show that fluid dominated arcs (low 87Sr/86Sr, high 143Nd/144Nd and 176Hf/177Hf) point towards high Nb/Ta, which was previously interpreted as the effect of residual rutile in equilibrium with metasomatic fluids, possibly at the supercritical state (e.g., W. Chen et al., 2018
Chen, W., Xiong, X., Wang, J., Xue, S., Li, L., Liu, X., Ding, X., Song, M. (2018) TiO2 Solubility and Nb and Ta Partitioning in Rutile-Silica-Rich Supercritical Fluid Systems: Implications for Subduction Zone Processes. Journal of Geophysical Research: Solid Earth 123, 4765–4782. https://doi.org/10.1029/2018JB015808
; T.-N. Chen et al., 2022Chen, T.-N., Chen, R.-X., Zheng, Y.-F., Zhou, K., Yin, Z.-Z., Wang, Z.-M., Gong, B., Zha, X.-P. (2022) The effect of supercritical fluids on Nb-Ta fractionation in subduction zones: Geochemical insights from a coesite-bearing eclogite-vein system. Geochimica et Cosmochimica Acta 335, 23–55. https://doi.org/10.1016/j.gca.2022.08.013
). Instead, sediment dominated arcs (high 87Sr/86Sr, low 143Nd/144Nd and 176Hf/177Hf) show either BSE-like Nb/Ta (Tuscan province, Santorini, Papua New Guinea, Cyprus) or shifts towards high Nb/Ta (Roman province, Stromboli, Bulgaria, Sunda rear-arc). Similar to what is observed in Bulgaria and in Roman and Tuscan volcanic rocks, at Sunda only silica-undersaturated samples have elevated Nb/Ta, whilst silica-saturated samples have BSE-like Nb/Ta. The high Nb/Ta magmatism of Sunda is only observed in the K-rich rear arc in the Eastern sector (Stolz et al., 1996Stolz, A.J., Jochum, K.P., Spettel, B., Hofmann, A.W. (1996) Fluid- and melt-related enrichment in the subarc mantle: Evidence from Nb/Ta variations in island-arc basalts. Geology 24, 587–590. https://doi.org/10.1130/0091-7613(1996)024<0587:FAMREI>2.3.CO;2
; Kirchenbaur et al., 2022Kirchenbaur, M., Schuth, S., Barth, A.R., Luguet, A., König, S., Idrus, A., Garbe-Schönberg, D., Münker, C. (2022) Sub-arc mantle enrichment in the Sunda rear-arc inferred from HFSE systematics in high-K lavas from Java. Contributions to Mineralogy and Petrology 177, 8. https://doi.org/10.1007/s00410-021-01871-9
). Importantly, at Sunda, different sediments are subducting, with a strong carbonate contribution only in the Eastern sector (House et al., 2019House, B.M., Bebout, G.E., Hilton, D.R. (2019) Carbon cycling at the Sunda margin, Indonesia: A regional study with global implications. Geology 47, 483–486. https://doi.org/10.1130/G45830.1
).In summary, in Italian collisional magmatism as well as in other melt-dominated arcs worldwide, the high Nb/Ta are associated with other evidence of recycling of carbonate-rich lithologies, such as the degrees of silica saturation. Silica saturation can also be affected by other factors, like degree and/or depth of partial melting, but which are not expected to account for the ubiquitously high Nb/Ta in such lavas. Therefore, in subduction zones affected by sediment melts, the high Nb/Ta of magmas represent a signature of recycling of carbonate-rich lithologies. Hence, Nb/Ta represents an important tool to constrain the role of subduction of recycled carbonates in the Earth’s carbon cycle.
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Acknowledgements
We thank Maria Kirchenbaur and Simone Tommasini for discussions and Frank Wombacher, Jonas Tusch, Josua Pakulla, and Mike Jansen for lab assistance. This research also used samples provided by the International Ocean Discovery Program (IODP). We thank the editor Horst Marschall, Dejan Prelević, and an anonymous reviewer for their valuable comments. This work was supported by UoC funding and ERC grant 669666 (to CM) and PRIN grants 20158A9CBM (to SC) and 2015EC9PJ5 (to RA).
Editor: Horst R. Marschall
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References
Altherr, R., Topuz, G., Siebel, W., Şen, C., Meyer, H.-P., Satır, M., Lahaye, Y. (2008) Geochemical and Sr–Nd–Pb isotopic characteristics of Paleocene plagioleucitites from the Eastern Pontides (NE Turkey). Lithos 105, 149–161. https://doi.org/10.1016/j.lithos.2008.03.001
Show in context In the Mediterranean area, leucitites and plagioleucitites occur also in the Pontides (Turkey) and the average Nb/Ta is slightly higher than the BSE (18 in Eastern Pontides; Altherr et al., 2008; 20 in Central Pontides; Gülmez et al., 2016).
View in article
Ammannati, E., Jacob, D.E., Avanzinelli, R., Foley, S.F., Conticelli, S. (2016) Low Ni olivine in silica-undersaturated ultrapotassic igneous rocks as evidence for carbonate metasomatism in the mantle. Earth and Planetary Science Letters 444, 64–74. https://doi.org/10.1016/j.epsl.2016.03.039
Show in context Other evidence for recycling of carbonates in the Roman but not in the Tuscan magma sources, includes i) 87Sr/86Sr buffered at a composition typical of carbonate-rich sediments for Roman, whilst reaching more radiogenic values for Tuscan lavas (e.g., Avanzinelli et al., 2009), ii) low Ni content and high Ca/Fe of high-Fo olivine within Roman lavas (Ammannati et al., 2016), iii) 238U-excess in Vesuvius magmas (Avanzinelli et al., 2018), iv) similar trace element patterns between Roman lavas and marls (Grassi et al., 2012), v) melt inclusions in the Roman lavas with high CaO (up 22 wt. %) and CaO/Al2O3 (Nikogosian and van Bergen, 2010) and vi) Ca isotopes of Roman leucitites (Ren et al., 2024).
View in article
Avanzinelli, R., Lustrino, M., Mattei, M., Melluso, L., Conticelli, S. (2009) Potassic and ultrapotassic magmatism in the circum-Tyrrhenian region: Significance of carbonated pelitic vs. pelitic sediment recycling at destructive plate margins. Lithos 113, 213–227. https://doi.org/10.1016/j.lithos.2009.03.029
Show in context Their trace element budget is dominated by a strong subduction signature (Conticelli and Peccerillo, 1992; Avanzinelli et al., 2009; Conticelli et al., 2015; Lustrino et al., 2019) as shown also by the very low Nb/Nb* (Fig. 1c).
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Previous studies discussed the differences between magmas from the Tuscan and Roman magmatic provinces, suggesting mantle metasomatism related to subducted Si-rich metapelites, in the former, and carbonate-rich metapelites (marls) in the latter (Avanzinelli et al., 2009; Frezzotti et al., 2009; Conticelli et al., 2015).
View in article
Other evidence for recycling of carbonates in the Roman but not in the Tuscan magma sources, includes i) 87Sr/86Sr buffered at a composition typical of carbonate-rich sediments for Roman, whilst reaching more radiogenic values for Tuscan lavas (e.g., Avanzinelli et al., 2009), ii) low Ni content and high Ca/Fe of high-Fo olivine within Roman lavas (Ammannati et al., 2016), iii) 238U-excess in Vesuvius magmas (Avanzinelli et al., 2018), iv) similar trace element patterns between Roman lavas and marls (Grassi et al., 2012), v) melt inclusions in the Roman lavas with high CaO (up 22 wt. %) and CaO/Al2O3 (Nikogosian and van Bergen, 2010) and vi) Ca isotopes of Roman leucitites (Ren et al., 2024).
View in article
Avanzinelli, R., Casalini, M., Elliott, T., Conticelli, S. (2018) Carbon fluxes from subducted carbonates revealed by uranium excess at Mount Vesuvius, Italy. Geology 46, 259–262. https://doi.org/10.1130/G39766.1
Show in context Other evidence for recycling of carbonates in the Roman but not in the Tuscan magma sources, includes i) 87Sr/86Sr buffered at a composition typical of carbonate-rich sediments for Roman, whilst reaching more radiogenic values for Tuscan lavas (e.g., Avanzinelli et al., 2009), ii) low Ni content and high Ca/Fe of high-Fo olivine within Roman lavas (Ammannati et al., 2016), iii) 238U-excess in Vesuvius magmas (Avanzinelli et al., 2018), iv) similar trace element patterns between Roman lavas and marls (Grassi et al., 2012), v) melt inclusions in the Roman lavas with high CaO (up 22 wt. %) and CaO/Al2O3 (Nikogosian and van Bergen, 2010) and vi) Ca isotopes of Roman leucitites (Ren et al., 2024).
View in article
Bragagni, A., Mastroianni, F., Münker, C., Conticelli, S., Avanzinelli, R. (2022) A carbon-rich lithospheric mantle as a source for the large CO2 emissions of Etna volcano (Italy). Geology 50, 486–490. https://doi.org/10.1130/G49510.1
Show in context For instance, the high Nb/Ta of some intraplate magmas was shown to derive from mantle metasomatism by carbonatite-like melts (Bragagni et al., 2022).
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The new data from Tuscan and Roman magmatic provinces and IODP drill cores of the Tyrrhenian Sea (representative of mantle sources not affected by subduction) were integrated with published data from Etna, Stromboli, Vulture, and Pantelleria (Bragagni et al., 2022).
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The high Nb/Ta of Etna and Vulture were previously explained by mantle-derived carbonatite-like metasomatism in the subcontinental lithospheric mantle (Bragagni et al., 2022).
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Brenan, J.M., Shaw, H.F., Phinney, D.L., Ryerson, F.J. (1994) Rutile-aqueous fluid partitioning of Nb, Ta, Hf, Zr, U and Th: implications for high field strength element depletions in island-arc basalts. Earth and Planetary Science Letters 128, 327–339. https://doi.org/10.1016/0012-821X(94)90154-6
Show in context However, it remains ambiguous why only some arc magmas show high Nb/Ta whilst the occurrence of residual rutile is rather ubiquitous, as suggested by the characteristic HFSE depletions of all subduction related magmas. Fractionation of Nb/Ta was ascribed to supercritical fluids (e.g., W. Chen et al., 2018; T.-N. Chen et al., 2022) or melts (Klemme et al., 2005; Stolz et al., 1996) in equilibrium with rutile, whereas aqueous fluids are not expected to significantly influence the bulk Nb/Ta due to their low HFSE abundance (e.g., Brenan et al., 1994).
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Chen, C., Förster, M.W., Foley, S.F., Shcheka, S.S. (2023) Carbonate-rich crust subduction drives the deep carbon and chlorine cycles. Nature 620, 576–581. https://doi.org/10.1038/s41586-023-06211-4
Show in context Subducting carbonate-rich sediments release minor CO2-rich melts/supercritical fluids (Chen et al., 2023) but in sufficient amounts to induce CO2-excess and produce silica-undersaturated magmas upon mantle partial melting (Conticelli et al., 2015; Gülmez et al., 2023 and references therein).
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Chen, T.-N., Chen, R.-X., Zheng, Y.-F., Zhou, K., Yin, Z.-Z., Wang, Z.-M., Gong, B., Zha, X.-P. (2022) The effect of supercritical fluids on Nb-Ta fractionation in subduction zones: Geochemical insights from a coesite-bearing eclogite-vein system. Geochimica et Cosmochimica Acta 335, 23–55. https://doi.org/10.1016/j.gca.2022.08.013
Show in context However, it remains ambiguous why only some arc magmas show high Nb/Ta whilst the occurrence of residual rutile is rather ubiquitous, as suggested by the characteristic HFSE depletions of all subduction related magmas. Fractionation of Nb/Ta was ascribed to supercritical fluids (e.g., W. Chen et al., 2018; T.-N. Chen et al., 2022) or melts (Klemme et al., 2005; Stolz et al., 1996) in equilibrium with rutile, whereas aqueous fluids are not expected to significantly influence the bulk Nb/Ta due to their low HFSE abundance (e.g., Brenan et al., 1994).
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This is because different melts/supercritical liquids interact, mix, and exsolve as function of the physical conditions of the mantle wedge (e.g., P–T–fO2) and chromatographic effects in the slab and within veined peridotite (e.g., Chen et al., 2022).
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Radiogenic isotope compositions (Sr-Nd-Hf), plotted against Nb/Ta (Fig. 3b–d), show that fluid dominated arcs (low 87Sr/86Sr, high 143Nd/144Nd and 176Hf/177Hf) point towards high Nb/Ta, which was previously interpreted as the effect of residual rutile in equilibrium with metasomatic fluids, possibly at the supercritical state (e.g., W. Chen et al., 2018; T.-N. Chen et al., 2022).
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Chen, W., Xiong, X., Wang, J., Xue, S., Li, L., Liu, X., Ding, X., Song, M. (2018) TiO2 Solubility and Nb and Ta Partitioning in Rutile-Silica-Rich Supercritical Fluid Systems: Implications for Subduction Zone Processes. Journal of Geophysical Research: Solid Earth 123, 4765–4782. https://doi.org/10.1029/2018JB015808
Show in context However, it remains ambiguous why only some arc magmas show high Nb/Ta whilst the occurrence of residual rutile is rather ubiquitous, as suggested by the characteristic HFSE depletions of all subduction related magmas. Fractionation of Nb/Ta was ascribed to supercritical fluids (e.g., W. Chen et al., 2018; T.-N. Chen et al., 2022) or melts (Klemme et al., 2005; Stolz et al., 1996) in equilibrium with rutile, whereas aqueous fluids are not expected to significantly influence the bulk Nb/Ta due to their low HFSE abundance (e.g., Brenan et al., 1994).
View in article
Radiogenic isotope compositions (Sr-Nd-Hf), plotted against Nb/Ta (Fig. 3b–d), show that fluid dominated arcs (low 87Sr/86Sr, high 143Nd/144Nd and 176Hf/177Hf) point towards high Nb/Ta, which was previously interpreted as the effect of residual rutile in equilibrium with metasomatic fluids, possibly at the supercritical state (e.g., W. Chen et al., 2018; T.-N. Chen et al., 2022).
View in article
Conticelli, S., Peccerillo, A. (1992) Petrology and geochemistry of potassic and ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources. Lithos 28, 221–240. https://doi.org/10.1016/0024-4937(92)90008-M
Show in context Here, chemical variations are well constrained and reflect different lithologies of the subducted sediments, being silicate-rich in the so called Tuscan magmatic province and carbonate-rich in the younger Roman magmatic province (e.g., Conticelli and Peccerillo, 1992; Conticelli et al., 2015).
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Their trace element budget is dominated by a strong subduction signature (Conticelli and Peccerillo, 1992; Avanzinelli et al., 2009; Conticelli et al., 2015; Lustrino et al., 2019) as shown also by the very low Nb/Nb* (Fig. 1c).
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This hypothesis is supported by the contrasting silica saturation, being saturated to oversaturated in the Tuscan and saturated to strongly undersaturated in the Roman volcanic rocks (Conticelli and Peccerillo, 1992).
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Conticelli, S., Avanzinelli, R., Ammannati, E., Casalini, M. (2015) The role of carbon from recycled sediments in the origin of ultrapotassic igneous rocks in the Central Mediterranean. Lithos 232, 174–196. https://doi.org/10.1016/j.lithos.2015.07.002
Show in context Here, chemical variations are well constrained and reflect different lithologies of the subducted sediments, being silicate-rich in the so called Tuscan magmatic province and carbonate-rich in the younger Roman magmatic province (e.g., Conticelli and Peccerillo, 1992; Conticelli et al., 2015).
View in article
Their trace element budget is dominated by a strong subduction signature (Conticelli and Peccerillo, 1992; Avanzinelli et al., 2009; Conticelli et al., 2015; Lustrino et al., 2019) as shown also by the very low Nb/Nb* (Fig. 1c).
View in article
Previous studies discussed the differences between magmas from the Tuscan and Roman magmatic provinces, suggesting mantle metasomatism related to subducted Si-rich metapelites, in the former, and carbonate-rich metapelites (marls) in the latter (Avanzinelli et al., 2009; Frezzotti et al., 2009; Conticelli et al., 2015).
View in article
Subducting carbonate-rich sediments release minor CO2-rich melts/supercritical fluids (Chen et al., 2023) but in sufficient amounts to induce CO2-excess and produce silica-undersaturated magmas upon mantle partial melting (Conticelli et al., 2015; Gülmez et al., 2023 and references therein).
View in article
Frezzotti, M.L., Peccerillo, A., Panza, G. (2009) Carbonate metasomatism and CO2 lithosphere–asthenosphere degassing beneath the Western Mediterranean: An integrated model arising from petrological and geophysical data. Chemical Geology 262, 108–120. https://doi.org/10.1016/j.chemgeo.2009.02.015
Show in context Previous studies discussed the differences between magmas from the Tuscan and Roman magmatic provinces, suggesting mantle metasomatism related to subducted Si-rich metapelites, in the former, and carbonate-rich metapelites (marls) in the latter (Avanzinelli et al., 2009; Frezzotti et al., 2009; Conticelli et al., 2015).
View in article
Grassi, D., Schmidt, M.W., Günther, D. (2012) Element partitioning during carbonated pelite melting at 8, 13 and 22 GPa and the sediment signature in the EM mantle components. Earth and Planetary Science Letters 327–328, 84–96. https://doi.org/10.1016/j.epsl.2012.01.023
Show in context Other evidence for recycling of carbonates in the Roman but not in the Tuscan magma sources, includes i) 87Sr/86Sr buffered at a composition typical of carbonate-rich sediments for Roman, whilst reaching more radiogenic values for Tuscan lavas (e.g., Avanzinelli et al., 2009), ii) low Ni content and high Ca/Fe of high-Fo olivine within Roman lavas (Ammannati et al., 2016), iii) 238U-excess in Vesuvius magmas (Avanzinelli et al., 2018), iv) similar trace element patterns between Roman lavas and marls (Grassi et al., 2012), v) melt inclusions in the Roman lavas with high CaO (up 22 wt. %) and CaO/Al2O3 (Nikogosian and van Bergen, 2010) and vi) Ca isotopes of Roman leucitites (Ren et al., 2024).
View in article
Green, T.H. (1995) Significance of Nb/Ta as an indicator of geochemical processes in the crust-mantle system. Chemical Geology 120, 347–359. https://doi.org/10.1016/0009-2541(94)00145-X
Show in context The relative concentrations of Nb and Ta remain nearly constant during most magmatic processes and, thus, variations of Nb/Ta in volcanic rocks can reveal specific processes. Among geological environments, the highest Nb/Ta values are observed in carbonatites, lithosphere-derived rocks, and subduction related rocks (e.g., Green, 1995; Münker et al., 2003; Klemme et al., 2005).
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The extremely high Nb/Ta of carbonatites can be used to recognise the involvement of carbonate-rich melts and fluids in different geological settings (e.g., Green, 1995).
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Green, T.H. (2000) New partition coefficient determinations pertinent to hydrous melting processes in subduction zones. In: Smith, I.E.M., Davidson, J.P., Gamble, J.A., Price, R.C. (Eds.) Processes and Time Scales in the Genesis and Evolution of ARC Magmas. Royal Society of New Zealand, Wellington, 92–95.
Show in context 4) Rutile, which likely controls HFSE in the subducting slab, shows the lowest DNb/DTa (0.35) when in equilibrium with a carbonatite melt (Green, 2000).
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Gülmez, F., Genç, Ş.C., Prelević, D., Tüysüz, O., Karacik, Z., Roden, M.F., Billor, Z. (2016) Ultrapotassic Volcanism from the Waning Stage of the Neotethyan Subduction: a Key Study from the Izmir–Ankara–Erzincan Suture Belt, Central Northern Turkey. Journal of Petrology 57, 561–593. https://doi.org/10.1093/petrology/egw021
Show in context In the Mediterranean area, leucitites and plagioleucitites occur also in the Pontides (Turkey) and the average Nb/Ta is slightly higher than the BSE (18 in Eastern Pontides; Altherr et al., 2008; 20 in Central Pontides; Gülmez et al., 2016).
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Gülmez, F., Prelević, D., Förster, M.W., Buhre, S., Günther, J. (2023) Experimental production of K-rich metasomes through sediment recycling at the slab-mantle interface in the fore-arc. Scientific Reports 13, 19608. https://doi.org/10.1038/s41598-023-46367-7
Show in context Subducting carbonate-rich sediments release minor CO2-rich melts/supercritical fluids (Chen et al., 2023) but in sufficient amounts to induce CO2-excess and produce silica-undersaturated magmas upon mantle partial melting (Conticelli et al., 2015; Gülmez et al., 2023 and references therein).
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Recently, Gülmez et al. (2023) showed that the reaction of carbonate-rich sediments with peridotites at 800–850 °C forms carbonatitic and K-rich silicic melts, explaining the genesis of ultrapotassic silica-undersaturated magmas, such as the Roman ones.
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House, B.M., Bebout, G.E., Hilton, D.R. (2019) Carbon cycling at the Sunda margin, Indonesia: A regional study with global implications. Geology 47, 483–486. https://doi.org/10.1130/G45830.1
Show in context Importantly, at Sunda, different sediments are subducting, with a strong carbonate contribution only in the Eastern sector (House et al., 2019).
View in article
Kirchenbaur, M., Münker, C. (2015) The behaviour of the extended HFSE group (Nb, Ta, Zr, Hf, W, Mo) during the petrogenesis of mafic K-rich lavas: The Eastern Mediterranean case. Geochimica et Cosmochimica Acta 165, 178–199. https://doi.org/10.1016/j.gca.2015.05.030
Show in context In the area, high precision HFSE data are available for the Rhodopes (Bulgaria) and Santorini (Kirchenbaur and Münker, 2015).
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Kirchenbaur, M., Schuth, S., Barth, A.R., Luguet, A., König, S., Idrus, A., Garbe-Schönberg, D., Münker, C. (2022) Sub-arc mantle enrichment in the Sunda rear-arc inferred from HFSE systematics in high-K lavas from Java. Contributions to Mineralogy and Petrology 177, 8. https://doi.org/10.1007/s00410-021-01871-9
Show in context Similar to what is observed in Bulgaria and in Roman and Tuscan volcanic rocks, at Sunda only silica-undersaturated samples have elevated Nb/Ta, whilst silica-saturated samples have BSE-like Nb/Ta. The high Nb/Ta magmatism of Sunda is only observed in the K-rich rear arc in the Eastern sector (Stolz et al., 1996; Kirchenbaur et al., 2022).
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Klemme, S., Prowatke, S., Hametner, K., Günther, D. (2005) Partitioning of trace elements between rutile and silicate melts: Implications for subduction zones. Geochimica et Cosmochimica Acta 69, 2361–2371. https://doi.org/10.1016/j.gca.2004.11.015
Show in context The relative concentrations of Nb and Ta remain nearly constant during most magmatic processes and, thus, variations of Nb/Ta in volcanic rocks can reveal specific processes. Among geological environments, the highest Nb/Ta values are observed in carbonatites, lithosphere-derived rocks, and subduction related rocks (e.g., Green, 1995; Münker et al., 2003; Klemme et al., 2005).
View in article
However, it remains ambiguous why only some arc magmas show high Nb/Ta whilst the occurrence of residual rutile is rather ubiquitous, as suggested by the characteristic HFSE depletions of all subduction related magmas. Fractionation of Nb/Ta was ascribed to supercritical fluids (e.g., W. Chen et al., 2018; T.-N. Chen et al., 2022) or melts (Klemme et al., 2005; Stolz et al., 1996) in equilibrium with rutile, whereas aqueous fluids are not expected to significantly influence the bulk Nb/Ta due to their low HFSE abundance (e.g., Brenan et al., 1994).
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Korsakov, A.V., Hermann, J. (2006) Silicate and carbonate melt inclusions associated with diamonds in deeply subducted carbonate rocks. Earth and Planetary Science Letters 241, 104–118. https://doi.org/10.1016/j.epsl.2005.10.037
Show in context 2) Natural melts produced from silica- and carbonate-rich lithologies, as observed in inclusions in high P–T metamorphic rocks, show variable enrichment in HFSE and K contents (Korsakov and Hermann, 2006).
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Lustrino, M., Fedele, L., Agostini, S., Prelević, D., Salari, G. (2019) Leucitites within and around the Mediterranean area. Lithos 324–325, 216–233. https://doi.org/10.1016/j.lithos.2018.11.007
Show in context Their trace element budget is dominated by a strong subduction signature (Conticelli and Peccerillo, 1992; Avanzinelli et al., 2009; Conticelli et al., 2015; Lustrino et al., 2019) as shown also by the very low Nb/Nb* (Fig. 1c).
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Even if these rocks were originally classified as lamproites, they are leucite-bearing and were later classified as plagioleucitites (Lustrino et al., 2019), making them comparable to Roman rocks.
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Münker, C., Pfänder, J.A., Weyer, S., Büchl, A., Kleine, T., Mezger, K. (2003) Evolution of Planetary Cores and the Earth-Moon System from Nb/Ta Systematics. Science 301, 84–87. https://doi.org/10.1126/science.1084662
Show in context The relative concentrations of Nb and Ta remain nearly constant during most magmatic processes and, thus, variations of Nb/Ta in volcanic rocks can reveal specific processes. Among geological environments, the highest Nb/Ta values are observed in carbonatites, lithosphere-derived rocks, and subduction related rocks (e.g., Green, 1995; Münker et al., 2003; Klemme et al., 2005).
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Tuscan and Tyrrhenian magmas have unfractionated Nb/Ta, similar to the BSE (14 ± 0.3; Münker et al., 2003), whilst Roman lavas display higher ratios (up to 24).
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Nikogosian, I.K., van Bergen, M.J. (2010) Heterogeneous mantle sources of potassium-rich magmas in central-southern Italy: Melt inclusion evidence from Roccamonfina and Ernici (Mid Latina Valley). Journal of Volcanology and Geothermal Research 197, 279–302. https://doi.org/10.1016/j.jvolgeores.2010.06.014
Show in context Other evidence for recycling of carbonates in the Roman but not in the Tuscan magma sources, includes i) 87Sr/86Sr buffered at a composition typical of carbonate-rich sediments for Roman, whilst reaching more radiogenic values for Tuscan lavas (e.g., Avanzinelli et al., 2009), ii) low Ni content and high Ca/Fe of high-Fo olivine within Roman lavas (Ammannati et al., 2016), iii) 238U-excess in Vesuvius magmas (Avanzinelli et al., 2018), iv) similar trace element patterns between Roman lavas and marls (Grassi et al., 2012), v) melt inclusions in the Roman lavas with high CaO (up 22 wt. %) and CaO/Al2O3 (Nikogosian and van Bergen, 2010) and vi) Ca isotopes of Roman leucitites (Ren et al., 2024).
View in article
Prelević, D., Foley, S.F., Romer, R., Conticelli, S. (2008) Mediterranean Tertiary lamproites derived from multiple source components in postcollisional geodynamics. Geochimica et Cosmochimica Acta 72, 2125–2156. https://doi.org/10.1016/j.gca.2008.01.029
Show in context Considering also conventional ICP-MS data, among potassic and ultrapotassic rocks of the circum-Mediterranean, the highest Nb/Ta was observed in ultrapotassic rocks from Macedonia (average of 20; Prelević et al., 2008).
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Importantly, other silica-saturated potassic and ultrapotassic rocks from Spain, Serbia and Turkey, have lower Nb/Ta, analytically indistinguishable from the BSE value (Prelević et al., 2008).
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Ravna, E.K., Zozulya, D., Kullerud, K., Corfu, F., Nabelek, P.I., Janák, M., Slagstad, T., Davidsen, B., Selbekk, R.S., Schertl, H.-P. (2017) Deep-seated Carbonatite Intrusion and Metasomatism in the UHP Tromsø Nappe, Northern Scandinavian Caledonides—a Natural Example of Generation of Carbonatite from Carbonated Eclogite. Journal of Petrology 58, 2403–2428. https://doi.org/10.1093/petrology/egy016
Show in context Interestingly, among these inclusions, the highest HFSE and K contents are recorded in melts with high Nb/Ta (∼30). 3) Carbonatite-like melts interpreted to derive from slab melting of carbonate-rich sediments also show elevated Nb/Ta (Ravna et al., 2017).
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Ren, H., Casalini, M., Conticelli, S., Chen, C., Foley, S.F., Feng, L., Liu, Y. (2024) Calcium isotope compositions of subduction-related leucite-bearing rocks: Implications for the calcium isotope heterogeneity of the mantle and carbonate recycling in convergent margins. Geochimica et Cosmochimica Acta 364, 100–113. https://doi.org/10.1016/j.gca.2023.11.022
Show in context Other evidence for recycling of carbonates in the Roman but not in the Tuscan magma sources, includes i) 87Sr/86Sr buffered at a composition typical of carbonate-rich sediments for Roman, whilst reaching more radiogenic values for Tuscan lavas (e.g., Avanzinelli et al., 2009), ii) low Ni content and high Ca/Fe of high-Fo olivine within Roman lavas (Ammannati et al., 2016), iii) 238U-excess in Vesuvius magmas (Avanzinelli et al., 2018), iv) similar trace element patterns between Roman lavas and marls (Grassi et al., 2012), v) melt inclusions in the Roman lavas with high CaO (up 22 wt. %) and CaO/Al2O3 (Nikogosian and van Bergen, 2010) and vi) Ca isotopes of Roman leucitites (Ren et al., 2024).
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Shu, Z.-T., Liu, S.-A., Prelević, D., Wang, Y., Foley, S.F., Cvetković, V., Li, S. (2023) Recycled Carbonate-Bearing Silicate Sediments in the Sources of Circum-Mediterranean K-Rich Lavas: Evidence From Mg-Zn Isotopic Decoupling. Journal of Geophysical Research: Solid Earth 128, e2022JB025135. https://doi.org/10.1029/2022JB025135
Show in context Zn-Mg isotope compositions in other circum-Mediterranean magmas suggests the recycling of marls (Shu et al., 2023).
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Stolz, A.J., Jochum, K.P., Spettel, B., Hofmann, A.W. (1996) Fluid- and melt-related enrichment in the subarc mantle: Evidence from Nb/Ta variations in island-arc basalts. Geology 24, 587–590. https://doi.org/10.1130/0091-7613(1996)024<0587:FAMREI>2.3.CO;2
Show in context However, it remains ambiguous why only some arc magmas show high Nb/Ta whilst the occurrence of residual rutile is rather ubiquitous, as suggested by the characteristic HFSE depletions of all subduction related magmas. Fractionation of Nb/Ta was ascribed to supercritical fluids (e.g., W. Chen et al., 2018; T.-N. Chen et al., 2022) or melts (Klemme et al., 2005; Stolz et al., 1996) in equilibrium with rutile, whereas aqueous fluids are not expected to significantly influence the bulk Nb/Ta due to their low HFSE abundance (e.g., Brenan et al., 1994).
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Similar to what is observed in Bulgaria and in Roman and Tuscan volcanic rocks, at Sunda only silica-undersaturated samples have elevated Nb/Ta, whilst silica-saturated samples have BSE-like Nb/Ta. The high Nb/Ta magmatism of Sunda is only observed in the K-rich rear arc in the Eastern sector (Stolz et al., 1996; Kirchenbaur et al., 2022).
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Supplementary Information
The Supplementary Information includes:
- Tyrrhenian Sea Samples Description
- Analytical Methods
- Quantifying the Role of Carbonate-rich Melts in Roman Magmatic Province
- Table S-1
- Figure S-1
- References for Literature Data in Figures 2 and 3
- Supplementary Information References
Download the Supplementary Information (PDF)
Download Table S-1 (xlsx)
Figures
Figure 1 Variations of Nb/Ta relative to other geochemical proxies expressing intraplate metasomatism of the lithosphere (K/K*), subduction affinity (K/K* and Nb/Nb*), K-enrichment (K/K* and K2O/Na2O) and degree of silica saturation (FSSI; Feldspathoid Silica Saturation Index). The K* and Nb* values are calculated from the geometrical mean of PM-normalised concentrations of Nb-U and La-U, respectively. “LD” refers to “Latium District”.
Figure 2 Nb/Ta against other proxies sensitive to carbonate-rich fluids/melts (e.g., carbonatites).
Figure 3 (a) Ba/Th vs. 143Nd/144Nd, showing the variable contribution of melts and (supercritical) liquids in different subduction related magmas. (b, c, d) Nb/Ta vs. Nd, Sr and Hf isotopes, respectively, in different subduction related magmas. Arc magmas dominated by sediment melts display the largest variations in radiogenic isotopes. Among them, silica-undersaturated volcanic rocks (highlighted by an open circle within the symbol) show elevated Nb/Ta due to recycling of subducting carbonate sedimentary lithologies. Only samples where Nb/Ta was determined by isotope dilution are plotted (see Supplementary Information for references).