Elemental abundances and isotopic composition of Italian limestones: Glimpses into the evolution of the Tethys

Abstract

Biogenic limestones from three sections (north, central, and south) across peninsular Italy have been analysed for major and trace elements and Nd, Pb, and Sr isotopic ratios. These data are used to monitor the evolution of the Tethys Ocean from the Triassic through to the Miocene. Limestones’ major, trace, and REE elements contents are consistent with their formation in seawater with little sign of crustal, volcanic, or hydrothermal input. V/Cr and Ce/Ce* ratios indicate their deposition in oxygenated waters. Rb-Sr-Ba discrimination diagram, consistent with the immobile trace element distribution, indicates that limestone deposition took place in either marginal or open ocean environments. Ages based on stratigraphy are in good agreement with the chronostratigraphic Sr curves implying that the Tethys ocean, throughout its history, was in contact with the open, global, ocean system. Although the isotopic values of Sr and Nd are relatively restricted, Pb is extremely variable and highly radiogenic. High Pb isotope ratios characterise limestones deposited during the rifting of the southern Tethyan ocean in the Lower Jurassic and in the Lower Cretaceous, suggesting stronger crustal inputs in small basins. The weighted average, present-day, isotope values (AIL = average Italian limestone) for the Italian limestones, excluding anomalous samples, are 87Sr/86Sr = 0.70785, 143Nd/144Nd = 0.51227, and 206Pb/204Pb = 18.94, 207Pb/204Pb = 15.69, 208Pb/204Pb = 38.66. These values are useful in monitoring the fate of limestones during orogenesis and the role that they may have played in magma genesis.

Keywords

• Limestone
• Isotopes
• Major and trace elements
• Tethys
• Italy