Constraints on the preservation of proxy data in carbonate archives – lessons from a marine limestone to marble transect, Latemar, Italy

Abstract

This work evaluates an exceptionally complex natural laboratory, the Middle Triassic Latemar isolated platform in the northern Italian Dolomite Mountains and explores spatial and temporal gradients in processes and products related to contact metamorphism, dolomitization and dedolomitization of marine limestones. The relation between petrographic change and re‐equilibration of geochemical proxy data is evaluated from the perspective of carbonate‐archive research. Hydrothermal dolomitization of the limestone units is triggered by dykes and associated hydrothermal fluids radiating from the nearby Predazzo Intrusion. Detailed petrography, fluid inclusion analysis, δ13C and δ18O data and 87Sr/86Sr isotope ratios shed light on the extreme textural and geochemical complexity. Metamorphic and diagenetic patterns include: (i) peak‐metamorphic and retrograde‐metamorphic phases including three dolomite marbles, two dedolomite marbles, brucite, magnesium silicates and late‐stage meteoric/vadose cement at the contact aureole; (ii) four spatially defined episodes of dolomitization, authigenic quartz, low magnesium calcite and late‐stage meteoric cement at the Latemar isolated platform; and (iii) kilometre‐scale gradients in δ13C values from the contact aureole towards the platform interior. Results shown here are relevant for two reasons: first, the spatial analysis of alteration products ranging from high‐grade metamorphic overprint of marbles at temperatures of 700°C in the contact aureole to moderately altered limestones in the platform interior at temperatures <100°C, allows the observation of processes that commonly occur along vertical (prograde) gradients from shallow burial to metamorphism at depths >20 km. Second, under rock‐buffered conditions, and irrespective of metamorphic to diagenetic fluid−rock interactions, both marbles, and low‐temperature hydrothermal dolomites have conservative marine δ13C and δ18O values. The fact that metamorphism and hydrothermal dolomitization of precursor limestones and early diagenetic dolostones did not per se reset environmental proxy data is of interest for those concerned with carbonate archive research in Earth’s deep time.

On the delimitation of the carbonate burial realm

“…If local mineral-fluid equilibrium is maintained, bulk carbonate 18 O-depletion of 5-8‰ in contact and regional metamorphic terranes points to fluid flow on the order of 2-50•10 4 mol/cm 2 in the direction of increasing temperature (Dipple & Ferry, 1992). Controversy, however, is found in the fact that virtually all marbles display a very significant petrographic and mineralogical overprint, but some retain what seem to be (primary) 'environmental' proxy data (Figure 15; Ferry et al, 2002;Melezhik et al, 2005Melezhik et al, , 2008Mueller et al, 2021). The problem at hand is that, before reaching the metamorphic domain, the precursors of these marbles have potentially been subjected to fluid-buffered diagenesis with a very significant potential to obliterate the marine geochemical properties (Dipple & Ferry, 1992;Ferry et al, 2002;Swart, 2015).…”

“…In many cases, individual flow mechanisms may not operate in isolation. The Mg‐species required for dolomitisation may be delivered from a variety of static and non‐static sources including (i) buried or convected (replenished) sea water (Given & Wilkinson, 1987; Ryan et al, 2020); (ii) via scavenging from clays (particularly desorption from clay mineral surfaces and magnesium expelled during the montmorillonite‐to‐illite transformation (Mavromatis et al, 2014); (iii) from the subsurface dissolution of Mg sulphate (brine reflux model; kieserite and epsomite; Machel, 2004; Wendte et al, 1998; Yao & Demicco, 1995) or (iv) from the (metamorphic) dedolomitisation of deep‐seated dolostones (Jacquemyn et al, 2014; Mueller et al, 2021).…”

“…Carbon isotope values (δ 13 C carb ) are often less susceptible to burial diagenesis and isotope fractionation (Buchardt & Nielsen, 1984; Immenhauser et al, 2002; Mueller et al, 2021). Burial LMC and saddle dolomite δ 13 C values may overlap with many biogenic and abiogenic marine carbonates (Figure 10) and often plot into a range of between −1 and +2‰.…”

“…Hence, the bulk geochemistry is not altered (Fantle et al, 2020). The processes involved have been established based on numerous experimental (Horita, 2014; Pederson et al, 2020; Ritter et al, 2017) and field studies (Ferry et al, 2002; Mueller et al, 2021). Hu et al (2018) describe the influence of hot, deep‐seated (6 km) hydrocarbon‐related fluids on carbonates (Figure 11).…”

“…In some cases, the rock‐dominated nature of the deep‐burial domain implies that even at elevated temperatures, the ratio of O in the fluid versus that in the carbonate is such that significant exchange of oxygen is lacking (Ferry et al, 2002), and mass‐dependent fractionation is suppressed (Mueller et al, 2021). The complexity of the processes involved is well exemplified in Geske et al (2012).…”