The geochemistry of carbonate diagenesis: The past, present and future

Abstract

Stable carbon and oxygen isotopes ( 18 O and  13 C values) and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years. As valuable as these insights have been, many problems regarding the interpretation of geochemical signals within mature rocks remain. For example, while the  18 O values of carbonate rocks are dependent both upon the temperature and the  18 O value of the fluid and additional information, including trace element composition aids in interpreting such signals, direct evidence of either the temperature or the composition of the fluids is required. Such information can be obtained This article is protected by copyright. All rights reserved.by analyzing the  18 O value of any fluid inclusions or by measuring the temperature using a method such as the 'clumped' isotope technique. Such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well-known tendency for  18 O values of carbonate rocks to decrease with increasing age. Unlike the  18 O,  13 C values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time. However, many studies have not sufficiently emphasized the effects of diagenesis and other post-depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored. Finally, many additional proxies are poised to contribute to the interpretation of carbonate diagenesis. Although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods, care should be taken to understand both old and new proxies before applying them to the ancient record.

What do we really know about early diagenesis of non-marine carbonates?

“…8 shows a compilation of carbon and oxygen stable isotopic compositions of carbonate sediments from the selected reference cases introduced in Table 1. An overview of carbonate stable isotopic compositions characteristic of vadose and phreatic, meteoric diagenesis can be found in numerous publications and books (Allan and Matthews, 1982;Morse and Mackenzie, 1990;Hoefs, 2004;Sharp, 2007;van Dongen et Moore and Wade, 2013;Swart, 2015) and several processes and trends are indicated in Fig. 8.…”

“…This subdivision was practical for marine carbonate fabric description, including porosity characteristics, but is no longer widely used. An alternative, recent approach involves subdivision based on the dominant fluid or fluid mixtures present (Flügel, 2004;Moore and Wade, 2013;Swart, 2015). This may be freshwater, marine, mixed Fig.…”

Origin of dolomite in the Middle Ordovician peritidal platform carbonates in the northern Ordos Basin, western China

“…Various models have been proposed to explain the origin of dolomite in carbonate platforms (Warren 2000;Machel 2004;Swart 2015), including regional subsurface flow models (sometimes referred to as burial-flow models) at elevated temperatures (Jones and Rostron 2000) and ''early'' synsedimentary models, such as those involving seepage reflux (Adams and Rhodes 1960). These models have been used to explain the massive dolomite in syndepositional evaporites (Jones and Rostron 2000;Qing et al 2001) or in seawater with elevated salinity (Rott and Qing 2013;Read et al 2012;Rivers et al 2012).…”

Geochemical signatures of pervasive meteoric diagenesis of Early Miocene syn-rift carbonate platform, Red Sea, NW Saudi Arabia

“…Many re search ers (e.g., Bathurst, 1975;Allan and Matthews, 1982;Tucker, 1986;Bishop et al, 2014;Swart, 2015) have highlighted the sen si tiv ity of metastable ar agon ite and high-Mg calcite (HMC) to the changes of pore wa ter chem is try. The me teoric en vi ron ment, in par tic u lar, plays a key role in the po ros ity de vel op ment in car bon ate res er voirs (e.g., Smalley et al, 1994;Bishop et al, 2014;Dick son and Kenter, 2014;Ma jor, 2014).…”

“…How ever, such stud ies have en coun tered many dif fi cul ties in rec og niz ing the fea tures and es tab lish the im pact of me te oric diagenesis from petrographic ev i dence solely. There fore, recent stud ies (e.g., Melim et al, 2002;Heba et al, 2009;Swart, 2015) have sug gested in te gra tion be tween petrographic data and sta ble ox y gen and car bon iso topes, in con junc tion with trace el e men tal anal y sis (Fe, Mn, Sr, Na) of the bulk car bon ate sam ples can be used to de ci pher the pres ence of me te oric diagenesis realm in an cient car bon ate se quences.…”