60th Annual Report on Research 2015

Funding generously provided by the ACS Petroleum Research Foundation has been used to study Mg and Ca isotope variability in early diagenetic dolomites.  From this work we have produced a large data set of paired Ca and Mg isotope measurements on Neogene dolomites from a wide range of environments and have developed a 1D numerical model for quantitatively exploring mass transfer of major and minor elements during dolomitization.  Each of these projects are discussed in more detail below. 

The numerical model we have developed consists of a series of boxes (n is variable but typically between 50-100) containing sediment and pore-fluid linked by fluid advection (Figure 1).  Within each box the sediment is partitioned into different primary carbonate minerals (aragonite and calcite) each of which reacts with the pore-fluid at a prescribed rate resulting in the formation of diagenetic calcite and/or dolomite.  The model predicts that d²⁶Mg and d⁴⁴Ca values in early diagenetic dolomite can exhibit a wide range of behavior depending on whether the dolomite formed under ‘open system’ or ‘closed system’ conditions.  The ‘open system’ end-member reflects a dolomite that has precipitated from essentially unaltered seawater at high fluid to rock ratios (Figure 1).  According to the model, dolomite precipitated under these conditions should have relatively homogeneous d²⁶Mg values (small amounts of dolomite quickly overwhelm the d²⁶Mg budget of the bulk sediment) and a range of d⁴⁴Ca values spanning the range from the initial d⁴⁴Ca of the sediment to the d⁴⁴Ca value predicted for precipitation from unaltered seawater.   In contrast, the 'closed system' end-member (Figure 1 – yellow and orange lines) reflects a dolomite that is precipitated from a fluid that has chemically evolved along the flow path.   In any particular dolomitizing system one might expect to find dolomites formed under both open and closed system conditions – indeed this appears to be exactly the case for the Miocene Monterey Formation – see Figure 2.  Going forward, our approach will be to combine model predictions of potential d²⁶Mg and d⁴⁴Ca values in dolomite with large datasets of sedimentary dolomite (and some limestone) to map out co-variation of d²⁶Mg and d⁴⁴Ca values for different discrete intervals in Earth history.

We have applied this approach (d²⁶Mg vs. d⁴⁴Ca co-variation) to two very different early diagenetic dolomites from the Neogene –dolomitized carbonate platforms and reefs from the Bahamas (Higgins, unpublished) and authigenic dolomites from the Monterey Formation.  Figure 2 shows that while in the Monterey dolomites (red diamonds) there is significant variation in both d⁴⁴Ca values and d²⁶Mg values, dolomites from the Bahamas are characterized by rather homogeneous d²⁶Mg values (-2.8 +/- 0.1ä N >40) but a range in d⁴⁴Ca values that is similar to dolomites from the Monterey.  Importantly, both of these ranges are consistent with outputs from the diagenetic model (red and blue lines in Figure 2) and can be described using a single set of values for the d²⁶Mg and d⁴⁴Ca value of the dolomitizing fluid and the Mg and Ca isotopic fractionation associated with dolomitization.  In the case of the Monterey dolomites, the model suggests that the range in d²⁶Mg and d⁴⁴Ca values is due to the degree to which the dolomite forms under ‘open system’ or ‘closed system’ conditions.   Heavy d⁴⁴Ca values and light d²⁶Mg values are the most ‘open system’ whereas heavy d²⁶Mg values associated with light d⁴⁴Ca values is consistent with dolomite precipitated under more ‘closed system’ conditions.  In contrast to the Monterey dolomites, the Bahamas dolomites (blue circles) sit along only the ‘open system’ model trajectory.

The power of the paired d²⁶Mg-d⁴⁴Ca measurements, as demonstrated by the Neogene sample suite, is that early diagenetic dolomites from different locales across the globe, if precipitated from seawater and seawater-derived fluids, should define a single field in a plot of d²⁶Mg vs. d⁴⁴Ca.  The range and end-members defined by this field provide robust constraints on the d²⁶Mg and d⁴⁴Ca of seawater.  For example, for an increase in the d²⁶Mg of seawater, we would expect a similar distribution of paired d²⁶Mg values and d⁴⁴Ca values with the minimum d²⁶Mg value (i.e. the open system trajectory) shifted higher by an amount equal to the increase in the d²⁶Mg value of seawater.  In addition, this approach can be applied to many of the carbonate-hosted geochemical proxies employed throughout the geologic record – C isotopes, Cr isotopes, I/Ca ratios, B isotopes, Mo isotopes, carbonate-associated sulfate, etc.  Evaluating the effects of diagenesis on these proxies is difficult and almost always qualitative.  Paired d²⁶Mg-d⁴⁴Ca measurements and diagenetic modeling can provide a quantitative framework to explore the effects of carbonate diagenesis on trace components of carbonate sediments that promises to improve our understanding and interpretation of the environmental information recorded in these proxies.