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| Titel |
Reactive transport modeling of ferroan dolomitization by seawater interaction with mafic igneous dikes and carbonate host rock at the Latemar platform, Italy |
| VerfasserIn |
Katreine Blomme, Sarah Jane Fowler, Pierre Bachaud |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250154162
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| Publikation (Nr.) |
 EGU/EGU2017-19230.pdf |
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| Zusammenfassung |
| The Middle Triassic Latemar carbonate platform, northern Italy, has featured prominently in
the longstanding debate regarding dolomite petrogenesis [1–4]. Recent studies agree that
ferroan and non-ferroan dolomite replaced calcite in limestone during reactive fluid flow at
<0.1 GPa and 40–80∘C. Regional igneous activity drove heating that provided kinetically
favorable conditions for the replacement reaction. However, the origin of the dolomitizing
fluid is unclear. Seawater may have been an important component, but its Fe concentrations
are insufficient to account for ferroan dolomite.
New field, petrographic, XRD, and geochemical data document a spatial, temporal,
and geochemical link between ferroan replacement dolomite and altered mafic
igneous dikes that densely intrude the platform. A critical observation is that ferroan
dolomite abundances increase towards the dikes. We hypothesize that seawater
interacted with mafic minerals in the dikes, leading to Fe enrichment in the fluid that
subsequently participated in dolomitization. This requires that dolomite formation was
preceded by dike alteration reactions that liberated Fe and did not consume Mg.
Another requirement is that ferroan and non-ferroan dolomite (instead of other Fe-
and Mg-bearing minerals) formed during fluid circulation within limestone host
rock.
We present reactive transport numerical simulations (Coores–Arxim, [5]) that predict
equilibrium mineral assemblages and the evolution of fluid dolomitizing potential from dike
crystallization, through dike alteration by seawater, to replacement dolomitization in
carbonate host rock. The simulations are constrained by observations. A major
advantage of the simulations is that stable mineral assemblages are identified based on a
forward modeling approach. In addition, the dominant igneous minerals (plagioclase,
clinopyroxene olivine and their alteration products) are solid solutions. Most reactive
transport simulations of carbonate petrogenesis do not share these benefits (e.g.
[6]).
Predicted alteration mineral assemblages are consistent with observations on dikes and
with ferroan and non-ferroan dolomite genesis. The simulation results also show that fluid
dolomitizing potential (Mg/Ca and Fe/Mg) increases during dissolution of igneous solid
solution minerals. Enrichment in fluid Fe concentration is sufficient to stabilize ferroan
replacement dolomite. Consistent with field observations, ferroan dolomite forms closest to
dikes due to the abundance of Fe associated with the dikes. This leads to depletion of Fe in
fluid flowing away from dikes and formation of non-ferroan replacement dolomite further
afield.
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