 |
| Titel |
Constraining serpentinization at mid-ocean ridges: effects on fluid flow and the global water budget |
| VerfasserIn |
Karthik Iyer, Lars Rüpke, Jason Phipps Morgan |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043889
|
|
|
|
|
|
| Zusammenfassung |
| Hydration of the oceanic lithosphere is an important and ubiquitous process which alters both
the chemical and physical properties of the affected lithologies. One of the most important
reactions that affect the mantle is the process of serpentinization which results in a drastic
decrease in the density (up to 40%), seismic velocity and brittle strength. More importantly,
serpentinization may result in the uptake of up to 13wt% of water and is an important
contributor to global water budget.
In this paper, we use numerical models to study the amount and extent of serpentinization
that may occur at mid-ocean ridges and its effects on fluid flow within the lithosphere. The
two dimensional, FEM model solves three coupled, time-dependent equations: (i)
mass-conserving Darcy flow equation, (ii) energy conserving heat transport equation and
(iii) serpentinization rate of olivine with feedbacks to temperature (exothermic
reaction), fluid consumption and variations in porosity and permeability (volume
changes).
The thermal structure of the ridge depends on the spreading velocity as well as
hydrothermal convection. Serpentinization of the oceanic mantle, in turn, depends on the
aforementioned, competing processes and may itself influence fluid flow due to the large
variations in porosity and permeability as a result of the associated volume changes.
Therefore, strong feedbacks exist between mantle hydration reactions and hydrothermal flow.
Increasing rates of serpentinization may enhance permeability by reaction-induced fracturing.
As serpentinization progresses, fracturing ceases and remaining pore space is reduced by the
precipitation of hydrous phases. Due to these variations in permeability/porosity controlled
by hydration reactions, in combination with differences in background permeabilities
between the crust and mantle, the Moho may be a self-limiting barrier to fluid flow due to the
formation of an impermeable serpentinized rock layer. We find that the variations
in background permeability and the relation between porosity/permeability and
serpentinization do not significantly affect the results of the model. The controlling factor,
however, is the considerable reduction of porosity/permeability as serpentinization
reaches completion. This is also supported by field observations where serpentinized
rocks consist of numerous veins clogged with hydrous phases and by the result that
unreasonable amounts of the mantle (up to ~13km) would be hydrated if this were not the
case.
The average water content of the mantle is a strong function of spreading rate.
Moreover, we find two distinct trends: slow spreading ridges show high degrees
of hydration that rapidly drop with increasing rate. Fast spreading ridges show
only limited water contents which do not greatly depend on the spreading rate. The
results suggest that fast and slow-spreading lithosphere are likely to have strikingly
different serpentinization characteristics when they reach a trench and begin to
bend and subduct. The total water budget of the mantle leaving the ridge has been
estimated to range between 0.18x105 kg/m2 and 2.52x105 kg/m2 depending on
the spreading rate. These values are consistent with independent estimates and
show that the mantle is an important reservoir in the global geological water cycle. |
|
|
|
|
|
|