Modeling the subsolidus evolution of melt-depleted peridotite residues beneath the continents
Abstract
The origin and stabilization of subcontinental lithospheric mantle is a consequence of 1) partial melting of peridotitic mantle in mid-ocean ridge, intraplate and/or subduction zone settings, 2) underplating of the melt-depleted residue beneath the continents as a consequence of either plume-head melting or tectonic mechanisms, 3) subsolidus evolution of the residual solid, and 4) metasomatic transformations. The evolution of phase compositions and modes within the lithospheric mantle as a consequence of these processes, and the resulting density structure, are essential to understanding the preservation or convective removal of lithosphere. Whereas melting experiments provide constraints on the range of compositions possible for either fertile or depleted peridotite, the effects of pressure and temperature changes below the solidus on peridotite mineralogy and density remain challenging to quantify. These difficulties have important implications, particularly with regard to the density structure of the lithosphere and the mechanisms enabling long-term stabilization of subcontinental lithospheric mantle. To calculate the effects of changes in P, T, and bulk composition on the subsolidus mineralogy and density of peridotite lithologies, we have coupled the algorithm of [1], which calculates modes and phase compositions of subsolidus peridotite by mass balance constrained by experimentally-determined mineral-mineral exchange and distribution coefficients with the algorithm of [2], which calculates densities at P and T conditions suitable to the upper mantle using mineral physics data. We benchmark our approach against the thermodynamic models pMELTS and PERPLE_X using recently published peridotite melting experimental data, and show that our estimates of mineral modes and compositions typically provide improvements to the fits of experimental results. We also show that our density calculations are similar to those predicted by these thermodynamic models when using their mineral modes and compositions as inputs. However, because our model provides a better fit to experimental mineral modes and compositions, our density estimates for any given bulk composition are likely to be more accurate than those predicted by either pMELTS or PERPLE_X. Given these differences, we apply our model to predict the density changes during the thermal evolution of melt-depleted peridotite forming subcontinental lithosphere and consider these relations in light of the isopynicity hypothesis [3]. [1] Baker et al, 2008, GCA, v. 72, p. A45; [2] Schutt & Lesher, 2006, JGR, v. 111; [3] Jordan, 1978, Nature, v. 274, p. 544-548
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V51D..04B
- Keywords:
-
- 8103 TECTONOPHYSICS / Continental cratons;
- 8120 TECTONOPHYSICS / Dynamics of lithosphere and mantle: general;
- 8412 VOLCANOLOGY / Reactions and phase equilibria