Program > Browse abstracts by speaker > Borghini Giulio

High-pressure crystallization experiments on a MORB-type basalt at 1-2.5 GPa: insights on clinopyroxenite generation
Giulio Borghini  1@  , Chiara Francesca Crotti  2@  , Patrizia Fumagalli  3@  
1 : Dipartimento di Scienze della Terra “Ardito Desio”, University of Milan  (UNIMI)
2 : Dipartimento di Scienze della Terra “Ardito Desio”, University of Milan  (UNIMI)
3 : Dipartimento di Scienze della Terra “Ardito Desio”, University of Milan  (UNIMI)

Pyroxenites represent widespread lithological heterogeneities found in the upper mantle in various geodynamical settings. Their origin can be attributed to different magmatic or metamorphic processes or a combination of them. Several mantle pyroxenites are believed to result from melt segregation at high-pressure conditions (P  1 GPa) of mantle-derived melts variably interacted with host peridotite. Parental melts having low XMg (XMg = 0.6-0.8) suggest that they could be derived either via peridotite-derived melt differentiation or partial melting of low-MgO heterogeneous mantle sources. We performed piston cylinder experiments at 1-2.5 GPa and 1100-1350°C to investigate the mineralogy and mineral chemistry of pyroxenites formed through high-pressure crystallization of a MORB-like basalt (XMg = 0.60). High temperature (T > 1200°C) isothermal experiments revealed that clinopyroxene is the liquidus phase at 1-2.5 GPa. Experiments cooled down to 1100 and 1150°C simulated isobaric close-system crystallization. During cooling, garnet precipitates after clinopyroxene from the basaltic melt at P  1.5 GPa and T > 1200°C. At 1 GPa, low-fosterite olivine (Fo76-79 at 1200°C and Fo65-68 at 1100°C) was observed in association with plagioclase An45-48 at 1100°C. At increasing pressure, clinopyroxenes exhibit progressively higher Na and Al contents (Jd up to 0.25 at 2.5 GPa) with decreasing Ca and Cr concentrations. Garnets record upon cooling increase of Grossular and Pyrope contents combined with a decrease of Almandine. Pyroxenites generated through infiltration and crystallization of MORB-type basalt at 1.5 to 2.5 GPa are garnet clinopyroxenites having garnet modal abundance increasing with pressure from 22 to 38 wt%. Olivine-bearing clinopyroxenite, with a low abundance (≈ 7 wt%) of Ab-rich plagioclase, forms at 1 GPa. Combining mineral compositions and modal abundances we estimated the bulk compositions of pyroxenites formed through high-pressure crystallization. Computed bulk-rock compositions well resemble those of clinopyroxenites documented in orogenic and ophiolitic ultramafic massifs.


Loading... Loading...