The time scales of magma mixing and mingling involving primitive melts and melt–mush interaction at mid-ocean ridges |
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| Authors: | Fidel Costa Laurence A Coogan Sumit Chakraborty |
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| Institution: | (1) Institut de Ciencies de la Terra ‘Jaume Almera’, CSIC, 08028 Barcelona, Spain;(2) Institut fuer Geologie, Mineralogie und Geophysik, Ruhr-Universitat Bochum, 44780 Bochum, Germany;(3) School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, V8W 3P6, Canada |
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| Abstract: | We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading
rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and
Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive
magma (~11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals
into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted
basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium
with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different
histories. Diffusion modelling shows that crystal residence times in the mush were <140 years, whereas the interval between
mush disaggregation and eruption was ≤1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa
Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some
crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration.
This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts
from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring
mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate-
and slow-spreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales;
(2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones
appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted
basalts. |
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