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Contributions to Mineralogy and Petrology

, Volume 159, Issue 3, pp 371–387 | Cite as

The time scales of magma mixing and mingling involving primitive melts and melt–mush interaction at mid-ocean ridges

  • Fidel Costa
  • Laurence A. Coogan
  • Sumit Chakraborty
Original Paper

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.

Keywords

Plagioclase Diffusion MORB Mixing 

Notes

Acknowledgments

Reviews by J. Sinton and P. Wallace are greatly appreciated and helped to clarify various parts of the manuscript. FC acknowledges contracts under project 526 of the German Science Foundation and a Ramon y Cajal Fellowship Spanish Ministry of Education. The ocean drilling program is acknowledged for providing access to the samples studied here. NSERC funded the LA-ICP-MS analyses at the University of Victoria.

Supplementary material

410_2009_432_MOESM1_ESM.xls (776 kb)
Supplementary material 1 (XLS 777 kb)
410_2009_432_MOESM2_ESM.xls (394 kb)
Supplementary material 2 (XLS 394 kb)

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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Fidel Costa
    • 1
    • 2
  • Laurence A. Coogan
    • 3
  • Sumit Chakraborty
    • 2
  1. 1.Institut de Ciencies de la Terra ‘Jaume Almera’, CSICBarcelonaSpain
  2. 2.Institut fuer Geologie, Mineralogie und GeophysikRuhr-Universitat BochumBochumGermany
  3. 3.School of Earth and Ocean SciencesUniversity of VictoriaVictoriaCanada

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