Textural and Compositional Variability of Phenocrysts from Plagioclase Ultraphyric Basalts: Evidence for a genetic magmatic suite at the SEIR
Abstract
Plagioclase ultraphyric basalts (PUBs) are common in many oceanic tectonic settings with a great abundance along slow to moderately fast spreading ridges, although their petrogenesis remain unclear. Plagioclase phenocrysts in these basalts tend to be modally dominant (>20%) and are often large (>5mm) with high An (>An88) contents. We have examined PUBs from the South East Indian Ridge (SEIR) along four segments leading to and on the Amsterdam-St. Paul Plateau (ASP). Our work focuses on measuring major- and trace-element compositions of plagioclase, olivine and spinel, the goal being to document the extent to which the phenocrysts represent a coherent suite of genetically related material. Furthermore, we use these analyses as a framework to understand the dynamics of magma storage, transport and evolution within the oceanic crust. Based on analyses of plagioclase from the SEIR, we see clear evidence of considerable inter- and intra-grain variability in major- and trace-element compositions. Plagioclase compositions range from An93 to An63 and major element variations demonstrate distinct trends at both the mineral and segment level. Plagioclase trace element data from segment J2 illustrates generally coherent correlations with the individual phenocrysts making up components of composite trends, which are interpreted as linking the phenocrysts to a suite of genetic magmas. However, large variations seen in some trace element compositions suggest that these magmas carried a diverse crystal cargo, which is supported by the varying textural patterns observed in plagioclase phenocrysts. Although not in cotectic proportions with plagioclase within these rocks, olivine is the second most abundant phase observed. Contacts between olivine and plagioclase exhibit characteristics of olivine resorption. Olivine is never observed to be in contact with high anorthite plagioclase (>An88). Olivine zoning profiles are often complex, and in cases where olivine and plagioclase are in contact, the plagioclase compositions are more sodic <An88, even in phenocrysts with high An cores (up to An93). Clinopyroxene occurs as a phenocryst phase rarely, (two samples from segment I2), and is always associated with <An86 plagioclase. From this work, it is clear that plagioclase megacrysts and associated phases in PUBs of the SEIR exhibit both textural and compositional heterogeneity. We believe that this is a direct result of differences in the crystallization, mixing and transport histories of each magma. This complexity is a fertile source of information on the details of heat and mass transfer in the oceanic crust.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V11A2247B
- Keywords:
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- 1021 GEOCHEMISTRY / Composition of the oceanic crust;
- 1032 GEOCHEMISTRY / Mid-oceanic ridge processes;
- 1042 GEOCHEMISTRY / Mineral and crystal chemistry;
- 3614 MINERALOGY AND PETROLOGY / Mid-oceanic ridge processes