Application of Sr-isotope and Trace Element Concentrations in Plagioclase to Understanding Melt Evolution in the Lower Crust at Mid-ocean Ridges
Abstract
After extraction from the mantle, melts below mid-ocean ridges must traverse through several kilometers of oceanic crust prior to eruption on the seafloor. During ascent through the lower crust, the chemistry of these melts may be greatly altered by crystal fractionation, reactive transport, and magma mixing. These processes can homogenize and obscure the original signature of the primary mantle melts. Our research presents a new data set utilizing plagioclase phenocrysts in mid-ocean ridge basalts. These phenocrysts preserve an array of geochemical signatures of lower crustal processes, including mixing of primitive melts and reactive transport. Plagioclase is particularly useful for the purpose of investigating crustal processes because it is an early crystallizing phase in the MORB crust and, unlike olivine, has measurable levels of a number of minor and trace elements (e.g. Ti, Sr, Ba, Ce, etc.), all of which have significantly different partitioning behavior and diffusion coefficients. The heterogeneity observed in plagioclase phenocrysts also suggests that these crystals typically grow prior to significant aggregation, allowing them to record processes that are more likely to have been erased in the host lava compositions. Basalts with abundant plagioclase phenocrysts are commonly found at seamounts and ridges with ultra slow to intermediate spreading rates. Major element, trace element and Sr-isotopic data were collected on plagioclase phenocrysts and host glasses from MORBs at ridges with a range of spreading rates and magma supplies including: Southwest Indian Ridge, Southeast Indian Ridge, Blanco Fracture Zone, Juan de Fuca Ridge, Mid-Atlantic Ridge and Axial Seamount. A single lava sample often contains plagioclase phenocrysts with trace element and isotopic characteristics consistent with growth from a range of parental melts. We find these phenocrysts often record the mixing of progressive mantle melts and variable degrees of melt aggregation and diffusive re-equilibration. Despite significant trace element and sometimes isotopic variability, many of the phenocrysts contain relatively constant major element concentrations. Constant anorthite profiles suggest that plagioclase grew in an environment where the major element chemistry was extensively buffered by the wallrock components. Such a process has a much smaller impact on concentration of the incompatible elements (e.g. Ti, Ba), preserving more of the primary magma signature.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T13G2722L
- Keywords:
-
- 1032 GEOCHEMISTRY / Mid-oceanic ridge processes;
- 1036 GEOCHEMISTRY / Magma chamber processes;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry