Generation of incompatible element depleted high-Al melts: Constraints from melt inclusions from the FAMOUS zone, mid-Atlantic Ridge
Abstract
High-Al, low-Si and high-Mg melts have been reported for various mid-ocean ridges (Eason and Sinton, EPSL, 2006; le Roux et al., Contrib. Min. Pet., 2002; Lissenberg and Dick, EPSL, 2008) and seem to occur preferentially at slow spreading ridges or close to fracture zones and ridge ends. Two hypotheses have been suggested to explain their chemistry: (1) high-pressure fractionation of clinopyroxene (Eason and Sinton, EPSL, 2006) and (2) a reaction between primitive cumulates and migrating melts in the lower crust (Lissenberg and Dick, EPSL, 2008). Our study of numerous lavas (>100) and melt inclusions (>335) along the FAMOUS segment (Northern mid-Atlantic Ridge) shows that high-Al melts occur both as melt inclusions and lavas, mostly found in the FAMOUS area near the center of the segment. The high-Al lavas studied for their melt inclusions host both high-Al and normal inclusions. No high-Al inclusion has been found in non-high Al basalts. High-Al melts are characterized by high Al2O3 (up to 18.4 wt%), high MgO (>8.3 wt% in the lavas; >10.3 wt% in the inclusions), low SiO2 (48.4-50.5 wt% in the lavas; 46.6-49.0 wt% in the inclusions) and low TiO2 (< 1 wt% in the lavas, < 0.73 wt% in the inclusions) contents. High-Al melt compositions are distinctively lower in highly incompatible elements (Ba as low as 18 ppm in the lavas and 1 ppm in the inclusions). With such low incompatible element concentrations, the high-Al melt inclusions constitute ultra-depleted melts (UDM). High-Al inclusions and lavas also display a relative enrichment in the HREE/MREE ratios compared to normal melts. Similarly, olivine phenocrysts that host high-Al melt inclusions present relatively higher Al2O3, NiO contents and Y/MREE and HREE/MREE than those with normal inclusions. High-pressure clinopyroxene fractionation is inconsistent with the relatively high Sc and very low SiO2 contents in High-Al inclusions. Reaction between a MORB and a plagioclase-rich cumulate results in trace element features as the increase in Sr with increasing Al2O3, opposite to the observations. Modeling of melt/mantle reequilibration between MORB and harzburgite also fails to reproduce the high-Al signature, in particular the low SiO2. The offset toward higher HREE in the high-Al inclusions and lavas is correlated with low SiO2 and melting calculations reproduce these features by high-pressure (~25kb) continuous melting of a very depleted source. The presence of a depleted mantle end-member defined by high-Al melts is confirmed by low Sr isotopes for most high-Al samples. Our results thus suggest a local mantle heterogeneity, with high-Al melts produced by high pressure melting of a more depleted mantle end-member. The UDMs produced at depth have to be transported through the mantle and the lithosphere without significant interaction. High-Al melt inclusions always display more primitive and extreme compositions than associated lavas, which suggests that high-Al lavas largely result from mixing of melts from high-Al and normal liquids.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V23B2059L
- Keywords:
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- 8410 VOLCANOLOGY / Geochemical modeling;
- 8416 VOLCANOLOGY / Mid-oceanic ridge processes