Our main objective is to determine to what extent the compositional variations observed in mid-ocean-ridge basalts either represent variations in the physical conditions of magma genesis from a homogeneous mantle composition (i.e., depth, temperature and extent of mantle melting), or alternatively, indicate mixing of melts originated from an heterogeneous mantle beneath the ridge. To address these issues, we decided to sample lavas from seamounts and from within transform faults near the ridge axis. Seamount and intra-transform lavas share a common mantle source with mid-ocean ridge basalts erupted at the ridge axis, but because they represent small melt volumes tapped locally (i.e., from areas lacking steady-state magma chambers where melt aggregate) they experience relatively less mixing and chemical differentiation than those erupted at the ridge axis. We have geochemically characterized more than 300 seamount and intra-transform lavas from the East Pacific Rise between 15° N and 23° S. The conclusions drawn from the geochemistry of seamount lavas is essentially identical to those obtained from the intra-transform lavas: Namely, the main process acting on the generation of mid-ocean ridge basalts is the melting-induced mixing of a two-component mantle, where the enriched component represent easily melted domains rich in volatiles elements within a more depleted and refractory matrix.
We are using the geochemical and petrological information of sampled lavas in conjunction with three-dimensional models of mantle flow, melt production and migration in a feedback loop to obtain a self-consistent geochemical and geophysical picture of mantle flow, mantle heterogeneity and melt production and migration in the Pacific Ocean. Models of ridge segmentation, mantle flow and melt focusing predict how the chemical compositions of mantle melts should vary along the mid-ocean ridge axis, meanwhile the petrology and geochemistry of basalts can provide a test to these model predictions. The main objective of our work is to provide, through the combination of physical models of melting and melt segregation and the geochemistry of the "zero age" lavas, an integral picture of mantle flow, melt production and migration, and mantle heterogeneity in the equatorial East Pacific Rise. Only the combination of geophysical and geochemical methodologies will give us a more integrated and realistic picture of the processes responsible for the formation of new crust at the mid-ocean ridges.
Pickle, R.C., Forsyth, D.W., Harmon, N., Nagle, A.N. and Saal A.E. - (2008) - Thermo-mechanical control of axial topography of intra-transform spreading centers. Earth and Planetary Science Letters, submitted.
Saal, A.E., Nagle, A.N., Myers, C., Hauri, E.H., Pickle, R., Forsyth, D., and Niu, Y. - (2008) - Evidence for a heterogeneous astenosphere from intra-transform and seamount lavas. EOS Trans. AGU 89, Fall Meet. Suppl., Abstract. Invited Talk.
Pickle, R., Forsyth, D., Saal, A.E., Nagle, A.N. and Perfit, M.R. - (2008) - Trace Elements in Basalts from the Siqueiros Fracture Zone: Implications for Melt Migration Models. EOS Trans.AGU 89, Fall Meet. Suppl., Abstract.
Nagle, A.N., Pickle, R.C., Saal, A.E., Hauri, E.H. and Forsyth, D.W. - (2007) - Geochemistry of basalts from intra-transform spreading centers: Implications for melt migration models. Geochimica et Cosmoquimica Acta 71, A700, Goldschmidt Conference, Cologne, Germany.
Nagle, A.N., Pickle, R.C., Saal, A.E., Hauri, E.H. and Forsyth, D.W. - (2007) - Volatiles in Basalts from Intra-Transform Spreading Centers: Implications for Melt Migration Models. EOS Trans. AGU 88, (52), Fall Meet. Suppl., Abstract DI43A-05.
Brown faculty collaborators:
Other project collaborators:
Y. Niu (Durham University, England); N. Geshi (Geological Survey of Japan); R. Hekinian (French Research Institute for Exploitation of the Sea, France); J. Sinton and J. Mahoney (University of Hawaii); E. Hauri (Carnegie Institution of Washington).
The analysis of volatiles in magmatic systems can be used to constrain the volatile content of the Earth's mantle and the influence that magmatic degassing has on the chemistry of the oceans and the atmosphere.
Most volatile elements have very low solubilities in magmas at atmospheric pressure, and therefore virtually all erupted lavas are degassed and do not retain their primary volatile signatures. We reported the undersaturated pre-eruptive volatile content for a suite of mid-ocean-ridge basalts from the Siqueiros intra-transform spreading centre. The undersaturation leads to correlations between volatiles and refractory trace elements that provide new constraints on volatile abundances and their behavior in the upper mantle. Our data generate improved limits on the abundances of carbon dioxide, water, fluorine, sulphur and chlorine in the source of normal mid-oceanridge basalt. The incompatible behavior of carbon dioxide, together with the CO2/Nb and CO2/Cl ratios, permit estimates of primitive carbon dioxide and chlorine to be made for degassed and chlorine-contaminated mid-ocean-ridge basalt magmas, and hence constrain degassing and contamination histories of mid-ocean ridges.
Saal, A.E., Hauri, E.H., Langmuir, C.H. and Perfit, M.R. - (2004) - Reply to “The role of fO2 in fluid saturation of oceanic basalt by B. Scaillet and M. Pichavant. Brief communication arising from A.E. Saal et al. Nature vol. 419, 451-455”. Nature 430, 7-29-04.
Saal, A.E., Hauri, E.H., Langmuir, C.H. and Perfit, M.R. - (2002) - Vapor undersaturation in primitive mid-ocean ridge basalt and the volatile content of the Earth’s Upper Mantle. Nature 419, 451-455.
Other project collaborators:
E. Hauri (Carnegie Institution of Washington); C. Langmuir (Harvard University); M. Perfit (University of Florida)
Uranium-series radioactive decay chains in young volcanic rocks have become a unique geochemical tool with the potential to constrain the physical processes involved in the generation and transport of magmas. Among the different isotopes of the decay series, the measured disequilibrium between 226Ra-230Th and between 210Pb-226Ra in basaltic melts has been crucial to determine the timescales of melting and melt transport in the Earth’s mantle. The key assumption proposed in all previous models, however, that the origin of 226Ra-230Th and 210Pb-226Ra disequilibrium in oceanic basalts is only produced during mantle melting processes, has remained largely unchallenged. We articulated an alternative process for the origin of the 226Ra-230Th and 210Pb-226Ra disequilibria in oceanic basalts. We argue that diffusive interaction of magmas with the oceanic crust may be responsible for the observed disequilibrium in oceanic basalts. We have developed a numerical model that considers the diffusion-controlled partitioning of parent and intermediate daughter nuclides between multiple minerals within the oceanic crust and the percolating basaltic melts. Hence, the inferences drawn from (226Ra)/(230Th) and (210Pb)/(226Ra) activity ratios concerning mantle melting and melt transport velocities are at best equivocal. The implications of our model are far reaching, and if proven correct, will completely modify the conclusions that scientists have drawn up to now from the U-series data in oceanic basalts.
Van Orman, J.A., Saal, A.E., Bourdon, B. and Hauri, E.H. - (2006) - Diffusive fractionation of U-series nuclides during mantle melting and shallow level melt-cumulate interaction. Geochimica et Cosmoquimica Acta 70, 4797-4812.
Saal, A.E. and Van Orman, J.A. - (2004) - The 226Ra enrichment in oceanic basalts: evidence for melt-cumulate diffusive interaction processes within the oceanic lithosphere. Geochemistry, Geophysics and Geosystems vol 5 (2) 2003GC000620.
Van Orman, J. and Saal, A.E., Reconciling 210Pb deficits with the physics of melt extraction. Geochimica et Cosmoquimica Acta 71, A1058, Goldschmidt Conference, Cologne, Germany. Keynote Talk.
Other project collaborators:
J. Van Orman (Case Western Reserve University)