Melt inclusions (minute pieces of melt trapped in crystals) have been used in several recent studies to determine the original composition of mantle derived magmas before degassing, magma mixing, crystal accumulation, contamination and alteration modified the original composition of the erupted lava. Melt inclusions, are commonly trapped at pressures exceeding the pressure of eruption, they form before magma aggregation, and they are protected by their crystal host from eruptive degassing and alteration. Therefore, the study of melt inclusions, as sealed samples of very small amounts of original melt, provides an advantage over the examination of whole rock samples from erupted lavas. There are, however, several serious disadvantages of using melt inclusions to evaluate mantle source compositions and melting processes: (A) the composition of melt inclusions can reflect localized, grain scale wall-rock reaction processes within the shallow magmatic plumbing system, (B) rapid crystal growth can lead to disequilibrium uptake of elements components whose diffusivities limit their dispersal near an advancing crystal interface, creating unusual melt inclusion compositions, and (C) the host crystal might not be an effective barrier to insure the inclusion remains a closed thermodynamic system (especially to volatile components, e.g., H2O, F). Several examples of our work indicate the importance and the challenges of working with melt inclusions to determine the composition of the lava mantle source:
- Using high-spatial-resolution secondary ion mass spectrometry (SIMS) we were able to measure for the first time Pb isotopes in 30 µm-size-melt inclusions from a single lava flow. The range of Pb isotopic compositions from individual melt inclusions from a single lava flow spans 50 percent of the worldwide range observed for oceanic basalts. Our data imply that magmas with different isotopic compositions (i.e., originate from different sources) existed in the volcanic plumbing system before or during melt aggregation. We favor a model where the Pb isotopic compositions of the melt inclusions reflect wall rock reaction of basaltic melts with the oceanic lithosphere.
- The composition of melt inclusions from oceanic islands have been previously used to demonstrate the presence of an ancient recycled plagioclase-rich cumulate within the mantle beneath oceanic islands such as Hawaii, Galapagos and Iceland. Our work on lavas and melt inclusions from the Galapagos Archipelago clearly indicates that their compositions reflect reaction processes between the magma and plagioclase-rich cumulates within the present-day oceanic lithosphere rather than the presence of an ancient recycled oceanic crust within the mantle beneath the Archipelago.
- In our new report on the composition of a suite of melt inclusions from the Galapagos Archipelago we determined (A) the distinctive volatiles contents of the heterogeneous mantle beneath the Galapagos Islands, and (B) that H2O and F contents did not remain as closed thermodynamic system in some of the melt inclusions. The variations in water and fluorine contents in these inclusions are primarily related to the fast diffusivity of H+ through the host crystal and, most likely, to the presence of F-rich clinohumite lamellae within in the crystal. Therefore, careful evaluation of the geochemical data from melt inclusions is required before making extraordinary claims about the mantle source compositions of the lavas.
Koleszar, A.M., Saal, A.E., Hauri, E.H., Nagle, A.N., Liang, Y. and Kurz, M.D. - (2008) - Melt inclusions, the volatiles contents of the Galapagos plume, and evidence of boundary layer reaction and open system behavior. Earth and Planetary Science Letters, submitted.
Saal, A.E., Hart, S.R., Shimizu, N., Hauri, E.H. and Layne, G.D. - (1998) - Pb isotopic variability in melt inclusions from oceanic island basalts, Polynesia. Science 282, 1481-1484.
Saal, A.E., Hart, S.R., Shimizu, N., Hauri, E.H., Layne, G.D. and Eiler, J.M. - (2005) - Pb isotopic variability in melt inclusions from the EMI-EMII-HIMU end members and the role of the oceanic lithopshere. Earth and Planetary Science Letters 240, 605-620.
Brown faculty collaborators:
Other project collaborators:
J. Eiler (California Institute of Technology); S. Hart, N. Shimizu and M. Kurz (Woods Hole Oceanographic Institution); E. Hauri (Carnegie Institution of Washington).
The main objective of our work is to develop a geodynamic picture of the mantle heterogeneities, their geographic distribution, upwelling velocities, and extent of melting below the Galapagos Archipelago. Previous works have been very successful in improving our knowledge of the Galapagos plume and its interaction with the Galapagos Spreading Center. There are a number of outstanding scientific issues that we are working on:
- Establishing the origin of the Archipelago. There is a long-standing model that oceanic islands such as Galapagos represent the surface expression of hot, buoyant upwelling mantle plumes sampling compositionally distinctive regions deep within our planet. However, this hypothesis has recently been questioned on the basis of geophysical, petrological and geochemical arguments. Whether mantle plumes exist or not is critical for our understanding of the thermal, dynamic and compositional evolution of the Earth’s mantle. We pursued full geochemical characterization (major, trace and volatile element contents, as well as Sr, Nd, Pb, Hf, O and U-series isotopes) of lavas from across the Galapagos Archipelago. The geochemical data allowed us to (A) confirm that the Galapagos volcanoes indicate the presence of a mantle plume beneath the Archipelago, (B) determine the extent and geographical distribution of crustal contamination experienced by the lavas, and (C) establish the volatile, trace element and isotopic composition of the different mantle components beneath the Archipelago
- Defining the compositions of the end-member components. It is fundamental to obtain a complete geochemical characterization of the different end-member components responsible for the compositional variation of the Galapagos magmatism. Previous studies of Galapagos basalts have shown that the mantle beneath the archipelago is isotopically heterogeneous. However, the nature, distribution, and scale of this heterogeneity remain uncertain. Our work on melt inclusion, submarine glasses and subaerial lavas from the Galapagos Archipelago is helping us to A) define the composition of the pre-aggregated end-member components, and B) estimate the primary volatile contents in those components. The volatile contents of melt inclusions and deep submarine glasses from the plume and from the depleted mantle will help us understand the effect of volatile versus temperature during mantle melting. In particular, it is of outmost importance to define the composition of the high 3He/4He component present in the Galapagos lavas. If high 3He/4He ratios is an indication of the presence of the undegassed lower mantle, the geochemical composition of the high 3He/4He component might provide the most direct way to learn about its major, trace, and volatile contents.
- Determining the origin of the different end-member components. We have to define what process or processes are responsible for the geochemical variation found in the Galapagos lavas. The Galapagos lavas provide an extensive record of chemical and isotopic variability; this variability may originate from several processes: A) heterogeneous distribution of ancient subducted oceanic crust ± sediments, B) mantle regions variably subject to internal differentiation and/or metasomatism, C) interaction and contamination of basalt with the oceanic lithosphere during aggregation, ascent and eruption. Oxygen isotopes, in combination with the already measured radiogenic isotopes, will provide the means to distinguish between the different processes responsible for the isotopic variation in the Galapagos lavas. Oxygen isotope will clearly discriminate between A) ancient subducted oceanic crust ± sediments or interaction and contamination of basalt with the oceanic lithosphere, and B) mantle regions variably subject to internal differentiation and/or metasomatism. Meanwhile, radiogenic isotopes will be useful to differentiate between ancient subducted oceanic crust and interaction and contamination with a young oceanic lithosphere.
Saal, A.E., Kurz, M.D., Hart, S.R., Busztajn, J., Blichert-Toft, J., Liang, Y. and Geist, D. - (2007) - The role of lithospheric gabbros on the composition of Galapagos Lavas. Earth and Planetary Science Letters 257, 391-406. 
Saal, A.E., Bourdon, B., Kurz, M.D., Hauri, E.H., Blusztajn, J.S., Blichert-Toft, J., Hart, S.R., Sims, K.W. - (2007) - Temperature versus Buoyant Mantle Heterogeneities, Evaluating the Origin of OIB Using the Galapagos Archipelago. EOS Trans. AGU 88, (52), Fall Meet. Suppl., Abstract V42B-05. Invited Talk.
Kurz, M.D., Rowland, S.K., Curtice, J., Saal, A.E. and Naumann, T. - (2008) - Eruption rates at Fernandina volcano, Galapagos archipelago, from cosmogenic helium. Geology, revision after reviews.
Bourdon, B., Ribe, N.M., Stracke, A., Saal, A.E. and Turner, S.P. - (2007) - Reply to “Evidence for mantle plumes? by D.L. Anderson and J.H. Natland. Brief communication arising from Bourdon et al. Nature 444, 713-717”. Nature 450, 11-22-07.
Bourdon, B., Ribe, N.M., Stracke, A., Saal, A.E. and Turner, S.P. - (2006) - Insights into the dynamics of mantle plumes from U-series geochemistry. Nature 444, 713-717.
Jackson, M.G., Hart, S.R., Saal, A.E., Shimizu, N., Kurz, M. D., Blusztajn, J. S. and Skovgaard, A.C., - (2008) - Globally elevated titanium, tantalum, and niobium (TITAN) in ocean island basalts with high 3He/ 4He. Geochemistry, Geophysics and Geosystems, 9 (4) doi:10.1029/2007GC001876.
Hart, S.R., Staudigel, H., Koppers, A.A.P., Blusztajn, J., Baker, E.T., Workman, R., Jackson, M., Hauri, E. , Kurz, M., Sims, K., Fornari, D., Saal, A.E. and Lyons, S. - (2000) - Vailulu'u Undersea Volcano: The New Samoa. Geochemistry, Geophysics and Geosystems GC000108, 1-13.
Brown faculty collaborators:
Other project collaborators:
B. Bourdon (ETH, Zurich); J. Eiler (California Institute of Technology); S. Hart and M. Kurz (Woods Hole Oceanographic Institution); E. Hauri (Carnegie Institution of Washington); K. Hoernle (Christian-Albrechts University of Kiel, Germany); Z. Wang (Yale University); D. Geist (U. Idaho); K. Harpp (Colgate University); J. Blichert-Toft (Ecole Normale Supérieure de Lyon, France).