I. Abstract The Galapagos Ridge System is one of the fastest spreading and most unique ridges on the Earth. Typical ridges spread at a rate of about mm/yr, while the Galapagos spreads at between mm/yr. Fast spreading rates are associated with increased magma output and with relatively thin crust beneath the ridge axis, as observed along the Galapagos Spreading Center (GSC). Spreading rate may also influence the depth of partial crystallization and the structure of magma plumbing systems beneath ridges. To constrain the depth at which partial crystallization of magmas occurs beneath the GSC, we have used a method to calculate the pressure of crystallization that is more accurate and reliable than similar methods such as that described by Claude Hertzberg (2004, Journal of Petrology). The method involves calculating the pressure at which a liquid, represented by volcanic glass, is chemically in equilibrium with olivine, plagioclase, and augite. Our data set is comprised of analyses of volcanic glass collected on scientific cruises along the Galapagos ridge. These analyses were downloaded from the RIDGE data base maintained by Lamont Earth Observatory. Filtering of the analyses was necessary to exclude glasses that were not in equilibrium with olivine, plagioclase, and augite. The 1,110 remaining glasses were then divided into twelve groups based on longitude. Results thus far indicate that Galapagos magmas crystallize over a range of pressure from kBar, equivalent to 0 to 30 km depth, but that most crystallization occurred between 2 and 6 kBar, or 8-18 km depth. This range of depth suggests that the magma plumbing system is complex and is likely composed of multiple, stacked chambers that are interconnected by dikes. Some of these chambers are probably located beneath the crust in the uppermost mantle. Further work is underway to determine whether there is any relation between magma chamber depth and other geochemical indicators of crustal thickness, such as Na concentrations (normalized to 8.0 wt % MgO). Emily Klein and co-workers have shown that Na 8 is a measure of crustal thickness with low values indicating higher mantle temperatures and greater degrees of melting. Implications for the interpretation of the Galapagos magma plumbing system include the possible influence of the Galapagos hotspot and transform faults near the western end of the ridge on magma plumbing systems. Magma Plumbing System Beneath the Fast-Spreading Galapagos Ridge Emily England and Michael Barton School of Earth Sciences, The Ohio State University, Columbus Ohio II. Locality Galapagos Ridge is located in the Eastern Pacific Ocean Ridge: ~ 2° N, 82° W - 98° W Galapagos Plume and Islands are located 200 km south at 0° N, 92° W Increase in number of fracture zones as move west (95 – 100º W) Figure 1. Bathymetry Map of the Galapagos Region. GSC is the Galapagos Spreading Center and WDL is the Wolf-Darwin lineament. Contours every 500 m. (Canales et al, 2002) Figure 3. III. Data Set All samples used are glass samples. This is important because glass analyses represent samples of quenched melts. Glasses formed from liquid in equilibrium with ol, plag, and cpx should have composition that lie exactly on the cotectic at the pressure of crystallization (Kelley and Barton 2008). Originally we downloaded 1246 analysis from RIDGE data base maintained by Lamont Earth Observatory. This data set had to be filtered. 1)After pressures were calculated for each sample, we deleted all samples with negative pressures (impossible!) 2) Then we deleted all sample that were not in equilibrium with olivine, plagioclase, and clinopyroxene (usually these sample had anomalously low CaO/ Al 2 O 3 ratio, no cpx). 3) Finally, samples with an uncertainty level greater than 1.2 for calculated pressure were deleted. Final set consisted of 1110 glasses from longitudes 84.77º W to 97.86º W. These were then sub-divided into 12 ridge segments for more detailed interpretation. V.Methods for Determining Pressure 1)Recasting melt compositions (aka glass analysis) into normative mineral components and projecting phase relations onto pseudoternary planes in the system CaO-MgO-Al 2 O 3 -SiO 2. 2)Projection of phase relationships from plag onto the plane ol-cpx-qtz using the recalculation procedure of Walker et al. (1979) shows the shift of the ol-plag-cpx cotectic towards ol with increasing P. 3)Used a computer program to quantitatively estimate the crystallization pressure based on such relationships. Figure 4. VI. Resulting Pressures Figure 5. Summary of all results obtained from glasses excluding those that were unrealistic or unreliable. Fig. 5a. Plot of T (ºC) calculated as described by Yang et al. (1996) versus P (GPa). Fig. 5b. Plot of P (GPa) versus MgO. Table 1. Summary of calculated pressures and temperatures displayed by ridge segment. Location 7 Location 8 VIII. Crustal Thickness The global average of mid-ocean ridge crustal thickness is 7.2 km. Seismic crustal thickness- Wide-angle refraction and multichannel reflection seismic data show that oceanic crust along the Galapagos Spreading Center (GSC) between 97°W and 91°25'W thickens by 2.3 km to a total of 8 km (Canales et al. 2002). Our estimates of average crustal thickness along the Galapagos is 10 ± 2.5 km, which is in reasonable agreement with seismic data. Multiple chambers:Single chamber: Location 2 IX. Conclusions 1) Average depth of crust inferred from depths of chambers is ~10 ± 2.5 km. 2)Evidence for single and multiple chambers indicates complex plumbing system and implies crustal accretion over range of depths 3)Na 8 values indicates higher degrees of melting and increased magma flux near 92°W, presumably due to influence by the plume 200 km south. XI. Acknowledgements I would like to give thanks to Dr. Barton for getting me involved in this research. Wendy Panero for her insight into Galapagos plume/ ridge interaction. Daniel Kelley for his previous work using our computer program. Jameson “Dino” Scott for his expertise using the CoPlot graphing program. X. Further Work Mid-ocean ridges, the structure of oceanic crust, and ophiolites have always amazed me! I would like to continue my graduate education is this field. Field work is something that I hope to experience. Figure 4. Incompatible elements, such as Na, enter melt first. Concentration of incompatible elements high at low degrees of melting and low at high degrees of melting (Klein, 1987). High Na 8 values at western segment of ridge linked with fracture zones, as observed elsewhere, along the ridge system. Positive correlation between crustal thickness and Na 8. The anomalous data point relates to the effect the Galapagos Plume has on the ridge. Bowen’s Reaction Series (Fichter, L. 2000) LocalitynAve PMaxMinDepth (km)Ave TMaxMin to 85.57ºW ± ± to 92.23ºW ± ± to 94.97ºW ± ± to 95.3ºW ± ± to 95.37ºW ± ± to 95.44ºW ± ± ºW ± ± to ºW ± ± to ºW ± ± ºW ± ± to 95.78ºW ± ± to 97.86ºW ± ± ALL84.77 to 97.86ºW ± ± VII. Depths of Magma Chambers Pressure of crystallization is converted to a depth by a simple conversion of multiplying pressure by 9.8 * 2900 * On the more eastern segment of the ridge there is evidence of a single magma chamber. On the more western edge of the ridge there is evidence for multiple magma chambers. This is associated with transform faults. IV. Sample Chemistry Figure 3. Variations of Al 2 O 3, CaO, and CaO/ Al 2 O 3 with MgO for entire data set. Graphs of this type allow for identification of the mineral phases that crystallized during magma evolution. The decrease in Al 2 O 3 with decreasing MgO is consistent with crystallization of olivine and plagioclase. The strong decrease in CaO with decreasing MgO indicates crystallization of clinopyroxene. The nearly horizontal trend of CaO/Al 2 O 3 with decreasing MgO indicates crystallization of clinopyroxene as well as pyroxene. Important Chemical Formulas: Olivine: (Mg,Fe) 2 SiO 4 Plagioclase: NaAlSi 3 O 8 - CaAl 2 Si 2 O 8 Clinopyroxene: Ca(Mg,Fe)Si 2 O 6 Pyroxene: CaAl 2 Si 2 O 6 Figure 5. Location 4