White Paper Title: 
U-series constraints on magma supply, mantle source and melt migration along mid-ocean ridge spreading centers

Chris J. Russo and Ken Rubin;

SOEST- Department of Geology and Geophysics, University of Hawaii, Honolulu 96822

U-series disequilibrium in volcanic rocks provides unique insight into magmatic processes and their timescales. There is a significant literature of applications to mid-ocean ridge basalts that can be applied to the Ridge 2000 Integration and Synthesis goals.  For instance, disequilibrium between relatively short-lived 210Pb (t1/2=22.3 yrs.) and 226Ra (t1/2=1600 yrs.) in MORB indicate rapid transport of melt from the mantle to the crust over timescales of 10's of years and equally short crustal magma chamber residence times (Rubin et al., Nature, 2005, vol. 437). Longer-lived 230Th (t1/2=75,000 yrs.) activities and (226Ra/230Th) and (230Th/238U) are more sensitive to melting rate and source lithology in MORB (e.g., Sims et al., GCA, 2002, vol. 77).  Disequilibrium amongst these longer-lived nuclides is sensitive to both elemental fractionation during melting and the melting rate (%melt/km upwelled), making it a unique and powerful tool for understanding mantle upwelling conditions and identifying the presence of even small amounts (≤5%) of enriched lithologies in mantle beneath spreading ridges (because the high modal abundance of clinopyroxene in most enriched mantle lithologies enhances their fusibility and thus melting rate).  As a result, high melt productivities of such lithologies produce magmas with low 230Th and 226Ra excesses and distinct trace element concentrations while only contributing a small amount to the total melt volume delivered to the ridge axis (e.g., as demonstrated along the Southeast Indian Ridge; see attached figure from Russo et al., EPSL, 2009, vol. 278).
U-series characteristics in MORB from both the 1991-92 and 2005-06 eruptions within the EPR ISS at 9°50'N has provided a unique opportunity to explore decadal variations in crustal level magmatic processes at, and melt supply to, this well characterized ridge segment.  Collectively, similar measured values of (230Th/232Th) and (238U/232Th) in lavas from both eruptions, higher measured Th, U, Ra, Pb, and Ba concentrations in 2005-06 Bullseye lavas, and smaller 210Pb-226Ra disequilibria in 2005-06 lavas compared to those erupted in 1991-92 are consistent with decay and differentiation occurring in a closed magma system over a 16 to 60 year time period.  Additionally, 2005-06 lavas extend to even higher incompatible element abundances at the northern and southern extremes of the eruption (locations that were not sampled or known to have experienced an eruption in 1991-92).  Collectively these observations are consistent with a 2005-06 eruption sequence supplied by either a chemically zoned magma body or a series of disconnected magma bodies that were transported and evolved over short (i.e. decadal) timescales (see also A. R. Goss, M. R. Perfit, W. I. Ridley, K. H. Rubin, G. D. Kamenov, S. A. Soule, A. Fundis, and D. J. Fornari, Geochem. Geophys. Geosyst., 11, Q05T09, doi:10.1029/2009GC002977, 2010).

It is our desire to integrate these observations with other datasets that describe hydrothermal variability, mantle source, magma temperature and melt distribution in an effort to produce a holistic view of the role magma plays in terms of supplying mass and heat to the crust of the ridge system within the EPR ISS.  An integrated view of this sort is crucial for the advancement of models describing the thermal evolution of the upper mantle and crust within this ISS and elsewhere along the global ridge system and represents one tangible product of a focused synthesis effort facilitated by this workshop.

Figure Caption: Along-axis variation in a) ridge depth/axial morphology, b) 230Th-excess, c) (230Th/232Th) and d) schematic representation of the competing effects mantle temperature and mantle lithology have on melting rate along the Southeast Indian Ridge from 88° to 118° E (modified from Figure 5 of Russo et al., EPSL, 2009, vol. 278).