V53A-3131
A long-term record of magma compositions at the Juan de Fuca ridge from analysis of sediment hosted volcanic glass: tests of the effects of sea level on melt production
Friday, 18 December 2015
Poster Hall (Moscone South)
David Ferguson1, Yinqi Li2, Charles H Langmuir2, Kassandra Costa3 and Jerry F McManus4, (1)Harvard University, Earth and Planetary Sciences, Cambridge, MA, United States, (2)Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA, United States, (3)Lamont -Doherty Earth Observatory, Palisades, NY, United States, (4)Columbia University of New York, Palisades, NY, United States
Abstract:
It is hypothesized that the pressure changes caused by sea level variations during glacial cycles can influence long-term rates of magmatism in ocean basins. This proposed coupling between oceanic magmatism and climate has important implications for temporal trends in rates of mid-ocean ridge (MOR) volcanism, the structure of the oceanic crust and the composition of oceanic magmas, including the flux of volatile elements from the mantle. Testing the extent to which climate cycles may or may not modulate the compositions of oceanic magmas is difficult, as it requires compositional records that stretch over timescales comparable to those of glacial cycles (i.e. 10s of kyrs), which do not currently exist. In this study we use fragments of volcanic glass found in sediment cores to construct compositional time-series for lavas erupted at the Juan de Fuca ridge, NW Pacific. These provide continuous records of magmatism over several 10s kyrs, with a temporal resolution of a few kyrs. The longest record in our current dataset spans ~80 kyrs and records two significant step-wise changes in the average compositions of the erupted magmas, linked to both variations in the extent of crustal fractionation (i.e. MgO) and also the composition of primary mantle melts (i.e. K2O/TiO2). These changes occur rapidly and are decoupled in time, with the MgO shift happening around ~20 kyrs before the change in K2O/TiO2. Compositional variations such as these, and the temporal offset between them, are generally consistent with models of the effect of sea-level variation on magma chemistry, which can modulate the melt flux to the ridge and the extent of mantle melting. To place our results in a chronological and climatic context we will compare our data to oxygen-isotope records collected from the same sediment horizons and chemical analysis of the host sediments. Our study demonstrates the potential of this method for investigating the magmatic behaviour of ridges over long timescales.
