A Coupled Geochemical and Geodynamical Approach for Mantle Melting Beneath Hawaii.

Abstract:

The presence of the Hawaiian plume is manifested by the Hawaiian swell [1] and voluminous eruption of Ni-rich lavas [2] with enriched isotopic compositions [3]. Here we estimate the conditions of melt generation needed to reproduce both features.

We used thermodynamic treatment for fractional melting [4] and melting parameterizations for pyroxenites [5] and peridotite [6] to determine pyroxenite contribution in magmas Xpx as functions of potential temperature T_P, pyroxenite abundance in the source P, radius of the melting zone R and distance to the plume axis. The final pressure of melting is set to correspond with the base of the lithosphere (3 GPa) at the plume axis and increases with the distance from the axis [7]. The Hawaiian plume axis is thought to be currently between Loihi (L), Kilauea (K) and Mauna Loa (ML), which are 25 km, 32 km and 44 km radially away from the plume axis, respectively [3]. To determine Xpx, we assumed that magmas are accumulated melts produced on a circular sampling zone of 50 km diameter centered beneath each volcano [8].

Preliminary calculations show that for T_P = 1525°C, P = 0.07 and R = 55 km, Xpx^ML = 0.59, Xpx^K = 0.49 and Xpx^L = 0.45. Xpx^ML and Xpx^K are similar to values suggested by [2]. Computed liquidus temperatures at 3 GPa are consistent with those of Hawaiian parental melts (1500-1520°C; [9]). Xpx^L is higher than suggested by [2] (Xpx^L = 0.09) but their estimate is based on only one glass analysis. Our model is also consistent with isotopic compositions: K and L have similar εNd, while ML is more enriched [3]. Finally, we can compute the density deficit using parameterization of [1] and relate it to the volume flux volume flux [10]: we obtain 3.2 km³/Yr, a value similar to the estimations based on the Hawaiian swell model [1].

1-Ribe & Christensen EPSL 1999; 2-Sobolev et al. Nature 2005; 3-DePaulo et al. GGG 2001; 4-Phipps Morgan GGG 2001; 5-Lambart et al. in prep; 6-Katz et al. GGG 2003; 7-Ito & Mahoney EPSL 2005; 8-DePaulo & Stolper JGR 1996; 9-Putirka Miner. Soc. Amer. Geochem. Soc. 2008; 10-Turcotte & Schubert Cambridge Press 2002