Understanding magmatism at the EPR ISS; the xenolith story

W.I. Ridley¹*, M.R. Perfit², V.D. Wanless², S.A. Soule³, D.J. Fornari³, E. Klein⁴, S.M. White⁵

Corresponding author: iridley@usgs.gov
¹USGS, Denver, CO 80207
²Dept of Geological Sciences, University of Florida, Gainesville, FL 32611
³Woods Hole Oceanographic Institution, Woods Hole, MA 02543
⁴Division of Earth and Ocean Sciences, Duke University, Durham, NC 27708
⁵Dept of Geological Sciences, University of South Carolina, Columbia, SC 29208

Abstract:
The chemistry of lavas erupted onto the seafloor at ridges integrates a variety of magmatic processes, including mantle source physicochemistry, crystal fractionation, in situ fractionation, magma mixing, and reactions between ascending melts and crystal mush of the nascent layer 3A crust. Recognizing the individual contribution of each process to lava chemistry is a challenge. In the lavas themselves, information regarding these processes is cryptic and often addressed through modeling of major, trace element, and isotopic compositions. Xenoliths captured by ascending melts testify to the various lithologies of the ridge crust and to the complexities of magmatic processes within the sub-crustal magma system. These properties provide the rationale for an ongoing study of a comprehensive suite of xenoliths, collected from the 9ºN03’ overlapping spreading center (OSC) and the 9º50’N sections of the EPR, and include xenoliths from the 1991-1992 and 2005-2006 lavas. Highlights are listed as follows. 1) The xenolith suite includes primitive, cumulate troctolites and evolved poikilitic gabbros (Figure 1). Both xenolith types are found in the 2005-2006 flow units. Troctolitic xenoliths also are found in the 1991-1992 lavas and in pre-1991 lavas (Bender et al. (1991). Poikilitic gabbros are also found in eastern limb lavas from the OSC. 2) The mineral phase chemistry invariably demonstrates lack of equilibrium with the host melts, so collectively the xenoliths are not simply connate solids. 3) Olivine melt inclusions in troctolites from the 1991-1992 and 2005-2006 flows have variable compositions and reflect the spatial association of a diverse population of melt. Under these conditions magma mixing must be a common process. 4) Precipitation and dissolution of olivine, plagioclase and clinopyroxene is a common process as melts migrate through the various zones of crystal mush and these processes must be reflected in the chemistry of erupted magmas. 5) Poikilitic gabbros, identical to those described by Lissenburg and Dick (2008) from the Kane Megamullion, are particularly susceptible to resorption of pyroxene and plagioclase. The resorption/recrystallization of pyroxene may account for the curious pyroxene signatures observed in some MORB lavas, which may therefore be of low pressure origin. Resorption of plagioclase must partly contribute to the trace element chemistry of MORB melts.

Keywords:
xenolith, troctolite, poikilitic gabbro, 2005-06 eruption, 1991-92 eruption, overlapping spreading center

Contributions to Integration and Synthesis:
Xenoliths provide glimpses of sub-crustal magma systems beneath the northern EPR and provide textural and phase chemical insights that are complimentary to the chemistry of lavas erupted at the EPR. Large data sets of chemical information from the EPR ISS form the basis of several additional contributions to this meeting and xenoliths inform the processes that control the compositional variability observed in lavas. In addition, xenoliths provide lithologic information that is useful in constraining seismic models of the sub-crustal magma systems.

References:
Bender, J.F., Miller, D.M., and Langmuir, C.H., 1991, Gabbroic xenoliths from 9º20’N – 10º00’N on the East Pacific Rise suggests in situ crystallization, EOS Trans. AGU, 72, 496.
Lissenburg, C.J., and Dick, H.J.B., 2008, Melt-rock reaction in the lower oceanic crust and its implications fro the genesis of mid-ocean ridge basalt, EPSL, 271, 311-25.

Figures:
Figure 1. Poikilitic gabbro (a) Ridley_fig1a.jpg and partly dismembered troctolite (b) Ridley_fig1b.jpg from the 2005-2006 flow at 9º50’N.