Supplementary Material
Wanless, V. D., M. R. Perfit, E. M. Klein, S. White, and W. I. Ridley (2012), Reconciling
geochemical and geophysical observations of magma supply and melt distribution at the
9_N overlapping spreading center, East Pacific Rise, Geochem. Geophys. Geosyst., 13,
QXXXXX, doi:10.1029/2012GC004168.
S1. Depths of Evolved Basalts Formation at the OSC
Ferrobasalts and FeTi basalts at the OSC require 30-55% fractional crystallization
from a relatively high-MgO parent (~8.5 wt. % MgO), however, where within the
magmatic system this crystallization occurs is uncertain. In principle, it may occur in
several locations as discussed by Natland and Dick [2009]: 1) in the shallow melt lens; 2)
within the uppermost mantle and 3) during magma ascent through a crystal-rich mush
zone in the deeper crust or some combination of these. Below we discuss each of these
hypotheses with respect to the formation of ferrobasalts and FeTi basalts at the 9° N
OSC.
The absence of high-MgO lavas at the OSC and the elimination of high-MgO
melt as a mixing end-member argue against the idea that all crystallization occurs in the
shallow melt lens (note, however, that we are not suggesting that there is no
crystallization occurring within this region). This conclusion is supported by petrologic
modeling [Danyushevsky and Plechov, 2011] that suggests that several OSC lavas have
undergone fractional crystallization at pressures greater than 1 kbar and thus at depths
greater than that of the melt lens (Figure 4). It is also clear from the range of
compositions of MOR gabbroic rocks (that include gabbronorites and ferrogabbros) and
their mineral phases that MORB melts can differentiate to high degrees in the lower parts
of the oceanic crust [Hekinian et al., 1993; Coogan, 2007; and Natland and Dick, 2009]
and that crystallization is not restricted to the melt lens.
The second hypothesis proposes that melts differentiate from primitive high-MgO
compositions to more typical MORB compositions (< 9 wt% MgO) in the upper mantle.
This may occur in sill-like bodies at the base of the crust below ridge axes [e.g., Natland
and Dick, 2009] or during transit to the ridge axis in the mantle [e.g., Toomey et al.,
2007]. There is abundant petrologic evidence for the variable degrees of differentiation
of primitive or high-MgO MORB melts in the mantle below the ridge axis prior to ascent
into the oceanic crust [e.g., Kelemen et al., 1995; 2007; Collier and Kelemen, 2010;
Wanless and Shaw, 2012]. However, none of these studies support the formation of
evolved ferrobasalt compositions in the mantle or Moho. Instead, examination of exposed
sections of oceanic lower crust and mantle in tectonic windows and in ophiolite sections
indicate that melts feeding the ridge axis are more primitive than or similar to high-MgO
MORB [e.g., Kelemen et al., 2007]. Therefore, there is little evidence to support a
hypothesis that crystallization in the mantle alone is effective in producing the
moderately to highly evolved ferrobasalts and FeTi basalts erupted at the eastern OSC.
The third hypothesis suggests that melts formed in the mantle below the OSC may
crystallize within [e.g., Sinton and Detrick, 1992; Wanless and Shaw, 2012] and/or
interact with the crystal-rich mush zones [e.g., Coogan et al. 2000; Ridley et al., 2006;
Lissenberg and Dick, 2008; Kvassnes and Grove, 2008] in the mid- to lower crust. Such
mush zones, postulated to compose most of layer 3 of nascent oceanic crust, have been
identified in seismic and compliance data from beneath the east limb melt lens [Crawford
et al., 1999; Crawford and Webb, 2002; Canales et al., 2006]. Thus, magmas evolve
during ascent through the mush zone either by increasing degrees of fractional
crystallization [e.g., Sinton and Detrick, 1992] or by melt-rock reaction [e.g., Lissenberg
and Dick, 2008]. This is further supported by petrologic modeling results from the OSC
lavas that suggest crystallization occurs at depths greater than that of the melt lens (as
discussed above). Thus, we believe that melt differentiation at the OSC likely begins in
the lower oceanic crust and continues into the shallow melt lens. Unfortunately, a
commonly used geochemical indicator of deeper crystallization (decreasing CaO/Al2O3
with decreasing MgO), cannot be used to support this conclusion because the trends of
decreasing CaO/Al2O3 can also be generated by mixing of high-silica magmas that have
very low CaO/Al2O3 with a variety of different basaltic melts with higher CaO/Al2O3
(Figure S1).
S2. Timescales of Decreased Magma Supply at the OSC
In the absence of any data suggesting increased cooling rates, we have argued that
melt supply to the shallow melt lens must decrease, at least temporarily, for high-silica
lavas to form at the OSC [Wanless et al., 2010]. Placing quantitative constraints on the
timescales over which this occurs at the OSC is difficult; however, there are several
studies that provide estimates for dacite formation in other settings. At the Juan de Fuca
Ridge, crystallization of a melt from a basalt to a dacite was estimated to have occurred
over ~10-20 ka at most [Schmitt et al., 2011], thus placing an upper bound on the time
required for dacite formation at MORs. In 2005, dacitic melt was discovered during
drilling of a hydrothermal well along the lower east rift zone of Kilauea (Puna Ridge) on
the Island of Hawai’i [Teplow et al., 2009]. Historically, the lower east rift zone of
Kilauea has only erupted basaltic compositions and ~80% of it’s surface is covered by
basalt lavas that are <500 years old [Teplow et al., 2009]. The last eruption that occurred
along this section of the rift zone was in 1955. Teplow et al. [2009] hypothesized that the
dacitic melt evolved via fractional crystallization of the 1955 basaltic magma injected
into the rift zone ~50 years ago. While this high-silica melt formed in the rift zone of a
subaerial volcano, it may be analogous to a propagating ridge of an OSC, which suggests
that very short timescales are possible for dacite formation on MORs. However, the
thermal conditions at the OSC may be quite warmer than this subaerial rift zone setting,
so this is likely an extreme.
At the OSC, the young dacitic pillow mounds on the floor of the east limb axial
graben are surrounded by ferrobasalts, suggesting that the magmatic conditions beneath
OSC can produce both high- and low silica lavas over relatively short timescales. This is
consistent with U-series disequilibria estimates indicating that basaltic and dacitic lavas
collected within the axial graben all erupted within the last 8 ka [Waters, 2010].
Additionally, older (based on seafloor imagery and relative positions; Nunnery, 2008)
dacitic lavas are located at the top of the axial graben while basaltic lavas were found at
the base indicating that high-silica lavas are not restricted to a single time period and that
basaltic volcanism commonly occurs between dacitic eruptions. Additionally, the
intermittent replenishment of basaltic melts to the system must occur over short enough
timescales that the dacitic melts do not completely crystallize prior to mixing with
ferrobasaltic melts. Therefore, we suggest that the decreased melt supply to the axial
melt lens occurs sporadically but over relatively short timescales. The relatively short
periods of decreased magmatism at the ridge axis are not great enough, however, to affect
overall crustal thicknesses at the OSC.
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Figure S1: CaO/Al2O3 versus MgO for OSC lavas. EPR lavas from 8° to 10°N are shown
for comparison. Dashed lines show calculated [Danyushevsky and Plechov, 2011]
fractional crystallization pathways at 0.5, 1 and 2 kbars (see text for details and starting
parameters). Solid lines are binary mixing lines between high-silica lavas erupted at the
OSC and several different basaltic compositions. The decreasing CaO/Al2O3 ratios may
be explained by high-pressure fractional crystallization or by mixing.