Saturday, March 31, 2012

Refertilization in the arc mantle: shallow vs. deep, pervasive vs. localized?

I meant to write a blog about the recent Rice Earth Science Dept. trip to Chile, but that never happened. However, Brandon Harper and I are working on a poster about the trip which will not only provide nice photos, but hopefully educate the viewer on some of the cool outcrops we saw, as well as give a big picture view of active continental margin geology.

I'm still trying to figure out the refertilization story with the Sierran peridotites. As we showed in our Journal of Petrology paper that was published just this month, many of the Sierran peridotites, particularly the deep, garnet-bearing ones, appear to be modally metasomatized. Whole-rock major element trends show excess CaO relative to primitive mantle and hypothetical melting trends, for instance, indicating the presence of excess cpx. We attributed these geochemical trends to influx of basaltic melts into depleted peridotites (we know they are depleted because Cr# in spinels are high, indicating high degrees of melting). It appears that melt infiltration overprinted a highly deformed matrix, as manifested by diffuse cpx and garnet-rich bands. The next step was to decipher 1) what the refertilizing agent was, and 2) at what conditions the refertilization occurred. 1) proved exceedingly difficult, if not impossible, because all the peridotites have undergone extensive cooling and subsolidus re-equilibration, thus any attempt to calculate a metasomatic melt in equilibrium with the cpx (if the cpx precipitated from such a melt) would be meaningless (unless there is some way to correct a cpx composition to solidus conditions, which I guess one could do using experimental D values, and then calculate a melt from there). To address 2), I think it would be interesting to look at the "other" Sierran peridotites - the shallower spinel peridotites. Why? First, a good number of the Sierran spinel peridotites appear refertilized themselves. That in itself is interesting, because we considered these to be true melt-depletion residues associated with arc magmatism. Also, the spinel peridotites may not have cooled as much as the garnet peridotites (some spinel peridotites preserve temperatures 200 degrees hotter than the garnet peridotites, ~1000 vs. 700 C). Most minerals in the spinel peridotites are unzoned chemically (although we need to verify this on the electron microprobe). The fact that the spinel peridotites have not undergone as much subsolidus cooling could be an advantage because the mineral compositions are closer to conditions under which refertilization (>1000 C) may have occurred. So, as a first step, I looked at some LA ICPMS data I had on cpx's in Sierran spinel peridotites:

Here, I plot cpx from 3 different spinel peridotites. The red line is a fine-grained, recrystallized sample, the orange line is a medium-grained, "transitional" (showing some recrystallization, but still preserving an original protogranular coarse texture), and the green line is a protogranular peridotite. CPX in the most deformed peridotite (the red one) appears to be most enriched in rare earth elements, whereas cpx in least deformed peridotite appears least enriched in REE. What would be interesting to plot next would be cpx mode vs. degree of enrichment of cpx. Could this suggest that more deformed samples experienced increased metasomatism/melt infiltration? For one thing, all the garnet peridotites (most of which are all refertilized) are extensively deformed, all having porphyroclastic textures and highly recrystallized matrix.

Anyhow, I think that revisiting (or visiting for the first time in detail, at least for me) the Sierran spinel peridotites could provide some more insights into the complex refertilization history of the Sierran lithosphere. We know that refertilization in the garnet peridotites had to have occurred prior to cooling, or else all textural disequilibrium features we observe (garnet coronas around spinel, exsolution lamellae, etc.) would be obliterated. But, the question is, at what depth did the refertilization occur? What is the mechanism: reactive fluid or melt percolation, trapped residual melt along deformation planes (causing "disequilibrium" crystallization of cpx and gt, a very localized phenomenon), a batch influx of melt? I compared the Sierran spinel and garnet peridotites (blue and yellow circles) to a suite of Fe-enriched lherzolites thought to have obtained Fe-enrichment by reactive melt percolation (Ionov et al., 2005).

This reactive percolation is expected to lead to low Mg# olivine, but the olivine in Sierran peridotites retained pretty refractory compositions. Another thing is that Cr#'s in spinel also still retain refractory compositions, despite the whole rock being decoupled from the melting trend. So, either something very localized within the sample is going on, maybe a trapped melt thing, because if melt was added wholesale to the peridotite, one should expect the Cr#'s and Mg#'s in olivine to be reset. Ack, it's hard to figure out!

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