Do you know a paper that describes the proportion of metasomatic veins (or of pyroxenites) in the lithospheric mantle based on field observations?
Hi Sarah,
Roughly 15 out of 100 abyssal peridotite samples I've looked at are crosscut by "gabbroic" veins. Most are smaller than 1cm wide, many smaller than 1mm (unpublished). Many are highly evolved, so from the perspective of whole rock analysis, this creates an ugly overprint. Also, because of their thinness, they will have a misleadingly high Mg#.
The more mantle-like metasomatic veins, like the kind of pyroxenites you may have seen discussed in a manuscript recently, are less common. Don't have a number for you, but I would guess less than 1 percent of the oceanic mantle peridotites are crosscut by pxites, or were collected along with pxites.
Henry would know. Henry? Is there a poke function on this thing?
One additional question on this topic is the origin of "dykes" observed in layered xenoliths. Do they represent injected material (into peridotite host) prior to eruption of the alkali-basaltc volcanism? Or something different?
what are the evidences for polybaric crystallization. What is the P-T range of this process? and what are the timescales?
Thank you for your answers.
The (quasi) absence of pyroxenites associated with abyssal peridotites is often explained by the fact that pyroxenites would be completely melted when the mantle reach the surface. As Martin suggest, I suspect that the same process can affect the proportion of pyroxenites veins in basalt-borne or kimberlite-borne xenolith suites. Also, the melting residue of pyroxenites at low/moderate pressures (1 GPa) is similar to a peridotite in terms of mineralogy. Hence, some pyroxenite residue may be interpreted as peridotite.
So, I'm looking for papers that gives the proportion of pyroxenites (ie., pyroxenite/peridotite ratio) in ultramafic massifs (often interpreted as subcontinental lithosphere) and in ophiolite. In the literature that I found, there are very precise petrologic and geochemical descriptions of these veins but there is no estimation in terms of proportions.
Salut Sarah,
The first estimate was made in 1969 by Jacques Kornprobst at Beni Bousera (~5%).
Kornprobst, J. (1969). Le massif ultramafique des Beni Bouchera (Rif interne, Maroc): Etude des péridotites de haute temperature et de haute pression et des pyroxenolites, à grenat ou sans grenat, qui leur sont associées. Contrib. Mineral. Petrol. 23, 283-322.
But there are several more recent ones with similar values.
à+
Lyde
Salut Lyde!
Thanks! Could you give one or two of these references please?
Salut,
Another one, slightly more recent is Pertermann and Hirschmann, 2003.
If I remember well, they proposed, based on isotopic considerations, that less than 2% of the mantle is made of pyroxenites.
Pertermann, M., & Hirschmann, M.M. (2003). Partial melting experiments on a MORB-like pyroxenite between 2 and 3 GPa: constraints on the presence of pyroxenite in basalt source regions from solidus location and melting rate. J. Geophys. Res. 108 (B2), 2125. doi:10.1029/2000JB000118.
Chers,
Lyde
I totally agree. It's what I try to do: compare results from geochemistry and experiments with actual observations.
I sense that there is a bit of confusion related to the term 'pyroxenites'. Most mantle geochemists would associate this lithology with some sort of recycled oceanic crust, after it's been through the subduction factory and possibly some Sobolevian reaction process between eclogite-derived melts and peridotites (e.g. Sobolev et al., Science 2007). There are several geochemical signals observed in erupted melts that are best explained by such a recycled component, which most of us would think of as (grt-) pxites. Experiments support this too.
... But...
The pyroxenites we can study in the mantle sections of ophiolites and on the ocean floor are probably not recycled pyroxenites. One notable exception may be the grt-pxite layers in the N-Apennines (Montanini et al., EPSL 2012).... The abyssal pxites are not, and they are not residual either, as they tend to be quite fertile in terms of major elements, with mildly LREE depleted traces. Geochemically, they could be cumulates, crystallized from a melt that is no longer there. Some could be reaction products between peridotite and a melt that was not trapped. Yet another way to potentially produce pyroxenites, proposed by Dick and Sinton in 1979, is pressure dissolution creep. I am not aware of a study that discounts that hypothesis.
In a nutshell, taking estimates from observed pyroxenite abundance in the mantle may not give you a reliable estimate to compare with the abundance required by geochemical models, I think. The different origins mentioned above would give you an upper bound. The preferential melting of pyroxenites before the onset of peridotite melting during upwelling would do the opposite.
Eric