Jakub,

It is rather difficult, at least for me, to reply to you answer, that appears to be too generic to be answered with a simple and clear statement on the state of art on you problem.

I give to you some informations that are based on my experience of the natural laboratory of the ophiolitic peridotites from the Alps and Apennines (N-W Italy), and, accordingly concern the rifting stages of opening of the fossil Jurassic slow-ultraslow Ligurian Tethys basin.

I can say that :

1) lherzolites (and particularly fertile spinel – to plagioclase lherzolites), most probably derived from the sub-continental mantle lithosphere during the rifting stages of the basin, are exhumed and eventually exposed at the sea-floor close to the Ocean-Continent Transition (OCT) Zones of the rifted margins;

2) harzburgites (and, particularly. depleted reactive spinel harzburgites) frequently associated to large volumes of impregnated plagioclase peridotites, progressively replace sub-continental lherzolites ocean-wards. They are interpreted as products of melt/peridotite reaction caused by porous flow infiltration and percolation of early asthenospheric melts during the pre-oceanic magmatic rifting stages of the future basin.

3) refractory harzburgites, formed by oceanic partial melting and parental to the subsequent oceanic MORB magmatism of the basin, are doubtly described at the sea-floor during the embryonic stages of oceanic spreading.

In our cases, evident chemical contrast exist between fertile lherzolites, reactively depleted harzburgites and refertilized plagioclase peridotites, depending on the processes of formation, both melting and melt/rock interaction.

I don’t know if more informations on these situations and geodynamic settings are useful to your problem.

In the case, write again and I’ll give you appropriated references.

Giovanni

I am sorry, Giovanni. Perhaps, I was not clear enough. I am refering to a mid-ocean ridge setting with abyssal peridotites, such as found in oceanic core complexes (such as the Atlantis Bank, the Kane Megamullion, the Atlantis Massif etc) or along rift valleys. I am interested how harzburgites and lherzolites are related spatially to each other. I can see that there is clearly a pattern in Your tectonic setting, which is an "embryonic oceanic lithospere within continental lithospere" as I suppose (many thanks for this interesting description!). However, I would appreciate any information on the spatial relationship between harburgites and lherzolites within the mature oceanic lithospere. 

Dear Jakub,

Well, this is a rather broad compositional spectrum of peridotites from lherzolite to harzburgite that form in ultra-slow spreading and ultra-fast spreading ridges, respectively. This spectrum therefore is also imprinted in their geochemical composition, for example from fayalitic (ultra-slow) to forsteritic (ultra-fast) composition of olivine. In ultra-slow spreading ridges the composition is almost enriched lherzolite coming from a deeper hot zone, but you can see the CPX-free depleted harzburgite from shallow reservoirs beneath the ridge and vice versa.

Accordingly, I think you can use a set of major, trace, and REE elements to distinguish them. You should also keep this in mind that in-situ analyses of a mineral proxy (e.g. OL, PX, and spinel group) would help you much more than the whole rock composition.

Serpentinization of these prodotites are also different that I think you are not interested in this issue. 

Kind regards,

Rasoul

Many thanks for all the contributions. The two references regarding the spatial distribution, given in the last post, have shed some light on my question.

Whereas it is true in general that lherzolite is associated to slow spreading centers, and harzburgite to fast spreading centers (answers by Rasoul and Martin pointing the 1985 Boudier & Nicolas EPSL paper), my understanding is that you are more specifically interested in the occurences of both rock types in slow spreading centers only; and it is true that eventhough slow spreading centers are dominated by lherzolite occurences, they do contain harzburgite.

I suppose that unfortunately it was difficult for you to know which of your samples is structurally above the others (they do not come from core logging at a core complex?). That would have been helpfull.

My understanding from ophiolite studies is that away from core complexes you might have any of the two types at shallow levels:

-harzburgite can be present because you end up with the most depleted rocks at the top of the melting column, or -where the Moho transition forms, 0-1 km beneath the Moho- because melt reacting with more fertile peridotite will dissolve cpx (forming harzburgite) and may even dissolve cpx + opx (forming dunite).

-lherzolite can be present because peridotite is not as depleted as in fast spreading centers (as was first thought in the 1985 paper aforementioned), or because  harzburgite is "refertilized" by impregnating melt (as more recent studies tend to show, and as is obviously the case in the shallowest mantle, in parts of the Moho transition zone).

So the situation is not simple...

And not simplier in core complexes which show several varieties (Atlantis Bank, SWIR with more gabbro; Atlantis Massif, MAR with more peridotite).

In the Mirdita ophiolite where a peridotite-rich core complex seems to be preserved, the detachment seems to root into the Moho transition zone; it is made of mylonite lherzolite (not primitive lherzolite, but melt impregnated peridotite) which is structurally above coarse grained harzburgite (which preserved asthenospheric flow).

Nicolas et al, 1999, JGR, V104, pp15,155-15,167 have precise maps of the Mirdita opholite.

Boudier and Nicolas, 1995, JPet, V36-3, pp 777-796 explain the Moho transition zone (in another context but the same processes occur at slow spreading centers)

LeRoux et al, 2007, EPSL, 259, pp599-612 shows an exemple of refertilized peridotite (Lherz !) which might help you.

Good luck with your study.

Jakub,

certainly the suggestion of Jean Louis represent an important starting point, but I want to recall that  more recent papers from swiss and italian teams evidenced that most of the peridotites exposed at South Lanzo (and even North Lanzo) are products of melt/peridotite interactions (both reactive spinel harzburgites and refertilized plagioclase peridotites) rather than variably refractory residua after partial melting.

And we are dealing again with an "embryonic oceanic lithospere within continental lithospere", as you properly noted before.

Moreover, I guess that accurate petrographic-structural-geochemical surveys are needed to reach a sufficient confidence to recognizing the genetic process, if partial melting or melt/peridotite interaction, before using bulk rock and even mineral geo-chemical data.

This seems to be relatively easier in on-land ophiolite peridotites, representing in nature former oceanic mantle lithosphere, and it will be certainly more difficult with oceanic samples, in most cases too altered to allow these investigations. But rather fresh samples have been drilled from oceanic core complexes, so knowledge should be improved for the needs of Jakub.

Good luck

Giovanni

Thanks Giovanni for this precision. When I  wrote "...or because harzburgite is "refertilized" by impregnating melt (as more recent studies tend to show..." that was exactely what I was thinking about. However I am not very familiar with those studies (except for the Leroux paper about Lherz).

Could you please point us one or 2 key paper(s) from the Swiss and Italian teams that precisely treat this aspect of lherzolite formation?

Best wishes

Certainly David,

and you can find more papers on this topic quoted in the reference list of the papers.

Best Regards

Giovanni

Thank you very much for this bunch of contributions, especially for David who noticed, what is essential for my issue, namely that it is difficult to know how harzburgites and lherzolites are spatially distributed within the mantle, based on dredges and dives. A borehole through the mantle in an OCC would be very helpful, but we probably need to wait for IODP expedition #360 (and surely not the first leg) to get first cores of the mantle in situ. Atlantis Massif, as pointed out by David, with more peridotites from the dives (the P/G ratio 2.1, Karson et al. 2006) than Atlantis Bank (the P/G ratio 0.6; Zhou and Dick 2013). However, cores from borehole 1309 exhibit only few harburgites. Some lherzolites which occur, contain much plagioclase (Tamura et al., 2008), what makes me believe they are refertilized harzburgites. My question regarded rather the distribution of primary lherzolites, formed directly after the partial melting process. Unfortunately, my study area, the Kane Megamullion, with 3 tons of rocks collected and peridotite dome in the middle part (Dick et al. 2008), has not been drilled at all.

Coming back to the issue of refertilization, I am happy this came out anyway. My lherzolites are spinel lherzolites, and I have believed so far that they would be primary. Nevertheless, considering your remarks and examples (thanks to Giovanni, Martin, and David again), I will reconsider that, based on their mineralogy and the references you delivered, in order to be sure whether or not my lherzolite are primary or formed by refertilization of harzburgites.

Thanks for all the wishes of good luck!