As suggested by the name, the zircon xenocrysts must have come from an external source, in this case, possibly from crustal country rock. This happens when the peridotite had crystallized as cumulate at low pressure within the continent with mineral assemblage olivine+orthopyroxene+clinopyroxene+plagioclase as opposed to other varieties which had crystallized at medium to high pressure at greater depths.

Thank you Uriah for your comment

Such an explanation may be perhaps reasonable  for layered intrusions in crustal domains.

My question was related to Alpine-type peridotites, where the possible source of zircon can be the crustal material recycled in mantle . But in this case the P/T conditions in such an environment is far beyond the stability field of zircon and I can not imagine that they survive.

Best

Goncuoglu

Dear Mehmet,

Several contributions to the Special Session 15 at the 20th CBGA Congress in Tirana (where we both had great time quite recently) have been dedicated to the problems of many different minerals enclosed into chromitites. Most important - the contributions by P. Robinson and Chinese colleagues. They speak even about "collections" of minerals in chromite - simply because you find together such minerals as diamond and coesite, on the one hand, and zircon, kyanite, etc. on the other. "Collections" of minerals that are normally formed in different associations at depths differing in tens to a few hundred kilometers! I wish to draw your attention also to the contribution by M. Ohnenstetter at the same session: chromitites that have been presetved as lenses within extremely sheared biotite migmatites etc. crustal rocks that underwent very strong shear at medium pressure - medium to high temperature conditions (amphibolite facies). 

For me, for to have a full recrystallization of a given rock (or mineral) included into a geologically incompatible medium, besides the P-T conditions, you must have penetrative deformations plus (when needed) fluid circulation. Ultrabasic  and basic rocks seem to act as a very good "insullator" in any environment, and tend to preserve included uncommon minerals. In the case of zircons, they testify for more ancient events that are entirely obliterated in cases of hosting acid gneisses and migmatites. Recently, chromitites within metaperidotites in the Rhodopes have been described with zircons dated at c. 3 Ga whereas the nearby migmatites are dated at c. 300 Ma. Gabbros and norites occurring amidst migmatites both in the Central and East Rhodopes and in SW Bulgaria preserved zircons dated at 570 - 610 Ma, and in some cases, even more ancient zircons dated at around 3 Ga. 

In the literature, there were recent publications reporting the presence of very old zircons in recent ophiolites at the ocean ridges. Thus, the interpretation of any such occurrence may be quite difficult and unique by its own.

Best regards, Ivan 

Dear Ivan,

many thanks for your comment. Actually, I was stimulated by the presentations in CBGA 2014 (by the way, thanks for your kind words) to start a discussion on this issue. 

The fact is as you clearly express: they are there!

But how?

regards

Goncuoglu

I do not think that zircon xenocrysts in chromitites are necessarily outside their stability field.  Si concentrations are low in these environments, solubility of zircon in these melts will also be low and crystalline zircon is stable to extremely high temperatures.  But I admit I do not know the actual data for the stability from memory.  The experimental literature on zircon and baddeleyite may help.

As briefly mentioned in Zheng and Hermann (2014), both newly grown and relict zircon domains have been found in orogenic peridotites from continental subduction zones. These relict zircons exhibit older U-Pb ages than the subduction-zone metamorphic ages. They were erratically interpreted as xenocrysts in some publications, but substantially they were inherited from the metasomatized mantle wedge peridotites and thus eventually derived from subducted crust-derived fluids/melts. In this process, there are both the chemical transport of dissolved Zr and the physical transport of crustal zircon xenocrysts by either metamorphic fluids or anatectic melts at the slab-mantle interface in subduction channels (Zheng, 2012). While U-Pb ages for the newly grown zircon directly date the metasomatic event, its Lu-Hf isotope composition is primarily inherited from the metasomatic agent and thus has little to do with the composition of the overlying lithosphere. The survival of subducted crust-derived zircon in the orogenic peridotites suggests that the crustal metasomatism beneath the mantle wedge would have proceeded at kinetically limited conditions, and thus it did not achieve the thermodynamic equilibrium between crustal and mantle components.

Zheng Y.-F. (2012) Metamorphic chemical geodynamics in continental subduction zones. Chem. Geol. 328, 5-48.

Zheng Y.-F., Hermann J., 2014. Geochemistry of continental subduction-zone fluids. Earth, Planets and Space 66, 93.

I am not comfortable to make proposals in this field; nevertheless, I like Y-F Zheng point of view.

Dear Zheng,

thank you for the comments. Any data from SSZ-type material, where no indication for continental subduction has been detected?

Best

Goncuoglu

You may see this from  our publications and references therein. 

I am currently working zircon megacrysts (1 to 5 mm) from western Mamfe gem placers( see Kanouo et al., 2012: in Geology Resource Journal; Kanouo et al., 2014: submitted in this same journal; and Kanouo, 2014 submitted PhD research Thesis). In our research we use zircon external morphology, trace element geochemistry and U-Pb geochronology for provenance study. Some of the zircon grains are plotted in kimberlitic field of Belousova et al. (2002) and Veevers et al.(2006) zircon trace and rare earth element  field discriminating diagrams. Simple gemological test shows magmatic corrosions on kimberlitic related zircons. So, it is possible for zircon xenocrysts to be found in peridotites. But? you have to verify if they are from mantle origin (with very low Th and U content) or  from crustal origin. If from crustal origin, then they were sorted from the crustal derived country rocks  during magmatic couling. If from mantle source you  have to understand the local and regional geological setting (interpretation not really easy).

Kanouo makes a good point.  If mantle derived these zircon should have very low uranium as is seen in zircon from kimberlites.  Is anything know about trace element composition of these reported zircon?

Dear Professor CEMAL,

Your topic is quite interesting. If you can determine the  trace element abundance, couple  with fluid/melt, and mineral inclusion analyses in those zircons, the petrogenetic history of their crystallization  would be understood.  Zircon study is my current research interest. Have you determine their temperature of crystallization? see Watson et al. (2006), and others recent papers on zircon thermometry evaluation.

The primitive mantle is characterized by low abundances of HFSE, including Zr. In other word, Zr is unsaturated in peridotite of the ptimitive mantle, making the growth of primary zircon impossible. As a result, no zircon occurs in the primitive mantle. Nevertheless, both element Zr and tinny zircon grains can be carried by subducted crust-derived fluids/melts into the mantle wedge peridotite. In other words, peridotite Zr abundances can be locally elevated by crustal metasomatism at the slab-mantle interface in subduction channels. With the local enrichment of Zr in the mantle peridotite, very local Zr saturation may be achieved in the metasomatized mantle domains. Then zircon growth becomes possible under the conditions of Si saturation. The Zr-rich peridotites can be recognized by their arc-like trace element compositions, exhibiting enrichment of at least LILE and sometimes LREE. Furthermore, Zr is able to become saturated in fluids/melts that were derived from partial melting of the metasomatized mantle domains, leading to zircon growth at mantle depths. On the other hand, the crust-derived zircon grains may survive during the two episodes of mantle tectonism, i.e. the crustal metasomatism and the dehydration melting of the metasomatized mantle domains. Therefore, the old zircons in ophiolites and chromitites are inherited from the subducted crust-derived fluids/melts rather than captured from their crustal wallrocks during their emplacement. The newly grown zircons in kimberlites were reequilibrated with the host mantle peridotite in the presence of fluids/melts.

Polychronous Formation of Mantle Complexes in Ophiolites

G. N. Savelievaa, V. G. Batanovab, c, N. A. Berezhnayad, S. L. Presnyakovd, A. V. Sobolevb, c, S. G. Skublove, and I. A. Belousov

Abstract—The paper presents new determinations of the U–Pb zircon age of highAl chromitite from dunite of the mantle section of the Voikar–Synya massif at the Kershor site in the boundary zone with rocks of the dunite–wehrlite–clinopyroxenite complex. The highCr chromitite from dunite in the central part of the same massif contains zircon dated at ca. 0.6 Ga [10]. It is suggested that Paleoproterozoic (2.0–1.9 Ga) zir cons from chromitites of the mantle section near the petrological Moho boundary were formed in the course of partial melting of peridotites and/or their interaction with migrating MORBtype melts. The occurrence of Vendian and Paleoproterozoic zircons in chromitites from different parts of the mantle section, as well as previously published petrological, geochemical, and geological data [2, 11, 22] allow us to suggest a complex multistage evolution of the mantle section in ophiolites. The arguments stated below show that chromitites and host dunites could have been formed at different times and were probably related to different processes. Thus, not only various complexes of the prePaleozoic oceanic crust reworked in the suprasubducti