Extraordinary deep origin (≥ 350 km) and hot melting (≥ 1800 °C) of an Archaean orogenic peridotite massif in Norway

Decompression microstructures preserved in the mineral garnet from a Norwegian orogenic peridotite massif exposed on Otrøy Island revealed that this mantle fragment was exhumed from depths ≥ 350 km close to that of the Mantle Transition Zone (410 km). This peridotite massif represents one of the deepest kilometre-size mantle rock units discovered at the Earth's surface. The recognition of this extraordinary depth of origin enables the detailed study of the deep Earth now at the surface, in terms of the spatial relationship of different rock types (garnet-peridotite, spinel-peridotite, garnet-pyroxenite, websterite) that may vary from the deep Upper Mantle. Such studies have been impossible to do so far on mantle xenoliths.

The peridotites contain coarse polycrystalline garnets with pyroxene in two microstructural positions, i.e., inter-crystalline small grains and intra-crystalline tiny needles. Both types of pyroxene have been previously interpreted to be exsolved from a garnet-like precursor mineral (majorite) that is stable only at confining ultra-high pressure conditions. These microstructures were studied in a collaborative project among Utrecht University, CNR-Istituto di Geoscienze e Georisorse-Unità di Pavia and Free University in Amsterdam.
Our results are:

1) The quantification of both microstructures yielded in one polycrystalline garnet sample > 20.6 vol% of pyroxene. This high amount of pyroxene - if interpreted as exsolved - corresponds to an unexsolved majoritic precursor that is stable at a minimum depth of 350 km. Such a claim needed a test.

2) An exsolution origin of pyroxene from a former majorite can be tested on the concentration and partitioning of Rare Earth Elements (REE: La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Yb) in clinopyroxene and associated garnet, because the REE are able to mirror the mineral origin and the initial temperature environment. A crucial part for the interpretation forms therefore the possibility to analyse the pyroxene needles. This microanalytical study was performed successfully by Secondary Ion Mass Spectrometry (SIMS) at CNR-IGG (Pavia). The investigations, carried out with an ion-probe Cameca IMS 4f, resulted to be for the needles at the limits of the instrumental capabilities in terms of both spatial (few-µm size) and compositional (<< 1 μg/g REE) resolution. The SIMS results are in excellent agreement with the REE contents measured in garnet and mm-size pyroxene grains by Laser Ablation Mass Spectrometry (LA-ICP-MS) at Utrecht University.

Inter-mineral REE partitioning provides evidence for a high-temperature origin of all clinopyroxene. The equilibrium REE partitioning between garnet and clinopyroxene is strongly dependent on atomic number, pressure and temperature with about one order of magnitude difference in light-REE between high (~ 1300 °C) and low (~ 950 °C) temperatures. Analysed clinopyroxenes in this work have Ce contents between 8-13 times those of the host garnet. These values are at the high-temperature end of both mineral-partitioning experiments and values recorded in high-temperature (≤ 1380 °C) natural peridotite xenoliths. Consequently, clinopyroxene in both microstructures demonstrates its initial equilibration with garnet at high temperatures (≥ 1300 °C).

3) Some garnets with relics of both microstructures have exceptionally poor concentrations of middle-REE (with chondrite normalised Dy/Yb of less than 0.07), an effect typical for the extraction of large amounts of melt (≥ 30%) during deep melting. The extremely light-REE-depleted nature of all analysed clinopyroxene minerals (with chondrite normalised Ce/Sm < 0.08) clearly establishes that no pyroxenes were in contact with a metasomatising melt or fluid, nor crystallised directly from a melt, all processes that would produce significantly less light-REE depletion. The pyroxenes have thus inherited their light-REE-depleted character from a majoritic precursor.

4) Mineral 143Nd isotopes were measured using thermal ionisation mass spectrometry (TIMS) at Free University in Amsterdam. The results enclose model ages of 2.5-2.9 billion years showing that the highly melt depleted garnet peridotites were formed in the Archaean era.
Formation of the highly melt depleted but garnet hosting peridotites in the Archaean is also supported by literature data on a whole rock Re-Os model age (3.3 Ga) from an Otrøy garnet peridotite. The Otrøy peridotites therefore appear to be the first recorded case of orogenic peridotites produced by extensive melt depletion in the garnet peridotite stability field during the Archaean era.

The simplest model, which includes all four results for the peridotite origin (deep, hot, melt depleted and old), requires peridotite melting during decompression like in a rising limb of a convecting Earth upper mantle. Melting started at enormously high temperatures (≥ 1800 °C) and depths (≥ 250 km) and continued until the ascending peridotites reached the lower levels of the oldest continents (~ 150 km). Residues remaining after such extreme conditions have not been reported before. The thermal evolution of the Earth shows that such assemblages were possible only when the Earth was young (hot) enough that melting could start very deeply. At the same time, the first continents (cooling already progressed) needed to exist that melting could stop during decompression at depth where garnet could not leave the system. If decompression continued garnet would be removed from the peridotites. Final tectonic transport of the ultra melt depleted mantle fragments from lower continental levels towards the surface occurred more than two billion years later during the continent-continent plate collision, which formed the Caledonian mountain chain (0.4 billion years ago) along Norway.

Consequently, the mineral microstructures in Norvegian peridotites are relicts that provide a remarkable 3-billion-year record of continental evolution.

Results are published in Nature, vol. 440, 13 April 2006

Spengler D., van Roermund H.L.M., Drury M.R, Ottolini L., Mason P.R.D., Davies G.R.: Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway. Nature, vol. 440, 913-917 (13 Apr 2006), Letter to Editor.