How do peridotites form

Indeed, a few peridotites from back-arc occurrences in Japan, the Cascades, and Patagonia are very similar to hot peridotite residues Fig. Indeed, xenoliths from Patagonia Laurora et al. Clearly, some peridotites from back-arc regions have survived the effects of melt—rock reaction. Peridotites from subduction zones that have the lowest FeO T and highest mg -numbers might be characteristic of first-stage residues before they become affected by melt—rock interaction.

As first-stage residues from subduction zones are similar in composition to hot residues expected in plumes, they might have formed in buoyant oceanic plateaux. Niu et al. Oceanic plateaux might become buoyant platforms on which new continental crust can accrete e. Jordan, , ; Ben-Avraham et al.

The petrology of arc peridotite residues reported here provides strong support for this class of model. However, buoyant residues might also have been produced in hot ridge type environments during the Archean, an alternate possibility that is examined below. Many peridotites from active subduction zones and cratonic mantle are very similar in displaying orthopyroxene enrichment relative to hot residues Figs 5 and 6. There is, however, an important difference as shown in Fig.

Opx-rich cratonic mantle tends to be low in FeO T Fig. In contrast, some Opx-rich subduction zone peridotites are rich in FeO T i. Itinome-gata , whereas others are not i. Luzon arc, Patagonia back-arc. Open symbols are model residues. Closed symbols are too Opx-rich to be residues.

See captions to Figs 5 and 6 for detailed explanation of symbols. Orthopyroxene-rich cratonic mantle has been interpreted as a mixture of residues and cumulus orthopyroxene Herzberg, , , and as a reaction product of residues with a silica-rich melt Kelemen et al.

Kelemen et al. Recent progress in numerical simulations indicates that elevated mg -number in peridotite might be a possible consequence of the interaction of a basaltic melt with a first-stage residue Bedini et al. Melt—rock reaction models for cratonic and subduction zone mantle must, therefore, differ in terms of the melt or fluid compositions that originate from the slab and mantle wedge; these differences have not yet been explored in forward numerical simulations.

If progress cannot be made in simulating the formation of FeO-depleted and Opx-rich cratonic mantle by melt—rock reaction, then alternative models must be preferred e. Herzberg, ; Francis, Plotted in Fig. Many ophiolitic peridotites differ from abyssal peridotites in being depleted in Al 2 O 3 and enriched in SiO 2. They are similar to peridotites from subduction zones; however, SiO 2 enrichments are not as extreme. Peridotites in ophiolite sections cannot, therefore, be single-stage residues of fertile peridotite.

They also cannot be second-stage residues of a depleted mantle source as shown in Electronic Appendix 4 http:www. These results favor models of Tethyan ophiolite formation in back-arc basins and subduction zones rather than open oceans e.

Robertson, Peridotites from the mantle sections of Tethyan ophiolites compared with model residues formed by fractional melting of fertile peridotite KR Many samples are too rich in SiO 2 and too poor in Al 2 O 3 to be residues of fertile peridotite, similar to peridotites from active subduction zones. CaO and Al 2 O 3 can be fractionated from each other in komatiites that had augite and garnet in the residue.

However, fractionation is not significant when melting occurs with residual harzburgite or dunite e. Nesbitt et al. Herzberg, Consequently, if we consider this parameter alone, the residue of aluminum-undepleted komatiites could be either harzburgite Nisbet et al.

To distinguish between these possibilities, an examination is made of FeO T and MgO contents of komatiites. Open crosses, peridotite compositions. Liquid and peridotite compositions are given again in Tables A1 and A3 in Electronic Appendix 1 http:www. Shore, personal communication, Shown in Fig. Olivine was incrementally added to and subtracted from representative sample Z5 from Belingwe Bickle et al. Compositions of liquids are shown as the gray and black trajectories, and these are connected to coexisting olivine compositions.

The coherent negative sloping trend displayed by aluminum-undepleted komatiites from three continents is coincident with the two trajectories, demonstrating that they can be successfully modeled by olivine fractionation. MgO and FeO T contents of aluminum-undepleted komatiites compared with magmas and their coexisting equilibrium olivine compositions.

Komatiite database is given in the Fig. Olivine was incrementally added to and subtracted from representative sample Z5 from Belingwe shown as the open cross Bickle et al. Tie-lines connect liquid compositions in each trajectory with coexisting olivine compositions. To evaluate the residuum mineralogy, the assumption is made that the estimated parental magma compositions shown in Fig. Primary magmas that exit the melting regime in the mantle must erupt directly to the surface without crystallization for this assumption to be valid.

Results in Fig. All liquids produced by equilibrium melting are too low in FeO T compared with the primary komatiite magma. The match is improved for the case of accumulated fractional melting where olivine is the only residuum phase Fig. There are, however, several possible difficulties with this model. Komatiites are rare in comparison with basalts in Archean greenstone belts, and it is reasonable to expect that residuum dunites would be even more rare, given that they resided in the mantle; they may also have been subjected to modification by iron-rich magmas in the mantle.

Alternatively, dunites having mg -numbers of 98 may not be observed because fractional melting of fertile peridotite may be an inappropriate model. Black trajectory is the range of derivative liquids produced by fractional crystallization of olivine from the primary magma shown, and describes the alumina-undepleted komatiites in Fig.

Another model is shown in Fig. The abyssal peridotite source used in Fig. Total iron contents of depleted and fertile source compositions can be very similar despite the large variations in Al 2 O 3 as shown in Fig. A depleted source is required from observed light rare earth element LREE and isotopic depletions Arndt, ; Bickle et al.

Inspection of Fig. Accumulated fractional melting is not a possible model solution because it would form liquids with FeO T contents higher than that of the komatiite primary magma.

The residue would have been pure dunite with an mg -number of 94; harzburgite is not an acceptible residue. Olivines approaching mg -numbers of 94 have been reported for cratonic mantle xenoliths Boyd et al. However, these are rare and they occur in harzburgites rather than dunites. The lack of dunite residues of aluminum-undepleted komatiites with mg -numbers of either 94 or 98 does not permit a determination to be made about whether melting was equilibrium or fractional. MgO and FeO T contents of potential primary magmas produced by equilibrium and accumulated fractional melting of a depleted peridotite composition given in the text.

High ratios must reflect the source composition when olivine and harzburgite are residues because CaO and Al 2 O 3 fractionation is not significant e. Parman et al. This model requires an unusual source composition that is not observed in fertile peridotites Fig. Other difficulties with this model have been discussed elsewhere Arndt et al. Open cross, peridotite KR This crystalline assemblage is likely to transform in the subsolidus to an assemblage of garnet harzburgite with minor augite.

Dots, Proterozoic mantle peridotites Herzberg, Barberton alumina-depleted komatiites are similar in composition to experimental anhydrous liquids in equilibrium with olivine, subcalcic clinopyroxene, and garnet [i. Residues of aluminum-depleted komatiites have not been reported, and this might again reflect the general rarity of komatiites in Archean greenstone belts.

There is a rough complementarity between the major element geochemistry of Archean komatiites and cratonic lithospheric mantle xenoliths, an observation that has led many workers to propose a genetic link O'Hara et al.

In particular, cratonic lithospheric mantle is highly depleted in incompatible elements, consistent with it being a residue from high degrees of partial melting. However, with the exception of Herzberg , none of the above workers provided quantitative mass balance calculations with respect to any particular mantle source.

Herzberg noted that equilibrium melting of fertile peridotite produces melts with FeO contents lower than those of Al-undepleted komatiites, a misfit that is reproduced in this study Fig.

The suggestion that this misfit might be explained by some form of fractional melting Herzberg, appears to hold Fig. Petrographic descriptions provided by these researchers are in good agreement with whole-rock data that plot in the compositional fields defined by harzburgite and lherzolite, not dunite Fig.

The few cases of dunite that have been reported are shown in Fig. In contrast, the inferred residue of aluminum-undepleted komatiite is dunite with mg -numbers of 94 and 98 for equilibrium and fractional melting models, respectively.

Residues of aluminum-depleted and -undepleted komatiites have never been observed as xenoliths of cratonic mantle peridotite. It is concluded, therefore, that cratonic lithospheric mantle did not form as a residue from Archean komatiite melt extraction. How then do we explain its geochemistry? One way to explore this problem is to seek alternatives to Al-depleted and -undepleted komatiites. This is examined in the next section.

Primary magmas are inferred from peridotite residues using equation 3 in the text. Primary magmas inferred from both peridotite residues and olivine-phyric eruptives are very similar.

Variations in the MgO contents of primary magmas are mostly produced by variations in mantle potential temperature, and those shown in Fig. This is another way of showing that there is no petrogenetic connection between Archean cratonic mantle and aluminum-undepleted komatiites see above. If cratonic peridotite is so similar in composition to residues expected from plume occurrences of Phanerozoic age, what does that say about the plume model of cratonic mantle formation Herzberg, , ?

These primary magmas are very similar to Phanerozoic plume-related magmas such as Kilauea, Gorgona, and early Tertiary Iceland, and to those that were complementary to cratonic mantle Fig.

Petrographic and geochemical characteristics were used to suggest that the Chukotat volcanics formed during lithosphere rifting and extension, and that they record a change from continental to oceanic environments Francis et al. If this is correct, then the geodynamic implications are significant.

The Cape Smith volcanics might have formed strictly in a hot ridge if extension was not associated with the impact of a plume. It further suggests that some cratonic mantle of Proterozoic and Archean age might be composed of residues that also formed in hot oceanic ridge environments, with mantle potential temperatures that are characteristic of modern plumes i.

This situation might be very different from the Tertiary North Atlantic igneous province, where rifting along the Greenland margin was associated with the eruption of picrites from the Icelandic plume e. Saunders et al. Indeed, it may be more analogous to the more common situation further to the south where rifting and formation of the Atlantic Ocean occurred without eruption of picrites and komatiites.

This is explored further in the next section. These primary magmas might have differentiated to common basalt that characterizes greenstone belts. Constraining the MgO content of the primary magma for aluminum-depleted komatiites is more difficult, and will be examined elsewhere. As aluminum-undepleted komatiites show no evidence for the involvement of significant amounts of magmatic water Arndt et al.

Unfortunately, the potential temperature is difficult to constrain. They are also conditions where the densities of komatiite and its residual olivine become similar, and this will favor equilibrium melting over fractional melting Arndt, At some stage, eruption of dense komatiite magma will probably follow the buoyancy laws of a feather captured in an upwelling thermal rather than an olivine crystal in a laboratory experiment Herzberg, , and equilibrium melting might dominate.

Fractional melting will gain in importance when the plume rises to higher levels, where magmas become significantly buoyant with respect to their coexisting crystals. The ways in which phase equilibria and density impose an imprint on the geochemistry of komatiites in deep plumes are likely to be complex, and readers are referred to Arndt for a discussion of some possibilities.

This paper ends with a conjecture. These temperatures would have been sufficient to produce deep and extensive melting that characterizes aluminum-undepleted komatiites. The petrology of peridotite residues varies according to the geodynamic setting in which fractional melting takes place.

Residues formed by high melt fractions and low final melting pressures are universally elevated in MgO. These effects have been calibrated by a parameterization of experimental data for fertile mantle peridotite, and the results have been applied to natural mantle peridotite samples.

The following are specific conclusions. More fertile peridotites had minimal melt extracted. A few peridotite xenoliths from the Japan, Cascades, and Chile—Patagonian back-arcs appear to be precursors that have escaped some of the effects of melt—rock reaction.

They are similar to hot residues formed in plumes, in agreement with the suggestion made by Niu et al. However, FeO T contents are lower, and it remains to be determined whether they can be successfully explained by melt—rock reaction models e. The primary magmas may have crystallized to common basalts in Archean greenstone belts. They left behind dunite residues with mg -numbers of 94 and 98 calculated for equilibrium and fractional melting, respectively.

Dunites with these compositions have not been reported, possibly reflecting the rarity of komatiites in Archean greenstone belts, or modification by iron-rich magmas in the mantle. These residues have also not been reported. Special thanks go to Mike O'Hara for a stimulating 30 year correspondence on the petrology of mantle peridotite.

Abbott, D. The structure and geochemical evolution of the continental crust: support for the oceanic plateau model of continental growth. Reviews of Geophysics , Supplement, — Agee, C. Crystal—liquid density inversions in terrestrial and lunar magmas. Physics of the Earth and Planetary Interiors , 63 — The growth of continental crust. Tectonophysics , 1 — Aoki, K. Pyroxenes from lherzolite inclusions of Itinome-gata, Japan.

Lithos 6 , 41 — Toggle navigation. Peridotite Facts Peridotite is an igneous rock, hard and rough, made up mostly of the minerals olivine, pyroxene, and amphibole. It is coarse grained, dark-colored, and ultramafic, which means it has very low silica content compared to other igneous rocks though is rich in minerals. Peridotites often contain chromite, and ore of chromium, and they can be a source for diamonds, which makes them economically important.

In addition, they have the potential to be used as a material for isolating carbo dioxide. A large portion of the upper part of the Earth's mantle is thought to be composed of peridotite.

Interesting Peridotite Facts: The word peridotite comes from the gemstone peridot consisting of pale green olivine. Wherlite with pyrope red and Cr-diopside green. Garnet lherzolite: xenolith from a kimberlite pipe, Kimberley, central South Africa.

Verde Sao Vicente C. Cumulate Adcumulate Orthocumulate. Larvik complex Ekerite Larvikite. Oka complex Niocalite carbonatite Monticellite carbonatite.

Accessory phases include garnet, spinel, plagioclase, ilmenite, chromite and magnetite. The aluminuous phase present in mantle peridotite changes with pressure with plagioclase present at low pressure, spinel at intermediate pressure, and garnet at high pressure. Ophiolites : An ophiolite is a large slab of oceanic crust, including part of the mantle, that has been overthrust onto continental crust at a convergent plate boundary. These structures bring large masses of peridotite up to Earth's surface and offer a rare opportunity to examine rocks from the mantle.

Studies of ophiolites have helped geologists better understand the mantle, the process of seafloor spreading, and the formation of oceanic lithosphere. Pipes : A pipe is a vertical intrusive structure that forms when a deep-source volcanic eruption brings magma up from the mantle. The magma often breaks through the surface, producing an explosive eruption and a steep-walled crater known as a maar. These deep-source eruptions are the origin for most of the Earth's primary diamond deposits.

The magma that forms the pipe is thought to ascend rapidly from the mantle, tearing rocks free from the mantle and from the walls of the pipe. These pieces of foreign rock are known as "xenoliths. Xenoliths provide the only way that diamonds can ascend from the mantle to the surface without being melted or corroded by the hot magma. Dikes and Sills : Dikes and sills are intrusive igneous rock bodies. Some of them are composed of peridotite that was sourced from deep within the Earth.

When they are exposed by erosion, they provide another way that peridotite from great depth can be observed at Earth's surface. Certain types of garnet , along with chromite and ilmenite , can be indicator minerals for diamond prospecting.

Public domain image by Woudloper. The formation of diamonds requires very high temperatures and pressures that only occur on Earth at depths of miles below the surface and at locations in the mantle where temperatures are at least degrees Fahrenheit. The diamonds are delivered to the surface in pieces of rock, known as xenoliths, which are torn from the mantle by deep-source volcanic eruptions.

When the mantle material approaches the surface, an explosive eruption occurs that forms a pipe-shaped structure that might be several hundred yards to over a mile in diameter. These "pipes," the rocks that are blasted from them, and the sediments and soils produced by their weathering are the source for most of Earth's natural diamonds. The best way to learn about rocks is to have specimens available for testing and examination. Some peridotites contain significant amounts of chromite.

Some of these form when a subsurface magma slowly crystallizes. During the early stages of crystallization, the highest-temperature minerals such as olivine, orthopyroxene, clinopyroxene, and chromite begin to crystallize from the melt.

The crystals are heavier than the melt and sink to the bottom of the melt. These high-temperature minerals can form layers of peridotite on the bottom of the magma body. These are known as "stratiform deposits.