Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds
Abstract
Occurrence and ClassificationFragments of the Earth's mantle are frequently transported to the surface via volcanic rocks that are dominantly alkaline in nature. These fragments range up to sizes in excess of 1 m across. The term "mantle xenoliths" or "mantle nodules" is applied to all rock and mineral inclusions of presumed mantle derivation that are found within host rocks of volcanic origin. The purpose of this contribution is to review the geochemistry of mantle xenoliths. For detailed petrological descriptions of individual locations and suites, together with their geological setting, the reader is referred to the major reference work by Nixon (1987).Despite peridotite xenoliths in basalts being recognized for several centuries and comparisons being made to lherzolite massifs (Lacroix, 1893), it was not until work on garnet peridotites and diamonds in kimberlites by Fermor (1913) and Wagner (1914) that such xenoliths were conceptually associated with a peridotite zone in the Earth beneath the crust, i.e., the zone that we now identify as the mantle. Mantle xenoliths provide snapshots of the lithospheric mantle beneath particular regions at the time of their eruption and hence are crucial direct evidence of the nature of the mantle beneath regions where no samples have been exposed by tectonic activity. As such, xenoliths are an essential compliment to tectonically exposed bodies of mantle (orogenic peridotites and ophiolites) that occur at plate boundaries (see Chapter 2.04). One obvious contrast between the mantle samples provided by xenoliths and those provided by peridotite massifs is the lack of field relationships available for xenoliths. Other drawbacks include the small size of many xenoliths. This makes accurate estimation of bulk compositions difficult and accentuates modal heterogeneities. The frequent infiltration of the host magma also complicates their chemical signature. Despite these drawbacks, xenoliths are of immense value, being the only samples of mantle available beneath many areas. Because they are erupted rapidly, they freeze in the mineralogical and chemical signatures of their depth of origin, in contrast to massifs which tend to re-equilibrate extensively during emplacement into the crust. In addition, many xenolith suites, particularly those erupted by kimberlites, provide samples from a considerably greater depth range than massifs. Over 3,500 mantle xenolith localities are currently known. The location and nature of many of these occurrences are summarized by Nixon (1987). A historical perspective on their study is given by Nixon (1987) and Menzies (1990a). Mantle xenoliths from any tectonic setting are most commonly described from three main igneous/pyroclastic magma types (where no genetic relationships are implied):(i) Alkalic basalts sensu-lato (commonly comprising alkali basalt-basanites and more evolved derivatives), nephelinites and melilitites.(ii) Lamprophyres and related magmas (e.g., minettes, monchiquites, and alnoites) and lamproites.(iii) The kimberlite series (Group I and Group II or orangeites; Mitchell, 1995).Although mantle xenoliths most commonly occur in primitive members of the above alkaline rocks, rare occurrences have been noted in more evolved magmas such as phonolites and trachytes (e.g., Irving and Price, 1981).To simplify matters and to circumvent the petrographic complexities of alkaline volcanic rocks in general, we will use the term "alkalic and potassic mafic magmas" to include alkalic basalts, nephelinites, melilitites, and lamprophyres. Occurrence of xenoliths in such magmas can be compared to those occurring in kimberlites and related rocks. As a general rule, the spectrum of mantle xenoliths at a given location varies with host rock type. In particular, alkalic and potassic mafic magmas tend to erupt peridotites belonging predominantly to the spinel-facies, whereas kimberlites erupt both spinel and garnet-facies peridotites (Nixon, 1987; Harte and Hawkesworth, 1989).Even within either "group" of volcanic rocks the variety of possible xenolith types is great. Table 1 presents a summary of the most common mantle xenolith groups that are found in kimberlitic hosts and within the alkalic and potassic mafic magmas. The significance and abundance of these groups will be discussed below. Table 1. Major groups of mantle xenoliths in kimberlite-related and alkali basalt series volcanic rocks (after Harte and Hawkesworth, 1989). Textural classification follows that of Harte (1977). Terminology for phlogopite-rich mafic mantle xenoliths from Gregoire et al. (2002). For supplementary data and classifications see Nixon (1987), table 62 TypeCharacteristicsExamplesMg# olivine (A) Cratonic/circum-cratonic xenoliths erupted by Kimberlite-related volcanics AI: Coarse Mg-rich, low-T peridotitesOften abundant. Mostly harzburgites and lherzolites with varying but low modal diopside and garnet. Wide range of orthopyroxene abundance, Kaapvaal examples notably enriched. Crystals typically 0.2 mm with equant or tabular shapes, irregular grain boundaries, rarely granoblastic (Harte, 1977). Bulk compositions typically highly depleted in Fe, Ca, and Al, enriched in Mg. Mineralogy: Cr-rich pyrope, Cr-diopside. Orthopyroxene in garnet facies characterized by >1.0 wt.% Al2O3. Cr-spinel sometimes evident. Minor phlogopite common grading into type VIII phlogopite peridotites. Phlogopite often surrounds garnet and is strongly correlated with the presence of diopside. Estimated equilibration temperatures less than 1,100 °C. Equilibration pressures can vary widely within a pipe and range from c. 2 GPa to >6 GPa. Rarely diamondiferous (e.g., Dawson and Smith, 1975), more commonly contain graphite ( Pearson et al., 1994).N. Lesotho (Nixon and Boyd, 1973a), Kaapvaal craton ( Gurney and Harte, 1980; Boyd and Nixon, 1978; Boyd and Mertzman, 1987; Nixon, 1987), Siberia ( Sobolev, 1974; Boyd et al., 1993); Jericho Slave craton ( Kopylova et al., 1999)Av 92.8 (91-95) Subcalcic garnet (high Cr-pyrope; knorringitic) bearing harzburgite varieties scarce but can contain diamond and graphite. Can be megacrystalline. Textures similar to type I. Equilibration temperatures and pressures intermediate between low-T and high-T lherzolites, i.e., 1,150 °C, 5-6 GPa, but vary widely.Udachnaya, Siberia (Sobolev et al., 1973; Pokhilenko et al., 1993), Kaapvaal ( Boyd et al., 1993)92-95.5 Spinel facies widespread but less abundant. Textures as for garnet variety, spinel texture symplectitic or irregular. Equilibration temperatures <800 °C. Can also be orthopyroxene enriched, like garnet facies. Spinel composition can vary widely in Cr# but mostly aluminous. Cr-rich spinels coexist with garnet. Orthopyroxenes in spinel facies have >1.0 wt.% Al2O3. Similar range in bulk composition to garnet facies.Kaapvaal craton (Carswell et al., 1984; Boyd et al., 1999)91.5-94 AII: coarse, Fe-rich low-T peridotites and pyroxenitesWidespread, normally rare but locally abundant. Mainly garnet lherzolites and garnet websterites but also clinopyroxenites and orthopyroxenites ("bronzitites"). Ilmenite can be present in pyroxenites. Coarse grained to "megacrystalline" (at Jericho). Textures and equilibration temperatures as for type I. Sometimes modally layered. Wide ranging bulk and mineral compositions, with high Fe, Ca, Al, and Na relative to type I. Rare fine-grained "quench textured" ilmenite/garnet pyroxenites.Matsoku, Kaapval craton (Gurney et al., 1975); Jericho, Slave craton ( Kopylova et al., 1999); Mzongwana, SE margin Kaapvaal craton ( Boyd et al., 1984a)83-89 AIII: dunitesWidespread, locally common. Two varieties: (i) Highly depleted, coarse to ultracoarse >50 mm olivine (megacrystalline) dunites, often containing chromite or sub calcic high-Cr pyrope and frequently diamondiferous. (ii) Often fine to medium grained more Fe-rich dunites, mineral zoning indicates "metasomatism." Mostly deformed textures. Orthopyroxene, garnet, phlogopite, diopside, chromite present.Siberia, notably Udachnaya (Pokhilenko et al., 1993)Kimberley ( Boyd et al., 1983; Dawson et al., 1981)93-9585-93 AIV: deformed low-T peridotites and pyroxenitesWidespread, locally common. Porphyroclastic or mosaic-porphyroclastic textures. Modal abundances, chemical characteristics and P-T equilibration conditions similar to those of type I.Jericho, Slave craton (Kopylova et al., 1999)91-95 AV: deformed high-T peridotitesWidespread but variable abundance in group I kimberlites, absent/scarce in group II kimberlites. Commonly deformed; porphyroclastic and mosaic-porphyroclastic textures with fine neoblasts of olivine. Although generally more depleted than pyrolite, bulk rocks and minerals generally enriched in Fe and Ti compared to type I (low-T) and significant compositional overlap of minerals with megacrysts (type X). Equilibration temperatures 1,100 °C to >1,500 °C, equilibration pressures generally 4.5 GPa to >6.5 GPa. Garnets and pyroxenes frequently zoned.N. Lesotho (Nixon and Boyd, 1973a); Jagersfontein, Kaapvaal craton ( Burgess and Harte, 1999); Siberia ( Sobolev, 1974; Boyd et al., 1993); Slave ( Kopylova et al., 1999); Somerset Island, Churchill Province ( Schmidberger and Francis, 1999)87-92 AVI: phlogopite-rich mafic mantle xenolithsWidespread and locally common. Olivine poor/absent rocks. Two main subdivisions of this group (Gregoire et al., 2002) are: (i) MARID suite (mica-amphibole-rutile-ilmenite-diopside) with accessory zircon common. Probable genetic link to group II kimberlites. Medium to coarse grained, undeformed to deformed, sometimes modal banding. Amphibole always K-richterite. (ii) PIC suite (phlogopite-ilmenite-clinopyroxene) with minor rutile. Diopside or Al- and Ti-poor augites. Probable genetic link to group I kimberlites. K-richterite is absent; grade to glimmerites as phlogopite mica reaches >90%. Coarse grained, variably deformed.Kimberley (Dawson and Smith, 1977; Gregoire et al., 2002)Kimberley ( Gregoire et al., 2002)NANA AVII: pyroxenite sheets rich in Fe and TiRestricted to Matsoku. Orthopyroxene and clinopyroxene rich rocks with widely variable olivine and garnet compositions, often with ilmenite and phlogopite (the IRPS suite: see type VIII). Bulk compositions Fe and Ti rich. Form magmatic intrusions (<16 cm thick) into type I rocks which become metasomatized.Matsoku (Gurney et al., 1975; Harte et al., 1975, 1987) AVIII: modally metasomatized peridotitesWide spread, variable abundance. Mostly metasomatized variants of type I. Diverse mineralogies, Two most commonly recognized groups are phlogopite peridotites (PP) and phlogopite-K-richterite-Peridotites (PKP) of Erlank et al. (1987). Can be harzburgite or lherzolite, typically coarse grained, undeformed but some display porphyroclastic textures. Assemblages vary with location. Cr-titanate "LIMA" minerals (Lindsleyite-Mathiasite) relatively common at Bultfontein; edenite-phlogopite association at Jagersfontein; ilmenite-rutile-phlogopite-sulfide (IRPS) suite at Matsoku associated with pyroxenitic sheets (type VII). Metasomatic clinopyroxene link to type AI.Matsoku (Gurney et al., 1975); Kimberley pipes ( Erlank et al., 1987; Gregoire et al., 2002); Jagersfontein ( Winterburn et al., 1990)Same as type I, to more Fe-rich. AIX: eclogites, grospydites, alkremites, and variantsVery widespread, rare to locally abundant. Eclogites (omphacite and pyrope-almandine garnet). Garnet composition widely variable, in grospydites garnet has a large grossular component. At some locations (e.g., Jagersfontein), unusual assemblages of garnet+spinel (Alkremites), garnet+corundum (Corganites) and corundum+garnet+spinel (Corgaspinites) occur (Mazzone and Haggerty 1989). Accessory phases in eclogites include kyanite, corundum, ilmenite, rutile, sanidine, coesite, sulfides, graphite and diamond. Eclogites classified on texture: group I large subhedral to rounded garnets in matrix of omphacite. High Cr, Ca, Fe, and Mn in omphacite. Garnets more Na (avg. 0.1 wt.% Na2O) and Mg rich. Group II have interlocking texture of anhedral garnet and omphacite and are less altered. Garnets are lower in Na2O (0.05 wt.%). Common hosts for diamond, especially group I. Not all eclogites of obvious mantle origin and some grade into garnet granulites and pyroxenites of crust origin.Roberts Victor (MacGregor and Carter, 1970; McCandless and Gurney, 1989); Jagersfontein ( Nixon et al., 1978; Mazonne and Haggerty, 1989); Orapa ( Robinson et al., 1984), all Kaapvaal craton. Udachnaya, Siberian craton ( Sobolev, 1974; Ponomarenko, 1975); Koidu, W. African craton ( Tompkins and Haggerty, 1984; Hills and Haggerty, 1989)NA AX: megacrysts (discrete nodules)Single crystals or monominerallic polycrystalline aggregates (sometimes exsolved) weighing up to 15 kg. Rare mutual lamellar or granular intergrowths. Large range in Mg#, Cr, and Ti in a given suite. Cr-poor variety: widespread, locally abundant (e.g., Monastery). Garnets, clino- and orthopyroxenes, phlogopite and ilmenite most common, zircon and olivine rarer. Debatable whether phlogopite and olivine are members of Cr-poor suite. Wide range in chemistry but Cr-poor, Fe-Ti-rich relative to type I (low-T) peridotite minerals. Mineral chemistry and estimated equilibration P/Ts overlap those of type V (high-T) lherzolites. Some Slave craton "Cr-poor megacrysts" show mineral chemistry links to type II megacrystalline pyroxenite xenoliths. See review of Schulze (1987).N. Lesotho (Nixon and Boyd 1973b); Monastery ( Gurney et al., 1979), Jagersfontein ( Hops et al., 1992), Kaapvaal craton; The Malaita megacryst suite ( Nixon and Boyd, 1979), occurs in an ocean plateau alnoite, but has many similarities with the kimberlitic low-Cr suite Cr-rich variety: (i) A suite comprising garnet plus ortho- and clinopyroxene, mostly restricted to kimberlites from Colorado-Wyoming. Mineralogically similar to type I lherzolites. (ii) "Granny Smith" diopsides; bright green Cr-diopside, may contain blebs/intergrowths of ilmenite and phlogopite. Can be polycrystalline.Colorado-Wyoming craton (Eggler et al., 1979)Kimberley and Jagersfontein ( Boyd et al., 1984b) Miscellaneous: mostly garnets and pyroxenes with no clear paragenetic association or links to other megacryst suites. May represent disrupted peridotites/eclogites/pyroxenites. AXI: polymict aggregatesPolymict aggregates of peridotite, eclogite and megacrysts, of variable grain size, some containing quenched melt. Mineral assemblages not in elemental or isotopic equilibrium.Bultfontein, De Beers and Premier mines, Kaapvaal (Lawless et al., 1979). MalaitaHighly variable AXII: diamond and inclusions in diamondsWidespread and closely related to cratons. Abundance varies from <1 ppm to 100 ppm by weight. Size <<0.1 g to c. 750 g. Type I diamonds contain abundant N, type II low N (Harris, 1987).All cratons (Harris, 1987; Meyer, 1987)93-96 Inclusion suites divided into peridotitic (P-type) and eclogitic (E-type) parageneses. P-type inclusions: high-Cr, low Ca garnet, Cr-diopside, Fo-rich olivine, orthopyroxene, chromite, wustite, Ni-rich sulfide, have restricted, high Mg, high Ni chemistry. Equilibration temperatures 900-1,100 °C. E-Type inclusions: pyrope-almandine, high Na garnet (>0.1 wt.%), omphacite, coesite, low-Ni sulfide. AXIII: ultra-deep peridotitesRare and restricted to Jagersfontein (Kaapvaal Craton) and Koidu (W. African craton). Four-phase garnet lherzolite. Close association of pyrope-garnet (∼70% py; 2 wt.% Cr2O3) and jadeite-rich clinopyroxene (20% Jd, & 4% Di). Clinopyroxene forms either orientated rods in garnet host or as discrete grains attached to garnet in cuspate contact. Both pyroxenes exsolved from garnet at 100-150 km depth. Recombination of garnet gives original depths of derivation of 300-400 km. Discrete garnets and "lherzolites" with eclogitic affinities also found (Sautter et al., 1991).All samples so far from the Jagersfontein kimberlite, S. Africa (Haggerty and Sautter, 1990; Sautter et al., 1991) and Koidu, Sierra Leone ( Deines and Haggerty, 2000)91.6 (B): Non-cratonic xenoliths erupted by alkalic and potassic mafic magmas sensu latoa BI: Cr-diopside lherzolite groupVery widespread and common in a variety of tectonic settings, off-craton. Dominantly spinel-facies (Al or Cr-spinel) lherzolites but can be garnet-facies and garnet-spinel facies (e.g., Vitim). Coarse grained, commonly little deformed, sometimes show preferred orientation. Include harzburgites, orthopyroxenites, clinopyroxenites, websterites, and wehrlites. Pargasite and phlogopite may also be common. Both low TiO2 and high TiO2 amphiboles can occur at the same locality. Accessory apatite, can be common locally (e.g., Bullenmerri, Victoria). Interstitial silicate glass can be present. Garnet and spinel facies significantly more olivine-rich and orthopyroxene poor than peridotites from cratons such as Kaapvaal and Siberia. Bulk rocks less depleted in Ca, Al, Fe, and lower in Mg than cratonic peridotites. Minerals generally higher Mg# and Cr# and lower Na and Ti than those of the Al-Augite group. Can be subdivided into type IA (LREE depleted clinopyroxene) and type IB (LREE enriched clinopyroxene).Victoria, SE Australia (Frey and Green, 1974); Vitim ( Ionov et al., 1993a); San Carlos and other W. USA localities ( Frey and Prinz, 1978; Wilshire and Shervais, 1975); Eifel ( Stosch and Seck, 1980); Hawaii ( Jackson and Wright, 1970); Scotland ( Menzies and Halliday, 1988)Garnet facies: Thumb, Navajo field ( Ehrenberg, 1982a, b); Pali-Aike, Patagonia ( Stern et al., 1989); Vitim, S. Siberia ( Ionov, 1993a, b)>0.85, Avg. ∼90 BII: Al-augite wehrlite-pyroxenite groupWidespread and common. Frequently clinopyroxene-rich rocks but widely variable: wehrlites, clinopyroxenites, dunites, websterites, lherzolites, lherzites, gabbros. Al-spinel is the typical aluminous phase but may contain plagioclase. Kaersutite common along with apatite, Fe-Ti oxides, and phlogopite. Some igneous and metamorphic textures. Composite xenoliths relatively common (in contrast to kimberlite-related xenoliths). Cross-cutting pyroxene-rich veins and layers may occur in olivine-rich hosts. Olivine-rich aggregates also found in pyroxene-rich xenoliths. Minerals generally lower Mg# and Cr#, higher Ti than those of the type I (Cr-diopside group).Victoria, SE Australia (Frey and Green, 1974); San Carlos and other W. USA localities ( Frey and Prinz, 1978; Wilshire and Shervais, 1975; Irving, 1980); Hawaii ( Jackson and Wright, 1970; Irving, 1980)<0.85 BIII: garnet pyroxenite groupWidespread but not abundant. Garnet clinopyroxenites and websterites plus clinopyroxenites and websterites where pyroxenes commonly show exsolution of garnet and/or spinel as well as Ca-rich or Ca-poor pyroxene. Accessory ilmenite and sometimes apatite. Coarse grained, undeformed textures, sometimes layered. "Basaltic" bulk compositions.Delegate, SE Australia (Lovering and White, 1969; Irving, 1974), Salt Lake Crater, Hawaii ( Beeson and Jackson, 1970)NA BIV: modal metasomatic groupWidespread varieties of the above groups showing evidence for modal (or "patent") metasomatism. Wehrlite-clinopyroxenites with mica, glimmerites. Typical metasomatic phases include pargasite/kaersutite, phlogopite, apatite, and grain-boundary oxides e.g., rutile. Apatite only in some cases. Silicate glass as melting product of amphibole, clinopyroxene, or phlogopite common. Composite xenoliths occur.Nunivak, Alaska (Francis, 1976), SE Australia ( O'Reilly et al., 1991), Menzies and Murthy, 1980a, Vitim ( Ionov et al., 1993a), Loch Roag and Fife Scotland ( Menzies et al., 1989)NA BV: megacrystsWidespread with variable abundance. Usually large (>1 cm) single crystals. Large range in Mg#, Cr, and Ti in a given suite.SE Australia (Binns et al., 1970; Irving and Frey, 1984; Schulze, 1987), Loch Roag, Scotland ( Menzies et al., 1989)NA Group A: Al-augite, Al-bronzite, olivine, kaersutite, pyrope, pleonaste, plagioclase; some of which may have crystallized from the host magma Group B: Anorthoclase, Ti-mica, Fe-Na salite, apatite, magnetite, ilmenite, zircon, rutile, sphene, and corundum, all of which are likely xenocrysts. Some coarse crystals are undoubtedly derived from disaggregated type I and type II xenoliths. a Based on Harte and Hawkesworth (1989) with nomenclature from Frey and Green (1974), Wilshire and Shervais (1975), Frey and Prinz (1978), Irving (1980), and Menzies (1983). Although widespread in the literature, classification of xenoliths on the basis of their host magma is not fully informative. It is more geologically useful to subdivide xenoliths in terms of their tectonic setting. A basic subdivision of xenolith occurrences is between those erupted in oceanic settings and those erupted in continental settings. The continental occurrences far outnumber the oceanic occurrences. The continental occurrences can be further subdivided depending on age of the crust and the tectonic history of the area being sampled. Xenoliths from stable cratonic and circum-cratonic regions are distinctly different in petrology from those occurring in areas that have experienced significant lithospheric rifting, generally in noncratonic crust, in the recent geological past. As such, we will utilize the terms cratonic/circum-cratonic to refer to xenoliths occurring on and around craton margins and the term noncratonic in referring to mantle sampled away from cratons, often in areas that have experienced recent lithospheric thinning. There is a link back to the host rock in that, as a general rule, cratonic and circum-cratonic xenoliths are erupted by kimberlites and noncratonic xenoliths are erupted by alkalic and potassic mafic magmas.2.05.1.1.1. Mantle xenoliths in continental volcanic rocksXenoliths found in Archean cratonic regions are characterized by the lithological types reported in Table 1(a). Garnet-facies peridotites dominate the peridotite xenolith inventory in these locations. In contrast, away from cratons, there is a scarcity of garnet-facies peridotites ( Table 1(b)). In addition, cratonic xenolith suites contain samples derived from depths ranging from crustal levels to >200 km, whereas noncratonic xensoliths come from less than 140 km deep. There can be distinct differences between xenoliths erupted on craton and those erupted in stable areas of Proterozoic crust marginal to cratons. For instance, subcalcic-garnet harzburgites occur on most cratons but do not occur in circum-cratonic suites ( Boyd et al., 1993). In addition, the maximum depths of equilibration of circum-cratonic peridotite suites are less than for cratonic peridotite suites (e.g., Finnerty and Boyd, 1987). These differences warrant the distinction between "cratonic" and "circum-cratonic" xenoliths. In addition, young rift-related magmatism, marginal to cratons, samples very thin lithosphere compared to cratonic and circum-cratonic lithosphere. The xenoliths sampled in this environment fall into the loose category of "noncratonic" xenoliths. A more detailed and complex tectonic classification is provided by Griffin et al. (1999a).2.05.1.1.2. Mantle xenoliths in oceanic volcanic rocksThe nature of the suboceanic mantle is largely constrained from geochemical studies of its partial melts (see Chapter 2.08) because occurrences of mantle xenoliths in the ocean basins are much rarer than on the continents. The host rocks for these xenoliths are exclusively alkalic and potassic mafic magmas. The xenolith suite of the Hawaiian volcanic chain is perhaps the best characterized (Jackson and Wright, 1970) of the ocean islands, while extensive suites have also been found in the Canary Islands ( Neumann et al., 1995), Samoa ( Hauri et al., 1993), Grande Comore ( Coltorti et al., 1999), and Tahiti. Most of these occurrences sample the oceanic lithosphere directly below the islands and those of type I ( Table 1) are proposed to be residues of partial melting that have been variably metasomatized, with carbonatite-like fluids frequently being invoked ( Hauri et al., 1993; Coltorti et al., 1999). The Hawaiian suite is more complex. Pyroxenites of type II and type III are common and iron-rich peridotites, some with garnet, are thought to be physical mixtures of spinel lherzolites with the pyroxenite suite ( Sen and Leeman, 1991).Some xenolith localities sample the mantle lithosphere beneath oceanic plateaux. The most extensive and varied xenolith suite in this regard is that from Malaita (Solomon Islands) on the margin of the Ontong Java Plateau (Nixon and Boyd, 1979). This locality is hosted by an alnoite and contains both garnet and spinel-facies lherzolites together with a spectacular megacryst suite. Although in an oceanic setting, the variety of the xenolith suite provided by the Malaita alnoite, in particular the megacrysts, show strong similarities to suites from kimberlites ( Nixon and Boyd, 1979).2.05.1.1.3. Mantle xenoliths in subduction zone environmentsAlthough xenoliths from subduction-related tectonic settings have been known for sometime, their detailed relationship to the subduction zone system has been a matter of debate. Most samples are type-I spinel lherzolites and modal metasomatic variants of type IV, most commonly kaersutite and phlogopite. Among the best-known examples are from Itinome-Gata, Japan (Aoki, 1968) and Simcoe, NW, USA ( Brandon et al., 1999), although spinel lherzolites from Grenada, Lesser Antilles, also occur. It is not well established whether these xenoliths actually represent parts of the metasomatized mantle wedge above the subduction zone, or simply mantle lithosphere not intimately related to the subduction zone process. McInnes and Cameron (1994) have reported xenoliths from the Tabar-Lihir-Tanga-Feni arc, Papua New Guinea, that are purported to be mantle wedge compositions.
- Publication:
-
Treatise on Geochemistry
- Pub Date:
- December 2003
- DOI:
- Bibcode:
- 2003TrGeo...2..171P