Spinel-bearing Peridotites Research Articles

UHP Magma Paragenesis, Garnet Peridotite, and Garnet Clinopyroxenite: An Example from the Dominican Republic

Spinel-bearing garnet peridotite and corundum-bearing variants in the Cuaba Gneiss of the Cretaceous Rio San Juan Complex show evidence for ultrahigh pressure (UHP) partial melting and magmatic fractionation (orthocumulate textures). The paragenesis involves the following sequence of assemblages (plus inferred melt) with declining T: (1) Grt + Ol + Spl + Cpx + Liq (partial melt assemblage); (2) Grt + Spl + Cpx + Liq (cumulate Cpx with interstitial Grt); (3) Grt + Spl + Crn + Cpx + Liq (pegmatite, Cpx with interstitial Grt and late Crn). Comparison with 3 GPa liquidus relationships in CMAS (Milholland and Presnall, 1998) and extrapolation to pressures above which sapphirine is not possible (> 3.4 GPa at ~1570°C) show that assemblage (1) is consistent with the equilibrium Spl + Cpx = Ol + Grt + Liq. Liquid fractionated from this assemblage crystallized in equilibrium with Grt + Cpx + Spl (2). Further fractionation resulted in the crystallization of Crn according to the equilibrium, Cpx + Grt + Crn = Spl + Liq (3). This last reaction is only possible at P > 3.4 GPa and T > 1550°C. The assemblages constrain a short but well-defined liquid line of descent. The inferred conditions of T are much higher than previous estimates that did not take melt into account. Previously estimated conditions (P = 2.8-3.4 GPa, T = 740-810°C) are presumed to reflect subsolidus reequilibration. Evidently, the rocks originated in the deepest part of the lithosphere, or shallowest part of the asthenosphere, and cooled more or less isobarically as they were delivered to the subduction zone, prior to ascent.

Mantle diversity beneath the Colombian Andes, Northern Volcanic Zone: Constraints from Sr and Nd isotopes

In order to provide mantle and crustal constraints during the evolution of the Colombian Andes, Sr and Nd isotopic studies were performed in xenoliths from the Mercaderes region, Northern Volcanic Zone, Colombia. Xenoliths are found in the Granatifera Tuff, a deposit of Cenozoic age, in which mantle- and crustal-derived xenoliths are present in bombs and fragments of andesites and lamprophyres compositions. Garnet-bearing xenoliths are the most abundant mantle-derived rocks, but websterites (garnet-free xenoliths) and spinel-bearing peridotites are also present in minor amounts. Amphibolites, pyroxenites, granulites, and gneisses represent the lower crustal xenolith assemblage. Isotopic signatures for the mantle xenoliths, together with field, petrographic, mineral, and whole-rock chemistry and pressure–temperature estimates, suggest three main sources for these mantle xenoliths: garnet-free websterite xenoliths derived from a source region with low P and T (16 kbar, 1065 °C) and MORB isotopic signature, 87Sr/ 86Sr ratio of 0.7030, and 143Nd/ 144Nd ratio of 0.5129. Garnet-bearing peridotite and websterite xenoliths derived from two different sources in the mantle: i) a source with intermediate P and T (29–35 kbar, 1250–1295 °C) conditions, similar to that of sub-oceanic geotherm, with an OIB isotopic signature ( 87Sr/ 86Sr ratio of 0.7043 and 143Nd/ 144Nd ratio of 0.5129); and ii) another source with P and T conditions similar to those of a sub-continental geotherm (>38 kbar, 1140–1175 °C) and OIB isotopic characteristics ( 87Sr/ 86Sr ratio=0.7041 and 143Nd/ 144Nd ratio=0.5135).

Re–Os systematics of UB-N, a serpentinized peridotite reference material

The reference material (RM) UB-N is a typical representative of earth's upper mantle. It is a serpentinized garnet and spinel-bearing peridotite (a metamorphosed lherzolite) from the Vosges mountains, France, that is well characterized for major and many trace elements. In order to test whether UB-N is a suitable Re–Os reference material, 32 digestions in three different laboratories (CRPG/CNRS, MPI (Mainz) and University of Leoben) with four different digestion techniques (low-temperature acid attack, Carius tube dissolution, high-pressure asher (HPA-S) acid attack and alkali fusion) were performed. The results show that the low-temperature acid attack is unsuitable for the study of the Re–Os systematics of UB-N. Surprisingly, the well-established Carius tube acid digestion technique also fails to completely digest all Os-bearing mineral phases. Only alkali fusion and HPA-S acid attack yield the highest Os concentrations. Though sample inhomogeneity has been recognized (approximately 6% RSD for 2-g sample aliquots), it is possible to determine a well-defined average Os concentration of 3.85±0.13 ng g −1 (95% confidence; 19 digestions, fusion and HPA-S only). Rhenium-bearing minerals are very homogeneously distributed and replicates within each laboratory yield highly reproducible results independent of the digestion technique. A value of 0.2095±0.0040 ng g −1 (95% confidence; n=24) is assigned to the Re concentration. The best estimate for the whole-rock 187Os/ 188Os is 0.1278±0.0002 (95% confidence; n=12). The UB-N reference material now has well-understood Re–Os systematics that are typical of fertile upper mantle rocks. Analysis of this standard is, thus, highly recommended for the validation of Re–Os analytical procedures.

Records of Crustal Metasomatism in the Garnet Peridotites of the Ulten Zone (Upper Austroalpine, Eastern Alps)

Peridotites in the Ulten Zone (Upper Austroalpine, Eastern Alps), occur as small bodies within lower-crustal rocks (gneisses and migmatites) subducted at eclogite-facies conditions during the Variscan orogeny. They record a complex metamorphic and deformation evolution as indicated by the transition from coarse-grained spinel-bearing peridotites to fine-grained garnet + amphibole-bearing peridotites, and are interpreted as portions of mantle wedge that were incorporated in a downgoing slab of cold continental crust. The transition from spinel- to garnet-bearing assemblage was accompanied by significant input of metasomatic agents, as shown by the crystallization of abundant amphibole. Here we present trace-element mineral chemistry data for selected Ulten peridotites, with the aim of unravelling the nature of the metasomatic processes. Amphiboles display significant light rare earth element (LREE) enrichment [CeN/YbN = 3·90–11·50; LREE up to (20–50) × C1], high Sr (150–250 ppm), K (1910–7280 ppm) and Ba (280–800 ppm) contents, and low concentrations of high field strength elements (HFSE) (Zr = 14–25 ppm, Y = 6·7–16 ppm, Ti = 1150–2500 ppm, Nb = 2–7 ppm). On the basis of (1) the evidence for modal orthopyroxene decrease as a result of the garnet-forming reaction rather than abundant orthopyroxene crystallization, (2) the high modal amounts of amphibole (up to 23%) in the most metasomatized peridotites and (3) the strong large ion lithophile element (LILE)/HFSE fractionation in amphiboles, we infer that the metasomatic agent was an H2O–CO2 fluid with a low CO2/H2O ratio. Petrological investigations and geochronological data indicate that the host metapelites experienced in situ partial melting and migmatization concomitantly with the garnet + amphibole-facies recrystallization in the enclosed peridotites. We infer that the metasomatizing hydrous fluids could represent the residual fluids left after the crystallization of leucosomes, starting from water-undersaturated melts produced during migmatization of the host gneisses.

Noble gases in pyroxenites and metasomatised peridotites from the Newer Volcanics, southeastern Australia: implications for mantle metasomatism

The elemental and isotopic compositions of five noble gases (He, Ne, Ar, Kr and Xe) have been determined in selected, well-documented, ultramafic xenoliths from southeastern Australia. These xenoliths include both spinel-bearing peridotites with an apparent metasomatic overprint and garnet-bearing pyroxenites. In general, helium, neon, argon and xenon isotopic ratios from gases trapped in fluid inclusions of the samples are mid-ocean ridge basalt (MORB)-like, as has previously been found in anhydrous lherzolite xenoliths from the same area. In addition to the MORB-like components, radiogenic ( 4He*) and nucleogenic ( 21Ne* and 22Ne*) components were found in the present samples, reflecting relatively high U and Th contents in the metasomatic minerals such as amphibole, apatite and clinopyroxene. These components are only released upon melting, thus are likely to be trapped in the crystal lattices of the minerals. The MORB-like noble gas component found in fluid inclusions (invariably CO 2-rich) of these samples probably was introduced into the lithospheric mantle by metasomatising melts derived from the upper mantle. The noble gases dissolved in the metasomatising melt are likely to be effectively decoupled from incompatible elements during segregation of a CO 2-rich fluid in the ascending melt. In the metasomatising melt, the noble gases would partition into the CO 2-rich fluid whereas the incompatible elements would remain in the melt phase. As a result, the noble gas composition in fluid inclusions of the minerals appears to be independent of their degree of metasomatism indicated by their host rock mineralogy and trace elemental geochemistry. Once noble gases are trapped in CO 2-rich fluid inclusions within the minerals, they would preserve their source signatures without being affected significantly by the ingrowth of radiogenic and nucleogenic products, because the U, Th and K would remain in the melt. The predominance of MORB-like noble gas signatures in fluid inclusions of the xenoliths suggests that the metasomatising components were derived from the asthenospheric mantle underlying southeastern Australia.

ENRICHMENT PROCESSES IN GARNET-BEARING MANTLE XENOLITHS FROM KIMBERLEY PIPES (SOUTH AFRICA)

ENRICHMENT PROCESSES IN GARNET-BEARING MANTLE XENOLITHS FROM KIMBERLEY PIPES (SOUTH AFRICA)

CRUSTAL METASOMATISM IN SUBDUCTED MANTLE: RECORDS FROM THE ULTEN PERIDOTITES (UPPER AUSTROALPINE, EASTERN ALPS)

CRUSTAL METASOMATISM IN SUBDUCTED MANTLE: RECORDS FROM THE ULTEN PERIDOTITES (UPPER AUSTROALPINE, EASTERN ALPS)

Petrology of Peridotite and Pyroxenite Xenoliths from the Jericho Kimberlite: Implications for the Thermal State of the Mantle beneath the Slave Craton, Northern Canada

We report comprehensive petrological and thermobarometric data for the following typesof mantle xenoliths from the Jericho kimberlite in the Slave craton: (1) coarse peridotite; (2) porphyroclastic peridotite; (3) megacrystalline pyroxenite; (4) ilmenite–garnet wehrlite and clinopyroxenite. The varieties of the upper-mantle xenoliths, and their proportions, petrography and mineralogy, mainly resemble those from kimberlites of other cratons. Unique characteristics of the north–central Slave mantle include the presence of high-temperature megacrystalline pyroxenite and an unusually high proportion of pyroxenitic magmatic rocks related in origin to megacrysts. Other unique aspects of Jerichoperidotitic mantle are: (1) a pronounced Cr enrichment in mineral chemistry within high-temperature peridotite; (2) an anomalously high proportion of chemically unequilibrated samples; (3) an uncommon pattern in silicate mineral chemistry in spinel-bearing peridotite that reflects equilibration with spinel. The central Slave mantle is colder than the mantle beneath the rest of the North American craton and the mantle beneath Kaapvaal and Siberian cratons. Thermobarometric data are fitted to a model steady-state geotherm with an exponential decrease in heat production with depth using a geologically constrained valueof surface heat production at Jericho. The estimated model values of surface heat flow (Q0 = 52–53 mW/m2) show an excellent agreement with two heat flow measurements available for the Slave craton (53–55 mW/m2). Thus, the cool upper mantle here coexists with a highly radiogenic crust.

Petrology of Peridotite and Pyroxenite Xenoliths from the Jericho Kimberlite: Implications for the Thermal State of the Mantle beneath the Slave Craton, Northern Canada

We report comprehensive petrological and thermobarometric data for the following typesof mantle xenoliths from the Jericho kimberlite in the Slave craton: (1) coarse peridotite; (2) porphyroclastic peridotite; (3) megacrystalline pyroxenite; (4) ilmenite–garnet wehrlite and clinopyroxenite. The varieties of the upper-mantle xenoliths, and their proportions, petrography and mineralogy, mainly resemble those from kimberlites of other cratons. Unique characteristics of the north–central Slave mantle include the presence of high-temperature megacrystalline pyroxenite and an unusually high proportion of pyroxenitic magmatic rocks related in origin to megacrysts. Other unique aspects of Jerichoperidotitic mantle are: (1) a pronounced Cr enrichment in mineral chemistry within high-temperature peridotite; (2) an anomalously high proportion of chemically unequilibrated samples; (3) an uncommon pattern in silicate mineral chemistry in spinel-bearing peridotite that reflects equilibration with spinel. The central Slave mantle is colder than the mantle beneath the rest of the North American craton and the mantle beneath Kaapvaal and Siberian cratons. Thermobarometric data are fitted to a model steady-state geotherm with an exponential decrease in heat production with depth using a geologically constrained valueof surface heat production at Jericho. The estimated model values of surface heat flow (Q0 = 52–53 mW/m2) show an excellent agreement with two heat flow measurements available for the Slave craton (53–55 mW/m2). Thus, the cool upper mantle here coexists with a highly radiogenic crust.

Reequilibration of Ultramafic Xenoliths from Namibia by Metasomatic Processes at the Mantle Boundary

Mantle xenoliths from the Hanaus and the Anis Kubub pipes in the Gibeon Kimberlite Province of southern Namibia show evidence for intensive heating in lower as well as intermediate lithospheric levels. The investigated samples are garnet- and spinel-bearing peridotites (as well as one orthopyroxenite) with granular, partly sheared, and sheared textures. While granular and partly sheared xenoliths reveal distinct zonation patterns, most of the sheared xenoliths display perfect mineral equilibria. Thermobarometric estimates for the primary mineral assemblages of these samples plot in the graphite stability field with maximum P-T conditions of about 1320°C at 44 kbar in garnet harzburgites and a minimum of about 630°C at 19 kbar in spinel lherzolites. The initial steady-state geotherm of $$44 mW/m^{2}$$ is only preserved in a few granular xenoliths, whereas the sheared samples plot along an elevated geothermal gradient of about $$50 mW/m^{2}$$, testifying to transient heating processes in the mantle coupled ...

Oxygen isotope composition of garnet and spinel peridotites in the continental mantle: Evidence from the Vitim xenolith suite, southern Siberia

Spinel and garnet fades peridotites from the Vitim Volcanic Field in Siberia comprise a suite of fertile mantle xenoliths, which are LREE depleted and have strongly depleted strontium and neodymium isotopic compositions. Determination of 18O16O ratios by conventional and laser-assisted fluorination techniques yield a very restricted range of whole-rock δ18O values for both spinel peridotites (+ 5.4 to + 5.8‰) and garnet lherzolites (+ 5.5 to + 5.8‰) equivalent to that observed for lunar rocks and midocean ridge basalts (MORB). Mineral δ18O values for the xenoliths are: olivine = + 5.1 to + 5.8%., orthopyroxene = + 5.7 to + 6.0%., clinopyroxene = + 5.5 to + 6.2%., garnet = + 5.5 to +6.0%., and spinel = + 4.9 to + 5.5%. Similarly, δ18O ranges for silicate mineral pairs vary from only 0.5 to 0.7%. The sixteen peridotite xenoliths studied exhibit equilibrium O-isotope fractionations between minerals of a magnitude expected from theoretical and experimental considerations, in particular a temperature-controlled 18O distribution between olivine and pyroxenes in spinel-bearing peridotites. Because the Vitim garnet and spinel peridotites are rich in basaltic components and show long-term depletion in incompatible elements, they are derived from a MORB-like mantle source which makes up, according to current geochemical models, a predominant part of the Earth's upper mantle. The Vitim xenolith suite may, therefore, reflect its bulk O-isotope character. With increasing temperature and degree of partial melting, such a mantle source could generate basaltic liquids with δ18O values varying from C. + 6.1 to + 5.7%. Thus, the broad δ18O range of c. + 4.5 to + 7.5%. reported in the literature for spinel peridotite xenoliths is probably a very restricted disequilibrium phenomenon that likely reflects recent metasomatic overprinting in highly localized zones of the uppermost continental mantle affected by basaltic magmatism and, therefore, should not be considered as representative of the upper mantle, in general.

Metamorphic evolution of the pre-Hercynian basement of the Schwarzwald (Federal Republic of Germany)

The high-grade gneisses and migmatites of the Central Schwarzwald display a polyphase metamorphic history with a dominating L P-H T metamorphism. Numerous small intercalations of eclogites (or eclogitic amphibolites), ultramafics (both garnet- and spinel-bearing peridotites and pyroxenites), and granulitic gneisses together with mineral relics in the high-grade gneisses are indicative of H P and M P metamorphic events predating the L P-H T metamorphism. The widespread occurrences of these rocks, their petrological evolution and their relationship to their gneissic country rocks strongly indicate that the metamorphic history of at least some of the gneiss complex is identical to that of the H and M P intercalations. Late Hercynian low-grade metamorphism is related to retrograde processes associated with the uplift history and with brittle deformation of the basement.

Solubility of alumina in orthopyroxene plus spinel as a geobarometer in complex systems. Applications to spinel-bearing alpine-type peridotites

The addition of Fe and Cr to the simple system MgO-SiO2-Al2O3 markedly affects the activities of phases involved in the equilibrium $$\begin{gathered} {\text{Mg}}_{\text{2}} {\text{SiO}}_{\text{4}} {\text{ + MgAl}}_{\text{2}} {\text{SiO}}_{\text{6}} {\text{ = MgAl}}_{\text{2}} {\text{O}}_{\text{4}} {\text{ + Mg}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{6}} \hfill \\ {\text{Olivine + Opx}}_{{\text{solid solution}}} {\text{ = Spinel + Opx}}_{{\text{solid solution}}} \hfill \\ \end{gathered}$$ . In particular, the activity of MgAl2O4 in many natural spinels is decreased, which gives lower calculated pressures when the formula $$P = 1 + \left\{ {(9,116 - 5.065{\text{ }}T_0 ) - {\text{RT}}_{\text{0}} \ln \left( {\frac{{(X_{{\text{Mg}}} )_{{\text{SP}}} (X_{{\text{Al}}} )_{{\text{SP}}}^{\text{2}} (X_{{\text{Mg}}}^{{\text{M1}}} )_{{\text{OPX}}} }}{{(X_{{\text{Mg}}} )_{{\text{OL}}}^{\text{2}} (X_{{\text{Al}}}^{{\text{M1}}} )_{{\text{OPX}}} }}} \right)} \right\}/\Delta V_\Gamma$$ derived from the experimental data of MacGregor (1974) is applied to complex systems containing Fe and Cr.