Mg-rich Rims Research Articles

Compositional Variations in Primitive Kimberlite Melts and Entrained Mantle Cargo from a Global Survey of Trace Element Compositions in Kimberlite Olivine

Abstract Olivines are the dominant phase of kimberlites and the majority of grains display distinct compositional zoning with xenocrystic cores surrounded by magmatic rims. Previous work has documented large variations in both core and rim major and minor element compositions in kimberlites globally, which has been related to variable entrainment and assimilation of sub-continental lithospheric mantle (SCLM) material. However, there is limited knowledge of trace element variations in olivine from kimberlites and it is unclear whether mantle assimilation has any effect on the trace element composition of kimberlite melts. To fill this gap, we present a global survey of olivine trace element compositions, along with previously reported major and minor element compositions, for samples representing the full spectrum of olivine compositional variations in kimberlites, including samples from the following: Lac de Gras, Canada (Grizzly, Koala); Kimberley, South Africa (Bultfontein, De Beers, Kimberley Big Hole, Wesselton); Kaalvallei, South Africa (New Robinson, Samada); and Alto Paranaiba, Brazil (Limpeza-18, Tres Ranchos-04). Trace element concentrations of olivine cores can discriminate between those derived from the disaggregation of peridotitic material and those related to the megacryst suite. The megacrystic olivine cores exhibit a negative correlation between Al and Mn, which is absent in peridotite-derived cores, and are characterised by high concentrations of temperature-dependent elements (e.g. Al, Na, V) as well as Zn, Ti, and Mn. Following pre-screening of cores for megacrystic and spinel peridotite-derived grains, we applied the Al-in-olivine thermometer to assess the P–T equilibration conditions of cores in equilibrium with garnet and estimate the sampling depth of kimberlite magmas in the lithospheric mantle. Our results are consistent with predominant entrainment of deep lithosphere xenocrysts in highly diamondiferous compared with diamond-poor kimberlites. Temperature-dependent elements display a gradational increase with depth due to higher T with Ca, Cu and, to a lesser extent, Zn and Ti being higher and Mg# being lower towards the base of the SCLM, which is consistent with melt modification of the lower lithosphere. The Zn, Ti, Co, Mn, Li, Al, Cr, Na, and V concentrations of magmatic olivine rims display systematic variations that have a negative correlation with Mg# (whereas Cr is positively correlated). Lac de Gras olivine feature Mg-rich rims (Mg# >90) and low concentrations of these trace elements, whereas the Fe-rich olivine rims (Mg# ~85) of the Kaalvallei kimberlites have higher concentrations of these elements, with the Kimberley and Alto Paranaiba kimberlites being intermediate. Direct correlations between average Ti, Zn, Co and Li compositions of olivine cores and rims suggests that the olivine rim (i.e. proxy for primitive melt) variations are related to variable assimilation of metasomatised SCLM and can be effectively used to track the composition of the lithospheric column that is traversed by kimberlite magmas. These observations further imply an intimate link between early proto-kimberlite melt, leading to formation of megacrystic olivine at the base of the SCLM, and the composition of kimberlite melts which entrain and assimilate these products. We conclude that lithospheric mantle assimilation has a major and previously overlooked influence on the trace element composition of kimberlite magmas.

Experimental Modeling of Silicate and Carbonate Sulfidation under Lithospheric Mantle P,T-Parameters

Interactions of mantle silicates with subducted carbonates, sulfides, and sulfur-rich fluids are experimentally simulated in the olivine-ankerite-sulfur and olivine-ankerite-pyrite systems using a multi-anvil high-pressure split-sphere apparatus at 6.3 GPa and range of 1050–1550 °C. Recrystallization of Fe,Ni-bearing olivine and ankerite in a sulfur melt was found to be accompanied by sulfidation of olivine and carbonate, involving partial extraction of metals, carbon, and oxygen into the melt, followed by the formation of pyrite (±pyrrhotite), diopside, and Fe-free carbonates. The main features of metasomatic alteration of Fe,Ni-olivine by a reduced sulfur fluid include: (i) a zonal structure of crystals (Fe-rich core, Mg-rich rim); (ii) inclusions of pyrite and pyrrhotite in olivine; (iii) certain Raman spectral characteristics of olivine. At T > 1350 °C, two immiscible melts, a predominantly sulfur melt with dissolved components (or a Fe–Ni–S–O melt) and a predominantly carbonate one, are generated. The redox interaction of these melts leads to the formation of metastable graphite (1350–1550 °C) and diamond growth (1550 °C). The studied olivine-ankerite-sulfur and olivine-ankerite-pyrite interactions may be considered as the basis for simulation of metasomatic processes accompanied by the formation of mantle sulfides during subduction of crustal material to the silicate mantle.

Origin of Reverse Zoned Cr-Spinels from the Paleoproterozoic Yanmenguan Mafic–Ultramafic Complex in the North China Craton

We conducted petrological and mineral chemistry investigations of Cr-spinel in ultramafic rocks of the Yanmenguan mafic–ultramafic complex in the North China Craton. The Cr-spinel grains occur as inclusions in enstatite, tschermakite, phlogopite, and olivine, or as interstitial grains among the aforementioned silicate minerals, and show concentric or asymmetrical textures. Back-scattered electron and elemental images and compositional profiles of the spinel grains indicate the presence of Cr- and Fe-rich cores and Al- and Mg-rich rims. The host silicate minerals display a decrease in Al and Mg contents accompanied by an increase in Cr and Fe away from the spinel. These textures and compositional variations suggest that subsolidus elemental exchange more likely gave rise to the compositional zonation, resulting in the transfer of Al and Mg from the silicate minerals to the spinel. The Mn, Ni, and Ti contents in spinel and the major elements of olivine-hosted spinel are relatively stable during subsolidus elemental diffusion and thus are more reliable tracers of primary high-temperature processes. The temperature estimates reveal that the subsolidus diffusion might have occurred at 600–720 °C, which could be linked to the regional metamorphic event.

An experimental study of $$ ^{{{\text{Fe}}^{2 + } {-}{\text{Mg}}}} K_{\text{D}} $$ between orthopyroxene and rhyolite: a strong dependence on H2O in the melt

The effect of temperature, pressure, and dissolved H2O in the melt on the Fe2+–Mg exchange coefficient between orthopyroxene and rhyolite melt was investigated with a series of H2O fluid-saturated phase-equilibrium experiments. Experiments were conducted in a rapid-quench cold-seal pressure vessel over a temperature and pressure range of 785–850 °C and 80–185 MPa, respectively. Oxygen fugacity was buffered with the solid Ni–NiO assemblage in a double-capsule assembly. These experiments, when combined with H2O-undersaturated experiments in the literature, show that $$ ^{{{\text{Fe}}^{2 + } {-}{\text{Mg}}}} K_{\text{D}} $$ between orthopyroxene and rhyolite liquid increases strongly (from 0.23 to 0.54) as a function of dissolved water in the melt (from 2.7 to 5.6 wt%). There is no detectable effect of temperature or pressure over an interval of 65 °C and 100 MPa, respectively, on the Fe2+–Mg exchange coefficient values. The data show that Fe-rich orthopyroxene is favored at high water contents, whereas Mg-rich orthopyroxene crystallizes at low water contents. It is proposed that the effect of dissolved water in the melt on the composition of orthopyroxene is analogous to its effect on the composition of plagioclase. In the latter case, dissolved hydroxyl groups preferentially complex with Na+ relative to Ca2+, which reduces the activity of the albite component, leading to a more anorthite-rich (calcic) plagioclase. Similarly, it is proposed that dissolved hydroxyl groups preferentially complex with Mg2+ relative to Fe2+, thus lowering the activity of the enstatite component, leading to a more Fe-rich orthopyroxene at high water contents in the melt. The experimental results presented in this study show that reversely zoned pyroxene (i.e., Mg-rich rims) in silicic magmas may be a result of H2O degassing and not necessarily the result of mixing with a more mafic magma.

Partial least squares-discriminant analysis of trace element compositions of magnetite from various VMS deposit subtypes: Application to mineral exploration

The petrography and mineral chemistry of magnetite from fifteen volcanogenic massive sulfide (VMS) deposits in Canada, and the Lasail VMS deposit in Oman, as well as from two VMS-associated banded iron formations (BIF), Austin Brook (New Brunswick, Canada) and Izok Lake (Nunavut, Canada), were investigated using optical microscopy, electron probe micro-analyzer (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The method of robust estimation for compositional data (robCompositions) was applied to investigate geochemical censored data. Among thirty-seven elements analyzed by EPMA and/or LA-ICP-MS in magnetite from the studied deposits/bedrock lithologies, only the results for Si, Ca, Zr, Al, Mg, Ti, Zn, Co and Ni contain <40% censored values, and thus could be imputed using robCompositions. Imputed censored data were transformed using centered log-ratios to overcome the closure effect on compositional data. Transformed data were classified by partial least squares-discriminant analysis (PLS-DA) to identify different compositional characteristics of magnetite from VMS deposits and BIFs. The integration of petrography and mineral chemistry identifies three types of magnetite in VMS settings: magmatic, hydrothermal, and metamorphic. Magmatic magnetite in VMS deposit host bedrocks is characterized by ilmenite exsolution and may be overprinted by metamorphism. Some VMS deposits contain hydrothermal magnetite, which is intergrown with sulfides, and shows a metamorphic overprint as it is partly replaced by common metamorphic minerals including chlorite, sericite, anthophyllite, and/or actinolite, whereas the majority of the deposits are characterized by metamorphic magnetite formed by replacing pre-existing sulfides and/or silicates, and is intergrown with metamorphic minerals. Among VMS deposits of the Noranda mining district, the West Ansil deposit is characterized by hydrothermal-metamorphic magnetite zoned by inclusion-free cores and Si- and Mg-rich rims. Magnetite from the studied VMS-associated BIFs is also metamorphic in origin. Aluminum, Ti and Zn contents of magnetite can separate BIF from the other mineralized and un-mineralized bedrock lithologies in the studied VMS settings.PLS-DA shows that variable compositions of magnetite slightly discriminate different studied deposits/bedrock lithologies. The geochemical observations suggest that the variation in magnetite chemistry from different VMS settings might be sourced from differences in: 1) the composition and temperature of parental magmas or hydrothermal fluids, 2) the composition of host bedrocks, 3) the composition of co-forming minerals, and 4) oxygen fugacity. PLS-DA distinguishes magnetite compositions from the studied VMS deposits and BIFs from that of the other ore deposit types including Ni–Cu, porphyry Cu-Mo-Au, iron oxide-copper- gold, iron oxide-apatite, and the Bayan Obo REE-Fe-Nb deposit. Magnetite from the VMS settings on average contains lower concentrations of Si, Zr, Al, Mg, Ti, Zn, Co and Ni relative to that from the other mineral deposit types. PLS-DA of magnetite data from VMS deposits and BIFs of the Bathurst mining camp as well as PLS-DA of magnetite compositions from various mineral deposit types yield discrimination models for application to mineral exploration for VMS deposits using indicator minerals in Quaternary lithified sedimentary rocks.

Corundum-bearing garnet peridotite from northern Dominican Republic: A metamorphic product of an arc cumulate in the Caribbean subduction zone

Garnet peridotite in oceanic subduction complexes has been reported only in two locations in the world. One of these examples occurs in the late Cretaceous to early Tertiary subduction complex in northern Dominican Republic. The garnet peridotite (wehrlite and olivine-bearing clinopyroxenite) occurs as large (≤ 4 m) boulders together with boulders of eclogites and serpentinites along a narrow (< 10 m) stream of the Rio Cuavas in the southern part of the Rio San Juan Complex. The peridotite is composed of garnet, diopsidic clinopyroxene (partially altered to magnesiohornblende), olivine (extensively altered to serpentine), Al-spinel and minor corundum; the latter two are mostly enclosed in garnet. Coarse-grained garnet also encloses small grains of early-formed garnet that contain Ca- and Al-rich cores and Mg-rich rims. The garnet peridotite contains low Cr, Ni, and Ir-group platinum group elements in bulk rock compared to primitive mantle values, low Mg (Mg#; 0.74–0.83) and NiO (< 0.1 wt.%) in olivine, and elevated concentrations of fluid-mobile elements (Sr, Pb and U) in clinopyroxene and bulk rock. Combined with the rare earth element data of bulk rocks and clinopyroxene, these data suggest that the peridotite originally solidified as a plagioclase-bearing cumulate of an arc melt at a shallow depth, < 35 km, in the mantle wedge. The cumulate was later dragged by mantle flow from the subarc mantle towards the subduction plane. Subsequent downward movement along the subduction plane resulted in the crystallization of corundum and Ca- and Al-rich garnet at the expense of plagioclase, at a depth of ~ 50 km. The garnet peridotite continued to be subducted to a depth of ~ 120 km, causing a progressive increase in pressure and temperature that resulted in the crystallization of Mg-rich garnet. The garnet periodotite was then exhumed in the serpentinite subduction channel. These events likely took place during the very early stages of the subduction system, where a strong mantle corner flow was likely produced by poor lubrication along the interface between the subducting plate and the overlying Caribbean Plate.

Deformation-controlled cation diffusion in compositionally zoned tourmaline

AbstractThe compositional zonation of both undeformed and plastically deformed tourmaline crystals from an amphibolite-facies mylonitic pegmatite from the Sierras Pampeanas (NW Argentina) has been investigated using electron microprobe analysis (EMPA). Undeformed tourmaline shows optical and compositional major and minor element growth zonation with a Ca- and Mg-rich rim zone and an Fe- rich core zone. The tourmaline population of the mylonite consists of crystals which appear undeformed at microscopic scale, and of weakly, moderately, and strongly deformed crystals. Depending on the intensity of plastic deformation, the optical zonation is blurred or absent, and the compositional zonation is less pronounced or destroyed. Plastic deformation mobilizes small cations (Fe2+, Mg2+) more efficiently and at lower deformation intensity than larger cations (Na+, Ca2+). In addition to intra-crystal homogenization, plastic deformation caused variable but generally minor Fe, Mg, Si, Al, Ca, and Na exchange between deformed tourmaline domains and co-existing fluid or solid phases. Dislocation creep is interpreted as the dominant deformation mechanism leading to the homogenization of the initial tourmaline growth zonation. The composition and the degree of homogeneity of deformed tourmaline domains depend on the initial composition of the growth zones, their initial volume ratio, the intensity and homogeneity of plastic deformation, and the size of the mobilized cation. Consequently, the composition of and the element distribution within plastically deformed crystals is not entirely controlled by intensive variables (P-T-X), and therefore not suitable for petrogenetic interpretation.

Metasomatism in Serpentinite Mélange Rocks from the High-Pressure Maksyutov Complex, Southern Ural Mountains, Russia

Metasomatized basalts are enclosed in antigorite serpentine mélange of the Upper Unit of the Maksyutov Complex, southern Ural Mountains, Russia. The structurally Lower Unit of this complex consists predominantly of mafic eclogite and sedimentary gneiss metamorphosed at ultrahigh pressures; rocks of the Upper Unit underwent blueschist-facies conditions. The metasomatized rocks are fine-grained blocks, variably composed of chlorite, garnet, lawsonite, white-mica, epidote, and titanite. Partial rodingitization accompanied serpentinization of the surrounding peridotite, and predated the high-pressure metamorphism that led to the formation of lawsonite. Extensive Mg metasomatism followed rodingitization, leading to prevalent Mg-rich rims around the mafic blocks. Potassium metasomatism, although minor compared to the rodingitization and Mg metasomatism, contributed to the formation of muscovite pseudomorphs after lawsonite during exhumation.

Patchy charnockites from Jenapore, Eastern Ghats granulite belt, India: Structural and petrochemical evidences attesting to their relict nature

The charnockite patches that occur within leptynite host, in and around Jenapore, northern sector of the Eastern Ghats granulite belt, are disposed in a linear fashion and generally have sharp lithological contact with the host leptynite. Sometimes the patches and foliations of the host are cofolded. Also, the patches sometimes have the internalS 1 foliation, while the host leptynite records onlyS2 foliation. Mineralogically and chemically patchy charnockites and host leptynites are distinct entities, and cannot be related by any prograde and retrograde reactions. Particularly important is the peraluminous granitic composition and high Rb/Sr ratios of the leptynites, presumably resulting from biotite-dehydration melting; as against metaluminous granodioritic to tonalitic composition and low Rb/Sr ratios of the patchy charnockites, presumably resulting from hornblende-dehydration melting. The charnockite patches here can be interpreted as caught up patches or xenolith within granitic melt (leptynite). Mg-rich rims of garnet in the charnockite patch were probably caused by heat from the crystallising melt or decompression during ascent of melt.

Garnet zoning and reaction textures in overprinted eclogites, Bohemian Massif, European variscides: a record of their thermal history during exhumation

Eclogites from different parts of the Variscan basement of the Bohemian Massif show a variation in temperatures recorded at peak pressures as well as during subsequent exhumation. These differences are reflected in the extent of modification of pre-, syn- and post-eclogite facies garnet compositions and by the types of reactions taking place at garnet rims. Münchberg Massif eclogites formed at conditions of 600–650°C, > 20 kbar, were mainly overprinted in the amphibolite facies field, and contain garnets with an old homogeneous core and often well preserved idiomorphic growth zoning to Mg-rich rims. In the Mariánské Lázně Complex the eclogite facies stage (640–715°C, < 19.5 kbar) was followed by a weak granulite facies stage, with the formation of orthopyroxene and sapphirine, and a more pervasive amphibolite facies overprint. Despite the slightly higher temperatures, strong compositional profiles in garnet have not been smoothed or homogenised. In the Moldanubian Zone (eclogite stage 700–800°C, > 15 kbar), many eclogites show well developed granulite facies overprints with widespread orthopyroxene growth in garnet and omphacite domains, spinel and/or sapphirine after kyanite, and in some cases even a low-pressure high-temperature overprint that produced Fe-rich olivine. The latter rocks clearly endured higher temperatures during breakdown but, remarkably, strong growth zoning and sharp overgrowth features are preserved in some cases. Diffusion modeling shows that the preservation of zoning in garnet of rocks that have experienced granulite facies temperatures is only possible if the time spent at these conditions is short ( < 1 Ma). This time scale is inconsistent with a smooth P-T-t path as expected from thermal modeling for erosion- or extension-driven exhumation and implies that these Variscan rocks experienced short-lived thermal events during their exhumation history.

Fassaite composition trends during crystallization of Allende Type B refractory inclusion melts

Fassaite is a major phase in Type B refractory inclusions and its wide compositional variability makes it an excellent recorder of crystallization-induced changes in liquid composition. Traverses across zoned crystals with electron and ion microprobes show that Ti, V, Sc, Zr and Hf decrease from core to rim, while Mg, Si, REEs, and other trace elements increase. Fassaite/liquid distribution coefficients derived from the compositions of early fassaite and calculated parent liquid compositions are higher than previously reported fassaite Ds, e.g., 0.2–0.8 vs. 0.08–0.5 for the trivalent REEs, possibly due to the high Ti and Al contents of the fassaite we analyzed. Log-log plots of concentrations of elements vs. those of Sc give linear relationships with slopes of (D̄ixl/L − 1)/(D̄Scxl/L − 1), where D̄ixl/L- is the bulk crystal/liquid distribution coefficient for element i. From this relationship, we derive effective fassaite/liquid Ds of 2.8 for Sc, 1.5 for Hf, 1.1 for Zr, 0.52 for Y, 0.34 for Ta, 0.29 for Nb, and 0.31–0.48 for trivalent REEs. The effective Ds are applicable to fassaite crystallizing from melts having the compositions and thermal histories of Allende Type B refractory inclusions, and incorporate any equilibrium and kinetic effects that modify Ds during the course of crystallization. Observed fassaite REE contents can be reproduced by a fractional crystallization model in which the effective Ds are used. Ti3+ is more compatible in fassaite than Ti4+ and, without equilibration of the residual liquid with an external reducing gas, Ti3+Titot ratios (where Titot = Ti3+ + Ti4+ should decrease from core to rim in fassaite crystals. Comparison of profiles of Ti3+Titot ratios in several crystals with those calculated for fractional crystallization suggests that the trend in one crystal was controlled by fractional crystallization alone, but in two others the virtually constant Ti3+Titot ratios may imply that the liquids in these inclusions were able to maintain equilibrium with the solar nebular gas throughout crystallization. Attempts to determine the ƒo2 of fassaite crystallization on the basis of Ti3+Titot ratios must be based on fassaite grains which are not zoned with respect to Ti3+Titot. The range of observed REE abundances in subliquidus fassaite (7–800 × Cl for La and 30–2000 × Cl for Lu) now encompasses those in grains previously identified as relict, removing a major reason for their identification as such. Rounded fassaite grains with fairly sharp Mg-rich rims and Ti-rich cores, poikilitically enclosed in mantle melilite in TS34 are candidates for relict grains. The grains are so Scrich that they would require extreme prior crystallization of spinel and melilite, which is inconsistent with their very low REE contents (e.g., La at 3–7 × Cl). We suggest that the cores of these grains are relict and were trapped with liquid during formation of the mantle.

Mineralogical relations and magma mixing in calc-alkaline andesites from lake Atitl�n, Guatemala

Quaternary calc-alkaline andesites erupted form three neighboring volcanoes along the Guatemalan volcanic front have mineralogic compositions and textures which show varying degrees of disequilibrium. Basaltic andesites and andesites (SiO2 % = 50−59), erupted from Atitlan volcano located nearer to the trench, have the lowest degree of disequilibrium. These lavas contain an anhydrous phenocryst assemblage of mildly bimodal plagioclase, olivine, augite opx, and magnetite. Orthopyroxene occurs at the expense of olivine with increasing whole rock SiO2. Most pyroxene phenocrysts show a trend of Fe enrichment. Andesites from Toliman (SiO2% = 53−62) and San Pedro (Si02% = 54−67) volcanoes, located further away from the trench, show comparatively high and moderate degrees of disequilibrium, respectively. Toliman andesites have bimodal plagioclase compositions and textures. Olivine persists with increasing whole rock Si02 and lacks clear modal relations with coexisting orthopyroxene and hornblende phenocrysts. When compared to Atitlan andesites, Toliman olivines are more forsteritic and pyroxenes contain higher proportions of Mg-rich rims, though normal zoned phenocrysts occur within the same rock. Toliman andesite also have lower proportions of phenocrysts to microphenocrysts, more calcic plagioclase groundmass compositions, and higher modal phenocrystic magnetite. San Pedro andesites have disequilibrium assemblages similar to Toliman andesites but are not as striking. Magma mixing is proposed as the dominant cause for observed disequilibrium. Disequilibrium features are preserved best in Toliman and San Pedro andesites because inferred durations between mixing and eruption are shortest, and consequently, these mixed andesites more clearly record mafic and silicic endmember compositions. The mafic component is a relatively high temperature, high-Al basalt containing phenocrysts of Mg-rich olivine (Fo = 78−80), calcic plagioclase (An 70−80), augite and titanomagnetite. The silicic component contains quartz, sodic plagioclase (An 40−50), Fe-rich orthopyroxene and titanomagnetite. Short durations between mixing and eruption produce petrographic features which, in part, mimic the effects of increasing PH20 in a fractionating magma. Inferred mixing durations for Atitlann andesites are longer and involve a less-silicic composition. The intervolcano disequilibrium relations suggest that as Si02 in a silicic endmember increases, the duration and efficiency of mixing decreases.

Formation of zoning of olivine with progressive metamorphism of serpentinite. An example from the Ryumon peridotite body of the Sanbagawa metamorphic belt, Kii peninsula.

Olivine formed by the reaction antigorite+brucite=2 olivine+3 water from the Ryumon peridotite body, a regionally metamorphosed serpentinite of the Sanbagawa metamorphic belt, exhibits zoning with Fe, Mn and Ni-rich core and Mg-rich rim. From the core to the rim, XMn decreases at much higher rate than XNi does. The chemical relations between olivine and coexisting antigorite and brucite suggest that the zoning was formed by fractional crystallization of olivine from antigorite and brucite at the time of progressive metamorphism. The higher decreasing rate of XMn than XNi can be explained by the smaller values of KDanti-ol Mn-Mg and KD bru-ol Mn-Mg than KDanti-ol Ni-Mg and KDbru-ol Ni-Mg, respectively. This type of zoning may be characteristic of the zoning of olivine crystallized from serpentine. The zoned olivine suggests that local equilibrium was maintained even at low-grade in metamorphosed serpentinite.