Silicic melts produced by reaction between peridotite and infiltrating basaltic melts: ion probe data on glasses and minerals in veined xenoliths from La Palma, Canary Islands

Mantle xenoliths hosted by the historic Volcan de San Antonio, La Palma, Canary Islands include veined spinel harzburgites and spinel dunites. Glasses and associated minerals in the vein system of veined xenoliths show a gradual transition in composition from broad veins to narrow veinlets. Broad veins contain alkali basaltic glass with semi-linear trace element patterns enriched in strongly incompatible elements. As the veins become narrower, the SiO₂-contents in glass increase (46 → 67 wt% SiO₂ in harzburgite, 43 → 58 wt% in dunite) and the trace element patterns change gradually to concave patterns depleted in moderately incompatible elements (e.g. HREE, Zr, Ti) relative to highly incompatible ones. The highest SiO₂-contents (ca. 68% SiO₂, low Ti-Fe-Mg-Ca-contents) and most extreme concave trace element patterns are exhibited by glass in unveined peridotite xenoliths. Clinopyroxenes shift from LREE-enriched augites in basaltic glass, to REE-depleted Cr-diopside in highly silicic glass. Estimates indicate that the most silicic glasses represent melts in, or near, equilibrium with their host peridotites. The observed trace element changes are compatible with formation of the silicic melts by processes involving infiltration of basaltic melts into mantle peridotite followed by reactions and crystallization. The Fe-Mg interdiffusion profiles in olivine porphyroclasts adjacent to the veins indicate a minimum period of diffusion of 600 years, implying that the reaction processes have taken place in situ in the upper mantle. The CaO-TiO₂-La/Nd relationships of mantle rocks may be used to discriminate between metasomatism caused by carbonatitic and silicic melts. Unveined mantle xenoliths from La Palma and Hierro (Canary Islands) show a wide range in La/Nd ratios with relatively constant, low-CaO contents which is compatible with metasomatism of “normal” abyssal peridotite by silicic melts. Peridotite xenoliths from Tenerife show somewhat higher CaO and TiO₂ contents than those from the other islands and may have been affected by basaltic or carbonatitic melts. The observed trace element signatures of ultramafic xenoliths from La Palma and other Canary Islands may be accounted for by addition of small amounts (1–7%) of highly silicic melt to unmetasomatized peridotite. Also ultramafic xenoliths from other localities, e.g. eastern Australia, show CaO-TiO₂-La/Nd relationships compatible with metasomatism by silicic melts. These results suggest that silicic melts may represent important metasomatic agents. Received: 15 November 1998 / Accepted: 17 May 1999     

Diamond precipitation and mantle metasomatism – evidence from the trace element chemistry of silicate inclusions in diamonds from Akwatia, Ghana

Trace element concentrations in the four principal peridotitic silicate phases (garnet, olivine, orthopyroxene, clinopyroxene) included in diamonds from Akwatia (Birim Field, Ghana) were determined using SIMS. Incompatible trace elements are hosted in garnet and clinopyroxene except for Sr which is equally distributed between orthopyroxene and garnet in harzburgitic paragenesis diamonds. The separation between lherzolitic and harzburgitic inclusion parageneses, which is commonly made using compositional fields for garnets in a CaO versus Cr₂O₃ diagram, is also apparent from the Ti and Sr contents in both olivine and garnet. Titanium is much higher in the lherzolitic and Sr in the harzburgitic inclusions. Chondrite normalised REE patterns of lherzolitic garnets are enriched (10–20 times chondrite) in HREE (La_N/Yb_N = 0.02–0.06) while harzburgitic garnets have sinusoidal REE_N patterns, with the highest concentrations for Ce and Nd (2–8 times chondritic) and a minimum at Ho (0.2–0.7 times chondritic). Clinopyroxene inclusions show negative slopes with La enrichment 10–100 times chondritic and low Lu (0.1–1 times chondritic). Both a lherzolitic and a harzburgitic garnet with very high knorringite contents (14 and 21 wt% Cr₂O₃ respectively) could be readily distinguished from other garnets of their parageneses by much higher levels of LREE enrichment. The REE patterns for calculated melt compositions from lherzolitic garnet inclusions fall into the compositional field for kimberlitic-lamproitic and carbonatitic melts. Much more strongly fractionated REE patterns calculated from harzburgitic garnets, and low concentrations in Ti, Y, Zr, and Hf, differ significantly from known alkaline and carbonatitic melts and require a different agent. Equilibration temperatures for harzburgitic inclusions are generally below the C-H-O solidus of their paragenesis, those of lherzolitic inclusions are above. Crystallisation of harzburgitic diamonds from CO₂-bearing melts or fluids may thus be excluded. Diamond inclusion chemistry and mineralogy also is inconsistent with known examples of metasomatism by H₂O-rich melts. We therefore favour diamond precipitation by oxidation of CH₄-rich fluids with highly fractionated trace element patterns which are possibly due to “chromatographic” fractionation processes. Received: 27 January 1996 / Accepted: 5 May 1997     

Carbonatite metasomatized peridotite xenoliths from southern Patagonia: implications for lithospheric processes and Neogene plateau magmatism

The mineral chemistry, major and trace element, and Sr–Nd isotopic composition of Cr-diopside, spinel peridotite xenoliths from the Estancia Lote 17 locality in southern Patagonia document a strong carbonatitic metasomatism of the backarc continental lithosphere. The Lote 17 peridotite xenolith suite consists of hydrous spinel lherzolite, wehrlite, and olivine websterite, and anhydrous harzburgite and lherzolite. Two-pyroxene thermometry indicates equilibration temperatures ranging from 870 to 1015 °C and the lack of plagioclase or garnet suggests the xenoliths originated from between ˜40 and 60 km depth. All of the xenoliths are LILE- and LREE-enriched, but have relatively low ⁸⁷Sr/⁸⁶Sr (0.70294 to 0.70342) and high ɛ_Nd (+3.0 to +6.6), indicating recent trace element enrichment (∼25 Ma, based on the low ⁸⁷Sr/⁸⁶Sr and high Rb concentrations of phlogopite separates) in the long-term, melt-depleted Patagonian lithosphere. Lote 17 peridotite xenoliths are divided into two basic groups. Group 1 xenoliths consist of fertile peridotites that contain hydrous phases (amphibole ± phlogopite ± apatite). Group 1 xenoliths are further subdivided into three groups (a, b, and c) based on distinctive textures and whole-rock chemistry. Group 1 xenolith mineralogy and chemistry are consistent with a complex metasomatic history involving variable extents of recent carbonatite metasomatism (high Ca/Al, Nb/La, Zr/Hf, low Ti/Eu) that has overprinted earlier metasomatic events. Group 2 xenoliths consist of infertile, anhydrous harzburgites and record cryptic metasomatism that is attributed to CO₂-rich fluids liberated from Group 1 carbonatite metasomatic reactions. Extremely variable incompatible trace element ratios and depleted Sr–Nd isotopic compositions of Lote 17 peridotite xenoliths indicate that the continental lithosphere was neither the primary source nor an enriched lithospheric contaminant for Neogene Patagonian plateau lavas. Neogene plateau magmatism associated with formation of asthenospheric slab windows may have triggered this occurrence of “intraplate-type” carbonatite metasomatism in an active continental backarc setting. Received: 26 January 2000 / Accepted: 1 March 2000     

The determination of partial melt compositions of peridotitic systems by melt inclusion synthesis

An experimental method of melt inclusion synthesis within olivine crystals has been developed to determine the composition of the melt present in a partially molten peridotite assemblage. Trace element doped peridotite was equilibrated with 5 wt% of a C-O-H volatile source at 20 kbar/1175 °C in a piston-cylinder apparatus under buffered oxygen and sulphur fugacity conditions [log(f _O2) ∼ IW +1 log unit, log (f _S2) ∼ Fe/FeS > +1 log unit]. A single crystal of olivine, which had been cut to a disc shape, was included in the sample capsule. At run conditions the peridotite charge formed olivine, orthopyroxene, clinopyroxene, Fe-Ni sulphide and a volatile-bearing melt. The melt phase is preserved as homogeneous glass inclusions up to 50 μm in size, trapped in situ in the olivine disc. The major element composition of the glass inclusions showed them to be of broadly basaltic character, but with a low Mg/(Mg + ΣFe), which is associated with precipitation of olivine from the melt inclusion onto the walls of the olivine disc during quenching. Thus the equilibrium melt composition has been calculated from the glass inclusion composition by addition of olivine component using the Fe/Mg exchange coefficient of Roeder and Emslie (1970); the desired Mg/(Mg + ΣFe) being determined from the composition of olivine formed at run conditions in the peridotite section of the charge. The melt composition obtained is close to the trend for dry melting established by Falloon and Green (1988), and it is evident that although the reduced volatiles in this case have induced a liquidus depression of some 250 °C, there has been only a small shift in melt composition. Trace element, carbon and hydrogen contents of thirteen melt inclusions have been determined by secondary ion mass spectrometry (SIMS). The trace element signature is consistent with ∼29% melting in equilibrium with a lherzolitic assemblage. The equilibrium melt has a C/H of 0.48 by weight. Carbon solubility in partial melts is thus significant under reducing conditions in the presence of dissolved “water components” and establishes a major melt fluxing role for carbon in the upper mantle. The ubiquitous presence of carbon and hydrogen in basaltic magmas underscores the importance of determining both the position of vapour-present solidi and the composition of melts generated, when developing petrogenetic models. Received: 1 July 1996 / Accepted: 25 June 1997     

Metasomatism of sub-arc mantle peridotites below southernmost South America: reduction of<Emphasis Type="Italic"> f</Emphasis>O<Subscript>2</Subscript> by slab-melt

Quaternary basalts in the Cerro del Fraile area contain two types of mantle xenoliths; coarse-grained (2–5 mm) C-type spinel harzburgites and lherzolites, and fine-grained (0.5–2 mm) intensely metasomatized F-type spinel lherzolites. C-type xenoliths have high Mg in olivine (Fo = 90–91) and a range in Cr# [Cr/ (Cr + Al) = 0.17–0.34] in spinel. Two C-type samples contain websterite veinlets and solidified patches of melt that is now composed of minute quenched grains of plagioclase + Cr-spinel + clinopyroxene + olivine. These patches of quenched melts are formed by decompression melting of pargasitic amphibole. High Ti contents and common occurrence of relic Cr-spinel in the quenched melts indicate that the amphibole is formed from spinel by interaction with the Ti-rich parental magma of the websterite veinlets. The fO₂ values of these two C-type xenoliths range from ΔFMQ −0.2 to −0.4, which is consistent with their metasomatism by an asthenospheric mantle-derived melt. The rest of the C-type samples are free of “melt,” but show cryptic metasomatism by slab-derived aqueous fluids, which produced high concentrations of fluid-mobile elements in clinopyroxenes, and higher fO₂ ranging from ΔFMQ +0.1 to +0.3. F-type lherzolites are intensely metasomatized to form spinel with low Cr# (∼0.13) and silicate minerals with low MgO, olivine (Fo = ∼84), orthpyroxene [Mg# = Mg/(Mg + ΣFe) = ∼0.86] and clinopyroxene (Mg# = ∼0.88). Patches of “melt” are common in all F-type samples and their compositions are similar to pargasitic amphibole with low TiO₂ (<0.56 wt%), Cr₂O₃ (<0.55 wt%) and MgO (<16.3 wt%). Low Mg# values of silicate minerals, including the amphibole, suggest that the metasomatic agent is most likely a slab melt. This is supported by high ratios of Sr/Y and light rare earth elements (REE)/heavy REE in clinopyroxenes. F-type xenoliths show relatively low fO₂ (ΔFMQ −0.9 to −1.1) compared to C-type xenoliths and this is explained by the fusion of organic-rich sediments overlying the slab during the slab melt. Trench-fill sediments in the area are high in organic matter. The fusion of such wet sediments likely produced CH₄-rich fluids and reduced melts that mixed with the slab melt. High U and Th in bulk rocks and clinopyroxene in F-type xenoliths support the proposed interpretation.     

Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgO-Al2O3-SiO2-CO2 from 3 to 7 GPa

We have experimentally determined the solidus position of model lherzolite in the system CaO-MgO-Al₂O₃-SiO₂-CO₂ (CMAS.CO₂) from 3 to 7 GPa by locating isobaric invariant points where liquid coexists with olivine, orthopyroxene, clinopyroxene, garnet and carbonate. The intersection of two subsolidus reactions at the solidus involving carbonate generates two invariant points, I_1A and I_2A, which mark the transition from CO₂-bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing lherzolite respectively. In CMAS.CO₂, we find I_1A at 2.6 GPa/1230 °C and I_2A at 4.8 GPa/1320 °C. The variation of all phase compositions along the solidus has also been determined. In the pressure range investigated, solidus melts are carbonatitic with SiO₂ contents of <6 wt%, CO₂ contents of ˜45 wt%, and Ca/(Ca+Mg) ratios that range from 0.59 (3 GPa) to 0.45 (7 GPa); compositionally they resemble natural magnesiocarbonatites. Volcanic magnesiocarbonatites may well be an example of the eruption of such melts directly from their mantle source region as evidenced by their diatremic style of activity and lack of associated silicate magmas. Our data in the CMAS.CO₂ system show that in a carbonate-bearing mantle, solidus and near-solidus melts will be CO₂-rich and silica poor. The widespread evidence for the presence of CO₂ in both the oceanic and continental upper mantle implies that such low degree SiO₂-poor carbonatitic melts are common in the mantle, despite the rarity of carbonatites themselves at the Earth's surface. Received: 9 April 1997 / Accepted: 25 November 1997     

Petrology of ultramafic lamprophyres from the Beaver Lake area of Eastern Antarctica and their relation to the breakup of Gondwanaland

Summary Mesozoic melilite-bearing ultramafic lamprophyres are developed as sill, dyke and plug-like intrusive bodies in the East Antarctic Beaver Lake area. They consist of varying amounts of olivine, melilite, phlogopite, nepheline, titanomagnetite and perovskite as major phases, accompanied by minor amounts of apatite, carbonate, spinel, glass and, rarely, monticellite. The rocks are mineralogically and geochemically broadly similar to olivine melilitites, differing in higher CO₂ and modal phlogopite and carbonate contents. The ultramafic lamprophyres are MgO-rich (13.4–20.5 wt%) and SiO₂-poor (32.8–37.2 wt%), indicative of a near-primary nature. Major and trace element features are consistent with minor fractionation of olivine and Cr-spinel from melts originating at depths of 130–140 km. Primary melts originated by melting of upper mantle peridotite which had been veined by phlogopite + carbonate + clinopyroxene-bearing assemblages less than 200 Ma before eruption. The presence of the veins and their time of formation is required to explain high incompatible trace element contents and growth of ⁸⁷Sr/⁸⁶Sr, leaving ¹⁴³Nd/¹⁴⁴Nd unaffected. The major element, compatible trace element, and most radiogenic isotope characteristics are derived from melting of the wall-rock peridotite. The depth of about 130 km is indicated by the presence of phlogopite rather than amphibole in the veins, by control of the REE pattern by residual garnet, by the high MgO content of the rocks, and by the expected intersection of the rift-flank geotherm with the solidus at this depth. The higher CO₂ contents than are characteristic for olivine melilitites favoured the crystallization of melilite at crustal pressures, and suppressed the crystallization of clinopyroxene. The Beaver Lake ultramafic lamprophyres are a distal effect of the breakup of Gondwanaland, too distal to show a geochemical signature of the Kerguelen plume. Upward and outward movement of the asthenosphere-lithosphere boundary beneath the Lambert-Amery rift led first to the production of phlogopite- and carbonate-rich veins, and later to the generation of the ultramafic lamprophyres themselves. Received March 31, 2000; revised version accepted September 3, 2001     

Carbonate xenoliths in La Palma: Carbonatite or alteration product?

Carbonate xenoliths containing olivine and rimmed by kaersutitic amphibole were collected in basaltic rocks of the Basal Complex of La Palma. The mineralogical composition and microscopic appearance may suggest a relationship with carbonatites in general, thus a major element, trace element and stable isotope study was conducted to investigate the origin of the carbonate formation. Based on electron microprobe analyses, the carbonate is calcite with up to 6.3 wt% MgO and 7.2 wt% SiO₂. The elevated SiO₂ content may suggest a melt origin for the carbonate. However, the C and O isotope compositions of the carbonate xenoliths (δ¹³C and δ¹⁸O around −1‰ and 13‰, respectively) are similar to those of calcite veins and amygdales in basaltic rocks of the Basal Complexes of La Palma and Fuerteventura and are interpreted as produced by fluid degassing and metasomatism by CO₂-H₂O fluid derived from mobilization of sedimentary material. Trace element contents determined by laser-ablation ICP-MS analyses support the assumed origin, thus, the relationship with carbonatitic melts can be excluded. Based on trace element compositions, the amphibole surrounding the xenoliths is not related genetically to the carbonate. The elevated SiO₂ content of the calcite can be attributed to submicron relics of pyroxene, thus, the use of this feature as an evidence for melt origin is questionable.     

Carbonate-bearing mantle peridotite xenoliths from Spitsbergen: phase relationships,mineral compositions and trace-element residence

 Carbonates of mantle origin have been found in xenoliths from Quaternary basaltic volcanoes in NW Spitsbergen. The carbonates range from dolomite to Mg-bearing calcite and have high Mg-numbers [Mg/(Mg+Fe)=(0.92–0.99)]. In some samples they occur interstitially, e.g. at triple junctions of silicate minerals and appear to be in textural and chemical equilibrium with host lherzolite. Most commonly, however, the carbonates make up fine-grained aggregates together with (Ca,Mg)-rich olivine and (Al,Cr,Ti)-rich clinopyroxene that typically replace spinel, amphibole, and orthopyroxene as well as primary clinopyroxene and olivine. Some lherzolites contain amphibole and apatite that appear to have formed before precipitation of the carbonates. In situ analyses by proton microprobe show very high contents of Sr in the clinopyroxene, carbonates and apatite; the apatite is also very rich in LREE, U, Th, Cl, Br. Disseminated amphibole in carbonate-bearing rocks is very poor in Nb and Zr, in contrast to vein amphibole and mica from carbonate-free rocks that are rich in Nb and Zr. Overall, the Spitsbergen xenoliths provide evidence both for the occurrence of primary carbonate in apparent equilibrium with the spinel lherzolites (regardless of the nature of events that emplaced them) and for the formation of carbonate-bearing pockets consistent with metasomatism by carbonate melts. Calcite and amorphous carbonate-rich materials occur in com- posite carbonate-fluid inclusions, veins and partial melting zones that appear to be related to fluid action in the mantle, heating of the xenoliths during their entrainment in basaltic magma, and to decompression melting of the carbonates. Magnesite is a product of secondary, post-eruption alteration of the xenoliths. Received: 6 October 1995/Accepted: 17 June 1996     

Revisiting the compositions and volatile contents of olivine-hosted melt inclusions from the Mount Shasta region: implications for the formation of high-Mg andesites

Primitive chemical characteristics of high-Mg andesites (HMA) suggest equilibration with mantle wedge peridotite, and they may form through either shallow, wet partial melting of the mantle or re-equilibration of slab melts migrating through the wedge. We have re-examined a well-studied example of HMA from near Mt. Shasta, CA, because petrographic evidence for magma mixing has stimulated a recent debate over whether HMA magmas have a mantle origin. We examined naturally quenched, glassy, olivine-hosted (Fo_87–94) melt inclusions from this locality and analyzed the samples by FTIR, LA-ICPMS, and electron probe. Compositions (uncorrected for post-entrapment modification) are highly variable and can be divided into high-CaO (>10 wt%) melts only found in Fo > 91 olivines and low-CaO (<10 wt%) melts in Fo 87–94 olivine hosts. There is evidence for extensive post-entrapment modification in many inclusions. High-CaO inclusions experienced 1.4–3.5 wt% FeO^T loss through diffusive re-equilibration with the host olivine and 13–28 wt% post-entrapment olivine crystallization. Low-CaO inclusions experienced 1–16 wt% olivine crystallization with <2 wt% FeO^T loss experienced by inclusions in Fo > 90 olivines. Restored low-CaO melt inclusions are HMAs (57–61 wt% SiO₂; 4.9–10.9 wt% MgO), whereas high-CaO inclusions are primitive basaltic andesites (PBA) (51–56 wt% SiO₂; 9.8–15.1 wt% MgO). HMA and PBA inclusions have distinct trace element characteristics. Importantly, both types of inclusions are volatile-rich, with maximum values in HMA and PBA melt inclusions of 3.5 and 5.6 wt% H₂O, 830 and 2,900 ppm S, 1,590 and 2,580 ppm Cl, and 500 and 820 ppm CO₂, respectively. PBA melts are comparable to experimental hydrous melts in equilibrium with harzburgite. Two-component mixing between PBA and dacitic magma (59:41) is able to produce a primitive HMA composition, but the predicted mixture shows some small but significant major and trace element discrepancies from published whole-rock analyses from the Shasta locality. An alternative model that involves incorporation of xenocrysts (high-Mg olivine from PBA and pyroxenes from dacite) into a primary (mantle-derived) HMA magma can explain the phenocryst and melt inclusion compositions but is difficult to evaluate quantitatively because of the complex crystal populations. Our results suggest that a spectrum of mantle-derived melts, including both PBA and HMA, may be produced beneath the Shasta region. Compositional similarities between Shasta parental melts and boninites imply similar magma generation processes related to the presence of refractory harzburgite in the shallow mantle.     

Experimental studies on three potassium-rich ultramafic rocks from Damodar Valley, East India

Summary An experimental study on the phase relationships of three potassium-rich ultramafic rocks from the Damodar Valley, Gondawana basins, has been performed under upper mantle P–T conditions (1.0–2.5 GPa, 700–1200 °C). The Mohanpur lamproite and Satyanarayanpur minette, both from the Raniganj basins, have been investigated with the addition of 15 wt% H₂O. No water was added in the experiments done on an olivine minette from the Jarangdih coal mine, Bokaro Basin, which originally contains 15 wt% CO₂ and 2.86 wt% H₂O. In all cases, olivine is the liquidus phase followed by phlogopite. The subsolidus assemblage for the three rocks is a phlogopite-bearing harzburgite, associated with apatite, Mg-ilmenite and carbonates for the Jarangdih rock; apatite, chromian spinel and carbonates and priderite (only between 1.0 and 1.2 GPa) in the case of the Mohanpur lamproite, and finally apatite, chromian spinel, rutile, and carbonate in the Satyanarayanpur sample. Although orthopyroxene is absent in the natural potassium-rich ultramafic rocks, its presence in the run products of the Jarangdih rock is possibly related to a reaction between olivine and a CO₂-bearing fluid phase. The presence of orthopyroxene in the run products of Mohanpur and Satyanarayanpur rocks may be due to a reaction between K-feldspar, olivine and a vapour phase to produce phlogopite and orthopyroxene. On the basis of present experimental investigation and isotopic studies made by previous investigators, it has been suggested that these K-rich rocks have crystallized from melts derived by vein-plus-wall-rock melting of a phlogopite-bearing harzburgite source rock. Received December 15, 1999; revised version accepted June 17, 2001     

Melts in the mantle modeled in the system CaO-MgO-SiO2-CO2 at 2.7 GPa

The effect of CO₂ on mantle peridotites is modeled by experimental data for the system CaO-MgO-SiO₂-CO₂ at 2.7 GPa. The experiments provide isotherms for the vapor-saturated liquidus surface, bracket piercing points for field boundaries on the surface, and define the positions and compositions of isobaric invariant liquids on the boundaries (eutectics and peritectics). CO₂-saturated carbonatitic liquids (>80% carbonate) exist through approximately 200 °C above the solidus, with a transition to silicate liquids (>80% silicate) within ∼75 °C across a plateau on the liquidus. Carbonate-rich magmas cannot cross the silicate-carbonate liquidus field boundary, so the carbonate liquidus field is therefore a forbidden volume for liquid magmas. This confirms the fact that rounded, pure carbonates in mantle xenoliths cannot represent original liquids. A P-T diagram is constructed for the carbonation and melting reactions for mineral assemblages corresponding to lherzolite, harzburgite, websterite and wehrlite, with carbonate, CO₂ vapor (V), or both. The changing compositions of liquids in solidus reactions on the P-T diagram are illustrated by the changing compositions of eutectic and peritectic liquids on the liquidus surface. At an invariant point Q (∼2.8 GPa/1230 °C), all peridotite assemblages coexist with a calcite-dolomite solid solution (75 ± 5% CaCO₃) and a dolomitic carbonatite melt [57% CaCO₃ (CC), 33% MgCO₃ (MC), 10% CaMgSi₂O₆ (Di)], with 63% CC in the carbonate component. At higher pressures, dolomite-lherzolite, dolomite-harzburgite-V, and dolomite-websterite-V melt to yield similar liquids. Magnesian calcite-wehrlite is the only peridotite melting to carbonatitic liquids (more calcic) at pressures below Q (∼70 km). Dolomitic carbonatite magma rising through mantle to the near-isobaric solidus ledge near Q will begin to crystallize, releasing CO₂ (enhancing crack propagation), and metasomatizing lherzolite toward wehrlite. Received: 20 March 1998 / Accepted: 7 July 1999     

Recycled crustal melt injection into lithospheric mantle: implication from cumulative composite and pyroxenite xenoliths

A rare composite xenolith and abundant cumulative pyroxenites obtained from the Mesozoic Fangcheng basalts on the eastern North China Craton record a complex history of melt percolation and circulation in the subcontinental lithospheric mantle. The composite xenolith has a dunite core and an olivine clinopyroxenite rim. The dunite is of cumulative origin and has a granular recrystallized texture and extremely low Mg# [100 Mg/(Mg + Fe) = 81–82] contents in olivines. The olivine clinopyroxenite contains larger clinopyroxene and/or orthopyroxene with a few fine-grained olivine and tiny phlogopite, feldspar, and/or carbonate minerals interstitial to clinopyroxene. The clinopyroxene has low Mg# (83–85). Compositional similarity between dunitic olivine and pyroxenitic one indicates a sequential crystallization of dunite and pyroxenite from a precursor melt. Pyroxenite xenoliths include olivine websterites and clinopyroxenites, both are of cumulative origin. Estimation of the melt from major oxides in olivines and REE concentrations in clinopyroxenes in these composite and pyroxenite xenoliths suggests a derivation from subducted crustal materials, consistent with the highly enriched EMII-like Sr and Nd isotopic ratios observed in the pyroxenites. Occurrence of phlogopite, feldspar and carbonate minerals in some xenoliths requires the melt rich in alkalis (K, Na), silica and volatiles (water and CO₂) at the latest stage as well, similar to highly silicic and potassic melts. Thus, the occurrence of these composite and pyroxenite xenoliths provides an evidence for voluminous injection of recycled crustal melts into the lithosphere beneath the southeastern North China Craton at the Late Mesozoic, a reason for the rapid lithospheric enrichment in both elemental and isotopic compositions.     

Trace Element Residence and Partitioning in Mantle Xenoliths Metasomatized by Highly Alkaline, Silicate- and Carbonate-rich Melts (Kerguelen Islands, Indian Ocean)

Mantle xenoliths in alkaline lavas of the Kerguelen Islandsconsist of: (1) protogranular, Cr-diopside-bearing harzburgite;(2) poikilitic, Mg-augite-bearing harzburgite and cpx-poor lherzolite;(3) dunite that contains clinopyroxene, spinel phlogopite, andrarely amphibole. Trace element data for rocks and mineralsidentify distinctive signatures for the different rock typesand record upper-mantle processes. The harzburgites reflectan initial partial melting event followed by metasomatism bymafic alkaline to carbonatitic melts. The dunites were firstformed by reaction of a harzburgite protolith with tholeiiticto transitional basaltic melts, and subsequently developed metasomaticassemblages of clinopyroxene + phlogopite ± amphiboleby reaction with lamprophyric or carbonatitic melts. We measuredtwo-mineral partition coefficients and calculated mineral–meltpartition coefficients for 27 trace elements. In most samples,calculated budgets indicate that trace elements reside in theconstituent minerals. Clinopyroxene is the major host for REE,Sr, Y, Zr and Th; spinel is important for V and Ti; orthopyroxenefor Ti, Zr, HREE, Y, Sc and V; and olivine for Ni, Co and Sc. KEY WORDS: mantle xenoliths; mantle metasomatism; partition coefficients; Kerguelen Islands; trace elements     

Dissolution of orthopyroxene in basanitic magma between 0.4 and 2 GPa: further implications for the origin of Si-rich alkaline glass inclusions in mantle xenoliths

A large body of recent work has linked the origin of Si-Al-rich alkaline glass inclusions to metasomatic processes in the upper mantle. This study examines one possible origin for these glass inclusions, i.e., the dissolution of orthopyroxene in Si-poor alkaline (basanitic) melt. Equilibrium dissolution experiments between 0.4 and 2 GPa show that secondary glass compositions are only slightly Si enriched and are alkali poor relative to natural glass inclusions. However, disequilibrium experiments designed to examine dissolution of orthopyroxene by a basanitic melt under anhydrous, hydrous and CO₂-bearing conditions show complex reaction zones consisting of olivine, ± clinopyroxene and Si-rich alkaline glass similar in composition to that seen in mantle xenoliths. Dissolution rates are rapid and dependent on volatile content. Experiments using an anhydrous solvent show time dependent dissolution rates that are related to variable diffusion rates caused by the saturation of clinopyroxene in experiments longer than 10 minutes. The reaction zone glass shows a close compositional correspondence with natural Si-rich alkaline glass in mantle-derived xenoliths. The most Si-and alkali-rich melts are restricted to pressures of 1 GPa and below under anhydrous and CO₂-bearing conditions. At 2 GPa glass in hydrous experiments is still Si-␣and alkali-rich whereas glass in the anhydrous and CO₂-bearing experiments is only slightly enriched in SiO₂ and alkalis compared with the original solvent. In the low pressure region, anhydrous and hydrous solvent melts yield glass of similar composition whereas the glass from CO₂-bearing experiments is less SiO₂ rich. The mechanism of dissolution of orthopyroxene is complex involving rapid incongruent breakdown of the orthopyroxene, combined with olivine saturation in the reaction zone forming up to 60% olivine. Inward diffusion of CaO causes clinopyroxene saturation and uphill diffusion of Na and K give the glasses their strongly alkaline characteristics. Addition of Na and K also causes minor SiO₂ enrichment of the reaction glass by increasing the phase volume of olivine. Olivine and clinopyroxene are transiently stable phases within the reaction zone. Clinopyroxene is precipitated from the reaction zone melt near the orthopyroxene crystal and redissolved in the outer part of the reaction zone. Olivine defines the thickness of the reaction zone and is progressively dissolved in the solvent as the orthopyroxene continues to dissolve. Although there are compelling reasons for supporting the hypothesis that Si-rich alkaline melts are produced in the mantle by orthopyroxene – melt reaction in the mantle, there are several complications particularly regarding quenching in of disequilibrium reaction zone compositions and the mobility of highly polymerized melts in the upper mantle. It is considered likely that formation of veins and pools of Si-rich alkaline glass by orthopyroxene – melt reaction is a common process during the ascent of xenoliths. However, reaction in situ within the mantle will lead to equilibration and therefore secondary melts will be only moderately siliceous and alkali poor. Received: 24 August 1998 / Accepted: 2 December 1998     

Spinel inclusions in olivine of peridotite xenoliths from TUBAF seamount (Bismarck Archipelago/Papua New Guinea): evidence for the thermal and tectonic evolution of the oceanic lithosphere

Olivine in spinel peridotite xenoliths from the Bismarck Archipelago northeast of Papua New Guinea, which were transported to the surface by Quaternary basalts, shows spinel inclusions up to 25 μm long and 200 nm wide. These inclusions mainly occur as inhomogeneously distributed needles and subordinately as octahedral grains in olivine of veined metasomatic peridotites as well as peridotites without obvious metasomatism. The needles very often occur in swarms with irregular spacing in between them. Similar spinel inclusions in olivine have only previously been reported from ultramafites of meteoritic origin. Composition and orientation of the spinel inclusions were determined by transmission electron microscopy (TEM) and analytical electron microscopy (AEM). Both the needles and the grains display a uniform crystallographic orientation in the host olivine with [001]^O1//[1ˉ10]_Spl and (100)_Ol// (111)_Spl. The needles eare elongated parallel [010] in olivine, which is the same in all olivine grains. As these needles have no relation to the metasomatic sections in the peridotite, it is concluded that they are primary features of the rock. Although the composition of the spinel needles is often very similar to the large chromian spinel octahedra in the matrix, the small octahedral spinel inclusions in olivine are in part Mg-rich aluminous spinel and sometimes almost pure magnetite. The spinel needles are suggested to have formed by exsolution processes during cooling of Al- and Cr-rich, high-temperature olivine during the initial formation of the lithospheric mantle at the mid-ocean ridge. The Al-rich spinel octahedra probably formed by the breakdown of an Al-rich phase such as phlogopite or by metasomatism, whereas the magnetite was generated by oxidizing fluids. These oxidizing fluids may either have been set free by dehydration of the underlying, subducted plate or by the Quaternary magmatism responsible for the transport of the xenoliths to the seafloor. Received: 25 May 2000 / Accepted: 12 July 2000     

Mobility of components in metasomatic transformation and partial melting of gneisses: an example from Sri Lanka

Reaction textures, fluid inclusions, and metasomatic zoning coupled with thermodynamic calculations have allowed us to estimate the conditions under which a biotite–hornblende gneiss from the Kurunegala district, Sri Lanka [hornblende (N_Mg=38–42) + biotite (N_Mg=42–44) + plagioclase + quartz + K-feldspar + ilmenite + magnetite] was transformed into patches of charnockite along shear zones and foliation planes. Primary fluid inclusion data suggest that two immiscible fluids, an alkalic supercritical brine and almost pure CO₂, coexisted during the charnockitisation event and subsequent post-peak metamorphic evolution of the charnockite. These metasomatic fluids migrated through the amphibolite gneiss along shear zones and into the wallrock under peak metamorphic conditions of 700–750 °C, 5–6 kbar, and a^fl _H2O=0.52–0.59. This resulted in the formation of charnockite patches containing the assemblage orthopyroxene (N_Mg=45–48) + K-feldspar (Or_70–80) + quartz + plagioclase (An₂₈) in addition to K-feldspar microveins along quartz and plagioclase grain boundaries. Remnants of the CO₂-rich fluid were trapped as separate fluid inclusions. The charnockite patches show the following metasomatic zonation patterns: – a transition zone with the assemblage biotite (N_Mg= 49–51) + hornblende (N_Mg = 47–50) + plagioclase + quartz + K-feldspar + ilmenite + magnetite; – a KPQ (K-feldspar–plagioclase–quartz) zone with the assemblage K-feldspar + plagioclase + orthopyroxene (N_Mg=45–48) + quartz + ilmenite + magnetite; – a charnockite core with the assemblage K-feldspar + plagioclase + orthopyroxene (N_Mg = 39–41) + biotite (N_Mg=48–52) + quartz + ilmenite + magnetite. Systematic changes in the bulk chemistry and mineralogy across the four zones suggest that along with metasomatic transformation, this process may have been complicated by partial melting in the charnockite core. This melting would have been coeval with metasomatic processes on the periphery of the charnockite patch. There is also good evidence in the charnockitic core that a second mineral assemblage, consisting of orthopyroxene (N_Mg= 36–42) + biotite (N_Mg=50–51) + K-feldspar (Or_70–80) + quartz + plagioclase (An_28–26), could have crystallised from a partial melt during cooling from 720 to 660 °C at decreasing a^fl _H2O from 0.67 to 0.5. Post-magmatic evolution of charnockite at T < 700 °C resulted in fluids being released during the crystallisation of the charnockitic core. These gave rise to the formation of late stage rim myrmekites along K-feldspar grain boundaries as well as late stage biotite, cummingtonite, and carbonates. Received: 15 September 1999 / Accepted: 8 June 2000     

Megacrysts and mafic–ultramafic xenolith-bearing ignimbrites from Sirwa Volcano, Morocco: phase petrology and thermobarometry

Summary A suite of clinopyroxene and amphibole megacrysts and mafic–ultramafic xenoliths are present in ignimbritic rocks of trachybasaltic–andesitic composition from the Sirwa volcanic district, Morocco. The stumpy prismatic and sometimes euhedral clinopyroxene megacrysts are Ti–Al-rich diopsides with mg values in the range 0.82–0.87 and Ca/(Ca + Mg) ratios in the range 0.53–0.54. The prismatic, elongated amphibole megacrysts are calcic kaersutites–kaersutites with a narrow mg range (0.66–0.68). The xenoliths are represented by gabbroic and pyroxenitic types. In the gabbroic xenoliths two distinct textural types can be distinguished: medium-sized granular and banded. The granular type is characterized by the mineral assemblage Pl + Amph + Spl + Ilm + Ap. The banded type is distinct for the absence of Ilm and the presence of Cpx and Opx and shows alternating bands enriched in Pl and Amph, respectively. The megacrysts and, probably, the xenoliths are considered not cognate with the present host rocks since the calculated liquids in equilibrium with clinopyroxene and amphibole megacrysts over a wide range of physical conditions have different trace and rare earth element contents. The observed phase relations and thermobarometric calculations indicate that the megacrysts and xenoliths crystallized from their parent melts at P ≥ 10 kbar and T ≤ 1160 °C, i.e., in the upper mantle or near the crust-mantle boundary. A deep ( ≥ 30 km) magmatic chamber, where the megacrysts and xenoliths originated, and a shallow volcanic chamber, energetically activated up to explosive conditions by injection of deep-originated melts, is suggested to explain the occurrence of high-pressure megacrysts and xenoliths in the Sirwa volcanic explosive products. Received October 8, 2000; revised version accepted September 9, 2001     

Multiphase carbonate-salt immiscibility in carbonatite melts: data on melt inclusions from the Krestovskiy massif minerals (Polar Siberia)

Minerals of olivine–melilite and olivine–monticellite rocks from the Krestovskiy massif contain primary silicate-salt, carbonate-salt, and salt melt inclusions. Silicate-salt inclusions are present in perovskite I and melilite. Thermometric experiments conducted on these inclusions at 1,230–1,250°C showed silicate–carbonate liquid immiscibility. Globules of composite carbonate-salt melt rich in alkalies, P, S, and Cl separated in silicate melt. Carbonate salt globules in some inclusions from perovskite II at 1,190–1,200°C separated into immiscible liquid phases of simpler composition. Carbonate-salt and salt inclusions occur in monticellite, melilite, and garnet and homogenize at close temperatures (980–780°C). They contain alkalies, Ca, P, SO₃, Cl, and CO₂. According to the ratio of these components and predominance of one of them, melt inclusions are divided into 6 types: I—hyperalkaline (CaO/(Na₂O+K₂O)≤1) carbonate melts; II—moderately alkaline (CaO/(Na₂O+K₂O)>1) carbonate melts; III—sulfate-alkaline melts; IV—phosphate-alkaline melts; V—alkali-chloridic melts, and VI—calc-carbonate melts. Joint occurrence of all the above types and their syngenetic character were established. Some inclusions demonstrated carbonate-salt immiscibility phenomena at 840–800°C. A conclusion in made that the origin of carbonate melts during the formation of intrusion rocks is related to silicate–carbonate immiscibility in parental alkali-ultrabasic magma. The separated carbonate melt had a complex alkaline composition. Under unstable conditions the melt began to decompose into simpler immiscible fractions. Different types of carbonate-salt and salt inclusions seem to reflect the composition of these spatially isolated immiscible fractions. Liquid carbonate-salt immiscibility took place in a wide temperature range from 1,200–1,190°C to 800°C. The occurrence of this kind of processes under macroconditions might, most likely, cause the appearance of different types of immiscible carbonate-salt melts and lead to the formation of different types of carbonatites: alkali-phosphatic, alkali-sulfatic, alkali-chloridic, and, most widespread, calcitic ones.     

Melt inclusions in pegmatite quartz: complete miscibility between silicate melts and hydrous fluids at low pressure

Fluorine-, boron- and phosphorus-rich pegmatites of the Variscan Ehrenfriedersdorf complex crystallized over a temperature range from about 700 to 500 °C at a pressure of about 1 kbar. Pegmatite quartz crystals continuously trapped two different types of melt inclusions during cooling and growth: a silicate-rich H₂O-poor melt and a silicate-poor H₂O-rich melt. Both melts were simultaneously trapped on the solvus boundaries of the silicate (+ fluorine + boron + phosphorus) − water system. The partially crystallized melt inclusions were rehomogenized at 1 kbar between 500 and 712 °C in steps of 50 °C by conventional rapid-quench hydrothermal experiments. Glasses of completely rehomogenized inclusions were analyzed for H₂O by Raman spectroscopy, and for major and some trace elements by EMP (electron microprobe). Both types of melt inclusions define a solvus boundary in an X_H2O–T pseudobinary system. At 500 °C, the silicate-rich melt contains about 2.5 wt% H₂O, and the conjugate water-rich melt about 47 wt% H₂O. The solvus closes rapidly with increasing temperature. At 650 °C, the water contents are about 10 and 32 wt%, respectively. Complete miscibility is attained at the critical point: 712 °C and 21.5 wt% H₂O. Many pegmatites show high concentrations of F, B, and P, this is particularly true for those pegmatites associated with highly evolved peraluminous granites. The presence of these elements dramatically reduces the critical pressure for fluid–melt systems. At shallow intrusion levels, at T ≥ 720 °C, water is infinitely soluble in a F-, B-, and P-rich melt. Simple cooling induces a separation into two coexisting melts, accompanied with strong element fractionation. On the water-rich side of the solvus, very volatile-rich melts are produced that have vastly different physical properties as compared to “normal” silicate melts. The density, viscosity, diffusivity, and mobility of such hyper-aqueous melts under these conditions are more comparable to an aqueous fluid. Received: 15 September 1999 / Accepted: 10 December 1999