Enrichment In Incompatible Elements Research Articles

Mechanisms for incompatible-element enrichment on the Moon deduced from the lunar basaltic meteorite Northwest Africa 032

The lunar meteorite Northwest Africa (NWA) 032 is a low-Ti basalt that has incompatible-element abundances and Th/Sm ratios characteristic of the involvement of late stage magma ocean crystallization products (urKREEP) in its petrogenesis. This sample is very fine-grained and contains terrestrial weather products. A progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain Rb–Sr and Sm–Nd ages. Although many of the leachates, as well as the unleached mineral and whole rock fractions contain terrestrial alteration products, selected residue fractions yield concordant Rb–Sr and Sm–Nd ages. Rubidium–Sr isotopic analyses yield an age of 2947 ± 16 Ma with an initial 87Sr/ 86Sr of 0.700057 ± 17. These characteristics indicate NWA 032 is derived from a source region with an 87Rb/ 86Sr ratio of 0.044 ± 0.001. This value is higher than all but those determined for KREEP basalts, and suggests that NWA 032 is derived from a source region that has higher incompatible-element abundances than other low-Ti basalts. Samarium–neodymium isotopic analysis yield a concordant age of 2931 ± 92 Ma and an initial ε Nd of +9.71 ± 0.74 corresponding to a source region with 147Sm/ 144Nd ratio of 0.246 ± 0.004. The initial Nd isotopic composition stands in contrast to the initial Sr isotopic composition by requiring NWA 032 to be derived from a source with lower incompatible-element abundances than most low-Ti basalts. The source of NWA 032 is therefore unlike those of other lunar basalts. Modeling of magma ocean cumulate formation demonstrates that unlike other low-Ti basalt source regions the NWA 032 source is a mixture of olivine, pigeonite, and clinopyroxene bearing cumulates and only a small amount of urKREEP. Furthermore, unlike other mare basalt sources, the NWA 032 source does not contain appreciable quantities of plagioclase. Partial melting models demonstrate that the incompatible-element characteristics of the NWA 032 result from formation by smaller degrees of partial melting than other mare basalts. Thus, the incompatible-element geochemical signature that is observed in NWA 032 appears to reflect the combined effects of generation from an unusual plagioclase-free incompatible-element-depleted source region by very small degrees of partial melting. This study demonstrates that both the presence of urKREEP in the source region and small degrees of partial melting generate magmas with similar, but not identical, incompatible-element characteristics. In addition, it underscores the fact that there is significantly more geochemical diversity on the Moon than is represented by samples collected by the American and Soviet lunar missions.

Alteration and metasomatism of the UG2 melanorite and its stratiform pegmatoids, Bushveld Complex, South Africa -characteristics, timing and origins.

The UG2 chromitite at Two Rivers Platinum Mine (eastern limb, Bushveld Complex) is fully enclosed within a UG2 melanorite that is here divided into five sub-units, all of which crystallised from the same parental magma. From top to bottom, these sub-units are the upper distal, upper stratiform pegmatoid, upper split, lower split and lower stratiform pegmatoid. The upper distal and lower split sub-units exhibit the most primary characteristics whereas hydrothermal fluids and/or hydrous melts infiltrated the remaining three sub-units, as well as the UG2 chromitite. Within the stratiform pegmatoid sub-units, network veins of amoeboidal troctolite, high abundances of secondary silicates, enrichment in incompatible elements and the coarsening of grain sizes indicate this infiltration of fluids and/or melts. The upper split sub-unit is chromite-rich and locally altered to amphibolite. This fluid- and melt-induced alteration/metasomatism occurred at high temperatures and is pervasive along at least 7 km of strike length, suggesting magmatic origins. These magmatic fluids are probably related to the compaction of underlying cumulates or the intrusion of chromite crystal slurries.

Petrogenesis and Geological Implications of the Tianheyong Cenozoic Basalts, Inner Mongolia China

The Tianheyong basalts are basanites, which at least can be subdivided into three types of mineral assemblages. In general, the Tianheyong basalts are characteristics of low silica ( w(SiO 2)=43.97%–45.68%), enriched alkali ( w(K 2O+Na 2O) =5.91%–7.65%), enriched potassium ( w(K 2O)=2.04%–2.89%), high contents of titanium ( w(TiO 2)=2.18%–2.37%), high magnesium value (Mg #=68–76), and high contents of REE (ΣREE=(246.62–329.82)×10 −6). The chondrite normalized patterns of REE show right inclined smooth lines, extremely enriched in LREE, significantly fractionated from LREE and HREE, without obvious anomaly of Eu (δEu=0.90–1.02) and Ce (δCe=0.96–1.00), and the high enrichment of incompatible elements, especially, the peak value in high field strength elements (HFSE) of Nb, Ta, and Th indicate the OIB-like distribution patterns of multi-elements. The Tianheyong basalts are also high in compatible elements: Co ((39.1–48.9)×10 −6), Ni ((130–257)×10 −6), and Cr ((138–320)×10 −6). All of the above characteristics as well as the low degree of crystallization, the abundance in peridotite inclusions, and minor amount of xenocrysts, and the variation trend of elements indicate that the basalts were solidified from primitive basaltic magma. Trace element ratio of Ba/Rb (12–35) and variation of alkali metals might indicate the metasomatism of fluids in a source region of enriched lherzolites. Petrogenesis simulation indicates that the Tianheyong primitive basaltic magma might be the product of polybaric melting column with non-modal partial melting processes of enriched lherzolite, which straddle the boundary between garnet- and spinel-facies lherzolites. Magma might originate from low partial melting of source rocks(∼<5%), which have∼1% partial melting of garnet lherzolite and 2%–5% partial melting of spinel lherzolite. The integrative analysis indicates that the triggering mechanism of source rock partial melting might be the edge-driven convection of mantle, which results in that the Tianheyong basalts have the deepest origin depth as compared with those of eastern Jining and Hannuoba basalts.

Peridotite-melt interaction under transitional conditions between the spinel and plagioclase facies beneath the Mid-Atlantic Ridge: Insight from peridotites at 13°N

Peridotites (diopside-bearing harzburgites) found at 13°N of the Mid-Atlantic Ridge fall into two compositional groups. Peridotites P1 are plagioclase-free rocks with minerals of uniform composition and Capyroxene strongly depleted in highly incompatible elements. Peridotites P2 bear evidence of interaction with basic melt: mafic veinlets; wide variations in mineral composition; enrichment of minerals in highly incompatible elements (Na, Zr, and LREE); enrichment of minerals in moderately incompatible elements (Ti, Y, and HREE) from P1 level to abundances 4–10 times higher toward the contacts with mafic aggregates; and exotic mineral assemblages Cr-spinel + rutile and Cr-spinel + ilmenite in peridotite and pentlandite + rutile in mafic veinlets. Anomalous incompatible-element enrichment of minerals from peridotites P2 occurred at the spinel-plagioclase facies boundary, which corresponds to a pressure of about 0.8–0.9 GPa. The temperature and oxygen fugacity were estimated from spinel-orthopyroxene-olivine equilibria. Peridotites P1 with an uniform mineral composition record the temperature of the last complete recrystallization at 940–1050°C and FMQ buffer oxygen fugacity within the calculation error. In peridotites P2, local assemblages have different compositions of coexisting minerals, which reflects repeated partial recrystallization during heating to magmatic temperatures (above 1200°C) and subsequent reequilibration at temperatures decreasing to 910°C and an oxygen fugacity significantly higher than FMQ buffer (Δlog \( f_{O_2 } \) = 1.3–1.9). Mafic veins are considered to be a crystallization product from basic melt enriched in Mg and Ni via interaction with peridotite. The geochemical type of the melt reconstructed by the equilibrium with Ca-pyroxene is defined as T-MORB: (La/Sm)N ≈ 1.6 and (Ce/Yb)N ≈ 2.3, which is well consistent with compositional variations of modern basaltic lavas in this segment of Mid-Atlantic Ridge, including new data on quenched basaltic glasses.

Mantle deformation and noble gases: Helium and neon in oceanic mylonites

In an effort to constrain the behavior of noble gases during mantle deformation, we present new helium and neon data in mylonites from subaerial St. Peter and St. Paul Archipelago (Mid-Atlantic Ridge) and the submarine Southwest Indian Ridge. Coupled vacuum crushing and melting experiments show that most of the helium and neon within the mylonites is contained in the mineral matrices rather than fluid or melt inclusions: only 5 to 18% of the total helium is released by crushing. The mylonites and ultramylonites have much higher total helium concentrations than expected, based on their small grain size. The St. Paul's Rocks mylonites have helium contents equivalent to gas-rich MORB glasses, ranging from 6 × 10 − 6 to 3.8 × 10 − 5 cc STP He/g. The submarine mylonites and ultramylonites have helium contents between 5 × 10 − 8 and 4.4 × 10 −7 cc STP He/g, compared to 6.2 × 10 − 9 to 3.6 × 10 − 8 for the protogranular/porphyroclastic peridotites. Although the dataset is small, it suggests a relationship between metamorphic texture and noble gas abundance, and that mylonitization introduces mantle helium into mineral matrices. The mylonites are extremely fine grained, with an average grain size of ~ 10 µm, so helium residence in grain boundaries is also plausible. St. Paul's Rocks have modal hornblende, extreme geochemical enrichments in incompatible elements, and high temperature alteration phases (e.g., talc) that are rare or absent in the other samples; mineralogy must also play an important role. The 3He/ 4He ratios in the peridotites are primarily mantle derived, based on comparison with MORB data, suggesting that peridotites reflect the source mantle isotopic compositions. Neon isotopes in St. Paul's Rocks are a mixture of air and normal mantle, and fall along the line defined by MORB glasses. The atmospheric neon signal is preferentially released by crushing in vacuum, suggesting it resides within weakly bound sites in cracks and grain boundaries. He/Ne ratios in St. Paul's Rocks vary widely (~ 20×) with deformation and mineralogy, with the highest He/Ne ratios (and helium concentration) found in the finest grained ultramylonite peridotite. The neon and helium isotopic data show that mantle gases are preserved in fine-grained mylonites at very high concentrations. The most likely mechanism is diffusive trapping within defects at pressure in the mantle. The relationship between texture and helium abundance in peridotites suggests that metamorphism is a potentially important control on noble gas distribution in the mantle and crust.

Late Cenozoic xenoliths as a guide to the chemical isotopic composition and thermal state of the upper mantle under northeast Africa

Spinel-lherzolite xenoliths from the upper lithospheric mantle from young, 1.5 Ma intra-plate magmatism of the Bayuda volcanic field (Sudan), ca. 500 km west of the Red Sea, cover a broad range from fertile to depleted lherzolite. The trace element patterns are rather uniformly depleted with respect to the primitive mantle with only few samples showing enrichment of incompatible trace elements. Mineral chemistry and fabrics indicate equilibrium in the rocks at temperatures between ~800 and 960 °C in most samples (range between 760 and 1190 °C) as determined from ortho- and clinopyroxene compositions. Sr, Nd, and Pb isotope compositions of clinopyroxene separates vary between values of old depleted mantle ( e.g. 87 Sr/ 86 Sr ~ 0.70178; 143 Nd/ 144 Nd ~ 0.51349; 206 Pb/ 204 Pb ~ 16.14; 207 Pb/ 204 Pb ~ 15.22; 208 Pb/ 204 Pb ~ 35.85) and mantle showing strong influence of metasomatism ( e.g. 87 Sr/ 86 Sr ~ 0.70315; 143 Nd/ 144 Nd ~ 0.51279; 206 Pb/ 204 Pb ~ 19.66; 207 Pb/ 204 Pb ~ 15.60; 208 Pb/ 204 Pb ~ 39.67). The old depleted mantle formed before the Pan-African cycle. The formation of the metasomatized lithospheric mantle beneath the Arabian Plate and NE-Africa is explained as the result of the Pan-African orogeny. The depleted trace element patterns are at odds with the isotope composition in many samples and indicate a relatively young melt extraction from the upper lithospheric mantle, likely in the Cenozoic. Therefore, the thermal state of the upper mantle lithosphere indicated by the xenoliths is likely Cenozoic.

Identification of a subduction zone component in the Higganum dike, Central Atlantic Magmatic Province: A LA‐ICPMS study of clinopyroxene with implications for flood basalt petrogenesis

Determining the primary compositions of continental flood basalts has proven allusive because of the masking effects of crustal contamination. The Higganum dike (Connecticut, USA) that fed the Talcott basalt of the Central Atlantic Magmatic Province hosts strongly zoned clinopyroxenes. The cores are Mg‐ and Cr‐rich, containing up to 1 wt % Cr2O3. The grains show optical and major, minor, and trace element continua between cores and their mantles and rims, indicating that the cores are early crystallizing phenocrysts, not xenocrysts. Laser ablation inductively coupled plasma mass spectrometry analyses of the cores were used to calculate liquid compositions of the most primitive component in the Higganum system and to make inferences about composition of continental flood basalt parental magmas. The Cr‐rich cores yield calculated liquids with Mg numbers of 63. The high‐Cr contents of these calculated liquids indicate the absence of significant clinoproxene or chromite fractionation. Olivine or orthopyroxene fractionation may have lowered the liquid Mg number but would not have significantly influenced the calculated incompatible trace element patterns. Extended rare earth element diagrams for the most Cr‐rich liquids show enrichment in incompatible elements, similar to arc basalts. Prominent negative Nb anomalies are present as are distinct positive Pb anomalies indicative of the influence of subduction zone fluids in the mantle source. Calculated liquid Ba/La values range from 3 to 30, also suggestive of fluid input. These data indicate that the mantle source experienced subduction zone fluid metasomatism. Subsequent crustal contamination is evident in bulk‐rock Nd and Sr isotopic compositions.

Variation of magma generation and mantle sources during continental rifting observed in Cenozoic lavas from the Eger Rift, Central Europe

The 400 km long Eger Rift is one of the major extensional structures in Central Europe and is associated with abundant Tertiary to Quaternary volcanic rocks occurring both within the rift and mainly on its NW uplifted flank. Primitive lavas predominate with compositions ranging from alkali basalts to nephelinites and melilitites. Lavas in the rift centre show similar Si-undersaturation, incompatible element enrichment, and Sr, Nd, and Pb isotope compositions as rocks on the flanks of the Eger Rift suggesting similar degrees of partial melting and mantle sources. Crustal assimilation is abundant and increases with evolution of the magmas. The Eger Rift crosses several different lithospheric units but no systematic changes of magma sources appear to exist across lithospheric boundaries. Three different mantle sources can be defined in the Eger Rift magmas. Two of these sources must be enriched in incompatible elements as they yield the most enriched magmas and probably reside in the lithosphere because they show evidence for residual amphibole. The most common of these end-members resembles the so-called FOZO mantle source and occurs in the asthenosphere. The (Ce/Yb) N in the central part of the Eger Rift are lower than at the western end of the rift suggesting higher degrees of partial melting in the central part which apparently correspond to the narrowing of the rift, i.e. less extension in its western part.

Trace elements and Sr–Nd–Pb isotopes of K-rich, shoshonitic, and calc-alkaline magmatism of the Western Mediterranean Region: Genesis of ultrapotassic to calc-alkaline magmatic associations in a post-collisional geodynamic setting

High-MgO ultrapotassic rocks are found in four different areas of the Western Mediterranean basin associated in space and time with shoshonitic and calc-alkaline rocks. They represent different magmatic events at the active continental plate margin from Oligocene to Pleistocene. These rocks are found within the Western Alps (Northern Italy), in Corsica (France), in Murcia-Almeria (South-Eastern Spain), and in Southern Tuscany (Central Italy). Ultrapotassic terms are mostly lamprophyres, but olivine latitic lavas with a clear lamproitic affinity are also found. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated, and they are characterised by low Al 2O 3, CaO, and Na 2O contents. They are plagioclase-free rocks, but K-feldspar is abundant beside other K-bearing phases. Shoshonitic and calc-alkaline rocks are invariably space associated to lamproites, and they either precede or follow them. High-Mg ultrapotassic rocks are characterised by strong enrichment of incompatible elements, which prevent further enrichment due to shallow level crustal contamination. K 2O and incompatible element contents decrease passing from high-Mg ultrapotassic to high-Mg shoshonitic and calc-alkaline rocks suggesting that K and incompatible trace elements enrichments are a primary characteristic. Ultrapotassic to calc-alkaline rocks from Western Mediterranean regions, in spite of their different age of emplacement, are characterised by similar incompatible trace elements distribution. Depletion of High Field Strength elements with respect to Large Ion Lithophile elements is observed. Positive spikes at Th, U, and Pb, with negative spikes at Ba, Nb, Ta, Sr, P, and Ti, are common characteristics of ultrapotassic (lamproitic) to high-K calc-alkaline rocks. Ultrapotassic rocks are extremely enriched in radiogenic Sr and unradiogenic Nd with respect to the associated shoshonitic and calc-alkaline rocks. Different isotopic values are distinctive of the different magmatic provinces irrespective of magmatic affinities. 87Sr/ 86Sr i ranges between 0.71645 and 0.71759 for Western Alps lamproites, between 0.71226 and 0.71230 for Corsica lamproite, between 0.71642 and 0.72259 for Murcia-Almeria lamproites, and between 0.71578 and 0.71672 for Tuscany lamproites. Radiogenic Sr decreases along with K 2O through shoshonitic to calc-alkaline rocks. Conversely 143Nd/ 144Nd i values increase with decreasing K 2O, with the highest value of 0.51243 found for the one samples from Murcia-Almeria. Contrasting trends are observed among initial values of lead isotopes, but all falling well within the field of upper crustal rocks. Different trends of 207Pb/ 204Pb i and 208Pb/ 204Pb i vs. 206Pb/ 204Pb i for samples from the different provinces are observed. Several evidences indicate that most of the magmas of the different provinces have been generated in a depleted upper mantle (i.e., lithospheric) modified by metasomatism, but an asthenospheric component is also recognised in Corsica. At least two different subduction-related metasomatic agents re-fertilised the depleted original upper mantle source. Carbonate-free siliciclastic sediments and carbonate-rich sediments have been recycled within the upper mantle through subduction and partial melting. Assuming that metasomatic component is concentrated in a vein network, in Tuscany and Corsica, time relationships indicate that low degree of partial melting of the pure vein produced lamproitic-like magmas, whereas an increase in the partial melting involve the surrounding upper mantle, then diluting the alkaline component and produced the entire spectra of magma observed. In South-Eastern Spain calc-alkaline magmatism preceded lamproitic ones, and might be generated by partial melting of mantle wedge metasomatised by fluids from oceanic slab prior to collision. Lamproitic magmas followed after melt-dominated metasomatic agents invaded the lithospheric upper mantle domain. Migration of the magmatism with time is the result of eastward migration of subduction with subsequent opening of Balearic, Ligure-Provençal, and Tyrrhenian basins.

Geochemistry of basalt from the North Gorda segment of the Gorda Ridge: Evolution toward ultraslow spreading ridge lavas due to decreasing magma supply

High‐density, precisely located, dive and rock‐corer basalt samples from the 65‐km‐long North Gorda ridge segment reveal compositional diversity as great as for the entire Gorda Ridge. Lava compositions along the ridge axis show considerable major and minor element diversity (MgO 9.2–4.4%, K2O 0.04–0.36%) for lavas erupted in close proximity. Although they form a near‐continuum in the higher MgO range, the samples can be separated into two groups; one is typical N‐type mid‐ocean ridge basalt (MORB) (K2O/TiO2 &lt; 0.09), and the other is a more enriched T‐MORB (K2O/TiO2 &gt; 0.09). Incompatible elements also reflect this grouping with (Ce/Yb)N &lt; 1 and Zr/Nb &gt; 20 for N‐MORB and (Ce/Yb)N &gt; 1 and Zr/Nb &lt; 20 for T‐MORB. Samples collected from off‐axis, over a distance of 4 km up the eastern rift valley wall, are all light rare earth element (LREE)‐depleted N‐MORB with a narrower compositional range (MgO of glasses 7.7 ± 0.3%, Zr/Nb = 38–50; (Ce/Yb)N &lt; 1), although isotopic ratios are comparable to those on‐axis. Lavas erupted in the past, before the present‐day deep axial valley formed on this part of the ridge, were more uniform N‐MORB, generated by larger degrees of melting when magma supply was greater. Basalts from the adjoining southern Juan de Fuca Ridge segment, with comparable spreading rate but distinctly different ridge morphology, are also all LREE‐depleted N‐MORB, but the narrow range of evolved compositions of the sheet flows covering the broad, U‐shaped valley suggests shallower, more steady state magma reservoirs underlying this ridge segment. Basalts from Escanaba, the slowest spreading segment of Gorda Ridge, include N‐, T‐, and E‐MORB that were erupted from isolated volcanic centers. The pattern of incompatible element enrichment, especially in LREE, K, Ba, and 87Sr/86Sr, with decreasing spreading rate and magma supply is even more pronounced at the ultraslow spreading Arctic ridges where most lavas are E‐MORB (Zr/Nb &lt; 10, (Ce/Yb)N &gt;1.0–3.0). Arctic E‐MORB compositions lie along a common mixing trend with those from North Gorda. As the magma budget and/or partial melting decreases, a similar enriched component, especially in K, Ba, and LREE, widely present in the oceanic mantle is apparently incorporated to a greater degree. At North Gorda, morphology and chemical characteristics appear to evolve with time toward that of ultraslow spreading ridges.

The fidelity of melt inclusions as records of melt composition

A series of experiments created melt inclusions in plagioclase and pyroxene crystals grown from a basaltic melt at 1,150°C, 1.0 GPa to investigate diffusive fractionation during melt inclusion formation; additionally, P diffusion in a basaltic melt was measured at 1.0 GPa. Melt inclusions and melts within a few 100 microns of plagioclase–melt interfaces were analyzed for comparison with melt compositions far from the crystals. Melt inclusions and melt compositions in the boundary layer close to the crystal–melt interface were similar, but both differ significantly in incompatible element concentrations from melt found greater than approximately 200 microns away from the crystals. The compositional profiles of S, Cl, P, Fe, and Al in the boundary layers were successfully reproduced by a two-step model of rapid crystal growth followed by diffusive relaxation toward equilibrium after termination of crystal growth. Applying this model to investigate possible incompatible element enrichment in natural melt inclusions demonstrated that at growth rates high enough to create the conditions for melt inclusion formation, ∼10−9–10−8 m s−1, the concentration of water in the boundary layer near the crystal was similar to that of the bulk melt because of its high diffusion coefficient, but sulfur, with a diffusivity similar to major elements and CO2, was somewhat enriched in the boundary layer melt, and phosphorus, with its low diffusion coefficient similar to other high-field strength elements and rare earth elements, was significantly enriched. Thus, the concentrations of sulfur and phosphorus in melt inclusions may over-estimate their values in the bulk melt, and other elements with similar diffusion coefficients may also be enriched in melt inclusions relative to the bulk melt.

Subduction recycling of continental sediments and the origin of geochemically enriched reservoirs in the deep mantle

Isotopic and trace element geochemical studies of ocean island basalts (OIBs) have for many years been used to infer the presence of long-lived (∼ 1–2 Ga old) compositional heterogeneities in the deep mantle related to recycling of crustal lithologies and marine and terrigenous sediments via subduction [e.g., Zindler, A., Hart, S.R., 1986. Chemical geodynamics. Annu. Rev. Earth Planet. Sci. 14, 493–571; Weaver, B.L., 1991. The origin of ocean island basalt end-member compositions: trace element and isotopic constraints. Earth Planet. Sci. Lett. 104, 381–397; Chauvel, C., Hofmann, A.W., Vidal, P., 1992. HIMU-EM: the French Polynesian connection. Earth Planet. Sci. Lett. 110, 99–119; Hofmann, A.W., 1997. Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219–229; Willbold, M., Stracke, A., 2006. Trace element composition of mantle end-members: Implications for recycling of oceanic and upper and lower continental crust. Geochem. Geophys. Geosyst. Q04004. 7, doi:10.1029/2005GC001005]. In particular, models for the EM-1 type (“enriched mantle”) OIB reservoir have invoked the presence of subducted, continental-derived sediment to explain high 87Sr/ 86Sr ratios, low 143Nd/ 144Nd and 206Pb/ 204Pb ratios, and extreme enrichments in incompatible elements observed in OIB lavas from, for example, the Pitcairn Island group in the South Pacific [Woodhead, J.D., McCulloch, M.T., 1989; Woodhead, J.D., Devey, C.W., 1993. Geochemistry of the Pitcairn seamounts, I: source character and temporal trends. Earth Planet. Sci. Lett. 116, 81–99; Eisele, J., Sharma, M., Galer, S.J.G., Blichert–Toft, J., Devey, C.W., Hofmann, A.W., 2002. The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot. Earth Planet. Sci. Lett. 196, 197–212]. More recently, ultrapotassic, mantle-derived lavas (lamproites) from Gaussberg, Antarctica have been interpreted as the product of melting of deeply recycled (subducted) Archean-age metasediments in the mantle transition zone [Murphy, D.T., Collerson, K.D., Kamber, B.S., 2002. Lamproites from Gaussberg, Antartica: possible transition zone melts of Archaean subducted sediments. J. Petrol. 43, 981–1001]. Here we report the results of phase equilibria experiments on two different natural sedimentary compositions (a high-grade metapelite with < 1 wt.% H 2O, and a marine “mud” with ∼ 8 wt.% H 2O) at 16–23 GPa. In both materials, the high-pressure mineral assemblages contain ∼ 15–30 wt.% K-hollandite (KAlSi 3O 8), in addition to stishovite, garnet, an Al-silicate phase (kyanite or phase egg), and a Fe–Ti spinel (corundum). Ion microprobe analyses of K-hollandite for a range of trace elements reveal that this phase controls a significant proportion of the whole-rock budget of incompatible, large-ion lithophile elements (LILEs, e.g., Rb, Ba, Sr, K, Pb, La, Ce and Th). Comparisons between the abundances and ratios of these elements in K-hollandite with those in EM-I type ocean–island basalts from Pitcairn Island and related seamounts, and with the Gaussberg lamproites, indicate the presence of deeply recycled, continent-derived sediments in these lavas' sources. Our results suggest that the incompatible trace-element signature of EM-I OIB reservoirs in general and of the Gaussberg lamproites in particular can be attributed to recycling of K-hollandite-bearing continental sediments to transition zone depths.

LA-ICP-MS study of apatite- and K feldspar-hosted primary carbonatite melt inclusions in clinopyroxenite xenoliths from lamprophyres, Hungary: Implications for significance of carbonatite melts in the Earth’s mantle

We report the results of LA-ICP-MS analyses of rock forming minerals in clinopyroxene-apatite-K feldspar-phlogopite (CAKP) metasomatic xenoliths and primary carbonatite melt inclusions (CMI) hosted in apatite (Ap) and K feldspar (Kfs). The xenoliths are from the Cretaceous lamprophyre dikes of the Transdanubian Central Range, Hungary. The CMI in Ap have phosphorus dolomitic composition as opposed to CMI in Kfs, which display dolomitic alkali-aluminosiliceous character. The melts found in CMI in Ap and in Kfs likely formed by liquid–liquid separation from an originally carbonate- and phosphorous-rich melt. Primitive mantle (PM) normalized trace element distributions of both Ap- and Kfs-hosted CMI ( n = 60 and 20, respectively) reveal a strong negative Ti-anomaly, and an extreme enrichment in incompatible elements (U, Th, LILE and LREE) relative to HREE, Sc, V, Ni and Cr. Rarely, apatites contain unique CMI, which show major- and trace-element signature transitional to K feldspar-hosted CMI. This is due to heterogeneous entrapment of an immiscible phosphorous-bearing carbonatite melt and a carbonate-bearing alkali aluminosiliceous melt, which is a further evidence for their co-existence. CMI reveal that U, Th, Pb, Nb, Ta, P, Sr, Y and REE partitioned into the phosphorous-bearing carbonatite melt, whereas Cs, Rb, Na, K, B, Al, Zr and Hf preferred the silicate-bearing liquid. PM normalized REE pattern (high LREE/HREE), elevated Zr and Hf contents and negative Ti anomaly of clinopyroxene (Cpx) indicate that its formation is genetically linked to carbonatite metasomatism attested by CMI. Trace element partitioning between the studied Cpx and CMI is in accordance with experimentally determined trace element distributions between Cpx and carbonatite melt. Cpx, which occur in samples with high modal proportion of apatite represent mantle section, which interacted with a higher amount of “initial” carbonatite melt than Cpx from apatite-poor xenoliths. This is confirmed by higher Cr, Ni, V, Sc, Ti and lower Zr, as well as Hf concentration in Cpx from xenoliths with low modal abundance of Ap. CMI reveal that Ti, V, Ni and Cr were in lower concentration in the “initial” carbonatite melt than in PM. Contrarily, Zr and Hf were more abundant in this melt than in PM. Consequently, a continuously migrating “initial” carbonatite melt, increased Zr and Hf concentration, and decreased Ti, Sc, V, Ni and especially Cr in the clinopyroxenes. Our findings suggest that the studied CAKP rocks were formed by carbonatite melt metasomatism, which occurred in an open system in the upper mantle.

Reconnaissance-scale conceptual fuzzy-logic prospectivity modelling for iron oxide copper – gold deposits in the northern Fennoscandian Shield, Finland

The conceptual approach used in this study incorporates spatial analysis techniques for data integration and analysis to perform reconnaissance-scale mineral prospectivity mapping for iron oxide copper – gold (IOCG) mineralisation in Finland. The known IOCG occurrences in Finland are characterised by the following features: (i) an epigenetic magnetite-rich host-rock; (ii) an association of Fe – Cu – Au ± Co ± U; (iii) ore minerals comprising magnetite, chalcopyrite, pyrite or pyrrhotite, and native gold; (iv) a gangue dominated by Ca-amphibole ± diopside, albite and biotite; (v) enrichment in Ag, Au, Bi, Ca, CO2, Cu, Fe, S, Te ± As, Ba, Cl, Co, K, LREE, Mo, Na, Pb, Rb, Sb, Se, U; (vi) multi-stage alteration; (vii) formation in the P – T range of 400 – 600°C, 150 – 350 MPa; and (viii) a distinct structural control in regions that have experienced both extensive compression and extension. The datasets used for the prospectivity analysis include a 1:1 000 000 scale geological map, high-resolution airborne geophysics, regional-scale multi-element till-geochemistry data, and a mineral occurrence database. The derived parameters used in the conceptual analysis include: (i) proximity to the craton margin; (ii) intersecting fault structures; (iii) presence of granitic intrusions particularly those with compatible and incompatible element enrichment; (iv) Cu, Co and Fe concentrations in till samples; (v) presence of hematite; and (vi) airborne magnetic highs and radiometric U data. A conceptual fuzzy-logic model was used to predict and locate the most prospective or favourable areas for IOCG exploration in the study area using the above-mentioned data layers. The models identify several permissive and high-potential areas within a significantly reduced potential exploration area. Validation of the modelling was conducted by quantifying the spatial association between the predicted endowment as favourability classes on the prospectivity map and the known mineral deposit sites with IOCG affinities using the Bayesian weights-of-evidence method.

The age of the martian meteorite Northwest Africa 1195 and the differentiation history of the shergottites

Samarium-neodymium isotopic analyses of unleached and acid-leached mineral fractions from the recently identified olivine-bearing shergottite Northwest Africa 1195 yield a crystallization age of 347 ± 13 Ma and an ε Nd 143 value of +40.1 ± 0.9. Maskelynite fractions do not lie on the Sm–Nd isochron and appear to contain a martian surface component with low 147Sm/ 144Nd and 143Nd/ 144Nd ratios that was added during shock. The Rb–Sr system is disturbed and does not yield an isochron. Terrestrial Sr appears to have affected all of the mineral fractions, although a maximum initial 87Sr/ 86Sr ratio of 0.7016 is estimated by passing a 347 Ma reference line through the maskelynite fraction that is least affected by contamination. The high initial ε Nd 143 value and the low initial 87Sr/ 86Sr ratio, combined with the geologically young crystallization age, indicate that Northwest Africa 1195 is derived from a source region characterized by a long-term incompatible-element depletion. The age and initial Sr and Nd isotopic compositions of Northwest Africa 1195 are very similar to those of Queen Alexandra Range 94201, indicating these samples were derived from source regions with similar Sr–Nd isotopic systematics. These similarities suggest that these two meteorites share a close petrogenetic relationship and might have been erupted from a common volcano. The meteorites Yamato 980459, Dar al Gani 476, Sayh al Uhaymir 005/008, and Dhofar 019 also have relatively old ages between 474 and 575 Ma and trace element and/or isotopic systematics that are indicative of derivation from incompatible-element-depleted sources. This suggests that the oldest group of meteorites is more closely related to one another than they are to the younger meteorites that are derived from less incompatible-element-depleted sources. Closed-system fractional crystallization of this suite of meteorites is modeled with the MELTS algorithm using the bulk composition of Yamato 980459 as a parent. These models reproduce many of the major element and mineralogical variations observed in the suite. In addition, the rare earth element systematics of these meteorites are reproduced by fractional crystallization using the proportions of phases and extents of crystallization that are calculated by MELTS. Other shergottites that demonstrate enrichments in incompatible-elements and have evolved Sr and Nd isotopic systematics have some geochemical systematics that are similar to those observed in the depleted group. Most notably, although they exhibit a very limited range of incompatible trace element and isotopic compositions, they have highly variable major element compositions. This is also consistent with evolution from a common mantle source region by variable amounts of fractional crystallization. If this scenario is correct, it suggests that the combined effects of source composition and fractional crystallization are likely to account for the major element, trace element, and isotopic diversity of all shergottites.

Geochemistry of mafic phenocrysts from alkaline lamprophyres of the Spanish Central System: implications on crystal fractionation, magma mixing and xenoliths entrapment within deep magma chambers

The Permian alkaline lamprophyres from the Spanish Central System (SCS) are highly porphyritic rocks which carry a heterogeneous population of clinopyroxene and kaersutite zoned phernocrysts. Clinopyroxene phenocrysts may show 1) normal zoning (Cpx-I), 2) reverse zoning with Fe-rich green cores (Cpx-II), and 3) reverse zoning with colourless Al-poor, silica-rich cores (Cpx-III). Kaersutite phenocrysts also show a slight reverse zoning. Major and trace element composition of Cpx-I suggests that their compositional variation is related to a crystal fractionation process from melts similar to the host lamprophyres. The Cpx-II cores represent crystallization from highly evolved melts (low Mg-Cr contents and incompatible element enrichment), genetically related with the SCS alkaline magmatism, and the growth or surrounding Mg-rich inner rims points to a magma mixing process. The major and trace element composition of Cpx-III cores supports derivation from a magma which has fractionated plagioclase. This characteristic, together with their similarities when compared to clinopyroxenes from charnockite xenoliths, suggests that they might be xenocrysts from deep calc-alkaline cumulates. The composition of melts in equilibrium with clinopyroxene and amphibole phenocrysts supports a model in which Cpx-II and Cpx-III cores would have been incorporated into a more primitive lamprophyric magma stagnated at lower crustal levels. The low pressure composition of all phenocryst outer rims indicates that they crystallised directly from the host alkaline magma at their subvolcanic emplacement levels.

Megacryst suites from the Lekkerfontein and Uintjiesberg kimberlites, southern Africa: evidence for a non-cognate origin

Research Article| December 01, 2007 Megacryst suites from the Lekkerfontein and Uintjiesberg kimberlites, southern Africa: evidence for a non-cognate origin Margaret Merry; Margaret Merry Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa, margaret.merry@erm.com; anton.leroex@uct.ac.za Search for other works by this author on: GSW Google Scholar Anton le Roex Anton le Roex Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa, margaret.merry@erm.com; anton.leroex@uct.ac.za Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2007) 110 (4): 597–610. https://doi.org/10.2113/gssajg.110.4.597 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Margaret Merry, Anton le Roex; Megacryst suites from the Lekkerfontein and Uintjiesberg kimberlites, southern Africa: evidence for a non-cognate origin. South African Journal of Geology 2007;; 110 (4): 597–610. doi: https://doi.org/10.2113/gssajg.110.4.597 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Garnet and clinopyroxene megacrysts found in the Uintjiesberg and Lekkerfontein kimberlites, southern Africa, have been analysed for their major and trace element compositions. Minerals from both megacrysts suites define coherent geochemical trends that can be attributed to differentiation via crystallisation in a closed system. Clinopyroxene megacrysts from both localities are of the subcalcic, low-Cr variety and have Mg#’s that range from 0.821 to 0.890, whereas garnets have Mg#’s in the range 0.702 to 0.824. Clinopyroxene megacrysts from both localities are strongly light rare earth element (REE) enriched when normalised to chondrites (La/Ybn ∼5), whereas garnets are all strongly LREE depleted (La/Ybn <0.014) with flat HREE patterns; both mineral types show strongly subparallel REE patterns. Primitive mantle normalised trace element patterns are likewise subparallel, with clinopyroxene megacrysts reflecting strong incompatible element enrichment and garnet megacrysts incompatible element depletion. Quantitative modelling of REE patterns in both garnet and clinopyroxene megacrysts provides compelling evidence that an ocean island basalt (OIB)-like parental magma, rather than a primary kimberlite magma, gave rise, over an extended (∼60 to 75%) crystallisation period, to the megacryst suite minerals from both localities. Variations in Sr and high field strength element abundances are consistent with this model. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Alkaline-ultrabasic mantle-derived magmas, their sources, and crystallization features: Data of melt inclusion studies

To find out the reasons responsible for the diversity of igneous rocks forming the alkaline-ultrabasic carbonatite Krestovskiy massif (the Maimecha–Kotui province, Russia) we have studied melt inclusions in clinopyroxene of trachydolerites, porphyric melanephelinites, and tholeiites. It was established that the homogenization temperatures of inclusions in these rocks are rather close: 1140–1180 °C, 1190–1230 °C, and 1150–1210 °C, respectively. Compositions of melt inclusions in clinopyroxenes from different rocks are significantly different. The chemical composition of clinopyroxene of trachydolerites corresponds to that of trachybasalts and their derivatives. The inclusions are enriched in Sr, Ba, P, and S and their total sum of alkalies (at K ≥ Na) is never less than 5–6 wt.%. Inclusions from the rims of clinopyroxene phenocrysts in porphyric melanephelinites are similar in composition also to inclusions in trachydolerites. But in the cores of clinopyroxene phenocrysts the composition of inclusions corresponds to nephelinite melt. The composition of some melt inclusions in the intermediate and cores zones of clinopyroxene from porphyric melanephelinite has high SiO 2 (53–55 wt.%), MgO (8–9 wt.%) and a low (1–2 wt.%) total sum of alkalies (at Na ≥ K) and is depleted in Al 2O 3 (6–7 wt.%), which is similar to the composition of basaltic komatiites. The composition of inclusions in tholeiites is also basic, highly magnesian, and low-alkaline, Sr and Ba are rare to absent. Compared to the inclusions of basaltic komatiite composition, the inclusions in tholeiites are enriched in Al and depleted in Ca, Ti, and P. The melts trapped in clinopyroxenes from different rocks contain low (0.014–0.018 wt.%) water but they are enriched in F: from 0.37 wt.% in nephelinite melts to 0.1–0.06 wt.% in tholeiite and basaltic komatiite melts. Inclusions in all the rocks under study, host clinopyroxene, and the rocks themselves are significantly enriched in incompatible elements (1–2 orders of magnitude relative to the mantle norm). In tholeiites, the partitioning of these elements is rather uniform, while in trachydolerites and especially in melanephelinites it is contrasting with a drastic depletion in HREE relative to LREE, MREE, and HFSE. A conclusion is made that the Krestovskiy massif was formed by no less than three mantle-derived magmas: melanephelinite, tholeiite and basaltic komatiite. Magmas were generated in different magma sources at different depths with various degrees of enrichment in incompatible elements. These magmas were, most likely, dominated by melanephelinite magma. In intermediate chambers this magma differentiated to form derivative melts of nephelinite, trachydolerite–trachyandesite–trachyte compositions. Komatiite-basalt melts were, most likely, derivatives of primitive meimechite magmas.

Origin of E-MORB in a fossil spreading center: the Antarctic-Phoenix Ridge, Drake Passage, Antarctica

The fossilized Antarctic-Phoenix Ridge (APR) with three segments (P1, P2, and P3), Drake Passage, is distant from the known hotspots, and consists of older N-MORB formed prior to the extinction of spreading and younger E-MORB after extinction. The older N-MORB (3.5–6.4 Ma) occur in the southeastern flank of the P3 segment (PR3) and the younger E-MORB (1.4–3.1 Ma) comprise a huge seamount at the former ridge axis of the P3 segment (SPR) and a big volcanic edifice at the northwestern flank of the P2 segment (PR2). The PR3 basalts have higher Mg#, K/Ba, and CaO/Al2O3 and lower Zr/Y, Sr, and Na80 (fractionation-corrected Na2O to 8.0% MgO) with slight enrichment in incompatible elements and almost flat REE patterns. The SPR and PR2 basalts are highly enriched in incompatible elements and LREE. The extinction of spreading at 3.3 Ma seems to have led to a temporary magma oversupply with E-MORB signatures. Geochemical signatures such as Ba/TiO2, Ba/La, and Sm/La suggest the heterogeneity of upper mantle and formation of E-MORB by higher contribution of enriched materials (e.g., metasomatized veins) to mantle melting than the N-MORB environment. E-MORB magmas beneath the APR seen to have been produced by low-degree melting at deeper regime, where enriched materials have preferentially participated in the melting. The occurrence of E-MORB at the APR is a good example to better understand what kinds of magmatism would occur in association with extinction of the ridge spreading.

Geochemistry of the Jurassic Mirdita Ophiolite (Albania) and the MORB to SSZ evolution of a marginal basin oceanic crust

The Middle Jurassic Mirdita Ophiolite in northern Albania is part of an ophiolite belt occurring between the Apulian and Pelagonian subcontinents in the Balkan Peninsula. The upper mantle and crustal units of the Mirdita Ophiolite show major changes in thickness, rock types, and chemical compositions from west to east as a result of its complex evolution in a suprasubduction zone (SSZ) environment. The ∼ 3–4-km-thick Western Mirdita Ophiolite (WMO) includes lherzolite–harzburgite, plagioclase–lherzolite, plagioclase–dunite in its upper mantle units and a plutonic complex composed of olivine gabbro, troctolite, ferrogabbro, and gabbro. These peridotites and gabbroic rocks are overlain directly by a ∼ 600-m-thick extrusive sequence containing basaltic pillow lavas and hyaloclastites. Sheeted dikes are rare in the WMO. The ∼ 12-km-thick Eastern Mirdita Ophiolite (EMO) includes tectonized harzburgite and dunite with extensive chromite deposits, as well as ultramafic cumulates including olivine clinopyroxenite, wehrlite, olivine websterite, and dunite forming a transitional Moho with the overlying lower crustal section. The plutonic rocks are made of pyroxenite, gabbronorite, gabbro, amphibole gabbro, diorite, quartz diorite, and plagiogranite. A well-developed sheeted dike complex has mutually intrusive relations with the underlying isotropic gabbros and plagiogranites and feeds into the overlying pillow lavas. Dike compositions change from older basalt to basaltic andesite, andesite, dacite, quartz diorite, to late-stage andesitic and boninitic dikes as constrained by crosscutting relations. The ∼ 1.1-km-thick extrusive sequence comprises basaltic and basaltic andesitic pillow lavas in the lower 700 m, and andesitic, dacitic and rhyodacitic massive sheet flows in the upper 400 m. Rare boninitic dikes and lavas occur as the youngest igneous products within the EMO. The basaltic and basaltic andesitic rocks of the WMO extrusive sequence display MORB affinities with Ti and Zr contents decreasing upsection (TiO 2 = 3.5–0.5%, Zr = 300–50 ppm), while ɛ Nd( T) (+ 8 to + 6.5) varies little. These magmas were derived from partial melting of fertile MORB-type mantle. Fractional crystallization was important in the evolution of WMO magmas. The low Ti and HREE abundances and Cs and Ba enrichments in the uppermost basaltic andesites may indicate an increased subduction influence in the evolution of the late-stage WMO magmas. Basaltic andesites in the lower 700 m of the EMO volcanic sequence have lower TiO 2 (∼ 0.5%) and Zr (∼ 50 ppm) contents but ɛ Nd( T) values (+ 7 to + 6.5) are similar to those of the WMO lavas. These rocks show variable enrichment in subduction-enriched incompatible elements (Cs, Ba, Th, U, LREE). The basaltic andesites through dacites and boninites within the upper 400 meters of EMO lavas show low TiO 2 (∼ 0.8–0.3%) and ɛ Nd( T) (+ 6.5 to + 3.0). The mantle source of these rocks was variably enriched in Th by melts derived from subducted sediments as indicated by the large variations in Ba, K, and Pb contents. EMO boninitic dikes and lavas and some gabbroic intrusions with negative ɛ Nd ( T) values (− 1.4 and − 4.0, respectively) suggest that these magmas were produced from partial melting of previously depleted, ultra-refractory mantle. The MORB to SSZ transition (from west to east and stratigraphically upwards in the Mirdita Ophiolite and the progression of the ɛ Nd( T) values from + 8.0 to − 4.0 towards the east resulted from an eastward shift in protoarc–forearc magmatism, keeping pace with slab rollback in this direction. The mantle flow above the retreating slab and in the arc-wedge corner played a major role in the evolution of the melting column, in which melt generation, aggregation/mixing and differentiation occurred at all levels of the sub-arc/forearc mantle. The SSZ Mirdita Ophiolite evolved during the intra-oceanic collapse and closure of the Pindos marginal basin, which had a protracted tectonic history involving seafloor spreading, protoarc rifting, and trench-continent collision.