Ti Oxide Minerals Research Articles

The Skaergaard liquid line of descent revisited

There is a fundamental conflict between the suggestion that the iron content of Skaergaard liquids increases during Fe–Ti oxide fractionation and the observation that at the same time oxygen fugacity (\( f_{{\text{O}_{\text{2}}}} \)) drops by two log-units below the fayalite-magnetite-quartz oxygen buffer (FMQ). A new petrographic study of average Skaergaard gabbros shows that the total modal content of Fe–Ti oxides is about 22% in the early LZc and markedly decreases to below 5% in the UZc. Forward modeling based on these modal constraints, as well as experimental results on Skaergaard-related dikes, predicts that fractionation of troctolitic LZa gabbros drives the derivative liquid towards a high-iron content. Strong iron enrichment continues, together with a small decline in silica, during LZb crystallization due to the appearance of augite as a fractionating phase. The fractionation of Fe–Ti oxides in the LZc initially suppresses iron enrichment and reverses the silica trend to one of slight enrichment. However, continued evolution into the UZ produces liquids with maximum UZc FeO* content of 23–25 wt.% and SiO2 content of 53 wt.% (FeO* is total iron as FeO). The maximum in FeO* is dependent on several factors of which the Fe–Ti oxide mode has the strongest effect. The \( f_{{\text{O}_{\text{2}}}} \) during crystallization of the LZc is widely thought to have been at, or slightly below, the fayalite-magnetite-quartz oxygen buffer (FMQ). Under closed system evolution, incorporation of ferric iron into augite during formation of the LZb restricts the increase in \( f_{{\text{O}_{\text{2}}}} \) to about 0.1 log-units above FMQ (=0.1 ΔFMQ). Likewise, crystallization of the LZc through the UZa, involving Fe–Ti oxide minerals, leads to a decline in \( f_{{\text{O}_{\text{2}}}} \) of less than 0.1 ΔFMQ. Crystallization of the UZb-c gabbros results in oxidation to a maximum of 0.5 ΔFMQ. This behavior can account for the iron-rich character of the UZ gabbros, as well as, the low modal content of Fe–Ti oxides. Thus, evolved Skaergaard liquids are high in iron and contain a modest amount of SiO2. Our modeling result do not account for a strong drop in \( f_{{\text{O}_{\text{2}}}} \) through the layered series. Such a drop would require an unacceptably high proportion of Fe–Ti oxides and high-magnetite content in the fractionating assemblage.

Principal component analysis (PCA) and mineral associations of litoraneous facies of continental shelf carbonates from northeastern Brazil

The distribution of mineral phases according to the provenance of carbonate and terrigenous facies of carbonate sediments from a large area of the continental shelf of northeast Brazil was investigated using a major element multivariate analysis approach. Heavy minerals such as ilmenite are restricted to the litoraneous facies of the continental shelf of the states of Paraíba and Pernambuco, and clay minerals are found in distal facies of the continental shelf of the State of Ceará. In the carbonate fraction, composed essentially by Mg-calcite and aragonite, there is co-variation between CaO/MgO and bathimetry in part of the studied continental shelf from depths between 15 and 20 m, apparently due to influence of the seawater temperature, degree of oxygenation and luminosity. The terrigenous facies are mainly composed of quartz, clay minerals, K-feldspars and micro-micaceous minerals, having Fe and Ti oxide and hydroxide minerals as major accessory phases. Major element behavior attests to the presence of arenaceous quartz-rich relict sediments in the 35, 60 and 80 m isobaths of the continental shelf of the state of Ceará which is here interpreted as a proxy of ancient coast lines during the Flandrian transgression.

THE Ta-Nb-Sn-Ti OXIDE-MINERAL PARAGENESIS FROM LA VIQUITA, A SPODUMENE-BEARING RARE-ELEMENT GRANITIC PEGMATITE, SAN LUIS, ARGENTINA

The paragenesis and chemical composition of the primary Ta–Nb–Sn–Ti oxide minerals in the La Viquita spodumene-bearing granitic pegmatite, San Luis, Sierras Pampeanas, Argentina, evolved from the border zone inward. Early phases consist of minor columbite-(Fe) in the border and wall zones. In the intermediate zones, formed by a coarse-grained association of Kfs + Qtz + Ms + Ab with inwardly increasing proportions of spodumene, homogeneous or oscillatory-zoned columbite-(Fe) crystals occur in the outer parts. They show a fractionation trend to tantalite-(Fe) with Ta/(Ta + Nb) ranging from 0.35 to 0.56 at an almost constant Mn/(Mn + Fe2+ + Fe3+) of ~0.33, and locally to rare columbite-(Mn). In the middle and inner intermediate zones, the dominant phases are cassiterite, wodginite with elevated Zr contents and ferrowodginite. All members of the wodginite group are structurally ordered and carry some signs of exsolution-induced lamellae of ferrotapiolite. Ferrotapiolite is dominant in small Ms + Ab + Qtz units of the core-margin association, with intergrowths of ferrotitanowodginite, inclusions of hafnian zircon, and late-stage cassiterite. From the wall zone to the core-margin assemblage, the suite of Ta–Nb–Sn–Ti minerals shows a fractionation path with a non-linear increase in Ta/(Ta + Nb), basically in three stages marked by sequential dominance of columbite-group minerals, wodginite-group minerals and ferrotapiolite + cassiterite. The phase changes and compositional variations are attributed mainly to the combined effects of increasing proportion of Sn, Fe and Ti (the latter two possibly introduced from the host rocks), and of increasing f (O2) in the melt.

Explosive eruptive record in the Katmai region, Alaska Peninsula: an overview

At least 15 explosive eruptions from the Katmai cluster of volcanoes and another nine from other volcanoes on the Alaska Peninsula are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mount Martin and from Shelikof Strait to Bristol Bay (∼8,500 km2). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes area, and 140 radiocarbon dates on enclosing soils have established a time framework for entire soil–tephra sections to 10 ka; the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and tephra characteristics have been combined with microprobe analyses of glass and Fe–Ti oxide minerals to correlate ash layers with their source vents. Microprobe analyses (typically 20–50 analyses per glass or oxide sample) commonly show oxide compositions to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event are so compositionally coherent that they have been used as internal standards throughout this study. Other than the Novarupta and Trident eruptions of the last century, the youngest locally derived tephra is associated with emplacement of the Snowy Mountain summit dome ( 6 ka), two tephras (∼3,700 and ∼2,700 14C years B.P.), and a summit scoria cone with a crater still steaming today. Mount Katmai has three times produced very large explosive plinian to sub-plinian events (in 1912; 12–16 ka; and 23 ka) and many smaller pyroclastic deposits show that explosive activity has long been common there. Mount Griggs, fumarolically active and moderately productive during postglacial time (mostly andesitic lavas), has three nested summit craters, two of which are on top of a Holocene central cone. Only one ash has been found that is (tentatively) correlated with the most recent eruptive activity on Griggs (<3,460 14C years B.P.). Eruptions from other volcanoes NE and SW beyond the Katmai cluster represented in this area include: (1) coignimbrite ash from Kaguyak’s caldera-forming event (5,800 14C years B.P.); (2) the climactic event from Fisher caldera (∼9,100 14C years B.P.—tentatively correlated); (3) at least three eruptions most likely from Mount Peulik (∼700, ∼7,700 and ∼8,500 14C years B.P.); and (4) a phreatic fallout most likely from the Gas Rocks (∼2,300 14C years B.P.). Most of the radiocarbon dating has been done on loess, soil and peat enclosing this tephra. Ash correlations supported by stratigraphy and microprobe data are combined with radiocarbon dating to show that variably organics-bearing substrates can provide reliable limiting ages for ash layers, especially when data for several sites is available.

Sediment provenance in the River Eden catchment, Scotland, using chemical characterization of Fe–Ti oxide minerals and magnetic measurements

Synopsis Sediment provenance in the River Eden catchment, Scotland, has been studied using chemical characterization of Fe–Ti oxide minerals and magnetic measurements. Results show that magnetic parameters lead to a better characterization and differentiation of rock sources and sediments than the chemical compositions of Fe–Ti oxide grains, thereby establishing a basis for provenance modelling. However, the chemical characterization of titanomagnetite grains allows a more accurate interpretation of the magnetic measurements. Discriminant function analysis of magnetic parameters yields an environmentally sensible, qualitative provenance model; but the intra-source magnetic variability and the similarity of the sources in terms of magnetic concentration, composition and grain size prevent successful quantitative provenance modelling. Tributary inputs are recognized downstream along the main channel of the River Eden, despite the tendency for the sediment characteristics to be homogenized during transport. This study highlights the potential for using a combination of Fe–Ti oxides chemical compositions together with magnetic parameters in sedimentary provenance studies.

Y,REE,Nb,Ta,Ti-oxide (AB 2O 6) minerals from REL–REE euxenite-subtype pegmatites of the Třebíč Pluton, Czech Republic; substitutions and fractionation trends

Aeschynite-group minerals (AGM) and euxenite-group minerals (EGM) occur in REL–REE euxenite-subtype pegmatites from the Třebíč Pluton, Czech Republic. They form strongly metamictized, light brown to black, equigranular to needle-like, subhedral to anhedral grains enclosed in blocky K-feldspar and less commonly in albite, and blocky quartz, and in the graphic unit (quartz and K-feldspar). Both AGM and EGM are homogeneous to slightly heterogeneous in BSE images. They are not commonly associated with the other primary Y,REE,Ti,Nb-bearing minerals, i.e. allanite-(Ce), monazite-(Ce), titanite, and ilmenite, which occur within the same textural-paragenetic unit. Aeschynite-(Y), aeschynite-(Ce), aeschynite-(Nd), nioboaeschynite-(Ce), tantalaeschynite-(Ce), vigezzite and polycrase-(Y) were identified using EMP and canonical discrimination analysis [Ercit, T.S., 2005a. Identification and alteration trends of granitic–pegmatite-hosted (Y,REE,U,Th)–(Nb,Ta,Ti) oxide minerals: a statistical approach. Can. Mineral. 43, 4 1291–1303.]. The exchange vector ACa B(Nb,Ta) A(Y,REE) − 1 BTi − 1 or its combination with the exchange vector ACa 2 B(Nb,Ta) 3 A(U,Th) − 1 A(Y,REE) − 1 BTi − 3 have been elucidated for the AGM. The exchange vector ACa A(U,Th) A(Y,REE) − 2 is predominant in the EGM. The AGM are enriched in HREE, whereas LREE are concentrated in the EGM. Weak to none-existent geochemical fractionations, as expressed by the U/(U + Th), Y/(Y + REE), Ta/(Ta + Nb) and (Nb + Ta)/(Ti + Nb + Ta) ratios, were noted for single grains from both the AGM and EGM, as well as in grains of polycrase-(Y) from four different textural-paragenetic units located in the Vladislav pegmatite. Simultaneous increase of U/(U + Th) and Y/(Y + REE) in the AGM during fractionation is typical. The Ta/(Ta + Nb) fractionation is usually weak and contradicts the Y/(Y + REE) and U/(U + Th) fractionation trends. This unusual behavior of Nb and Ta may be controlled by associated Ti-rich minerals (titanite, ilmenite, rutile), the composition of parental melt and/or by elevated F activity. The AGM and EGM from pegmatites of the Třebíč Pluton are quite similar in composition to those from REL–REE euxenite-subtype pegmatites in the Trout Creek Pass, Chaffee County, Colorado, USA, which are generally Ca,U,Th-depleted, show lower Ta/(Ta+Nb), and lower variation in HREE/LREE.

High-field-strength elements in carbonatitic rocks: Geochemistry, crystal chemistry and significance for constraining the sources of carbonatites

Carbonatites and related rocks exhibit a significant variation in their content of high-field-strength elements (HFSE), i.e. Ti, Nb, Ta, Zr and Hf. The average abundances of these elements (calculated for 119 localities worldwide) decrease from phoscorites and silicocarbonatites to calciocarbonatites to magnesiocarbonatites. The analytical data currently available for ferrocarbonatites are insufficient to establish if this trend persists through the entire carbonatitic series. In comparison with the primitive mantle, the average carbonatite has significantly higher Nb/Ta and Zr/Hf ratios, and lower Zr/Nb and Zr/Ta ratios (35, 60, 1 and 29, respectively). Alkali-ultramafic rocks associated with carbonatites exhibit, on average, higher Zr/Nb ratios and no (or little) depletion in heavy HFSE (Ta and Hf). Implications of these data for constraining the source(s) of intracratonic carbonatites are discussed using the Kola Alkaline Province (NW Russia and Finland) as an example. The bulk of incompatible HFSE (> 85%) is estimated to reside in Ti, Nb and Zr oxide minerals (in particular, perovskite, pyrochlore, ilmenite, baddeleyite and zirconolite) and zircon, whereas a significant proportion of Ti is contained in magnetite and ferromagnesian silicates. The lowest recorded Nb/Ta and Zr/Hf ratios in the primary HFSE oxides and titanite (but not zircon) are significantly below the primitive-mantle values, which makes these phases an effective vehicle of heavy-HFSE fractionation at crustal levels. The HFSE geochemistry of carbonatites en masse is not consistent with their derivation from a hypothetical carbonate-rich alkali-ultramafic (e.g., nephelinitic) magma by either liquid immiscibility or crystal fractionation. The bulk of carbonatites are interpreted to originate by low-degree partial melting of a metasomatized HFSE 5+-enriched mantle source with elevated Nb/Ta and Zr/Hf ratios. The trace-element signature of carbonatites cannot be adequately explained by clinopyroxene-controlled decoupling of HFSE 5+ and HFSE 4+ during partial melting, and requires the presence of amphibole and, possibly, refractory phlogopite in their mantle source. The HFSE geochemistry, field relations and relative timing of Kola carbonatites and associated silicate rocks are consistent with their derivation by low-degree melting of a vertically heterogeneous amphibole-bearing mantle reservoir, followed by differentiation of the primitive magmas. The previously established volume, tectonic and geochemical constraints suggest that the metasomatic enrichment of the mantle may have resulted from reaction of garnet lherzolite with carbonate melts of asthenospheric provenance, which is in agreement with the published experimental and mantle-xenolith studies. There is no evidence for the presence of refractory rutile (i.e., subducted crust) in the postulated mantle source.

Experimental constraints on the Skaergaard liquid line of descent

New experimental information permits a forward approach to modeling the liquid line of descent of the Skaergaard intrusion. A series of melting experiments on chilled margins of evolved tholeiitic and ferrobasaltic dikes associated with the intrusion is, in combination with existing data, used to develop quantitative crystallization models that allow liquid and solid compositions to be predicted for initial magma compositions and crystallization conditions open or closed with respect to oxygen. The new experimental results comprise 6 experiments with melts coexisting with plagioclase and olivine, 29 experiments in addition containing augite, 14 experiments in addition containing ilmenite and/or magnetite, and 6 experiments in addition containing pigeonite and sometimes lacking olivine. All melting experiments were done at atmospheric pressure and with a furnace gas mostly controlled to the fayalite-magnetite-quartz oxygen buffer (FMQ). Using these experimental results, the melt evolution can be constrained for the layered series of the Skaergaard intrusion. Fractionation of a LZa troctolitic assemblage drives the residual liquid toward increasing iron with slight increase in silica. The appearance of augite as an abundant mineral phase in the LZb and the fractionation of a gabbroic assemblage adjust the liquid trend to one of slightly decreasing silica with continued strong increase in iron. Silica decline is principally dependent on the crystallization of augite and restricted to LZb. The appearance of Fe–Ti oxide minerals and the fractionation of Fe–Ti oxide gabbroic assemblages in LZc deflect the evolution trends of iron and silica. The modeling based on the experimental results suggests marked LZc–MZ silica enrichment concurrently with increasing iron content until upper MZ and thereafter relatively constant or slightly decreasing iron. The iron concentration level at which the deflection in iron and silica contents occurs is dependent on several factors of which the oxygen fugacity ( f O 2 ) has the strongest effect. Because of the restricted variation in f O 2 modeled in LZ (∼ 0.1 log unit above FMQ), the saturation of oxide minerals and the liquid line of descent are unlikely to deviate strongly from the predicted variation based on open system experimental conditions. For the same reason, there is no support for the suggestion that widely different LZc oxide mineral modes will result from crystallization conditions closed with respect to oxygen as opposed to the experimental conditions. Modeling based on the new experimental result suggests that iron can continue to increase through LZc, past the appearance of Fe–Ti oxides, and supports the possibility that iron may have continued to increase well into MZ. However, the forward modeling supports only a modest MZ and UZ decrease in f O 2 (< 1 log unit below FMQ). This result is supported by a good correspondence between the experimental modes and the actual observed gabbro modes. A marked UZ drop in f O 2 (∼ 2–3 log units below FMQ), as has been suggested, requires relatively high total modal content of Fe–Ti oxides (> 20 wt.%) and dominating magnetite over ilmenite not permitted based on the experimental observations. Such high oxide modes will always result in liquid lines of descent that are characterized by strong enrichment in silica with strong depletion in iron. The forward modeling illustrates that only for unrealistic small amounts of Fe–Ti oxide minerals will iron enrichment accompany silica depletion into UZ.

Heavy mineral characteristics of the Neogene–Quaternary sediments from the southern part of the Blagoevgrad Basin, SW Bulgaria

The Blagoevgrad Basin, a graben filled with Neogene–Quaternary continental sedimentary successions, is one of the most prospective placer-gold bearing regions in southwestern Bulgaria. The Quaternary sediments have been well explored and economically prospective areas have been identified. Previous studies indicated very limited occurrences of placer gold in the Neogene sediments, the genesis of which is connected with resedimentation of Paleogene sediments from the adjacent Bobov dol Basin to the north. From these previous studies, a detailed allo- and lithostratigraphic scheme, and paleogeographic models were developed. The present study aimed to evaluate the plausibility of the existing stratigraphic schemes and paleogeographic models for the area, through detailed study of the heavy mineral fraction of the Neogene and part of the Quaternary sediments from the southern part of the Blagoevgrad Basin. Additional objectives were to determine the mode of occurrence and the stratigraphic position of the placer gold in the Neogene sediments, and to investigate possible source areas. Special attention was paid to the mineral composition and the structure of the opaque detrital Fe–Ti oxide minerals, and the typologic features of the zircons. The results obtained could be used as a provenance indicator and for determination of the nature of the source area and the processes that operated therein. A very important finding is the discovery of placer gold in an additional three Neogene units. The morphologic features of the gold grains indicate short transport or proximity to the primary source. This interpretation is inconsistent with existing hypotheses on the origin and presence of the gold in the sediments of the basin. The characteristics of the magnetic heavy mineral fraction, and the morphologic features of zircon grains in auriferous samples, indicate the diorites and gabbro-diorites from the Strouma Diorite Group, which outcrop along the eastern edge of the graben, as a possible source. The stratigraphic distribution of the heavy minerals fully supports the plausibility of the existing detailed allo- and lithostratigraphic subdivision, and the paleogeographic model for the development of the southern part of the basin. Also, the results provide support for the soundness of an allostratigraphic approach for the study and exploration of placer deposits in basins similar to the Blagoevgrad.

IDENTIFICATION AND ALTERATION TRENDS OF GRANITIC-PEGMATITE-HOSTED (Y,REE,U,Th) (Nb,Ta,Ti) OXIDE MINERALS: A STATISTICAL APPROACH

The (Y,REE,U,Th)–(Nb,Ta,Ti) oxide minerals in REE-enriched granitic pegmatite and A- and I-type granites are primarily comprised of the fergusonite, samarskite, euxenite, aeschynite and pyrochlore groups. They are typically metamict and altered, and can be difficult to identify on a structural basis (XRD); consequently, examination of sample composition may be the most reliable approach to their identification. Canonical discriminant analysis can be used to discriminate among the various groups, and to assess the style and degree of alteration affecting samples. Identification of individual species should be made on the basis of previously published guidelines or, in their absence, on the basis of the “50% rule” of the IMA. Vector analysis shows that alteration typically pushes the compositions of samarskite-, euxenite- and aeschynite-group minerals in a direction that causes them to resemble pyrochlore-group minerals. For this reason, it has been historically difficult to determine the exact nature of “viethofingite”, “hjelmite” and “ampangabeite”; all are shown to be altered varieties of samarskite-group minerals. Yttrotantalite-(Y) is shown to be a member of the samarskite group, not the fergusonite group.

In situ cosmogenic 10Be and 21Ne in sanidine and in situ cosmogenic 3He in Fe–Ti-oxide minerals

We report concentrations of in situ cosmogenic 10Be and 21Ne from coexisting quartz and sanidine separates and of cosmogenic 3He in coexisting Fe–Ti-oxide minerals from ignimbritic successions of northern Chile (Oxaya and Lauca ignimbrites). New mineral-isotope pairs such as sanidine and Fe–Ti-oxide minerals are helpful in quantitative geomorphology for geological settings where the lithology lacks the commonly used minerals quartz, pyroxene or olivine. Production rates in sanidine and Fe–Ti-oxide minerals were determined by normalizing nuclide concentrations to established production rates in quartz. The experimentally determined production rates are compared to model production rates calculated with new cross-sections for 3He, 21Ne, and 10Be production from the individual target elements. The mean experimental 21Ne production rate for five sanidine samples is 30.4 ± 3.7 atoms g − 1 yr − 1 (30.4 ± 5.4 atoms g − 1 yr − 1 including the uncertainty of the 21Ne production rate in quartz, P 21Ne qtz). This is in excellent agreement with the modelled value of 28.3 atoms g − 1 yr − 1 , which has an estimated uncertainty of 20%. The 21Ne production rate in sanidine is thus about 50% higher than that in quartz. The cosmogenic neon in sanidine is entirely released in low temperature steps (400–600 °C) and no signs of an interfering nucleogenic neon component were observed. This is in stark contrast to quartz and makes sanidine an attractive mineral for terrestrial cosmogenic neon studies. 3He diffuses out of the sanidine structure. Preliminary results also indicate that sanidine is well suited for 10Be studies. The mean experimental 10Be production rate from two sanidine samples is 4.45 ± 0.38 atoms g − 1 yr − 1 (4.45 ± 0.42 atoms g − 1 yr − 1 including uncertainty of P 10Be qtz), very close to the modelled value of 4.55 atoms g − 1 yr − 1 , which has an estimated uncertainty of 20%. We emphasize that 10Be and 21Ne production in sanidine is composition dependent. Therefore, major element analyses should be carried out and production rates calculated on a sample by sample basis. Fe–Ti-oxide minerals retain 3He quantitatively. Experimentally derived production rates are in excellent agreement with new values derived from physical modelling. The mean experimental 3He production rate for five Fe–Ti-oxide minerals samples is 120 ± 11 atoms g − 1 yr − 1 (120 ± 12 atoms g − 1 yr − 1 including the uncertainties of P 21Ne qtz), compared to a mean modelled value of 124 atoms g − 1 yr − 1 , which has an estimated uncertainty of 20%. The variable chemical and structural composition of the solid solution lines of Fe–Ti-oxide minerals has little effect on the total 3He production rate. Cosmogenic 21Ne is not produced in significant quantities in Fe–Ti-oxide minerals due to the absence of suitable target elements.

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway). Part I: Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent

Whole-rock major element compositions are investigated in 99 cumulates from the Proterozoic Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway), which results from the crystallization of a jotunite (Fe–Ti–P-rich hypersthene monzodiorite) parental magma. The scattering of cumulate compositions covers three types of cumulates: (1) ilmenite–leuconorite with plagioclase, ilmenite and Ca-poor pyroxene as cumulus minerals, (2) magnetite–leuconorite with the same minerals plus magnetite, and (3) gabbronorite made up of plagioclase, Ca-poor and Ca-rich pyroxenes, ilmenite, Ti-magnetite and apatite. Each type of cumulate displays a linear trend in variation diagrams. One pole of the linear trends is represented by plagioclase, and the other by a mixture of the mafic minerals in constant proportion. The mafic minerals were not sorted during cumulate formation though they display large density differences. This suggests that crystal settling did not operate during cumulate formation, and that in situ crystallization with variable nucleation rate for plagioclase was the dominant formation mechanism. The trapped liquid fraction of the cumulate plays a negligible role for the cumulate major element composition. Each linear trend is a locus for the cotectic composition of the cumulates. This property permits reconstruction by graphical mass balance calculation of the first two stages of the liquid line of descent, starting from a primitive jotunite, the Tjörn parental magma. Another type of cumulate, called jotunite cumulate and defined by the mineral association from the Transition Zone of the intrusion, has to be subtracted to simulate the most evolved part of the liquid line of descent. The proposed model demonstrates that average cumulate compositions represent cotectic compositions when the number of samples is large (> 40). The model, however, does not account for the K 2O evolution, suggesting that the system was open to contamination by roof melts. The liquid line of descent corresponding to the Bjerkreim–Sokndal cumulates differs slightly from that obtained for jotunitic dykes in that the most Ti-, P- and Fe-rich melts (evolved jotunite) are lacking. The constant composition of the mafic poles during intervals where cryptic layering is conspicuous is explained by a compositional balance between the Fe–Ti oxide minerals, which decrease in Fe content in favour of Ti, and the pyroxenes which increase in Fe.

Compositional variation of Fe−Ti oxides from the Sokli complex, northeastern Finland

The phoscorite-carbonatite complex at Sokli, northeastern Finland, is composed of five stages of intrusions of phoscorites and carbonatites (P1-C1, P2-C2, P3-C3 phoscorites and calcite carbonatites, D4 and D5 dolomite carbonatites) which are cut by numerous lamprophyric dikes. Magnetite is ubiquitous in all constituent rock units of the complex and frequently associates with ilmenite. Most ilmenite intergrowths from the Sokli phoscorite-carbonatite complex occur as internal and/or external granules (composite type exsolutions) or sandwich type exsolution lamellae in the host magnetites. Discrete ilmenite crystals are found only in P3 and C3. On the other hand, ilmenites in the ultramafic lamprophyres occur as discrete crystals as well as trellis and sandwich type exsolution lamellae or composite type exsolutions in magnetite. Electron microprobe analyses reveal that magnetite of the Sokli complex belongs essentially to the magnetite-ulvospinel solid solution series with a small proportion of magnesioferrite. The compositions of magnetite from the Sokli phoscorite-carbonatite complex vary systematically from stage to stage with increasing Fe2+/(Fe2++Mg) ratio, and decreasing Al, Mn and Ti contents. Magnetite from the ultramafic lamprophyre is characterized by large compositional ranges owing to the large amount of mantle-derived xenocrysts. Some grains are particularly high in Cr (up to 21.0 wt.% Cr2O3). Aluminous magnesian titanomagnetites are also found, but most magnetites in the ultramafic lamprophyres are pure magnetite. The compositional variation of ilmenite from the Sokli complex is mainly caused by the substitution of Fe2+ into Mg and Mn, and partly Ti into Nb. Mg-and Mn-rich ilmenites in the early stage P1-C1 rocks evolve towards pure FeTiO3 composition in the latest D5 dolomite carbonatite. Ilmenites from the ultramafic lamprophyres are relatively poor in Mn compared to those from the phoscorite-carbonatite complex. In the coexisting magnetite and ilmenite, Mg and Mn cations preferentially partition into ilmenite rather than magnetite. In spite of the mineralogical and geochemical contrasts between the phoscorites and associated carbonatites, compositions of Fe−Ti oxide minerals from the two paired rocks at a given stage are basically the same, and evolve systematically from stage to stage. This supports the hypothesis that the phoscorites and conjugate carbonatites are derived from common parental melts. Furthermore, the intrusions of the Sokli phoscorite-carbonatite complex are considered to have either crystallized from successive batches representing a continuum in time and magmatic evolution, or that the P1-C1 parental magma has simply differentiated to form the later stage rocks.

A source for Icelandic magmas in remelted Iapetus crust

The geochemistry and large melt volume in the Iceland region, along with the paucity of evidence for high, plume-like temperatures in the mantle source, are consistent with a source in the extensive remelting of subducted Iapetus crust. This may have been trapped in the Laurasian continental mantle lithosphere during continental collision in the Caledonian orogeny at ∼420–410 Ma, and recycled locally back into the asthenosphere beneath the mid-Atlantic ridge by lithospheric delamination when the north Atlantic opened. Fractional remelting of abyssal gabbro can explain the major-, trace- and rare-earth-element compositions, and the isotopic characteristics of primitive Icelandic tholeiite. An enriched component already present in the recycled crustal section in the form of enriched mid-ocean-ridge basalt, alkalic olivine basalt and/or related differentiates could contribute to the diversity of Icelandic basalts. Compositions ranging from ferrobasalt to olivine tholeiite are produced by various degrees of partial melting in eclogite, and the crystallization of ferrobasalt as oxide gabbro, i.e., containing the magmatic Fe–Ti oxide minerals, ilmenite and magnetite, may explain the anomalously high density of the Icelandic lower crust. The very high 3He/ 4He ratios observed in some Icelandic basalts may derive from old helium preserved in U+Th-poor residual Caledonian oceanic mantle lithosphere or olivine-rich cumulates in the crustal section. The persistence of anomalous volcanism at the mid-Atlantic ridge in the neighborhood of Iceland suggests that in the presence of lateral ridge migration, the shallow fertility anomaly must be oriented transverse to the mid-Atlantic ridge. The Greenland–Iceland–Faeroe ridge is co-linear with the western frontal thrust of the Caledonian collision zone, which may thus be associated with the fertility source. The fertile material beneath the Iceland region must lie at a steep angle or be thickened by deformation or imbrication to supply the large volumes of basalt required to build the thick crust there. “Hot spot” volcanism and large-igneous-province emplacement often occurs within or near to old suture zones and similar processes may thus explain anomalous magmatism elsewhere that is traditionally attributed to plumes.

Re-Os study of Fe-Ti-V oxide and Fe-Cu-Ni sulfide deposits, Suwałki Anorthosite Massif, northeast Poland

The Suwalki anorthosite massif, located in extreme northeast Poland beneath more than a kilometer of Phanerozoic cover, hosts major Fe-Ti-V deposits. These deposits, discovered in 1962, are contained in Fe and Ti oxide minerals that coexist with subordinate quantities of Fe, Cu, Ni, and Co sulfides in massif-style anorthosites, norites, and gabbronorites. Accessibility and other considerations preclude development of this natural resource in the present economic climate. Detailed work by Polish geologists during the last 35 years provides a sound geologic framework for this Re-Os study of the age and origin of oxide and sulfide deposits associated with a major, but lesser known anorthosite massif. Rhenium and osmium abundances and Os isotopic compositions were measured for nine sulfides and four titanomagnetites from the Suwalki anorthosite massif. The titanomagnetites are over an order of magnitude lower in Re (0.4–1.5 ppb) and Os (0.036–0.144 ppb) concentrations than co-precipitated pyrrhotite, pyrite, and chalcopyrite that yield consistent concentrations for Re (30–55 ppb) and Os (1–6 ppb). Parallel lines connecting co-existing titanomagnetite and sulfides have slopes of ∼1 on Re versus common Os concentration plots, indicating that both Re and Os behave similarly during crystallization in their high preference for any sulfide phase over magnetite. Samples from three deposits within the anorthosite massif were analyzed. An age of 1559 ± 37 Ma (n=10) with an initial 187Os/188Os of 1.16 ± 0.06 for the Jezioro Okrągle and Krzemianka deposits is essentially identical to an age of 1556 ± 94 Ma (n=3) for the Udryn deposit. Udryn, however, yielded a marginally lower initial 187Os/188Os of 0.87 ± 0.20. The high initial 187Os/188Os combined with the Proterozoic Re-Os age indicates that the source for Suwalki oxides-sulfides is older crust, and hypothetically, could involve Archean rocks. An average crustal value of 50 for 187Re/188Os yields a 2777 Ma age for Suwalki source rocks. Widespread Phanerozoic cover severely limits knowledge of basement rocks in Poland, however, and no Archean rocks are known in the immediate region. More likely, 187Re/188Os ratios may be higher than average continental crust, reflecting mafic crust in the source, and may move the source age for Suwalki anorthosite and mineral deposits toward younger values that easily include ∼2.0 Ga Proterozoic rocks. This more favorable case also accommodates Paleoproterozoic Nd model ages. Regardless of Archean or Proterozoic source age, the high initial 187Os/188Os ratios derived from the Re-Os isochron indicate that the source for the oxide-sulfide mineral deposits is more likely the crust and not the mantle. Given that these deposits are clearly magmatic, the Re-Os results add a new dimension to the long-standing “origin of anorthosite” problem, implying a crustal source for the anorthosite as well. The 1559 Ma Suwalki age is compatible with a well-exposed east-west band of 1530-1660 Ma rapakivi granite-anorthosite magmatism to the immediate north, transecting western Russia, southern Finland, Estonia and Latvia, and central Sweden. In particular, the age and isotopic character of Suwalki are not unlike those of the well-studied Salmi rapakivi granite-anorthosite batholith in western Russia (Karelia).

Trace element geochemistry of a thick till and clay-rich aquitard sequence, Saskatchewan, Canada

Samples collected from a 170 m borehole penetrating till, bentonitic clay, and sand at the Birsay site, Saskatchewan, Canada, has been studied as a first step for understanding the controls on dissolution and transport of trace elements, including REEs, in porewaters of a clay-rich aquitard. The REE budgets of the till, clay, and sand are dominated by apatite and monazite, with sporadic zircon and Y-phosphate. Apatites contain 0.3 to 1.7 wt.% REE, and monazites 12 to 55 wt.%. All three units are characterized by unusual bulk sample REE patterns, with maxima at Eu–Gd, convex down patterns from La–Sm with minima at Ce, and variably fractionated HREE. Apatites feature maxima at Eu–Gd. Consequently the complex patterns arise from variable modal proportions of apatite, monazite, and plagioclase feldspar. Relative to Post-Archean Average Shale (PAAS), the till is SiO 2 and CaO rich with about half the contents of Al 2O 3, TiO 2, and Fe 2O 3. These compositional relationship stem from relatively high proportions of quartz and carbonate, but low modal abundances of ferromagnesian and Ti-oxide minerals. Similarly, the till has Cs, Co, Ni, Sc, V, Cu, Pb and Zn contents at about half PAAS. The bentonite clay has slightly higher SiO 2 and CaO contents than PAAS, with lower abundances of most other trace elements, including REE which plot at 0.7 to 0.9 PAAS. The major and trace element composition is similar to continental arc andesites, and the clay is interpreted to have a mixed provenance of arc andesite and eroded underlying sands. The composition of the till appears to reflect mixed provenance: erosion of the underlying Cretaceous clay and sand, Paleozoic carbonate rocks, and the Precambrian Shield. The silicious Ardkenneth sand has lower concentrations of REE and most trace elements compared to the clay, but the most fractionated REE patterns. The clay and till have inherited more subdued REE patterns in part from the underlying sand unit. Compositional variations between the units are marked by an upturn of Nb, Th, U and REE concentrations at the till–clay interface (80.2 m), and decrease of these elements at the clay–sand boundary (156.2 m). Ce and Eu anomalies vary erratically with depth: the anomalies appear to reflect the degree of sedimentary sorting, which in turn controls the modal abundances of detrital monazite, apatite, zircon, and plagioclase feldspar. Zr/Zr*, Hf/Hf* and Zr/Hf ratios are higher in the till than the clay and sand, indicating an additional Zr (Hf)-bearing phase with a distinctively high Zr/Hf ratio. The observed small compositional variations in the till and clay are not reflected in variations of geotechnical properties. Given that the REE budget of the till and clay are dominated by highly insoluble trace minerals, it is unlikely that the REE patterns of porewaters will be controlled by bulk sample REE concentrations. Weathering of the upper 3 m of the till has oxidized Fe 2+-bearing silicates, oxides, and sulphides, and dissolved carbonates relative to the underlying unoxidized till, weathering has not disturbed major, REE or other trace element compositions.

Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong area, southern China

The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar–Ar and K–Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO 2 with increasing SiO 2 content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe–Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895–986°C and oxygen fugacities range from 10 −13.4 to 10 −10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.

Dielectric constant as a predictor of porosity in dry volcanic rocks

Measurements of dielectric constant ( K′) are made on 34 samples of volcanic rocks at frequencies of 0.01 to 10 MHz under ambient atmospheric conditions. Bulk density ( ρ T), total porosity ( Φ T) and connected porosity ( Φ Conn) are also measured. The samples derive from two dacitic lava flows (∼60–62 and 68 wt.% SiO 2), dacitic pyroclastic deposits (∼66–68 wt.% SiO 2) and two basalt lava flows (∼49–52 wt.% SiO 2). Each locality provided a suite of samples with similar mineralogy and composition but a range of porosities. Porosity measurements indicate that as much as 17% of pumice pore space can be unconnected. The data show a strong correlation between K′ and Φ T and the dacitic rocks show a 2.5-fold decrease in K′ over a porosity range of 8–79%. The data are fitted to a time propagation (TP) model and to a more general two-parameter model based on the Lichtenecker–Rother equation. For dacitic rocks, the dielectric constant is best related to porosity by: (K′) 0.96=Φ+6.51(1−Φ). K′ and ρ T are also strongly correlated in these sample sets. The trend formed by samples of dacite in ( K′, ρ T) space is linear and the data compare well with published values for other non-basaltic rocks. Samples of basalt show greater variance in measured values of K′, due perhaps to higher and more variable modes of Fe–Ti oxide minerals. These new data suggest the possibility of inverting radar velocity data to obtain estimates of porosity in dry volcanic successions. Inversion of radar data for porosity could be useful in discriminating between units of an eruption cycle (e.g., lava flow, pyroclastic flow, airfall) and mapping porosity variations within deposits such as welded pyroclastic flows.

Emplacement, petrological and magnetic susceptibility characteristics of diverse magmatic epidote-bearing granitoid rocks in Brazil, Argentina and Chile

Magmatic epidote (mEp)-bearing granitoids from five Neoproterozoic tectonostratigraphic terranes in Northeastern (NE) Brazil, Early Palaeozoic calc-alkalic granitoids in Northwestern (NW) Argentina and from three batholiths in Chile have been studied. The elongated shape of some of these plutons suggests that magmas filled fractures and that dyking was probably the major mechanism of emplacement. Textures reveal that, in many cases, epidote underwent partial dissolution by host magma and, in these cases, may have survived dissolution by relatively rapid upward transport by the host magma. In plutons where such a mechanism is not evident, unevenly distributed epidote at outcrop scale is armoured by biotite or near-solidus K-feldspar aggregates, which probably grew much faster than epidote dissolution, preventing complete resorption of epidote by the melt. Al-in-hornblende barometry indicates that, in most cases, amphibole crystallized at P≥5 kbar. Kyanite-bearing thermal aureoles surrounding plutons that intruded low-grade metamorphic rocks in NE Brazil support pluton emplacement at intermediate to high pressure. mEp show overall chemical variation from 20 to 30 mol% (mole percent) pistacite (Ps) and can be grouped into two compositional ranges: Ps 20–24 and Ps 27–30. The highest Ps contents are in epidotes of plutons in which hornblende solidified under P<5 kbar. The percentage of corrosion of individual epidote crystals included in plagioclase in high-K calc-alkalic granitoids in NE Brazil, emplaced at 5–7 kbar pressure, yielded estimates of magma transport rate from 70 to 350 m year −1. Most of these plutons lack Fe–Ti oxide minerals and Fe +3 is mostly associated with the epidote structure. Consequently, magnetic susceptibility (MS) in the Neoproterozoic granitoids in NE Brazil, as well as Early Palaeozoic plutons in Argentina and Late Palaeozoic plutons in Chile, is usually low (<0.50×10 −3 SI), which is typical behavior of plutons which crystallized under low fO 2 (ilmenite-series granitoids), although Fe/(Fe+Mg) ratios in hornblende (0.40–0.65) indicate crystallization under high fO 2. Mesozoic to Tertiary calc-alkalic plutons in Chile, however, exhibit iron oxide minerals and MS values >3.0×10 −3 SI, typical of magnetite-series granitoids crystallized under higher oxygen fugacity. In NE Brazil, Argentina and Chile, it seems that mEp is more common in Precambrian to Palaeozoic ilmenite-series granitoids, while its occurrence in magnetite-series granitoids is more restricted to Mesozoic to Tertiary granitoids.

Shoshonitic liquid line of descent from diorite to granite: the Late Precambrian post-collisional Tismana pluton (South Carpathians, Romania)

The post-collision late-kinematic Tismana pluton belongs to the shoshonitic series. It is part of a Late Precambrian basement within the Alpine Danubian nappes of the South Carpathians (Romania). This pluton displays an exceptionally complete range of compositions from ultramafic to felsic rocks (granites). Widespread mingling/mixing relationships at all scales give rise to a variety of facies. A liquid line of descent from the diorites to the granites is reconstructed by considering the variation in major and trace elements (REE, Sr, Rb, Ba, Nb, Zr, Hf, Zn, V, Co, Cr, U, Th, Ga, Pb) from 33 selected samples as well as mineral/melt equilibrium relationships. The first step of fractional crystallization is the separation from a monzodioritic parent magma of a peridotitic cumulate similar to the ultramafic rock found in the massif. A possible contamination by lower crustal mafic component takes place at this stage. The second step marks the appearance of apatite and Fe–Ti oxide minerals as liquidus phases, and the third step, saturation of zircon. Mixing by hybridisation of magmas produced at different stages of the evolution along the liquid line of descent is also operating (endo-hybridisation). As depicted by Nd and Sr isotopes, fractional crystallization was combined to an important early contamination by a mafic lower crust in a deep-seated magma chamber and to a later and mild contamination by a felsic medium crust in an intermediate chamber. The mingling essentially occurred during the final emplacement in the high-level magma chamber. The monzodioritic parent magma, identified by major and trace element modelling, is shown by Sr and Nd isotopes to have its source in the lithospheric mantle or in a juvenile mafic lower crust derived from it. The necessarily recent enrichment in K 2O and associated elements of the lithospheric mantle is likely to be related to the preceding Pan-African subduction period. The partial melting of this newly formed deep source has to be linked to a major change in the thermal state of the plate.