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Within the wide compositional spectrum of potassium-rich magmatism in Central Italy, kamafugitic rocks in the Intra-Apennine Volcanic Province constitute the most alkalic and silica-undersaturated end-member, showing the highest enrichment in many incompatible trace elements. A close association with carbonatitic rocks has posed questions concerning the ambiguous origin of carbon, which may have been derived from the mantle source or from interaction with Mesozoic carbonates residing in the crust. We explored the crystallisation history of olivines and their melt inclusions (MI), separated from a representative rock specimen from San Venanzo, which enabled us to (1) determine the primary composition(s) of mantle-derived kamafugite melts and (2) trace the effects of crustal interaction on melt composition and magma evolution. Complex textures of olivine phenocrysts and their trace element compositions provide a framework for the sequence in which melt evolved within a single magmatic plumbing system. Olivine crystals in the studied kamafugite show complex textures between core and rim parts. There are strong indications of drastic late-stage changes in the crystallisation regime, including irregular and patchy compositional zoning and increasing amounts of inclusions in the rims, as well as rare primary melt inclusions in pristine parts of the olivines (Fig. 1). Pristine core parts, characterised by high Fo (93–90 mol%), low CaO (0.2–0.3 wt%) and Cr-spinel inclusions (Cr# ~0.7), are considered to have crystallised from a primary, mantle-derived melt. Compositions of homogenised melt inclusions in these core parts are consistent with derivation from high-MgO melts, but span a continuous range for K2O, Na2O, P2O5, TiO2 and CaO content. Contrastingly, the rim parts show a strong compositional gradient of decreasing forsterite (down to Fo70) and increasing CaO (up to 1.8 wt%). Profiles of phosphorous contents in the olivines point to steep increases in the rim parts, indicating that they originated as rapid overgrowths onto phenocryst cores. Homogenised MI from the rim parts are compositionally close to evolved carbonate-bearing volcanic products, natural skarns and glasses generated in carbonate assimilation experiments. These combined signatures suggest that the rim parts crystallised from an evolved melt that was contaminated through interaction with carbonate-rich lithologies. Fluid inclusions in the rim parts indicate that this interaction occurred at relatively shallow crustal levels (2–10 km; Fig. 1). From the major and trace element content of core-hosted MI we infer that the kamafugite represents an assembly of primary melts with different compositions, controlled by low-degree melt extraction from a mantle source with mineralogical variations. Both the major and trace element content of core-trapped MI furthermore demonstrate an important role for apatite in the mantle source during primary melt generation of the San Venanzo kamafugites. Our observations are consistent with a mantle source affected by siliceous K2O-rich and carbonate/apatite-rich metasomatic agents derived from subducted carbonate-bearing metapelites.

This article investigates the origin of a rare occurrence of kyanite quartzites in the Palaeoproterozoic greenstone belt of Suriname. The rocks form elongated hills in the Bosland area, Brokopondo district, where they are associated with meta-sedimentary, meta-volcanic and granitic lithologies. Their mineral content and unusual Si- and Al-rich chemical composition are inferred to be the result of advanced argillic alteration of felsic volcanic tuffs and a later overprint by regional metamorphism up to lower amphibolite facies during the Trans-Amazonian orogeny. Structurally, the Bosland area seems centred within a contractional strike-slip duplex of a major dextral fault system. The alteration was probably associated with a high-sulphidation environment and involved significant to almost complete removal of alkali and alkaline earth elements. Pseudosection modelling and textures suggest that the precipitation–temperature (P–T) history of the kyanite quartzites started with shallow (T>350°C), and ended in peak metamorphic conditions in the kyanite–staurolite stability field (P>4kbar andT=500–650°C). Alteration events that preceded the peak of Trans-Amazonian metamorphism may be more common in the rock record of Suriname's greenstone belt, which lends support to the hypothesis that gold mineralisations in the region can be pre-orogenic.

Quantification of trace element contents in bauxite by solution ICP-MS and other analytical methods that rely on conventional dissolution techniques can be challenging, because the refractory nature of common host minerals complicates complete digestion. Fusion with lithium borate, frequently used as a sample preparation method for XRF analysis of solid materials, avoids these problems. This manuscript documents that subsequent analysis of the low-dilution glass beads by laser ablation ICP-MS is a fast, accurate and precise method for determining trace element mass fractions in samples of bauxite and similar natural materials. The method was validated by determining thirty trace elements, including thirteen rare earth elements, in international reference materials for bauxite (ANRT BX-N, NIST SRM 69b, NIST SRM 696, NIST SRM 698) and iron formations (CCRMP FeR-2). Trace elements were typically measured to within 20% of reference values with an 'external' precision of < 20% RSD. Measurement results from various deposits in Suriname illustrate the procedure's effectiveness for studies concerning chemical properties and origin of bauxite.

The alteration mineralogy in the crater area of Poás volcano (Costa Rica) has been studied to constrain acid fluid-rock interaction processes and conditions relevant for the formation of sulphate-bearing mineral assemblages found on the surface of Mars. Individual sub-environments, which include the hyperacid lake (Laguna Caliente), ephemeral hot springs, fumarole vents and areas affected by acid rain and/or spray from the lake, are marked by distinct secondary mineral associations, with sulphates commonly as prevailing component. The sulphates occur in a wide mineralogical diversity comprising gypsum/anhydrite, various polyhydrated Al-sulphates, alunite-jarosite group minerals, halotrichite-, voltaite- and copiapite-group minerals, epsomite and römerite. Depending on the sub-environment, they are variably associated with clay minerals (kaolinite-group and smectite-group), zeolites, SiO2-polymorphs, Fe-(hydro)oxides, Ti-oxides, native sulphur, sulphides, chlorides, fluorides, phosphates and carbonates. Geochemical modelling was performed to identify mechanisms responsible for the formation of the secondary minerals found in the field, and to predict their possible stability under conditions not seen at the surface. The results indicate that the appearance of amorphous silica, hematite, anhydrite/gypsum, pyrite, anatase and kaolinite is relatively insensitive to the degree of acidity of the local aqueous system. On the other hand, alunite-jarosite group minerals, elemental sulphur and Al(OH)SO4 only form under acidic conditions (pH < 4). The presence of polyhydrated Mg- and Fe2 +-sulphates is restricted to olivine-bearing rocks exposed to acid rain or brine spray. Modelling suggests that their formation required a repetitive sequence of olivine dissolution and evaporation in an open system involving limited amounts of fluid. The mineral variety in the crater of Poás is remarkably similar to sulphate-bearing assemblages considered to be the product of acid-sulphate alteration on Mars. The analogy suggests that comparable fluid-rock interaction controls operated in Martian volcanic environments.

Lateritic bauxite deposits in Suriname rest on a variety of metamorphic, igneous and sedimentary parent rocks. Remnants of multiple planation surfaces with duricrusts that mark the tropical landscape are associated with recurrent episodes of bauxite formation since Late Cretaceous times. Plateau-type bauxites at the highest topographic levels developed on a range of Proterozoic crystalline bedrocks on the northern edge of the Guiana Shield in the country's interior. Weathering profiles of the Bakhuis Mountains, Nassau Mountains, Lely Mountains and Brownsberg largely correspond to the classical sequence of an iron-rich cap on top of a bauxite layer that covers a clay-rich saprolite interval grading into weathered and fresh bedrock. All of the investigated profiles are consistent with in-situ formation of the bauxite and are marked by Si-Al-Fe relationships indicative of medium to strong lateritization, with fresh bedrock being poorly exposed. The bauxite deposits contain gibbsite as the dominant Al-bearing phase, whereas boehmite is locally present in subordinate quantities. Their ferruginous character is expressed by relatively abundant goethite and hematite in the top layers. Kaolinite is the main mineral in the saprolite. Anatase and zircon are the most detected minor phases. The investigated bauxite deposits are generally of a medium-grade (average Al2O3 contents 33–49 wt%) but have variable chemical compositions according to exploration drilling results. Average Fe2O3 contents (13–34 wt%) show inverse relationships with Al2O3. Despite this overall conformity of the deposits, their thicknesses, textures, mineralogy and geochemistry are distinct in detail, reflecting contrasts in the nature of the parent rock and weathering history. Inter-element relationships show conspicuous differences between SiO2-poor (< 5 wt%) upper parts and SiO2-richer (> 5 wt%) lower parts of profiles in the Bakhuis Mountains, which developed on high-grade metamorphic pyroxene amphibolites and gneissic granulites. The bauxites of the Nassau Mountains and the other areas in eastern Suriname are marked by higher TiO2 contents (average > 3.9 wt%) and dissimilar profiles that reflect their development on variety of a low-grade metamorphic volcanic parent rocks. Weathering-induced redistribution of rare-earth and other trace elements affected even the least mobile elements. Differences in distribution patterns between individual profiles can be attributed to a combination of primary compositional differences of parent rocks, the nature and content of accessory mineral phases, and unequal responses to multiple bauxitization cycles.

Sr and Nd isotopes were determined using new thermal ionisation mass spectrometry (TIMS) techniques for a suite of 21 olivine-hosted (85-92mol% Fo) melt inclusions selected from potassic and ultra-potassic lavas from the Italian peninsula. Sr isotopes were measured using default 1011Ω resistors, whereas Nd isotope compositions were determined using new 1013Ω resistors mounted in the amplifiers' feedback loop. Compared to default 1011Ω resistors, use of 1013Ω resistors results in a 10-fold improvement of the signal-to-noise ratio and more precise data when analysing small ion beams (

Utrecht University, The Netherlands Universidad Nacional, Costa Rica Universidade Técnica de Lisboa, Portugal