Exploration in the Naica mine (Chihuahua, Mexico) recently unveiled several caves containing giant, faceted, and transparent single crystals of gypsum (CaSO 4 •2H 2 O) as long as 11 m. These large crystals form at very low supersaturation. The problem is to explain how proper geochemical conditions can be sustained for a long time without large fl uctuations that would trigger substantial nucleation. Fluid inclusion analyses show that the crystals grew from low-salinity solutions at a temperature of ~54 °C, slightly below the one at which the solubility of anhydrite equals that of gypsum. Sulfur and oxygen isotopic compositions of gypsum crystals are compatible with growth from solutions resulting from dissolution of anhydrite previously precipitated during late hydrothermal mineralization, suggesting that these megacrystals formed by a self-feeding mechanism driven by a solution-mediated, anhydrite-gypsum phase transition. Nucleation kinetics calculations based on laboratory data show that this mechanism can account for the formation of these giant crystals, yet only when operating within the very narrow range of temperature identifi ed by our fl uid inclusion study. These singular conditions create a mineral wonderland, a site of scientifi c interest, and an extraordinary phenomenon worthy of preservation.

No abstract

ABSTRACT. A new dynamic model of dolomitization predicts a multitude of textural, paragenetic, geochemical and other properties of burial dolomites. The model is based on two postulates, (1) that the dolomitizing brine is Mg-rich but undersaturated with both calcite and dolomite, and (2) that the dolomite-for-calcite replacement happens not by dissolution-precipitation as usually assumed, but by dolomite-growth-driven pressure solution of the calcite host. Crucially, the dolomitefor-calcite replacement turns out to be self-accelerating via Ca 2؉ : the Ca 2؉ released by each replacement increment accelerates the rate of the next, and so on. As a result, both pore-fluid Ca 2؉ and replacement rate grow exponentially. As brine enters and infiltrates a limestone, water/rock disequilibrium plus the self-accelerating feedback inevitably yield a process that is self-organized, both in time (as repeated dolomite growth pulses per slice of limestone) and in space (as successive slices). Self-organization in pulses and slices accounts for several properties of burial dolomites: (1) generation of dissolution porosity and its spatially periodic distribution; (2) dolomitization affects only limestones; (3) sharp field contacts between dolomitized and undolomitized limestone; (4) formation of both saddle dolomite and "late-stage" calcite near the end of each growth pulse, accompanied by MississippiValley-type ores if the brine also contains Zn, Pb, Ba, sulfate, and other relevant elements; (5) "sweeping" of ores downflow with accumulation in the last position of the dolomitization front.In addition, the combination of the self-accelerating feedback via Ca 2؉ with the known strain-rate-softening rheology of crystalline carbonates leads to another suite of predictions that are strikingly confirmed by observation. If the dolomite-for-calcite replacement becomes fast enough to lower the local rock viscosity sufficiently, then the dolomite growth will pass spontaneously from replacive to displacive. This is when thin, self-organized, displacive zebra veins form (Merino and others, 2006), indeed displaying seamless contacts with their replacive walls and consisting of curved, or saddle, dolomite crystals. Serendipitously, both the deformation of the dolomite crystals (produced by Ca-for-Mg substitution driven by the huge pore-fluid Ca 2؉ ) and the seamless rheological transition result from the self-accelerating feedback via Ca 2؉ itself; that is why they are always associated. This detail alone strongly suggests that the new model captures the chemistry, drives, mechanisms, and feedbacks that lend burial dolomitization and its often associated MVT ore deposits their geological uniqueness.

A detailed isotopic characterization (delta15N(Ntotal), delta15N(NO3), delta18O(NO3), delta34S(SO4), delta18O(SO4), (delta13C(Ctotal), and 87Sr/86Sr) of 27 commercial fertilizers used in Spain is presented in this paper. Results together with a compilation of fertilizer isotopic published data are used for two purposes: (i) to identify the origin of the primary constituents and raw materials used in fertilizer manufacture and relate these data with their heavy metals and rare earth elements (REE) contents; (ii) to compare the fertilizer isotopic signatures with natural values and other anthropogenic pollutants and evaluate the usefulness of multi-isotopic analyses to trace fertilizer contaminations in future study cases. Isotope data permit us to know, in most cases, the origin of the primary constituents of fertilizers, and the 87Sr/86Sr ratio distinguishes the origin of the phosphate content--phosphorites or carbonatites--which in turn implies a qualitatively defined and potentially contaminant presence of REE and heavy metals in fertilizers. Delta15N, delta34S, and 87Sr/86Sr have already been used to trace fertilizer contaminations. Their utility can be improved by the coupled use of delta15N(NO3)-delta18O(NO3) and delta34S(SO4)-delta18O(SO4) to evaluate the fractionation processes that can affect contaminants. Moreover, multi-isotopic analyses, using heavy isotopes, allow us to see beyond the fractionation effects to the fertilizer stable isotope signatures and a better distinction from other anthropogenic contaminants.

Detailed structural cross‐sections, analysis of extensional structures and palaeotemperatures obtained from primary fluid inclusions in quartz and calcite veins from the extensional Cameros Basin (N Spain) allow an interpretation of its thermal evolution and its geometric reconstruction to be constrained. The Cameros Basin underwent an extensional stage during the Late Jurassic to Early Cretaceous, with a maximum preserved thickness of Mesozoic deposits of about 9000 m. During the Tertiary, the basin was inverted, allowing a large part of the sedimentary sequence to be exposed. Extensional deformation in individual beds created N120E‐striking tension gashes in the synrift sequence, parallel to the master normal faults limiting the basin and dipping perpendicular to bedding. The extensional strain calculated from tension gashes varies between 4 and 12%. The number and thickness of veins increases the lower their position in the stratigraphic section. Palaeotemperatures were obtained from samples along a stratigraphic section comprising a thickness of 4000 m synrift deposits. Homogenization temperatures range from 107 to 225 °C. Palaeothermometric data and geometric reconstruction give a geothermal gradient of 27–41 °C km−1 during the extensional stage and allow an eroded section of at least 1500 m to be inferred. Low‐grade metamorphic assemblages in lutitic rocks of the deepest part of the basin presently exposed at surface imply P–T conditions of 350–400 °C and less than 2 kbar, which implies a geothermal gradient of about 70 °C km−1. Since the metamorphic thermal peak is dated at 100 Ma, the P–T path indicates a heating event during the late Albian, probably linked to the reaching of thermal equilibrium of the continental crust after extension. The results obtained support the hypothesis of a synclinal basin geometry, with vertical superposition of Lower Cretaceous sedimentary units rather than a model of laterally juxtaposed bodies onlapping the prerift sequence.