Kinetic model of carbonate dissolution in Martian meteorite ALH84001

The magnetites and sulfides located in the rims of carbonate globules in the Martian meteorite ALH84001 have been claimed as evidence of past life on Mars. Here, we consider the possibility that the rims were formed by dissolution and reprecipitation of the primary carbonate by the action of water. To estimate the rate of these solution-precipitation reactions, a kinetic model of magnesite-siderite carbonate dissolution was applied and used to examine the physicochemical conditions under which these rims might have formed. The results indicate that the formation of the rims could have taken place in < 50 yr of exposure to small amounts of aqueous fluids at ambient temperatures. Plausible conditions pertaining to reactions under a hypothetical ancient Martian atmosphere (1 bar CO₂), the modern Martian atmosphere (8 mbar CO₂), and the present terrestrial atmosphere (0.35 mbar CO₂) were explored to constrain the site of the process. The results indicated that such reactions likely occurred under the latter two conditions. The possibility of Antarctic weathering must be entertained, which, if correct, would imply that the plausibly biogenic minerals (single-domain magnetite of characteristic morphology and sulfide) reported from the rims may be the products of terrestrial microbial activity. This model is discussed in terms of the available isotope data and found to be compatible with the formation of ALH84001 rims. Particularly, anticorrelated variations of radiocarbon with δ¹³C indicate that carbonate in ALH84001 was affected by solution-precipitation reactions immediately after its initial fall (∼13,000 yr ago) and then again during its recent exposure prior to collection.     

Ion microprobe measurements of O/O ratios of phosphate minerals in the Martian meteorites ALH84001 and Los Angeles

Oxygen isotope ratios of merrillite and chlorapatite in the Martian meteorites ALH84001 and Los Angeles have been measured by ion microprobe in multicollector mode. δ¹⁸O values of phosphate minerals measured in situ range from ∼3 to 6‰, and are similar to Martian meteorite whole-rock values, as well as the δ¹⁸O of igneous phosphate on Earth. These results suggest that the primary, abiotic, igneous phosphate reservoir on Mars is similar in oxygen isotopic composition to the basaltic phosphate reservoir on Earth. This is an important first step in the characterization of Martian phosphate reservoirs for the use of δ¹⁸O of phosphate minerals as a biomarker for life on Mars. Cumulative textural, major-element, and isotopic evidence presented here suggest a primary, igneous origin for the phosphates in Los Angeles and ALH84001; textural and chemical evidence suggests that phosphates in ALH84001 were subsequently shock-melted in a later event.     

Isotope geochemistry of caliche developed on basalt

Enormous variations in oxygen and carbon isotopes occur in caliche developed on < 3 Ma basalts in 3 volcanic fields in Arizona, significantly extending the range of δ¹⁸O and δ¹³C observed in terrestrial caliche. Within each volcanic field, δ¹⁸O is broadly co-variant with δ¹³C and increases as δ¹³C increases. The most ¹⁸O and ¹³C enriched samples are for subaerial calcite developed on pinnacles, knobs, and flow lobes that protrude above tephra and soil. The most ¹⁸O and ¹³C depleted samples are for pedogenic carbonate developed in soil atmospheres. The pedogenic caliche has δ¹⁸O fixed by normal precipitation in local meteoric waters at ambient temperatures and has low δ¹³C characteristic of microbial soil CO₂. Subaerial caliche has formed from ¹⁸O-rich evapoconcentrated meteoric waters that dried out on surfaces after local rains. The associated ¹³C enrichment is due either to removal of ¹²C by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation. Caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over-layered during burial by calcite carrying the isotopic signature of the soil environment. The large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if Mars had a “warm, wet” early period and photosynthesizing microbes were present in the early soils. The approach can be similarly applied to terrestrial Precambrian paleocaliche in the search for the earliest record of life on land. Large variations reported for δ¹⁸O of carbonate in Martian meteorite ALH84001 do not necessarily require high temperatures, playa lakes, or flood runoff if the carbonate is an example of altered martian caliche.     

Petrography and bulk chemistry of Martian orthopyroxenite ALH84001: implications for the origin of secondary carbonates

New petrologic and bulk geochemical data for the SNC-related (Martian) meteorite ALH84001 suggest a relatively simple igneous history overprinted by complex shock and hydrothermal processes. ALH84001 is an igneous orthopyroxene cumulate containing penetrative shock deformation textures and a few percent secondary extraterrestrial carbonates. Rare earth element (REE) patterns for several splits of the meteorite reveal substantial heterogeneity in REE abundances and significant fractionation of the REEs between crushed and uncrushed domains within the meteorite. Complex zoning in carbonates indicates nonequilibrium processes were involved in their formation, suggesting that CO2-rich fluids of variable composition infiltrated the rock while on Mars. We interpret petrographic textures to be consistent with an inorganic origin for the carbonate involving dissolution-replacement reactions between CO2-charged fluids and feldspathic glass in the meteorite. Carbonate formation clearly postdated processes that last redistributed the REE in the meteorite.     

The role of vaterite and aragonite in the formation of pseudo-biogenic carbonate structures: implications for Martian exobiology.

A simple synthesis of various forms of calcium carbonate with spherical and 'floral' morphologies is reported. Vaterite formation occurs at approximately 25 degrees C, aragonite at approximately 70 degrees C and calcite at about approximately 80 degrees C. These are produced when CO2 is reacted with an aqueous solution of calcium chloride in the presence of ammonia. These conditions may have existed at the surface of Mars in the past, leading us to conclude that such mineral formations may be common there. Although the initial phases are modified over time with changing temperature and pressure conditions, they still influence the final morphology of the carbonates observed. A comparison of these structures with those found in the Martian meteorite ALH84001 suggests, but does not confirm, a non-biogenic origin for the ALH84001 carbonates.     

Polycyclic aromatic hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar ice

Recent analyses of the carbonate globules present in the Martian meteorite ALH84001 have detected polycyclic aromatic hydrocarbons (PAHs) at the ppm level (McKay et al., 1996). The distribution of PAHs observed in ALH84001 was interpreted as being inconsistent with a terrestrial origin and were claimed to be indigenous to the meteorite, perhaps derived from an ancient martian biota. We have examined PAHs in the Antarctic shergottite EETA79001, which is also considered to be from Mars, as well as several Antarctic carbonaceous chondrites. We have found that many of the same PAHs detected in the ALH84001 carbonate globules are present in Antarctic carbonaceous chondrites and in both the matrix and carbonate (druse) component of EETA79001. We also investigated PAHs in polar ice and found that carbonate is an effective scavenger of PAHs in ice meltwater. Moreover, the distribution of PAHs in the carbonate extract of Antarctic Allan Hills ice is remarkably similar to that found in both EETA79001 and ALH84001. The reported presence of L-amino acids of apparent terrestrial origin in the EETA79001 druse material (McDonald and Bada, 1995) suggests that this meteorite is contaminated with terrestrial organics probably derived from Antarctic ice meltwater that had percolated through the meteorite. Our data suggests that the PAHs observed in both ALH84001 and EETA79001 are derived from either the exogenous delivery of organics to Mars or extraterrestrial and terrestrial PAHs present in the ice meltwater or, more likely, from a mixture of these sources. It would appear that PAHs are not useful biomarkers in the search for extinct or extant life on Mars.     

Isotopic composition (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genesis of carbonates from the famennian manganiferous formation of Pai-Khoi

The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ¹³C values from–6.6 to 1.3‰ and δ¹⁸O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ¹³C from–5.2 to–2.9 and δ¹⁸O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ¹³C from–16.4 to–13.1 and δ¹⁸O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ¹³C from–13.0 to–10.4‰ and δ¹⁸O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ¹⁸O from 18.7 to 17.6‰ and δ¹³C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO₂. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.     

Two populations of carbonate in ALH84001: geochemical evidence for discrimination and genesis

We present major and trace-element, oxygen isotope, textural, and structural data for carbonates and related phases in the SNC meteorite ALH84001. These data document the existence of at least two distinct carbonate populations: one composed of finely zoned, chemically and isotopically heterogeneous concretions of magnesio-siderite with distinct white magnesite rims, and a second composed of relatively homogeneous, isotopically and compositionally simple domains of ankeritic carbonate and intimately intergrown glass and fine-grained pyroxene. We suggest on the basis of textural evidence and geochemical systematics that the first population consists of low-temperature aqueous precipitates, and the second is produced by shock melting of the first. Values of δ¹⁸O and Sr/Ca ratios are correlated with one another in magnesio-siderite concretions; the trend formed by these data is consistent with the predicted relationship for inorganic precipitation of carbonate from a solution of constant composition between temperatures of ∼190°C (for concretion cores) to 20°C (for magnesite-rich concretion rims). Given the assumptions inherent in this temperature estimate, the aqueous fluid parental to carbonate concretions is constrained to have a δ¹⁸O of −5‰ VSMOW (significantly mass fractionated compared with expected juvenile martian volatiles) and minor-element abundances broadly similar to terrestrial seawater.     

An Evaporation Model for Formation of Carbonates in the ALH84001 Martian Meteorite

Small, discoid globules and networks of magnesium-iron-calcium carbonates occur within impact-produced fracture zones in the ALH84001 Martian meteorite. Because these carbonates contain or are associated with the hydrocarbons, single-domain magnetite and iron-sulfide grains, and purported microfossils that collectively have been cited as evidence for ancient Martian life, it is critically important to understand their formation. Previous hypotheses for the origin of the carbonates involve either alteration of the rock by hydrothermal fluids at relatively low temperatures, or formation from a CO₂-rich vapor at high temperatures. This paper explores an alternative mechanism–direct precipitation from a ponded evaporating brine infiltrating into fractures in the floor of an impact crater. Such a model can be reconciled with the observed carbonate compositional zoning and extreme stable-isotopic fractionations. If the carbonates formed in this manner, this removes a possible obstacle to the proposed existence of microbial remains in ALH84001; however, the cited evidence for life can be better explained by inorganic processes expected from brines in an evaporating alkaline lake, with an overprint of shock metamorphism and subsequent contamination by organic matter after falling to Earth.     

Origins of magnetite nanocrystals in Martian meteorite ALH84001

The Martian meteorite ALH84001 preserves evidence of interaction with aqueous fluids while on Mars in the form of microscopic carbonate disks. These carbonate disks are believed to have precipitated 3.9 Ga ago at beginning of the Noachian epoch on Mars during which both the oldest extant Martian surfaces were formed, and perhaps the earliest global oceans. Intimately associated within and throughout these carbonate disks are nanocrystal magnetites (Fe₃O₄) with unusual chemical and physical properties, whose origins have become the source of considerable debate. One group of hypotheses argues that these magnetites are the product of partial thermal decomposition of the host carbonate. Alternatively, the origins of magnetite and carbonate may be unrelated; that is, from the perspective of the carbonate the magnetite is allochthonous. For example, the magnetites might have already been present in the aqueous fluids from which the carbonates were believed to have been deposited. We have sought to resolve between these hypotheses through the detailed characterization of the compositional and structural relationships of the carbonate disks and associated magnetites with the orthopyroxene matrix in which they are embedded. Extensive use of focused ion beam milling techniques has been utilized for sample preparation. We then compared our observations with those from experimental thermal decomposition studies of sideritic carbonates under a range of plausible geological heating scenarios. We conclude that the vast majority of the nanocrystal magnetites present in the carbonate disks could not have formed by any of the currently proposed thermal decomposition scenarios. Instead, we find there is considerable evidence in support of an alternative allochthonous origin for the magnetite unrelated to any shock or thermal processing of the carbonates.     

Siderophile elements in Martian meteorites and implications for core formation in Mars

Noble metals, Mo, W, and 24 other elements were determined in six SNC meteorites of presumably Martian origin. Based on element correlations, representative siderophile element concentrations for the silicate mantle of Mars were inferred. From a comparison with experimentally determined metal/silicate partition coefficients of the moderately siderophile elements: Fe, Ni, Co, W, Mo, and Ga, it is concluded that equilibrium between core forming metal and silicates in Mars has occurred at high temperatures (around 2200°C) and low pressures (<1 GPa). This suggests that metal segregation occurred concurrently with rapid accretion of Mars, which is consistent with the inference from excess ¹⁸²W in Martian meteorites (Lee and Halliday, 1997). Concentrations of Ir, Os, Ru, Pt, and Au in the analyzed Martian meteorites, except ALH84001, are at a level of approximately 10⁻²–10⁻³ × CI. The comparatively high abundances of noble metals in Martian meteorites require the addition of chondritic material after core formation. The similarity in Au/La and Pt/Ca ratios between ALH84001 and the other Martian meteorites suggests crystallization of ALH84001 after complete accretion of Mars.     

Terrestrial alteration of carbonate in a suite of Antarctic CM chondrites: Evidence from oxygen and carbon isotopes

The oxygen (δ¹⁸O, δ¹⁷O) and carbon (δ¹³C, FMOD¹⁴C-the fraction of modern ¹⁴C) isotopic compositions of carbonate were measured for a set of paired Antarctic CM chondrites (EET 96006, EET 96016, EET 96017, and EET 96019). While the oxygen isotopic compositions do not plot on the terrestrial fractionation line and indicate that a component of the carbonate minerals has an extraterrestrial origin, they also do not fall on the array defined for carbonates by CM falls and are thus consistent with the presence of a terrestrial carbonate component. The δ¹³C and FMOD¹⁴C measurements of carbonate suggest the presence of at least two carbon sources: carbonate derived from atmospheric CO₂ that is inferred to have been produced as a result of silicate weathering reactions and carbonate derived from another carbon source that is either old or non-atmospheric. The relationships between oxygen and carbon isotope data provide additional constraints on the weathering process, and allow the possibility that rock-dominated weathering of the meteorite caused the oxygen isotopic composition of Antarctic water added to the meteorite to evolve away from the terrestrial mass-fractionation array, leading to formation of low temperature terrestrial alteration products that do not lie on the terrestrial fractionation line.     

Stable isotope studies and processes of carbonate formation in Hungarian alkali basalts and lamprophyres: evolution of magmatic fluids and magma-sediment interactions

Processes of carbonate formation have been related to C and O isotopic compositions in the Mesozoic alkali basalt (Mecsek Mts.) and lamprophyre (Transdanubian Range) suites of Hungary. In the studied magrnatic rocks, carbonates are present as ocelli, amygdales, xenoliths, veins and groundmass carbonate. C and O isotope studies of these types of carbonate have yielded information on the origin of the carbonates and indicated the following processes of formation that determined the δ¹³C and δ¹⁸O values of the carbonates:(1)Crystallization of magmatic carbonate. Textural characteristics and δ¹³C values suggest formation of magmatic carbonate in alkali basalt and lamprophyre dikes, whereas the δ¹⁸O compositions of these carbonates indicate low temperature oxygen isotope exchange with magmatic fluids.(2) Assimilation of sedimentary carbonate by silicate magmas. Even completely recrystallized amygdales and ocelli of basalts and lamprophyres have preserved their sedimentary δ¹³C values. In contrast, variations in the extent of mobilization and isotope exchange with magmatic fluids are reflected in differences in the ranges of the δ¹⁸O values of amygdales, ocelli and veins, and can be attributed to different amounts of fluids involved in the magmatic events.(3) Low temperature alteration of magmatic rocks caused only ¹⁸O-enrichment in the carbonate amygdales of basalts and the groundmass carbonates of lamprophyres, indicating that no externally-derived CO₂ was present in the alteration fluids.(4) Degassing of magma and magmatic fluid. Correlations between δ¹³C and δ¹⁸O data, magma crystallization depths and amygdale sizes in the alkali basalts suggest that CO₂ degassing has been responsible for the negative δ¹³C and positive δ¹⁸O shifts observed. A similar trend was found in the lamprophyres, but the extent of the δ¹⁸O shift indicates that in these rocks H₂O degassing also played an important role.     

Negative δO values in Allan Hills 84001 carbonate: Possible evidence for water precipitation on mars

The Martian meteorite ALH84001 contains ∼1% by weight of carbonate formed by secondary processes on the Martian surface or in the shallow subsurface. The major form of this carbonate is chemically and isotopically zoned rosettes which have been well documented elsewhere. This study concentrates upon carbonate regions ∼200 μm across which possess previously unobserved magnesium rich inner cores, interpreted here as rosette fragments, surrounded by a later stage cement containing rare Ca-rich carbonates (up to Ca₈₁Mg₀₇Fe₀₄Mn₀₇) intimately associated with feldspar. High spatial resolution ion probe analyses of Ca-rich carbonate surrounding rosette fragments have δ¹⁸O_V-SMOW values as low as −10‰. These values are not compatible with deposition from a global Martian atmosphere invoked to explain ALH84001 rosettes. The range of δ¹⁸O values are also incompatible with a fluid that has equilibrated with the Martian crust at high temperature or from remobilisation of carbonate of rosette isotopic composition. At Martian atmospheric temperatures, the small CO₂(gas)-CO₂(ice) fractionation makes meteoric CO₂ an unlikely source for −10‰ carbonates. In contrast, closed system Rayleigh fractionation of H₂O can generate δ¹⁸O_H2O −30‰, as observed at high latitudes on Earth. We suggest that atmospheric transport and precipitation of H₂O in a similar fashion to that on Earth provides a source of suitably ¹⁸O depleted water for generation of carbonate with δ¹⁸O_V-SMOW = −10‰.     

Carbon,oxygen and strontium isotope geochemistry of Proterozoic carbonate rocks of the Vindhyan Basin,central India

Carbon, oxygen and strontium isotope compositions of carbonate rocks of the Proterozoic Vindhyan Supergroup, central India suggest that they can be correlated with the isotope evolution curves of marine carbonates during the latter Proterozoic. The carbonate rocks of the Lower Vindhyan Supergroup from eastern Son Valley and central Vindhyan sections show δ¹³C values of ∼0‰ (V-PDB) and those from Rajasthan section are enriched up to +2.8‰. In contrast, the carbonate rocks of the Upper Vindhyan succession record both positive and negative shifts in δ¹³C compositions. In the central Vindhyan section, the carbonates exhibit positive δ¹³C values up to +5.7‰ and those from Rajasthan show negative values down to –5.2‰. The δ¹⁸O values of most of the carbonate rocks from the Vindhyan Supergroup show a narrow range between –10 and –5‰ (V-PDB) and are similar to the ‘best preserved’ ¹⁸O compositions of the Proterozoic carbonate rocks. In the central Vindhyan and eastern Son Valley sections, carbonates from the Lower Vindhyan exhibit best-preserved ⁸⁷Sr/⁸⁶Sr compositions of 0.7059±6, which are lower compared to those from Rajasthan (0.7068±4). The carbonates with positive δ¹³C values from Upper Vindhyan are characterized by lower ⁸⁷Sr/⁸⁶Sr values (0.7068±2) than those with negative δ¹³C values (0.7082±6). A comparison of C and Sr isotope data of carbonate rocks of the Vindhyan Supergroup with isotope evolution curves of the latter Proterozoic along with available geochronological data suggest that the Lower Vindhyan sediments were deposited during the Mesoproterozoic Eon and those from the Upper Vindhyan represent a Neoproterozoic interval of deposition.     

Authigenic Carbonate Formation in the Ocean

Oceanic authigenic carbonates are classified according to the origin of carbonate carbon source using a complex methodology that includes methods of sedimentary petrography, mineralogy, isotope geochemistry, and microbiology. Mg-calcite (proto-dolomite) and aragonite predominate among the authigenic carbonates. All authigenic carbonates are depleted in heavy carbon isotope ¹³ and enriched in heavy oxygen isotope ¹⁸O (in PDB system), indicating biological fractionation of isotopes during the carbonate formation. Results obtained show that authigenic carbonate formation is a biogeochemical (microbial) process, which involves carbon from ancient sedimentary rocks, abiogenic methane, and bicarbonate-ion of hydrothermal fluids into the modern carbon cycle.     

The sources of water in Martian meteorites: clues from hydrogen isotopes

H isotope measurements of carbonate, phosphate, feldspathic and mafic glasses, and post-stishovite silica phase in the shergottites Zagami, Shergotty, SaU 005, DaG 476, ALHA 77005 and EETA 79001, as well as in Chassigny and ALH 84001, show that all these phases contain deuterium-enriched water of extraterrestrial origin. The minerals and glasses analyzed may contain an initial primary hydrogen component, but their isotopic composition was modified to varying degrees by three different processes: interaction with a fractionated exchangeable water reservoir on Mars, hydrogen devolatilization by impact melting, and terrestrial contamination. Positive correlations between δD and water abundance in feldspathic glass and post-stishovite silica in Zagami, Shergotty, and SaU 005 is indicative of mixing of a high δD component (3000-4000‰) and a less abundant, low δD component (∼0‰). The high δD component is primarily derived from the Martian exchangable reservoir, but may also have been influenced by isotopic fractionation associated with shock-induced hydrogen loss. The low δD component is either a terrestrial contaminant or a primary “magmatic” component. The negative correlation between δD and water abundances in mafic and feldspathic glasses in ALH 84001, ALHA 77005, and EETA 79001 is consistent with the addition of a low δD terrestrial contaminant to a less abundant high-deuterium Martian component. The low δD of magmatic glass in melt inclusions suggests that the δD of Martian parent magma was low and that the initial H isotope signature of Mars may be similar to that of Earth.     

火星生命研究的进展与前景

关于火星是否存在或曾经存在生命的争论由来已久。有人以ALH84001火星陨石新鲜破裂面上的大量碳酸盐小球体和多环芳香烃（PAHs）为主要依据，推论火星至少在13～36亿 aBP前很可能有生命形态存在。然而，很多人认为ALH84001陨石的各种特性可以是非生物成因的。由于地球上的生物在超过115℃的温度下很难存活（火星可与之类比），争论的焦点逐渐集中在碳酸盐球体的形成温度上。也有研究者关注该陨石上有机物质的来源问题。对ALH84001陨石的综合学科研究提出了互相矛盾的证据。综述了自1996年以来在国外各种主要期刊上发表的关于 ALH84001陨石与火星生命的研究成果（也包括了一些对其他火星陨石的研究），认为目前尚不能断言火星生命存在与否。对火星继续深入探索以获取进一步的证据是十分必要的。以美国国家航空和宇航局（NASA）Odys sey宇宙飞船起始的火星探测计划将引发新一轮火星生命研究的热潮。     

Formation of meteorite hydrocarbons from thermal decomposition of siderite (FeCO3)

Thermal decomposition of siderite has been proposed as a source of magnetite in martian meteorites. Laboratory experiments were conducted to evaluate the possibility that this process might also result in abiotic synthesis of organic compounds. Siderite decomposition in the presence of water vapor at 300°C generated a variety of organic products dominated by alkylated and hydroxylated aromatic compounds. The results suggest that formation of magnetite by thermal decomposition of siderite on the precursor rock of the martian meteorite ALH84001 would have been accompanied by formation of organic compounds and may represent a source of extraterrestrial organic matter in the meteorite and on Mars. The results also suggest that thermal decomposition of siderite during metamorphism could account for some of the reduced carbon observed in metasedimentary rocks from the early Earth.     

Sedimentology and isotope geochemistry of lacustrine carbonates of the Oligocene Campins Basin, north-east Spain

The non-marine Campins Basin developed in the Oligocene, during a period of early rifting of the Catalan Coastal Ranges. Lacustrine deposits, interbedded between two alluvial units, comprise shallow and deep lacustrine facies. The lower, shallow lacustrine facies are made up of microbialite buildups and thin limestone beds. In the studied area, these facies are overlain by deep lacustrine facies which consist of alternations of several, metre-thick carbonate- and mudstone-dominated intervals. In addition to calcite, which is characteristic of the shallow lacustrine facies, aragonite and abundant dolomite are present in the deep lacustrine facies. This mineralogical change in the sequence reflects an overall increase in the Mg/Ca ratio of the lake waters. The deep lacustrine sequences are interpreted as having formed in a hydrologically closed basin that was subject to changes in the Mg/Ca ratio of the water, probably related to variations in the evaporation/precipitation rate. The sedimentological, mineralogical and isotopic characteristics of the Campins Basin dolomites suggest that, in general, they are primary in origin. The stable isotope data show an approximate covariance between δ¹³C and δ¹⁸O in the lower shallow lacustrine carbonates (calcite) which suggests that they formed during the onset of closure of the lake. The δ¹³C and δ¹⁸O values of the deep lacustrine carbonates display three different clusters that are roughly related to the carbonate mineralogy. Normalisation with respect to calcite of the isotopic compositions of dolomite and aragonite from the deep lacustrine carbonates allows the integration of all these isotope values into one covariant trend. The sequential appearance of different carbonate minerals and the isotopic covariant trend may indicate an overall evaporative concentration of the lake waters. The change in slope of the covariant trend for the isotope values between the shallow and the deep lacustrine carbonates might reflect the change in the waterbody morphology recorded in the basin fill sequence.