Reconstruction of Late Paleozoic heat flows and burial histories at the Rhenohercynian-Subvariscan boundary, Germany

The thermal and burial history of the Herzkamp syncline, located in the transition zone between the Variscan Rhenish Massif and the Ruhr foreland basin (western Germany), was reconstructed using PDI/PC-1D-basin modelling software (IES). The models were calibrated with new vitrinite reflectance data measured on Palaeozoic outcrop samples. High sample density and quality of the calibration data allowed a 3D reconstruction of the heat flow as well as of burial and erosion history. Vitrinite reflectance values range from 0.8 to 4.9%R_r and generally increase with increasing stratigraphic age. The coalification pattern confirms pre-tectonic maturation, especially in the western part of the study area. A "low-coalification zone" showing stagnating/decreasing coalification with increasing stratigraphic age exists, however, northeast of the Ennepe thrust, indicating synorogenic coalification. This anomaly is explained by early thrusting in the northern Rhenish Massif resulting in restricted burial/early uplift and thus lower thermal maturity. One result of numerical modelling is that palaeo-heat flows during maximum burial (Westphalian or post-Westphalian) decreased southwards from approximately 65 to less than 50 mW/m². Maximum burial depths for the base and top of the Namurian also decrease southwards from 7000 to 3600 m and 4600 to 1800 m, respectively, resulting in southwards-decreasing coalification of the respective stratigraphic horizon. Eroded overburden increases southwards (3100-5700 m), with the exception of the low-coalification zone, which is characterised by lower amounts of eroded overburden (1300-2900 m) and an earlier onset of erosion, i.e. in the Westphalian B rather than Westphalian D or post-Westphalian.     

Coalification pattern and thermal modelling of the Permo-Carboniferous Saar Basin (SW-Germany)

Palaeo-heat flow values and thicknesses of eroded Permo-Carboniferous sediments in the Saar Basin were evaluated using one dimensional thermal modelling techniques. Thermal, burial and erosion histories for 16 wells were calibrated by comparing measured and calculated vitrinite reflectance using the kinetic EASY%R_o algorithm and by comparing measured and calculated temperature data. On the basis of 37 wells, coalification maps were constructed revealing a syn-kinematic coalification pattern. Thermal maturity of the sediments can only be explained by deep burial and moderate heat flows during time of maximum burial, i.e., in the Permo-Carboniferous. Calculated heat flow data range between 50 and 75 mW/m², which implies a crustal thickness between 30 and 40 km during the time of maximum burial. These values are in accordance with the geodynamic setting of the basin. The influence of the Permo-Carboniferous volcanism on the palaeo-temperature distribution was overwhelmed by the subsequent deep burial. During Permian times, between 1800 and 3000 m of Permo-Carboniferous sediments were eroded. Different sedimentation and erosion histories are characteristic for anticlines and synclines, respectively.     

Coal rank variation in the Intrasudetic Basin,SW Poland

The Upper Carboniferous, coal-bearing sequence of the Intrasudetic Basin (SW Poland) includes coals ranging from high-volatile bituminous to anthracitic rank. The lowest values of reflectance are recorded around the basin margins (0.6% R₀ max), the highest ones appear in the center of the basin (exceeding 4% R₀ max). Reflectance gradients are very high, reaching 0.6%/100 m in the centre of the basin.A comparison of the isoreflectance maps for three lithostratigraphical units—the Walbrzych, Bialy Kamien and Zacler Formations, with the present-day burial depth and the depth of burial during the Westphalian B/C—indicates that there is a strong relationship between reflectance and the sediment cover during the Westphalian B/C, particularly in the vicinity of Walbrzych and Lubawka. This suggests that the increase in coal rank is related to the increase in cover which permitted the temperature to build-up to high values.In the eastern and central parts of the basin and the Nowa Ruda area, higher reflectance than that derived from burial depth is observed which is believed to result from higher heat flow from the basement. The volcanic rocks of the Intrasudetic Basin appear to have little effect on coal rank and are not considered to be a significant contributor to the heat flow of the region.During coalification, the oldest Westphalian coal seams were buried to about 700 m and the youngest seams of the Walbrzych Formation to 900 m. Around the basin margins the coals had reached their present-day rank by the Westphalian B/C and in the central part probably by the end of the Stephanian. Most effective coalification took place during the Westphalian A,B occupying a period of less than 20 million years. The coalification temperature is calculated to be 160–170°C with a geothermal gradient of 8–10°C/100 m. These geothermal conditions support the suggestion of a volcanic origin for the Intrasudetic Basin.     

Palaeo‐climate controlled diagenesis of the Westphalian C & D fluvial sandstones in the Campine Basin (north‐east Belgium)

The Westphalian C and D fluvial sandstones in the Campine Basin (north‐east Belgium) are potential reservoirs for the sequestration of CO₂ and interesting analogues of the hydrocarbon reservoirs in the Southern North Sea. Although these sandstones were deposited in a relatively short period of time, their reservoir properties and mineralogical compositions are very different. A petrographic study complemented with stable isotope analyses, fluid inclusion microthermometry and X‐ray diffraction analyses of the clay fractions of the sandstones, which were sampled from deep boreholes (>1000 m) in the Campine Basin, revealed that these differences are related mainly to the climate at the time of deposition. The most important eogenetic processes affecting the Westphalian sandstones were the generation of a pseudomatrix by physical compaction of Al‐silicates and lithic fragments that were strongly altered by extensive meteoric leaching, kaolinitization of unstable silicates and precipitation of siderite. These processes had a detrimental influence on the reservoir properties of Westphalian C sandstones, but their impact on the Westphalian D sandstones was minimal. The difference is assumed to be related to the climate at the time of deposition, which changed from tropical humid in the Westphalian C to semi‐arid/arid during the Late Westphalian D. Both the Westphalian C and D sandstones were affected by similar mesogenetic processes. Mesogenetic quartz cementation resulted from chemical compaction and illitization of kaolinite, K‐feldspar and smectitic clays. Illitization of kaolinite was controlled by the available quantities of co‐existing kaolinite and K‐feldspar and mainly affected the Westphalian D sandstones. Illitization of K‐feldspar was controlled by the K‐feldspar content. It had a much larger impact on the reservoir properties of the Westphalian D as, in these sandstones, K‐feldspar was less affected by eogenetic alteration. The illitization of smectitic clays resulted in illite, quartz and ankerite cementation in both reservoirs. This process had a more important impact on the Westphalian C reservoir, since cementation here also resulted from smectite to illite conversion in the interbedded and underlying shales. The effect of mesogenetic alterations on the reservoir properties was much less drastic than the impact of eodiagenesis. Mesogenetic alterations do exert a significant control on the properties of the Westphalian D. The vast impact of eodiagenesis on the Westphalian C sandstones made them less susceptible to mesogenetic alteration. The effect of telogenetic processes on the porosity and permeability of the Westphalian sandstones was small and restricted to the top reservoir intervals that directly underlie the Cimmerian Unconformity. No significant telogenetic alterations related to the Variscan Unconformity were observed.     

Coalification history of the Stephanian Ciñera-Matallana pull-apart basin,NW Spain: Combining anisotropy of vitrinite reflectance and thermal modelling

The Stephanian Ciñera-Matallana Basin of NW Spain comprises 1,500 m of alluvial to lacustrine coal-bearing sediments, which were deposited in a late Variscan transtensional/transpressional pull-apart setting. The relationship between coalification pattern and rock deformation was evaluated by measurements of the anisotropy of vitrinite reflectance (AVR). The AVR ellipsoids reveal both pre-tectonic elements related to the bedding fabric and syn-tectonic elements related to folding, producing biaxial ellipsoid shapes with the maximum reflectance parallel to fold axes. The mean coalification gradient for the Stephanian succession is about 0.62 %Rr/km. Calculations of the mean palaeo-geothermal gradient are presented on the basis of three different empirical equations. A palaeo-geothermal gradient of 85 °C/km is considered the most realistic, with an overburden of about 1,000 m. 1-D numerical modelling of the burial history results in two possible scenarios, the most preferable involving a palaeo-heat flow of 150 mW/m² and an overburden of ca. 1,050 m. These results indicate that maximum coalification was related to a localised but high palaeo-heat flow/-geothermal gradient. The anisotropy of vitrinite reflectance highlights the interactive and transitional nature of sedimentary compaction and rock deformation on the maturation of organic material within strike-slip fault zones.     

Geothermal regime and Jurassic source rock maturity of the Junggar basin, northwest China

We analyze the thermal gradient distribution of the Junggar basin based on oil-test and well-logging temperature data. The basin-wide average thermal gradient in the depth interval of 0–4000 m is 22.6 °C/km, which is lower than other sedimentary basins in China. We report 21 measured terrestrial heat flow values based on detailed thermal conductivity data and systematical steady-state temperature data. These values vary from 27.0 to 54.1 mW/m² with a mean of 41.8 ± 7.8 mW/m². The Junggar basin appears to be a cool basin in terms of its thermal regime. The heat flow distribution within the basin shows the following characteristics. (1) The heat flow decreases from the Luliang Uplift to the Southern Depression; (2) relatively high heat flow values over 50 mW/m² are confined to the northern part of the Eastern Uplift and the adjacent parts of the Eastern Luliang Uplift and Central Depression; (3) The lowest heat flow of smaller than 35 mW/m² occurs in the southern parts of the basin. This low thermal regime of the Junggar basin is consistent with the geodynamic setting, the extrusion of plates around the basin, the considerably thick crust, the dense lithospheric mantle, the relatively stable continental basement of the basin, low heat generation and underground water flow of the basin. The heat flow of this basin is of great significance to oil exploration and hydrocarbon resource assessment, because it bears directly on issues of petroleum source-rock maturation. Almost all oil fields are limited to the areas of higher heat flows. The relatively low heat flow values in the Junggar basin will deepen the maturity threshold, making the deep-seated widespread Permian and Jurassic source rocks in the Junggar basin favorable for oil and gas generation. In addition, the maturity evolution of the Lower Jurassic Badaowan Group (J_1b) and Middle Jurassic Xishanyao Group (J_2x) were calculated based on the thermal data and burial depth. The maturity of the Jurassic source rocks of the Central Depression and Southern Depression increases with depth. The source rocks only reached an early maturity with a R₀ of 0.5–0.7% in the Wulungu Depression, the Luliang Uplift and the Western Uplift, whereas they did not enter the maturity window (R₀ < 0.5%) in the Eastern Uplift of the basin. This maturity evolution will provide information of source kitchen for the Jurassic exploration.     

Burial and thermal history of the West Bashkirian sedimentary basins

The GALO system is applied to the numerical reconstruction of burial and thermal histories of the West Bashkirian lithosphere from the Riphean to the present. An analysis of the variation in tectonic subsidence of the basin during its development is utilized to estimate approximately the mantle heat flow variations. Our variant of basin evolution suggests that after cooling in the Early Riphean, the rather weak thermal reactivations have not led to considerable heating of the lithosphere in the study region. Surface heat flow decreased from relatively high values in the Early Riphean (60–70 mW/m² in the eastern area and 40–50 mW/m² in the western part) to present-day values of 32–40 mW/m². In spite of the relatively low temperature regime of the basin as a whole, a syn-rifting deposition of more than 10 km of limestone, shale and sandstone in the Riphean resulted in rather high temperatures (180–190 °C) at the base of present-day sedimentary blanket in the eastern area. In agreement with the observed data, computed present-day heat flow through the sediment surface increases slightly from 32 to 34 mW/m² near the west boundary of the region to 42 mW/m² near the boundary of the Ural Foldbelt, whereas the heat flow through the basement surface decreases slightly from 28–32 to 24–26 mW/m² in the same direction. The mantle heat flow is only 11.3–12.7 mW/m², which is considerable lower than mean heat flow of the Russian Platform (16–18 mW/m²) and comparable with the low heat flow of Precambrian shields.     

New heat flow data from the immediate vicinity of the Kola super-deep borehole: Vertical variation in heat flow confirmed and attributed to advection

We present new heat flow values and other geothermal data in the upper crystalline crust in the immediate vicinity of the 12.4-km deep Kola super-deep borehole, NW Russia. Our results show a systematic vertical increase in geothermal gradient and heat flow density as deep as we could measure (1.6 km). Our results confirm earlier results on vertical heat flow trends of in the uppermost part of the Kola super-deep hole, and imply that the thermal regime is not in steady-state conductive conditions. In an area of 3-km × 5-km measurements were performed in 1–2-km deep boreholes surrounding the Kola super-deep hole and on core samples from these holes. Temperature logs are available from 36 holes. Core data exists from 23 boreholes with a total length of 11.5 km at a vertical resolution of 10 m. We carried out a very detailed study on thermal conductivity with regard to anisotropy, inhomogeneity and temperature dependence. Tensor components of thermal conductivity were determined on 1375 core samples from 21 boreholes in 3400 measurements. Additionally, we measured specific heat capacity, heat generation rate, density, porosity, and permeability on selected subsets of core samples. Heat flow from 19 boreholes varies between 31 and 45 mW m⁻² with an average value of 38 mW m⁻². In most boreholes the vertical heat flow profiles show a considerable variation with depth. This is consistent with observations in the upper part of the Kola super-deep borehole. We conclude that this variation is not caused by technical operations but reflects a natural process. It is considered to be due to a combination of advective, structural and paleoclimatic effects. Preliminary 3-D numerical modeling of heat and flow in the study area provides an indication of relative contributions of each of these factors: advective heat transfer turns out to have a major influence on the vertical variation of heat flow, although transient changes in surface temperature may also cause a significant variation. Heterogeneity of the rocks in the study area is less important.     

Synthesis and upper stability of paragonite

The mineral paragonite, NaAl₂[AlSi₃O₁₀ (OH)]₂, has been synthesized on its own composition starting from a variety of different materials. Indexed powder data and refined cell parameters are given for both the 1M and 2M₁ polymorphs obtained. The upper stability limit of paragonite is marked by its breakdown to albite + corundum + vapour. The univariant equilibria pertaining to this reaction have been established by reversing the reaction at six different pressures, the equilibrium curve running through the following intervals: 1 kb: 530°–550° C 2 kb: 555°–575° C 3 kb: 580°–600° C 5kb: 625°–640° C 6 kb: 620°–650° C 7 kb: 650°–670° C.Comparison with the upper stability limit of muscovite (Velde, 1966) shows that paragonite has a notably lower thermal stability thus explaining the field observation that paragonite is absent in many higher grade metamorphic rocks in which muscovite is still stable.The enthalpy and entropy of the paragonite breakdown reaction have been estimated. Since intermediate albites of varying structural states are in equilibrium with paragonite, corundum and H₂O along the univariant equilibrium curve, two sets of data pertaining to the entropy of paragonite (S ₂₉₈ ⁰ ) as well as the enthalpy ( H _f,298 ⁰ ) and Gibbs free energy ( G _f,298 ⁰ ) of its formation were computed, assuming (1) high albite and (2) low albite as the equilibrium phase. The values are: (1) (2) S ₂₉₈ ⁰ 67.8±3.9 cal deg^–1 gfw^–1 63.7±3.9 cal deg^–1 gfw^–1 H _f,298 ⁰ –1417.9±2.7 kcal gfw^–1 –1420.2±2.6 kcal gfw^–1 G _f,298 ⁰ –1327.4±4.0 kcal gfw^–1 –1328.5±4.0 kcal gfw^–1.Adapted from a part of the author's Habilitationsschrift accepted by the Ruhr University, Bochum (Chatterjee, 1968).     

Mg–Fe interdiffusion rates in wadsleyite and the diffusivity jump at the 410-km discontinuity

Mg–Fe interdiffusion rates have been measured in wadsleyite aggregates at 16.0–17.0 GPa and 1230–1530 °C by the diffusion couple method. Oxygen fugacity was controlled using the NNO buffer, and water contents of wadsleyite were measured by infrared spectroscopy. Measured asymmetric diffusion profiles, analyzed using the Boltzmann–Matano equation, indicate that the diffusion rate increases with increasing iron concentration and decreasing grain size. In the case of wadsleyite containing 50–90 weight ppm H₂O, the Mg–Fe interdiffusion coefficients at compositions of Mg/(Mg + Fe)=0.95 in the coarse-grained region (about 60 m) and 0.90 in the fine-grained region (about 6 m) were determined to be a D_Xmg = 0.95 (m² s^–1)=1.24 × 10^–9 exp[–172 (kJ mol^–1)/RT] and D_Xmg = 0.90 (m² s^–1)=1.77 × 10^–9 exp[–143 (kJ mol^–1)/RT], respectively. Grain-boundary diffusion rates were estimated to be about 4 orders of magnitude faster than the volume diffusion rate. Grain-boundary diffusion dominates when the grain size is less than a few tens of microns. Results for the nominally dry diffusion couple in the present study are roughly consistent with previous studies, taking into account differences in pressure and grain size, although water contents of samples were not clear in previous studies. We observed that the diffusivity is enhanced by about 1 order of magnitude in wadsleyite containing 300–2100 wt. ppm H₂O at 1230 °C, which is almost identical to the enhancement associated with a 300 °C increase in temperature. It is still not conclusive that a jump in diffusivity exists between olivine and wadsleyite because water contents of olivine in previous diffusion studies and effects of water on the olivine diffusivity are uncertain.     

Effect of the thermal gradient variation through geological time on basin modeling; a case study: The Paris basin

Many studies investigated the thermal modeling of the Paris basin for petroleum interests during the 1970s. Most of the softwares developed by oil companies or research centers were based on the assumption of a constant thermal gradient. In order to take into consideration the variation of the thermal gradient during basin evolution, we developed the TherMO's Visual Basic 1D program. We applied our model to twenty boreholes located along a cross-section roughly running EW over 150 km in the center of the Paris basin. The numerical results were calibrated with organic matter maturity data. TherMO's simulates the amount of heat provided to the sedimentary organic matter. The heat parameter simulated shows lateral variation along the cross-section. It decreases from Rambouillet to Trou Aux Loups boreholes (87–66 mW/m²) at about 100 km more to the east whereas the heat flux value simulated is 73 mW/m² in St. Loup borehole. The mean thermal gradient calculated for liassic horizons at 87 My for the Rambouillet well is 50.4 °C/km. This value is similar to previously published results. By integrating the calculation of the thermal gradients and conductivities related to the burial of each stratigraphic sequence, our approach points out variations in the thermal regimes the sedimentary organic matter (SOM) has been subjected to through geological time.     

Order-disorder kinetics in orthopyroxenes of ophiolite origin

The kinetics of the Fe–Mg order-disorder reaction in four low-Fe orthopyroxenes (Fs_7–14) has been studied by use of Mössbauer spectroscopy, to determine cooling rates for the host rocks. The obtained Fe²⁺ distribution data were corrected for the thickness effect. The sample set was selected from rocks expected to have distinctly different cooling histories. Equilibrium data for the exchange reaction were determined for the temperature range 600–800°C, and kinetic data at 650°C. The results indicate that the degree of ordering in low-Fe orthopyroxene is substantially lower than estimated by thermodynamic models. The cooling rates obtained seem realistic for most samples, ranging from 19°C/min at 760°C for a boninite pillow lava to 2°C/million years at 267°C for a regionally metamorphosed olivinite. The errors in obtained cooling rates are estimated to be near one order of magnitude. An Fe³⁺ containing sample yields an unreasonably slow cooling rate which may be due to oxidation of Fe²⁺, occurring after the quenching point of the order-disorder reaction was reached.     

Comparison of the high-pressure schist belts of New Caledonia and Sanbagawa,Japan

The high-pressure schist terranes of New Caledonia and Sanbagawa were developed along the oceanic sides of sialic forelands by tectonic burial metamorphism. The parent rocks were chemically similar, as volcanic-sedimentary trough or trench sequences, and metamorphic temperatures in both belts were 250° to 600° C. From phase equilibria curves, total pressures were higher for New Caledonia (6–15 kb) than for Sanbagawa (5–11 kb) and the estimated thermal gradients were 7–10° C/km and 15° C/km respectively.PT paths identify the higher pressure in New Caledonia (P differences 2 kb at 300° C and 4 kb at 550° C) with consequent contrast in progressive regional metamorphic zonation for pelites in the two areas: lawsonite-epidote-omphacite (New Caledonia) and chlorite-garnet-biotite (Sanbagawa). In New Caledonia the Na-amphibole is dominantly glaucophane and Na-pyroxenes associated with quartz are Jadeite (Jd_95–100) and omphacite; in Sanbagawa the amphibole is crossite or riebeckite and the pyroxene is omphacite (Jd₅₀). For both areas, garnet rims show increase in pyrope content with advancing grade, but Sanbagawa garnets are richer in almandine. Progressive assemblages within the two belts can be equated by such reactions as:New Caledonia Sanbagawa glaucophane+paragonite+H₂Oalbite+chlorite+quartz glaucophane+epidote+H₂Oalbite+chlorite+actinolite and the lower pressure Japanese associations appear as retrogressive phases in the New Caledonia epidote and omphacite zones.The contrasts inPT gradient, regional zonation and mineralogy are believed due to differences in the tectonic control of metamorphic burial: for New Caledonia, rapid obduction of an upper sialic plate over an inert oceanic plate and sedimentary trough; and for Sanbagawa, slower subduction of trench sediments beneath a relatively immobile upper plate.     

Geothermometry of exsolved augites from the Laramie Anorthosite Complex,Wyoming

Exsolved augite pyroxenes from the ferromonzonite border facies of the ferrosyenite in the Laramie Anorthosite Complex have been studied with the transmission electron microscope and the electron microprobe to determine their exsolution histories. The Lindsley and Andersen (1983) geothermometer gives initial crystallization temperatures of 1000° C for the bulk augite crystal (Wo₃₂ En₂₂ Fs₄₆). Exsolved lamellae are predominantly pigeonites with very low calcium contents (Wo_1–3 En_23–24 Fs_71–74) and have formation temperatures estimated to be in the range of 600 to 975° C. The uniform compositions of lamellae and hosts, despite the range in lamellar size and orientation, suggest that either 1) the ferromonzonite experienced an extended plateau in cooling or a reheating event at 600 to 650° C or 2) the pyroxenes recorded a blocking temperature. Two-feldspar geothermometry on exsolved feldspars also records 600° C and suggests that these low temperatures are not blocking temperatures.     

Mobility of metamorphic fluids inferred from infrared microspectroscopy on natural fluid inclusions: the example of Tinos Island,Greece

OH structure of metamorphic fluids has been studied by high temperature infrared (IR) microspectroscopy on natural fluid inclusions contained in quartz veins, over the temperature range 25–370 °C. Blueschist-facies veins from Tinos island core complex (Cyclades, Greece) display H₂O–NaCl–CaCl₂–CO₂ inclusions whereas greenschist-facies veins contain H₂O–NaCl ± CO₂ inclusions. From 25 to 370 °C, peak positions of OH stretching IR absorption bands increase quasi-linearly with slopes of 0.25 and 0.50 cm^–1 °C^–1 for inclusions trapped under blueschist and greenschist conditions, respectively. Extrapolation to 400 °C yield peak positions of 3,475 cm^–1 for blueschist inclusions and 3,585 cm^–1 for greenschist inclusions. Because the smaller wave number indicates the shorter hydrogen-bond distance between water molecules, fluids involved in the greenschist event have a loose structure compared with blueschist fluids. We suggest that these properties might correspond to a low wetting angle of fluids. This would explain the high mobility of aqueous fluids suggested by structural observation and stable isotope analysis.Editorial responsibility: J. Hoefs     

Coalification and graphitization in high-pressure schists in New Caledonia

The northern portion of the Tertiary high-pressure schist belt in New Caledonia contains, from west to east, a metamorphic progression from lawsonite-albite facies through glaucophanitic greenschists to eclogitic albite-epidote amphibolites. This belt is flanked to the west by Upper Cretaceous-Eocene metasediments, of prehnite-pumpellyite grade. Paraschists throughout this whole sequence contain abundant carbonaceous material which shows a progressive metamorphism from coal to graphite. Structural analysis of lithostatic load and oxygen isotope data have provided a PT profile for the carbon metamorphism. In the prehnite-pumpellyite metasediments, phytoclasts were progressively coalified to anthracite rank under PT conditions which extended up to 3 kb/255 ° C at the lawsonite isograd where graphite first appears. On the high grade side of the lawsonite isograd a transitional mixed zone of continued coalification and graphitization occurred within the PT range 3 kb/255 ° C to 5.5 kb/335 ° C which included the ferroglaucophane isograd. Immediately beyond this zone all phytoclasts were completely graphitized before the epidote isograd was reached at 6.3 kb/ 390 ° C. The prevailing metamorphic environment retarded coalification, but accelerated graphitization, under conditions of high pressure and a low temperature gradient (7 ° C/km) that had been generated within the sedimentary pile by rapid tectonic thickening and consequent deep burial.     

Stable isotope evidence for multiple fluid regimes during carbonate cementation of the Upper Tertiary Hazeva Formation, Dead Sea Graben, southern Israel

Stable carbon- and oxygen-isotope compositions of calcite and dolomite cements have been used to understand porewater evolution in the Upper Tertiary Hazeva Formation within the Dead Sea Graben, southern Israel. Sandstone samples were obtained from four boreholes in three tectonic blocks of the graben over depths of 253–6448 m, a variation that largely reflects differential subsidence of individual fault-bounded blocks. Early carbonate cements dominate diagenesis. Calcite occurs at <1600 m, but was replaced by dolomite at greater depths. Dolomite at 1600–2700 m is Fe-poor (<0.8 mol% FeCO₃), and at 4700–6200 m, Fe-rich (0.5–7.2 mol% FeCO₃). Magnesite, anhydrite and halite are the final diagenetic phases. Calcite has positively correlated δ¹⁸O (+21‰ to +25‰) and δ¹³C (−6‰ to −2‰) values that generally decrease with depth. Dolomite has a wider variation in δ¹⁸O (+18‰ to +30‰) and δ¹³C (−8‰ to −1‰) values, which also generally are lower with increasing depth. However, the δ¹³C and δ¹⁸O values of dolomite from the uppermost 400 m of the Hazeva Formation in the Sedom Deep-1 borehole are anomalous in spanning the entire range of stable carbon and oxygen isotopic compositions over this relatively small interval.The decreasing dolomite δ¹³C values likely indicate an increased contribution of carbon from organic sources with increasing depth. Except for the uppermost 400 m, Hazeva Formation dolomite in the Sedom Deep-1 borehole has stable carbon-isotope compositions that imply initial dolomitization at much shallower levels, prior to the preferential subsidence of this tectonic block. The oxygen isotopic compositions of the calcite cement are best explained by equilibration at present burial temperatures (≤55 °C) with porewater of meteoric origin. Its δ¹⁸O values increased from −5‰ at the shallowest depths to 0‰ at 1600 m. The dolomite oxygen isotopic compositions also reflect equilibration at present burial temperatures with porewaters ranging from 0‰ at 1600 m to +7‰ at 3600 m (100 °C). In the deepest fault block (Sedom Deep-1 borehole), however, increasingly Fe-rich dolomite has (re)equilibrated with porewater whose δ¹⁸O values decreased from +9‰ at 4750 m (120 °C) to +1‰ to +2‰ by 6200 m (150 °C).Much of the dolomite likely formed at relatively shallow depths from saline brines derived from precursors to the Dead Sea. These infiltrated the Hazeva Formation, mixing with and largely displacing meteoric water, and dolomitizing calcite. Rock–water ratios tended to be high during these processes. However, the upper 400 m of the Hazeva Formation in the deepest fault block were likely deposited during its rapid tectonic subsidence, and largely escaped the initial style of dolomitization pervasive elsewhere in the study area. These sediments were also capped by evaporites. This relatively thin interval likely became a preferential conduit for brines that escaped underlying and overlying strata, including the Fe-rich, lower ¹⁸O fluids (evolved seawater?) present in the deepest part of the graben. These rocks present the most promising target for the passage and accumulation of hydrocarbons in the study area.     

The upper stability limit of the assemblage paragonite + quartz and its natural occurrences

The assemblage paragonite + quartz is encountered frequently in low- to medium-grade metamorphic rocks. With rising grade of metamorphism they react mutually to yield the condensed assemblage albite + Al₂SiO₅.The univariant curve pertaining to the equilibrium paragonite + quartz=albite + andalusite + H₂O has been located experimentally. The reversed P _{H 2 O}-T data are: 1 kb: 470–490° C 2 kb: 510–530° C 3 kb: 540–560° C 4 kb: 560–580° C 5 kb: 590–600° C The univariant curve pertaining to the equilibrium paragonite + quartz=albite + kyanite + H₂O runs through the following P _{H 2 O}-T-intervals: 5 kb: 570–625° C 6 kb: 600–630° C 7 kb: 620–640° C Thermodynamic calculations of S ₂₉₈ ⁰ , H _f,298 ⁰ and G _f,298 ⁰ of the phase paragonite from the experimental data presented above and those obtained from the equilibria of the reaction paragonite=albite + corundum + H₂O (Chatterjee, 1970), agree within the limits of uncertainty. This prompts the idea that Zen's (1969) suggestion of a possible error of approximately 7 kcal in G _f,298 ⁰ of the Al₂SiO₅ polymorphs may in fact be due to an error of similar magnitude in G _f,298 ⁰ of corundum.A best estimate of S ₂₉₈ ⁰ , H _f,298 ⁰ and G _f,298 ⁰ of paragonite based on these considerations yield: S ₂₉₈ ⁰ : 67.61±3.9 cal deg^–1 gfw^–1 H _f,298 ⁰ : –1411.4±2.7 kcal gfw^–1 G _f,298 ⁰ : –1320.9±4.0 kcal gfw^–1 These numbers will be subject to change when better thermochemical data on corundum and albite are available.In medium-grade metamorphic rocks the assemblage paragonite + quartz is commonly found in stable coexistence with such other phases as muscovite, staurolite, andalusite, kyanite, but not with cordierite or sillimanite. However, the assemblage paragonite-sillimanite has been reported to be stable in the absence of quartz. All these petrologic observations can be explained on the basis of the stability data of the phases and phase assemblages concerned.     

Enhanced late gas generation potential of petroleum source rocks via recombination reactions: Evidence from the Norwegian North Sea

Gas generation in the deep reaches of sedimentary basins is usually considered to take place via the primary cracking of short alkyl groups from overmature kerogen or the secondary cracking of petroleum. Here, we show that recombination reactions ultimately play the dominant role in controlling the timing of late gas generation in source rocks which contain mixtures of terrigeneous and marine organic matter. These reactions, taking place at low levels of maturation, result in the formation of a thermally stable bitumen, which is the major source of methane at very high maturities. The inferences come from pyrolysis experiments performed on samples of the Draupne Formation (liptinitic Type II kerogen) and Heather Formation (mixed marine-terrigeneous Type III kerogen), both Upper Jurassic source rocks stemming from the Norwegian northern North Sea Viking Graben system. Non-isothermal closed system micro scale sealed vessel (MSSV) pyrolysis, non-isothermal open system pyrolysis and Rock Eval type pyrolysis were performed on the solvent extracted, concentrated kerogens of the two immature samples. The decrease of C₆₊ products in the closed system MSSV pyrolysis provided the basis for the calculation of secondary gas (C_1-5) formation. Subtraction of the calculated secondary gas from the total observed gas yields a “remaining” gas. In the case of the Draupne Formation this is equivalent to primary gas cracked directly from the kerogen, as detected by a comparison with multistep open pyrolysis data. For the Heather Formation the calculated remaining gas formation profile is initially attributable to primary gas but there is a second major gas pulse at very high temperature (>550 °C at 5.0 K min⁻¹) that is not primary. This has been explained by a recondensation process where first formed high molecular weight compounds in the closed system yield a macromolecular material that undergoes secondary cracking at elevated temperatures. The experiments provided the input for determination of kinetic parameters of the different gas generation types, which were used for extrapolations to a linear geological heating rate of 10⁻¹¹ K min⁻¹. Peak generation temperatures for the primary gas generation were found to be higher for Heather Formation (T_max = 190 °C, equivalent to R_o appr. 1.7%) compared to Draupne Formation (T_max = 175 °C, equivalent to appr. R_o 1.3%). Secondary gas peak generation temperatures were calculated to be 220 °C for the Heather Formation and 205 to 215 °C for the Draupne Formation, respectively, with equivalent vitrinite reflectance values (R_o) between 2.4% and 2.0%. The high temperature secondary gas formation from cracking of the recombination residue as detected for the Heather Formation is quantitatively important and is suggested to occur at very high temperatures (T_max approx. 250 °C) for geological heating rates. The prediction of a significant charge of dry gas from the Heather Formation at very high maturity levels has important implications for petroleum exploration in the region, especially to the north of the Viking Graben where Upper Jurassic sediments are sufficiently deep buried to have experienced such a process.     

Temperatures of serpentinization of ultramafic rocks based on O18/O16 fractionation between coexisting serpentine and magnetite

Five lizardite-chrysotile type serpentinites from California, Guatemala and the Dominican Republic show oxygen isotope fractionations of 15.1 to 12.9 per mil between coexisting serpentine and magnetite (O¹⁸ magnetite=–7.6 to –4.6 per mil relative to SMOW). Nine antigorites (mainly from Vermont and S. E. Pennsylvania) show distinctly smaller fractionations of 8.7 to 4.8 per mil (O¹⁸ magnetite=–2.6 to +1.7 per mil). Two lizardite and chrysotile serpentinites dredged from the Mid-Atlantic Ridge exhibit fractionations of 10.0 and 12.4 per mil (O¹⁸ magnetite=–6.8 and –7.9 per mil, respectively), whereas an oceanic antigorite shows a value of 8.2 per mil (O¹⁸ magnetite=–6.2). These data all clearly indicate that the antigorites formed at higher temperatures than the chrysotilelizardites. Electron microprobe analyses of magnetites from the above samples show that they are chemically homogeneous and essentially pure Fe₃O₂. However, some magnetites from certain other samples that show a wide variation of Cr content also give very erratic oxygen isotopic results, suggesting non-equilibrium. An approximate serpentine-magnetite geothermometer curve was constructed by (1) extrapolation of observed O¹⁸ fractionations between coexisting chlorites and Fe-Ti oxides in low-grade pelitic schists whose isotopic temperatures are known from the quartz-muscovite O¹⁸ geothermometer, and (2) estimates of the O¹⁸ fractionation factor between chlorite and serpentine (assumed to be equal to unity). This serpentine-magnetite geothermometer suggests approximate equilibrium temperatures as follows: continental lizardite-chrysotile, 85° to 115° C; oceanic lizardite and chrysotile, 130° C and 185° C, respectively; oceanic antigorite, 235° C; and continental antigorites, 220° to 460° C.Contribution No. 2029 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91109.