Origin of Drusy Dolomite Cement in Permo ‐ Triassic Dolostones, Northern United Arab Emirates

: While the characteristics and origin of drusy calcite cement in carbonate deposits is well constrained in the literature, little attention is paid to drusy dolomite cement. Petrographic obser ‐ vations, stable isotopes, and fluid ‐ inclusion microthermometry suggest that drusy dolomite cement in Permo ‐ Triassic conglomerate/breccia dolostone beds in northern United Arab Emirates has pre ‐ cipitated as cement and not by dolomitization of drusy calcite cement. The low  18 O VPDB ( − 9.4‰ to − 6.2‰) and high homogenization temperatures of fluid inclusions in drusy dolomite (T h = 73–233 °C) suggest that dolomitization was caused by hot basinal brines (salinity = 23.4 wt% NaCl eq.). The  13 C VPDB values (+0.18‰ to +1.6‰) and 87 Sr/ 86 Sr ratio (0.708106 to 0.708147) indicate that carbon and strontium were derived from the host marine Permo ‐ Triassic carbonates. Following this dolomiti ‐ zation event, blocky calcite (T h = 148 °C; salinity = 20.8 wt% NaCl eq.) precipitated from the hot basinal brines. Unravelling the origin of drusy dolomite cement has important implications for ac ‐ curate construction of paragenetic sequences in carbonate rocks and decipher the origin and chem ‐ istry of diagenetic waters in sedimentary basins. improvements in the definition of paragenetic sequences in carbonate rocks and related reservoir ‐ quality features. There are two possible interpre ‐ tations of the formation of drusy dolomite cement: (1) Precipitation as cement, and (2) fabric ‐ preserving dolomitization of precursor drusy mosaic calcite. Evidence supporting precipitation as drusy dolomite includes the sector cathodoluminescence zoning and its close association with the latter precipitated blocky calcite.


Introduction
Calcite cement with a drusy mosaic texture, displaying crystal size growth from pore wall to pore center, is widely distributed in limestones [1,2]. Such cement commonly fills moldic pore and hence has been interpreted to be sourced by dissolution of aragonitic allochems [3][4][5][6][7]. The stable isotopic composition of drusy mosaic calcite cement coupled with the presence of moldic porosity have been interpreted to indicate a meteoric origin [8][9][10]. Dolomite cement in carbonate rocks commonly develops rhombic crystals that do not show systematic size variation within the pores [11]. Nevertheless, rare drusy mosaic dolomite cement has been reported in carbonate deposits too [12][13][14][15]. The origin of such dolomite is not fully explored, particularly regarding whether it is a primary precipitate or formed by dolomitization of precursor drusy calcite cement. The fluid flow and related diagenetic evolution, particularly dolomitization of carbonate succession in northern United Arab Emirates (UAE), is interpreted to be controlled by the tectonic evolution of the region [16,17].
In this paper, we constrain the origin of drusy mosaic dolomite cement in conglomerates/breccia of the Bih Formation (Permo-Triassic), Ras Al Khaima, northern UAE (Figure 1), using petrographic, stable isotopic, and fluid-inclusion microthermometric analyses. This study provides important insights into the conditions of formation of drusy dolomite cement and its associated diagenetic fluids. The presented data and discussions help in constructing more accurate paragenetic sequences in Permo-Triassic dolostones, northern UAE, and in other carbonate successions with similar depositional and diagenetic settings.

Geological Setting
The Bih Formation (Permo-Triassic) crops out in the Musandam Peninsula, Ras Al Khaimah, northern United Arab Emirates (Figure 1). The formation is suggested to be ageequivalent to the Khuff Formation [19] which is an important hydrocarbon reservoir in the Arabian Gulf region. Deposition of the Bih Formation occurred in subtidal to intertidal (shoals, lagoon, and mudflats) environments on a passive margin of the Arabian plate [20].
The formation is divided into two units separated by a marker interval, which is called mid-Bih conglomerates/breccias [21]. The origin of this interval is uncertain, being interpreted to have resulted from the collapse of evaporite beds [20] tectonic deformation [20] or are lag deposits formed owing to marine transgression [22]. Strohmenger et al. [20] proposed that the Bih Formation corresponds to sequence KS4 through KS7 of the Khuff Formation ( Figure 2). Two main tectonic events have affected the study area [23] includ-ing: (i) The obduction of Oman ophiolite on the Arabian platform during the Late Cretaceous. During this tectonic phase, the passive northeastern margin of the Tethys ocean became compressive and resulted in the close of the Tethys ocean [24]; (ii) the Zagros orogeny (Paleocene to Middle Miocene) [25][26][27][28] which caused large scale folding and thrusting [26].

Samples and Methods
Fifty-six samples were collected from the conglomerate/breccia beds and associated dolostone beds. Thin sections were prepared for all the samples after impregnation with blue epoxy. Representative polished uncovered thin sections were prepared and examined by a Technosyn Cold cathodoluminescence microscope (CL), backscattered electron imaging (BSEI), and energy dispersive X-ray analyses (EDS) attached to a Quanta scanning electron microscope. Fluid-inclusion microthermometry was conducted on two representative double polished 50 μm thick sections using a pre-calibrated LINKAM THMS600G stage fitted onto the Nikon E600 microscope.
Synthetic pure H2O and CO2 inclusions standard were used to calibrate the thermocouple. Both homogenization (Th) and final ice melting temperatures (Tmice) were measured whenever the size of the inclusions permitted. The former is the minimum entrapment temperature of the inclusion (i.e., the temperature of mineral precipitation) [29]. The latter is the temperature at which the last ice melts in an aqueous fluid inclusion and reflects the trapped diagenetic fluid salinity [29]. The fluid salinity (wt% NaCl eq.) was inferred from Tmice using the Bodnar equation [30]. In order to avoid overheating, low heating rates were used for measuring homogenization temperatures (Th).
Stable carbon and oxygen isotopes were obtained for the 36 micro-drilled drusy dolomite cement with a GVI IsoPrime continuous flow isotope ratio mass spectrometer system (CF-IRMS). The  13 C and  18 O isotopic values are determined on carbon dioxide (CO2) from carbonate minerals by the reaction with 100% phosphoric acid (H3PO4) at 90 °C in a vacuum using standard procedures based on the principles reported in [31]. Samples and standards are weighed into Exetainer™ septum vials, sealed and flushed with pure helium. In addition, 100% phosphoric acid is injected into the vial, which is then placed in an aluminum tray maintained at 90 °C for more than 1 h. The CO2 is extracted automatically with a double-hole needle Gilson auto-sampler connected to a GVI MultiFlow. The MultiFlow contains a 500-ul sample loop, and a GC column that separates CO2, N2, O2, and H2O. Helium carrier gas then transports the purified CO2 into the GVI IsoPrime continuous flow isotope ratio mass spectrometer system (CF-IRMS) that measures the isotope ratios. Four representative micro-drilled drusy dolomite cement samples were analyzed for Sr isotopes. One mg of each sample is collected to react with suprapure HCl for 24 h. Purified Sr is obtained by chromatographic separation with 2.5 mL of AGW 50 × 8 (Biorad) cation exchange resin [32]. The analysis equipment is a Finnigan MAT 262 7-collector solid-source mass spectrometer with a single Re filament applying 1 μL of ionization enhancing solution [32].

Field Observations
The base of the Bih Formation does not crop out, and the lowest stratigraphic part consists of a medium-to-thick-bedded brown dolomitized packstone-grainstones with locally well-developed coarse-crystalline dolostone and vuggy textures. About 180 m above the base of the exposed Bih Formation, the mid-Bih conglomerate/breccia occurs (approx. 20 m thick; Figures 2B and 3A). The conglomerate/breccia beds of Wadi Bih, which are interbedded with dolograinstones, are massive overall but locally show a low-angle, through cross stratification ( Figure 3B). The beds are cross cut by a series of normal faults ( Figure 3A). The basal contact between conglomerate/breccia and underlying dolograinstone beds is sharp and erosional ( Figure 3C). The lower contact between dolograinstone beds with the conglomerate/breccia beds is marked by the presence of rip-up dolomudstone intraclasts ( Figure 3D). Stylolites occur within as well as along boundaries between the conglomerate/dolostone beds. Above the mid-Bih breccia, there are cycles of interbedded dolostones and dolomitic limestones, with algal laminations, bird-eye structures, and dissolution molds. The succession continues with 90 m cliff-forming, dark gray coarse-crystalline dolostones.

Petrography of the Drusy Dolomite
The conglomerate/breccia dolostone beds are cemented by drusy mosaic dolomite with a crystal size increasing progressively from 20 μm along the pore wall to 2 mm in the pore center ( Figure 4A-D). In some cases, the centermost parts of the pore are occupied by blocky calcite (500-700 μm across). Dolomite crystals adjacent to this blocky calcite are partly calcitized ( Figure 5A,B). The drusy dolomite crystals display sector CL zoning with alternating non-luminescent and bright orange luminescent zones ( Figure 5C,D). The microcrystalline equant to bladed dolomite crystals (20-100 μm across) along the pore walls are dull luminescent. The blocky calcite crystals are dull luminescent with a local thin orange luminescent zone. Dull luminescent calcite occurs along stylolites and fills fractures varying in width from 5 to 60 μm ( Figure 5E,F). The stylolites (amplitudes up to 2 cm) and dissolution seams cut across the dolostone pebbles and have bed-parallel and bed-oblique orientations. Some of the stylolites are cutting across both pebbles and dolomite between pebbles ( Figure 6A-D). The drusy mosaic dolomite has Fe and Mn contents below the EDS detection limits. Fe and Mn contents as low as few ppm can still generate zoning in dolomite [33][34][35].

Discussion
Constraining the origin of the drusy dolomite in terms of whether it has been precipitated as cement, which is a common cement in limestones [13,14] or formed by the replacement of precursor drusy calcite [15] has implications for unravelling the role of diagenetic paleofluids, as well as for improvements in the definition of paragenetic sequences in carbonate rocks and related reservoir-quality features. There are two possible interpretations of the formation of drusy dolomite cement: (1) Precipitation as cement, and (2) fabric-preserving dolomitization of precursor drusy mosaic calcite. Evidence supporting precipitation as drusy dolomite includes the sector cathodoluminescence zoning and its close association with the latter precipitated blocky calcite.
Fluid-inclusion microthermometry of drusy mosaic dolomite and the subsequently precipitated calcite cement indicate formation from hot brines during compressional tectonic events in conjunction with the obduction of Oman ophiolites [16,36]. However, it is not immediately clear whether the drusy mosaic dolomite has been precipitated as cement [13] or formed by dolomitization of drusy mosaic calcite by the hot basinal brines [15,37]. Drusy calcite is a common cement in limestones [1,[5][6][7]38]. Thus, the common presence of drusy mosaic dolomite in totally dolomitized limestones may suggest formation by fabricpreserving dolomitization [15]. The latter process suggests similar dissolution rates of drusy calcite and precipitation rate of dolomite during the dolomitization process. A similar fabric-preserving dolomite of Bih Formation has been reported in earlier studies of carbonate reservoirs in the world [39][40][41].
The high Th (73 to 233 °C) and salinity (20.8 to 25.1 wt% NaCl eq.) of drusy mosaic dolomite and subsequently precipitated cement indicates that they have precipitated either during deep-burial diagenesis or by upward flux of hydrothermal/hot basinal fluids [42]. The increase in Th with the increase in dolomite crystal size (av. 118 to 175.5 °C) is interpreted to indicate that the drusy dolomite precipitated with the increase in temperature during progressive burial. The maximum burial temperatures reached by the Bih Formation is approximately 190 to 200 °C [22]. The wide range of Th of different carbonate phases indicates that some fluid inclusions have been subjected to re-equilibration during burial processes [29]. The relatively systematic Th zoning from pore walls inwards (i.e., increase in crystal size) supports the formation of drusy dolomite by precipitation from pore fluids than by dolomitization of precursor drusy calcite cement. Apparently, the chemistry of the dolomitizing fluids has changed with time and reflected on the variation of Fe-rich and Fe-poor zoning of CL pattern of the drusy dolomite cement (cf., [2,36]).
The similar Th (148 °C) and salinity values (20.8 wt% NaCl eq.) of late blocky calcite and fracture-filling medium to coarse calcite cement is attributed to the circulation of hot brines [17]. The considerably lower  13 CVPDB values (−3.8‰ and −0.9‰) of fracture-filling calcite cement than drusy dolomite samples might indicate the deviation of dissolved carbon from degradation of hydrocarbons or organic matter [42]. Using the  18 OVPDB values (−9.4‰ to −6.2‰) and Th (73 to 233 °C) of drusy dolomite, the fractionation equation of Land [43] it is inferred that precipitation has occurred from evolved brines ( 18 OVSMOW = +0.29‰ to +10.96‰). The wide extent of variations in the degree of geochemical evolution of the brines could be related to variable fluid sources and variable degrees of interaction with the host rocks in the basin. The positive correlation between C and O isotopes of drusy mosaic dolomite (Figure 7) suggests increasing input of 12 C into the formation waters with the increasing temperature [44]. The small variations in Sr isotopic compositions between the various types of carbonate cements suggest a related origin ( Figure 8A,B). The 87 Sr/ 86 Sr ratios in one of the pebbles (0.708202) is within the isotopic range of Permian to Triassic seawater values (0.7069 to 0.7083) [31]. The slightly higher ratio in another pebble sample (0.708424) than the range for Permo-Triassic seawater ( Figure 8B) suggests that the hot basinal fluids have interacted with siliciclastic rocks [45,46]. The similar carbon isotope and 87 Sr/ 86 Sr ratio of drusy dolomite and coeval marine carbonates ( [31]; 0.708106 and 0.708147, respectively) suggests that dissolved carbon and Sr were derived from the dissolution of marine carbonates and/or marine pore waters. The slightly lower oxygen isotope value of drusy dolomite than coeval marine carbonates ( Figure 8A) is attributed to precipitation at elevated temperatures.
The development of stylolites and dissolution seams along the rim of pebbles or cutting across both pebbles and dolomite between pebbles indicate that stylolitization postdates the drusy dolomite cement. This complex relationship between fracturing, stylolitization, and dolomite cementation may be caused by multiple phases of tectonic movements. The cross-cutting of low amplitude seams by high amplitude stylolites in conglomerate/breccia samples could be a result of increasing compressive stress by tectonic movements generated during the obduction of Oman ophiolites [47,48]. The presence of dolomite cement (Th up to 190 °C; salinity up to 20.8 wt% NaCl eq) along the stylolites suggests the flux of hot basinal brines and may indicate that the dolomite was precipitated by dolomitization of the host limestones prior to stylolitization. Dolomite cement apparently has been concentrated in the vicinity of the stylolitization surface as an insoluble residue (cf., [49]). Similar observations and interpretations have been presented for carbonate successions elsewhere [49][50][51]. The presence of clay minerals (locally pigmented by TiO2) along stylolites suggests that these minerals promoted the pressure dissolution of carbonates [52,53].
Constraining the depths at which the flux of hot dolomitizing basinal brines occurred is difficult without radiometric dating of dolomite [54,55]. However, the cross-cutting relationship between the stylolites and dolomite cement suggests that dolomitization took place at shallow depths [36].

Conclusions
The puzzling origin of pore-filling drusy mosaic dolomite cement in Permo-Triassic conglomerate/breccia beds outcropping in northern United Arab Emirates is constrained using the integrated field, petrographic, isotopic, and fluid-inclusion microthermometric analyses. There are two possible interpretations of the formation of drusy dolomite cement: (1) Precipitation as cement, and (2) fabric-preserving dolomitization of precursor drusy mosaic calcite. Fluid-inclusion microthermometry indicates the formation of this dolomite from hot basinal brines (Th = 73 to 233 °C; salinity = 21.8% to 25.1% wt% NaCl eq.). Using the average homogenization temperature and oxygen isotope of dolomite  18 OVPDB (−9.4‰ to −6.2‰) the brines are inferred to be geochemically evolved with  18 OVSMOW (+0.29‰ to +10.96‰). Lines of evidence supporting precipitation as drusy dolomite from pore waters include the sector cathodoluminescence zoning, the relatively systematic Th zoning, and with the presence of the latter precipitated blocky calcite, which has also precipitated from hot basinal brines (Th = 148 °C salinity = 20.8 wt% NaCl eq.). The construction of accurate paragenetic sequences is important for establishing exploration and production models for carbonate reservoirs. To achieve this goal, diagenesis should be linked to fluid history and tectonic evolution of the basin.