Stable-isotope and fluid inclusion constraints on the timing of diagenetic events in the dolomitized Dolomia Principale inner platform (Norian, Southern Alps of Italy)
Stable-isotope and fluid inclusion constraints on the timing of diagenetic events in the dolomitized Dolomia Principale inner platform (Norian, Southern Alps of Italy)
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DOI：
10.1016/j.marpetgeo.2020.104615
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发表时间：
2020-11
影响因子：
4.2
通讯作者：
F. Berra;K. Azmy;G. Porta
中科院分区：
文献类型：
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作者：
F. Berra;K. Azmy;G. Porta
意大利北部南阿尔卑斯山诺里安(上三叠统)白云化碳酸盐岩渗透台地相的岩石学、阴极发光(CL)、稳定同位素和显微测温研究确定了两个主要的白云岩相。早期的交代保留组构的白云岩(D1和D2，分别取代了从窗口泥岩/板岩到生物碎屑盖岩、泥岩和叠层岩的沉积相和早期的海相胶结物)，随后是晚期的充填洞穴和裂缝的白云岩胶结阶段(D3，平面S结构)。组构保留的白云岩碳酸盐(<；4μm−30μm，D1)和海相纤维胶结物(30μm−250μm，D2)在CL下表现出暗淡的发光，而埋藏的白云石胶结物(200μm−500μm，D3)则显示出带状发光。D3原生两相流体包裹体的平均均一温度为112.4℃±88.2℃，假设在导致阿尔卑斯特提斯裂谷开放的早侏罗世裂谷期间，表层海水温度至少为20~25℃，地温梯度约为40℃/km，这样的温度可能在合理的埋藏深度2.0~2.5℃之间达到。根据沉降曲线，该深度在早侏罗世达到，与流体包裹体记录的温度一致，D3胶结沉淀导致孔隙度显著降低。所研究的白云岩(D1、D2和D3)的δ13C值变化范围很小(2.0~3.1‰VPDB)，指向流体缓冲体系。不同的是，δ18O值变化很大，范围从−12.8到+1.9‰VPDB。在该范围内，相对于d1和d2，D3值在−6.0vpdb和−12.8vpdb之间明显地聚集，这两个值聚集在大约‰4vpdb到+1.9vpdb之间。D1和D2母质流体的氧同位素组成(~0~−5‰VSMOW)表明，可能来自海洋对微蒸发海水的贡献，这与早期研究提出的干旱气候和一些盆地限制相一致，而D3(+1~+5‰VSMOW)的氧同位素组成与埋藏流体一致。
The petrographic, cathodoluminescence (CL), stable isotope, and microthermometry investigation of inner platform facies of the pervasively dolomitized carbonate platform of the Norian (Upper Triassic) Dolomia Principale (Southern Alps, N Italy) identified two major dolomite phases. An early replacement fabric-retentive dolomite (D1 and D2, respectively replacing the sedimentary facies - from fenestral mudstone/wackestone to bioclastic packstone and mudstone and stromatolites- and early marine cements) is followed by a later, vug- and fracture-filling dolomite cement phase (D3, planar-s texture).The fabric-retentive dolomitized carbonates (<4 μm−30 μm, D1) and marine fibrous cements (30 μm−250 μm, D2) exhibit dull luminescence under CL whereas the burial dolomite cement (200 μm−500 μm, D3) exhibits zoned luminescence. Microthermometric measurements of the primary two-phase fluid inclusions in D3 yielded a mean homogenization temperature of 112.4 ± 8.2 °C. Supposing a surface seawater temperature of at least 20–25 °C and a geothermal gradient of about 40 °C/km during the Early Jurassic rifting that led to the opening of the Alpine Tethys, such temperature might be reached at reasonable burial depth ranging from 2.0 km to about 2.5 km. According to the subsidence curve this depth, compatible with the temperature recorded by the fluid inclusions, was reached during the Early Jurassic, with D3 cement precipitation causing a significant reduction of the porosity.The δ13C values of the investigated dolomites (D1, D2, and D3) show a narrow range of variation (2.0–3.1‰ VPDB), pointing toward a fluid-buffered system. Differently, the δ18O values largely varies, ranging from −12.8 to +1.9‰ VPDB. In this range, D3 values cluster distinctively between −6.0 to −12.8‰ VPDB with respect to D1 and D2, which cluster from about −4 to +1.9‰ VPDB. The estimated oxygen isotope composition of the parent fluid of D1 and D2 (~0 to −5‰ VSMOW) suggests possible contributions from marine to slightly evaporated seawater, which is consistent with the arid climate and some basin restrictions suggested by earlier studies, whereas that of D3 (+1 to +5‰ VSMOW) is consistent with burial fluids.The followed integrated approach highlights the possibility to better characterize the occurrence of multiple diagenetic events in time and space, as well as their feedback effects on the porosity evolution of dolomitized carbonate reservoirs, framing the diagenetic events at regional scale.