Petrography

About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.

Petrographic and isotopic characterization of the marble of the island of tinos (greece)

Abstract: Most outcrops of good-quality crystalline marble in Mediterranean countries were exploited in Greek and Roman times. Hence, a wide variety of marble is now found in archaeological excavations. The precise determination of the provenance of a marble object is of great archaeological importance, and is now mostly obtained by coupling the petrographic study of a thin section with the analysis of the C and O stable isotopes, or by stereomicroscopy and EPR. The existing databases are considerable but still insufficient, because our knowledge about and study of ancient Mediterranean quarries remain incomplete. The contribution of this research is to add new petrographic and isotopic data on small quarries on the Greek island of Tinos that were exploited in antiquity. This marble belongs to the same geological horizon; it is quite pure, sometimes dolomitic and characterized by a low-T, high-P metamorphism that produced a limited recrystallization (MGS varying from 0.64 to 2.50 mm) on marine limestone protoliths. The petrographic features are quite distinctive: the fabric is strongly lineated and often stressed; and the accessory minerals are ubiquitary quartz, ore minerals, graphite and muscovite. As far as the isotopic data are concerned, δ 13 C PDB varies from 1.1 to 2.7, and δ 18 O PDB from -1.7 to -11.4. Both the features and the data have been compared with those of similar marbles used in antiquity, showing that their combination mirrors the fingerprint of the Tinos marble.

Late-stage crystallization history of the jurassic north mountain basalt, nova scotia, canada. i. textural and chemical evidence for pervasive development of silicate-liquid immiscibility

Abstract: The Jurassic (201 Ma) quartz-normative continental tholeiitic basalts of the North Mountain Basalt (NMB) Formation of southern Nova Scotia, Canada, record evidence of pervasive silicate-liquid immiscibility. The basalts, up to 400 m thick, typically with phenocrysts of plagioclase (An50–70) and clinopyroxene (Wo40 En40Fs20), are subdivided into lower, middle, and upper units on the basis of the nature of the flows and petrographic features. Petrographic observations combined with image analysis indicate that the middle and upper units are characterized by an abundance of mesostasis material (i.e., quenched residual melt) that contains skeletal clinopyroxene, acicular plagioclase, skeletal Fe–Ti oxides, Fe–Ti–P-rich globules, and andesitic to rhyolitic glass. Raster analysis of the mesostasis tracks an in situ fractionation that culminated in formation of an interstitial felsic glass (i.e., 74 wt.% SiO2) that contains skeletal apatite and Fe-rich clinopyroxene. The composition of the skeletal clinopyroxene, ca. Wo20En15Fs65, acicular plagioclase (An35–55, ≤2.5 wt.% FeO), Fe–Ti–P-rich globules, and intergranular glass of the mesostasis are consistent with formation from an interstitial melt that evolved in a disequilibrium environment promoted by rapid cooling of the lavas. The mineralogical and chemical features are consistent with the process of silicate-liquid immiscibility within the residual liquids of basaltic systems, in this case late-stage intergranular melts. The presence within the NMB of (1) Fe-rich clinopyroxene-bearing mafic pegmatite, (2) thin (i.e., 1–2 cm) seams of rhyolite associated with the mafic pegmatites, and (3) locally abundant segregation pipes of mixed mafic–felsic composition, is considered to reflect mobilization of the Fe- and silica-rich immiscible melts.

Fluid inclusions record thermal and fluid evolution in reservoir sandstones, Khatatba Formation, Western Desert, Egypt: A case for fluid injection

Abstract: A fluid inclusion and petrographic study, focused on quartz overgrowths, was performed in reservoir sandstones from the Jurassic Khatatba Formation (Salam oil field, Egypt's Western Desert). The combination of detailed fluid inclusion petrography and scanning electron microscope (SEM) cathodoluminescence imaging has allowed us to relate individual fluid inclusion assemblages, that is, the most finely discriminated groups of petrographically associated fluid inclusions, to specific growth zones of authigenic quartz, establishing the relative timing of entrapment of the inclusions. After entrapment, fluid inclusions in authigenic quartz have been preserved without reequilibration, as indicated by the narrow ranges of homogenization temperatures (<4-5 degreesC) in most fluid inclusion assemblages. Three main growth zones are distinguished under SEM cathodoluminescence in the quartz overgrowths and are termed Q1, Q2, and Q3 from the oldest to the youngest. Zone Q1 is further subdivided into three subzones and contains abundant primary aqueous inclusions. Their homogenization temperatures range from 162 to 130 degreesC, with the earliest assemblages having the highest temperatures and with some large temperature fluctuations indicated between successive assemblages. Most Q1 inclusions have salinities in the freshwater to seawater range, with a trend toward increasing salinity through time. Zone Q2 contains primary aqueous inclusions with homogenization temperatures (overall range 148-125 degreesC) also recording large temperature fluctuations and cooling events. The Q2 fluid inclusions have high salinities (~5-20 wt. % NaCl equivalent), with salinity increasing through time. Zone Q3 contains both aqueous and oil inclusions of primary origin. The Q3 aqueous inclusions have homogenization temperatures (overall range 134-112 degreesC) recording overall cooling and high salinities (21-24 wt. % NaCl equivalent). In early Q3 subzones, oil inclusions appear to be of medium gravity, undersaturated with respect to gas. In later Q3 subzones, oil inclusions are of gas-saturated lighter oil. Our results indicate that fluid flow, involving drastic changes in temperature and salinity, was responsible for the precipitation of some of the quartz cement. The earliest quartz (Q1) precipitated from freshwater and seawater at temperatures significantly higher than those expected from the burial history and thermal maturity of these rocks. This quartz is interpreted to have precipitated during cooling of injected fluids that originated as hot connate fluids from deeper parts of the basin. The Q2 precipitation is interpreted to have resulted from episodes of injection of hot saline brines from below. Late quartz cement (Q3) precipitated during oil charge, from progressively cooler and more saline brines interpreted to have refluxed from the surface; it preserves a record of increasing oil maturity and gas saturation through time.

Potassium-Argon mineral dates on intrusive rocks from the Foveaux Strait area

Abstract: Potassium-argon dates on biotite and hornblende separated from six intrusive rocks from the north shore of Foveaux Strait (between Pahia Peninsula and Bluff) and Ruapuke Island range from 253 m.y, (late Permian) to 133 m.y, (early Cretaceous). The younger dates, however, are discarded because of wide discordance between the biotite and hornblende determinations, and petrographic evidence of alteration. The accepted range of ages is from 250 m.y. to 217 m.y., although the younger of these, corresponding to about middle Triassic, are regarded with caution because of minor discordance between the biotite and hornblende dates in the two rocks concerned. Nevertheless, the possibility that intrusion of some of the rocks extended into the Triassic is recorded and briefly discussed.