Geological radioactive clocks which rely on the known rate of decay of radioactive elements start ticking when these radioactive elements gets trapped in growing minerals and the mineral stops exchanging elements with its surroundings. This commonly occurs in minerals crystallizing out of magma or lava or when a mineral recrystallizes during metamorphism. A radioactive clock from a mineral in an igneous rock tells us the date of the solidification of the magma or lava while a radioactive clock from a metamorphic rock may time the thermal event during which the host igneous or metamorphic rock recrystallized.
What about directly dating sedimentary rocks using radioactive clocks? Rocks like sandstones for example are made up of pieces of eroded igneous or metamorphic or sedimentary rocks. They may contain minerals like zircon that contain uranium or feldspars that contain radiogenic potassium, but dating these sedimentary particles means finding out the age of the source rocks and not neccessarily the timing of deposition of sediment.
One can indirectly date sedimentary rocks using radioactive clocks based on their geological relationship with associated igneous rocks. For example; 1) a sandstone sequence may unconformably overlie a granite. The age of the granite tells us that the sandstone sequence is younger than a particular date. 2) a sedimentary sequence may be intruded by an igneous body. The age of that igneous body if established tells us that the sequence is older than a particular date and 3) a sedimentary sequence may be bounded by igneous bodies, example volcanic eruptions many deposit lava or ash at various intervals synchronous with sediment deposition. In this case, the dates if established tell us that sedimentation occurred between two particular dates.
Often though a fortuitous association with igneous rocks is absent or saying "younger than", or "older than" or "between" leaves tens of millions or even hundreds of millions of years unaccounted for. In a study published a few years ago in GSA Bulletin, James E. Conrad and colleagues use another approach to directly date when sedimentation occurred. Although sedimentary rocks like sandstones are mostly made up of eroded particles they also occasionally contain minerals that grow on the sea floor or a few cms below the sediment water interface. Such new minerals that form during or just after sedimentation are called diagenetic or authigenic minerals. Glauconite is one such diagenetic mineral which grows on the sea floor. It is a iron potassium (K) silicate and the radiogenic K40 decays into Argon40. Using the ratio of Ar40 to non radiogenic Ar39, the age of glauconite formation and thus sediment deposition was established.
Indian Proterozoic basins are just beginning to get their chronology established more robustly and diagenetic glauconite which is present in many different Indian basins offers another tool besides associated igneous rocks to more firmly establish the timing of basin formation, sedimentation, sequence evolution and changes in ocean geochemical conditions (glauconite forms under reducing conditions on the sea floor) during Proterozoic times.
Abstract:
Ages of some key stratigraphic sequences in central Indian Proterozoic basins are based predominantly on lithostratigraphic relationships that have been constrained by only a few radioisotopic dates. To help improve age constraints, single grains of glauconitic minerals taken from sandstone and limestone in two Proterozoic sequences in the Pranhita-Godavari Valley and the Chattisgarh basin were analyzed by the 40Ar/39Ar incremental heating method. Analysis of the age spectra distinguishes between ages that are interpreted to reflect the time of glauconite formation, and anomalous ages that result from inherited argon or postcrystallization heating. The analyses indicate an age of 1686 ± 6 Ma for the Pandikunta Limestone and 1566 ± 6 Ma for the Ramgundam Sandstone, two units in the western belt of Proterozoic sequences in Pranhita-Godavari Valley. Glauconite from the Chanda Limestone, in the upper part of this sequence, contains inherited 40Ar but is interpreted to reflect an age of ca. 1200 Ma. Glauconite from the Somanpalli Group in the eastern belt of the Pranhita-Godavari Valley gives an age of 1620 ± 6 Ma. In the Chattisgarh basin, glauconite from two units gives disturbed ages that suggest a period of regional heating in the Chattisgarh basin at ca. 960–1000 Ma. These new ages indicate that these sequences are 200–400 m.y. older than previously recognized, which has important implications for geochemical studies of Mesoproterozoic ocean redox conditions in addition to providing important constraints on regional tectonics and lithostratigraphy.
What about directly dating sedimentary rocks using radioactive clocks? Rocks like sandstones for example are made up of pieces of eroded igneous or metamorphic or sedimentary rocks. They may contain minerals like zircon that contain uranium or feldspars that contain radiogenic potassium, but dating these sedimentary particles means finding out the age of the source rocks and not neccessarily the timing of deposition of sediment.
One can indirectly date sedimentary rocks using radioactive clocks based on their geological relationship with associated igneous rocks. For example; 1) a sandstone sequence may unconformably overlie a granite. The age of the granite tells us that the sandstone sequence is younger than a particular date. 2) a sedimentary sequence may be intruded by an igneous body. The age of that igneous body if established tells us that the sequence is older than a particular date and 3) a sedimentary sequence may be bounded by igneous bodies, example volcanic eruptions many deposit lava or ash at various intervals synchronous with sediment deposition. In this case, the dates if established tell us that sedimentation occurred between two particular dates.
Often though a fortuitous association with igneous rocks is absent or saying "younger than", or "older than" or "between" leaves tens of millions or even hundreds of millions of years unaccounted for. In a study published a few years ago in GSA Bulletin, James E. Conrad and colleagues use another approach to directly date when sedimentation occurred. Although sedimentary rocks like sandstones are mostly made up of eroded particles they also occasionally contain minerals that grow on the sea floor or a few cms below the sediment water interface. Such new minerals that form during or just after sedimentation are called diagenetic or authigenic minerals. Glauconite is one such diagenetic mineral which grows on the sea floor. It is a iron potassium (K) silicate and the radiogenic K40 decays into Argon40. Using the ratio of Ar40 to non radiogenic Ar39, the age of glauconite formation and thus sediment deposition was established.
Indian Proterozoic basins are just beginning to get their chronology established more robustly and diagenetic glauconite which is present in many different Indian basins offers another tool besides associated igneous rocks to more firmly establish the timing of basin formation, sedimentation, sequence evolution and changes in ocean geochemical conditions (glauconite forms under reducing conditions on the sea floor) during Proterozoic times.
Abstract:
Ages of some key stratigraphic sequences in central Indian Proterozoic basins are based predominantly on lithostratigraphic relationships that have been constrained by only a few radioisotopic dates. To help improve age constraints, single grains of glauconitic minerals taken from sandstone and limestone in two Proterozoic sequences in the Pranhita-Godavari Valley and the Chattisgarh basin were analyzed by the 40Ar/39Ar incremental heating method. Analysis of the age spectra distinguishes between ages that are interpreted to reflect the time of glauconite formation, and anomalous ages that result from inherited argon or postcrystallization heating. The analyses indicate an age of 1686 ± 6 Ma for the Pandikunta Limestone and 1566 ± 6 Ma for the Ramgundam Sandstone, two units in the western belt of Proterozoic sequences in Pranhita-Godavari Valley. Glauconite from the Chanda Limestone, in the upper part of this sequence, contains inherited 40Ar but is interpreted to reflect an age of ca. 1200 Ma. Glauconite from the Somanpalli Group in the eastern belt of the Pranhita-Godavari Valley gives an age of 1620 ± 6 Ma. In the Chattisgarh basin, glauconite from two units gives disturbed ages that suggest a period of regional heating in the Chattisgarh basin at ca. 960–1000 Ma. These new ages indicate that these sequences are 200–400 m.y. older than previously recognized, which has important implications for geochemical studies of Mesoproterozoic ocean redox conditions in addition to providing important constraints on regional tectonics and lithostratigraphy.
No comments:
Post a Comment