INVESTIGADORES
BOLMARO Raul Eduardo
artículos
Título:
Cryptic Diagenetic Changes in Quaternary Aragonitic Shells: A Textural, Crystallographic, and Trace-Element Study on Amiantis Purpurata (Bivalvia) From Patagonia, Argentina
Autor/es:
MARÍA SOL BAYER; FERNANDO COLOMBO; NATALIA S. DE VINCENTIS; GONZALO DUARTE; RAÚL E. BOLMARO; SANDRA GORDILLO
Revista:
PALAIOS
Editorial:
SEPM-SOC SEDIMENTARY GEOLOGY
Referencias:
Lugar: Lawrence; Año: 2013 vol. 28 p. 438 - 451
ISSN:
0883-1351
Resumen:
Modern to Pleistocene Amiantis purpurata shells collected in Bahý´a San
Antonio (Patagonia, Argentina) were studied by X-ray diffraction (XRD),
optical and electron microscopy, electron microprobe analyses, and
microindentation, in order to characterize early diagenetic changes and
mechanical resistance. The sole crystalline phase is twinned aragonite
showing pseudohexagonal symmetry. The regularity of the crystallographic
texture decreases in older samples, but average crystallite size
does not increase. The microstructure, which is dominantly crossed
lamellar, is progressively replaced by a more randomly oriented grain
aggregate. Compositional profiles across the shell show gradients in Sr,
Na, S, and Cl, whereas Mg and P are more evenly distributed. Each shell
layer has a distinct chemical signature. A marked decrease in the
concentration of all of these elements, along with flattening of profiles, is
evident as age increases. Vickers microhardness is lowest in modern
specimens, showing at the same time the least chipped regions; older shells
become harder and more fragile. All of these changes are attributed to
postdepositional modifications by dissolution-recrystallization processes
mediated by a thin film of water in a vadose environment. Microstructural
adjustments are more sluggish than chemical modifications produced by
diagenetic processes, whereas microhardness rapidly reaches high values,
probably due to the early degradation of organic compounds from the
shell. Our study shows that aragonitic shells that retain their primary
mineralogical composition have undergone subtle chemical and microstructural
changes. A very small amount of calcite was produced during
grinding for XRD. Care should therefore be taken when seeking calcite as
evidence of diagenetic changes.
Antonio (Patagonia, Argentina) were studied by X-ray diffraction (XRD),
optical and electron microscopy, electron microprobe analyses, and
microindentation, in order to characterize early diagenetic changes and
mechanical resistance. The sole crystalline phase is twinned aragonite
showing pseudohexagonal symmetry. The regularity of the crystallographic
texture decreases in older samples, but average crystallite size
does not increase. The microstructure, which is dominantly crossed
lamellar, is progressively replaced by a more randomly oriented grain
aggregate. Compositional profiles across the shell show gradients in Sr,
Na, S, and Cl, whereas Mg and P are more evenly distributed. Each shell
layer has a distinct chemical signature. A marked decrease in the
concentration of all of these elements, along with flattening of profiles, is
evident as age increases. Vickers microhardness is lowest in modern
specimens, showing at the same time the least chipped regions; older shells
become harder and more fragile. All of these changes are attributed to
postdepositional modifications by dissolution-recrystallization processes
mediated by a thin film of water in a vadose environment. Microstructural
adjustments are more sluggish than chemical modifications produced by
diagenetic processes, whereas microhardness rapidly reaches high values,
probably due to the early degradation of organic compounds from the
shell. Our study shows that aragonitic shells that retain their primary
mineralogical composition have undergone subtle chemical and microstructural
changes. A very small amount of calcite was produced during
grinding for XRD. Care should therefore be taken when seeking calcite as
evidence of diagenetic changes.
Amiantis purpurata shells collected in Bahý´a San
Antonio (Patagonia, Argentina) were studied by X-ray diffraction (XRD),
optical and electron microscopy, electron microprobe analyses, and
microindentation, in order to characterize early diagenetic changes and
mechanical resistance. The sole crystalline phase is twinned aragonite
showing pseudohexagonal symmetry. The regularity of the crystallographic
texture decreases in older samples, but average crystallite size
does not increase. The microstructure, which is dominantly crossed
lamellar, is progressively replaced by a more randomly oriented grain
aggregate. Compositional profiles across the shell show gradients in Sr,
Na, S, and Cl, whereas Mg and P are more evenly distributed. Each shell
layer has a distinct chemical signature. A marked decrease in the
concentration of all of these elements, along with flattening of profiles, is
evident as age increases. Vickers microhardness is lowest in modern
specimens, showing at the same time the least chipped regions; older shells
become harder and more fragile. All of these changes are attributed to
postdepositional modifications by dissolution-recrystallization processes
mediated by a thin film of water in a vadose environment. Microstructural
adjustments are more sluggish than chemical modifications produced by
diagenetic processes, whereas microhardness rapidly reaches high values,
probably due to the early degradation of organic compounds from the
shell. Our study shows that aragonitic shells that retain their primary
mineralogical composition have undergone subtle chemical and microstructural
changes. A very small amount of calcite was produced during
grinding for XRD. Care should therefore be taken when seeking calcite as
evidence of diagenetic changes.