TRIASSIC ARAGONITE IN CARBONATE SOURCE-ROCK FROM RAGUSA BASIN (SICILY) - GEOCHEMISTRY, COMPARISON WITH RECENT SEDIMENTS AND ORIGIN

Aragonitic muds from the upper Trias occur in laminites between two laminae rich in clays and/or organic matter. Two types of aragonite are identified. The first shows rod or needle morphology, with Sr content (9,300 ppm), delta(18)0 (-1.1 to -1.7) and delta(13)C (+2.1 to +2.8) mostly similar to aragonite of Recent sediments. It is not biodetrital in origin but results from direct precipitation at 22-30 degrees C in sea water with a Sr-m(2+)/Ca-m(2+) ratio very near to Recent values. The second aragonite showing greater prismatic crystals with inclusions of relics of rods and needles, a high content in strontium (15,800 ppm) and a negative delta(13)C (-13.0 a -14.4), is diagenetic.