Ash Layers from Deep Sea Drilling Project Leg 84: Middle America Trench Transect

Abstract

Three Leg 84 sites provided a good record of explosive volcanism onshore (in Central America). Ash layers and many ashy pods are interbedded in Recent to Oligocene strata. Evidence of the main periods of activity was noted in Recent to upper Pleistocene, Pliocene-Pleistocene, lower Pliocene to upper Miocene, lower Miocene, and upper Oligocene. Noticeable traces of older volcanism were found in upper Eocene strata.The chemical analyses of glass shards show a dacitic to rhyolitic composition with a low to moderate calc-alkalinity. A preliminary distinction of samples in three geochemical groups according to their K 2 O/SiO 2 contents is done to test a magmatic evolution. Comparisons are made with Leg 67 and on-land data.

Petrography and Geochemistry of Volcanic Ash Layers from Leg 87A, Nankai Trough (South Japan)

“…The deficit is due to hydration of glass in seawater (Sheidegger et al, 1978). Ninkovitch (1979) demonstrates that hydration in glass shards reaches a saturation point of 4-5% in 250,000 to 400,000 yr. Our recent studies on samples from DSDP Legs 67 (Cadet et al, 1982) and 84 (Pouclet et al, 1985) show that hydration occurs more rapidly and leads to a greater content in acidic glass (5-8%) than in basic glass (3-5%). However, the initial water content is unknown, and magmatic acidic glasses may be richer in H 2 O (5% for rhyolitic glass in the Osaka group in Japan; Yoshikawa, 1978).…”

Volcanic Ash from Leg 112 off Peru

“…data). Previous studies (Scheidegger et al, 1978;Ninkovich, 1979;Cadet et al, 1982;Pouclet et al, 1985aPouclet et al, , 1985b showed that hydration occurs rapidly (less than 400,000 yr) to reach a saturation point of 4% to 5% in a transparent glass. Beyond this point, phyllitic secondary products developed into the glass.…”

“…For example, in Central American deep-sea tephra (DSDP Legs 66, 67, and 84;Cadet et al, 1981Cadet et al, , 1982Pouclet et al, 1985a), the CAF displays a lower calc-alkaline character. This character ranges from -14 to -12.8 in early Miocene time and from -13.3 to-9.2 in mid-Miocene to Holocene time.…”

“…Acidic glasses partly belong to the same series, but also to the shoshonitic series. A schematic separation giving three categories, lowpotassium (A), medium-potassium (B), and high-potassium (C) was determined, using volcanic ashes of Central America from Legs 66, 67, and 84 (Cadet et al, 1981(Cadet et al, , 1982Pouclet et al, 1985a). The glasses from Leg 112 are more alkalic and more potassic.…”

“…Many scientists assume that the record of explosive volcanic activity on land can be read, with certain limitations, in the distribution of ash in deep-sea sediments (Donnelly, 1974;Kennett and Thunell, 1975;Ninkovich and Donn, 1976;Cadet and Fujioka, 1980;Cadet et al, 1981 andPouclet et al, 1985a and1985b).…”