Geochemistry and minerals of glass shards at DSDP Leg 66 Holes

Deep Sea Drilling Project Leg 66 drilled eight sites along a transect across the Middle America Trench off Mexico, including continental (Sites 493 and 489), oceanic (Site 487), and trench (Site 486) reference sites and four sites (490, 492, 491, 488) in the trench inner wall. Because of their location - close to volcanic sources and subject to prevailing winds and marine currents (N to S, NW to SE) - analysis of airborne ashes intercalated within the sediments at these sites provides a reliable record of explosive volcanism in the area. Intense onshore volcanic activity in Mexico during the Oligo-Miocene has been well documented by the andesites and ignimbrites of the Sierra Madre Occidental and Sierra Madre del Sur and in the Plio-Quaternary by the andesites and basalts from the Trans-Mexican Neovolcanic Belt and the eastern border of Baja California.深海钻探计划（Deep Sea Drilling Project）第66航次沿横跨墨西哥外海中美洲海沟（Middle America Trench）的断面钻探了8个站位，涵盖大陆站位（continental sites，对应493、489站位）、洋壳站位（oceanic sites，对应487站位）、海沟参照站位（trench reference sites，对应486站位），以及海沟内壁（trench inner wall）的4个站位（490、492、491、488站位）。由于这些站位临近火山源区，且受盛行风与海流（北→南、西北→东南）影响，对各站位沉积物中夹藏的火山空降灰（airborne ashes）开展分析，可获取该区域爆发性火山活动（explosive volcanism）的可靠记录。渐新世-中新世（Oligo-Miocene）时期墨西哥境内强烈的陆上火山活动，已通过西马德雷山脉（Sierra Madre Occidental）与南马德雷山脉（Sierra Madre del Sur）产出的安山岩（andesites）及熔结凝灰岩（ignimbrites）得到充分证实；上新世-第四纪（Plio-Quaternary）时期的火山活动，则由跨墨西哥新火山带（Trans-Mexican Neovolcanic Belt）与下加利福尼亚半岛（Baja California）东部边界出产的安山岩与玄武岩（basalts）得以佐证。