Paper No. 5-2
Presentation Time: 8:55 AM
CHARACTERISTICS OF MAFIC TO INTERMEDIATE VOLCANICLASTIC SAND AND ITS DIAGENETIC TRANSFORMATION TO SANDSTONE IN MARINE BASINS ON PLANET EARTH: LESSONS FOR MARS?
Petrographic observations of suites of sand samples derived from mafic to intermediate volcanic sources (reworked epiclastic, hydroclastic and pyroclastic debris) around the globe indicate that mafic volcanic clasts can differ from their intermediate counterparts in terms of their phenocryst types, proportions, fabrics and sizes. Mafic sand exhibits higher proportions of fragments with lathwork textures and black tachylitic glass. When buried in the marine realm, there is first hydration and dissolution of glass; clay mineral coatings preserve fragment shapes and textures, including vesicles and suspended microlites. These pores are then infilled by various authigenic phases such as clay minerals, carbonates, and zeolites. Direct replacement of glass by clay minerals also occurs. Labile phenocryst populations may be only partly preserved during diagenesis. Impregnation with blue-dyed epoxy helps define porosity relationships and staining helps define zeolite mineralogy. Patterns of cementation in circum-Pacific arc-related basins based on ~600 samples from Deep Sea Drilling Project cores show that: 1) sand in forearc regions is first cemented at greater burial depths than in backarc regions (>400m vs. >200m), 2) backarc basin sand intervals are more thoroughly cemented, and 3) cements are present only in Pliocene or older units. In backarc regions there is a steady down-hole increase in authigenic zeolite and clay minerals, likely as a function of geothermal gradient. Subsequent Ocean Drilling Program (ODP) and International Ocean Discovery Program (IODP) cruises have provided information on early cementation of young (Quaternary) sediments associated with spreading ridge subduction (ODP Leg 141), and distinct patterns of cementation in intraoceanic forearc and backarc successions in the Izu-Bonin system (ODP Leg 126 and IODP Expedition 351). Insights into burial diagenesis are facilitated because coring was continuous, physical properties measurements were routine, and pore-water geochemistry was analyzed until induration thwarted fluid extraction. Porewaters can be significantly altered brines in zeolite-bearing intervals. Studies are underway to potentially link volcaniclastic sediment composition with authigenic zeolite occurrence and mineralogy at these sites.