Paleomagnetism of ash‐flow tuffs: Microanalytical recognition of TRM components

Abstract

A microanalytical technique, employing a cryogenic magnetometer and progressive alternating field demagnetization, is described and used to investigate the natural remanent magnetization of individual components (groundmass, crystals, and lithic and pumice fragments) in seven ash‐flow tuff lithologies and to determine if subblocking temperature flow‐or compaction‐related movements took place in the units on scales ranging from the volume of the individual microspecimen (1–2 mm in diameter) to that of a standard paleomagnetic core specimen. Closely grouped directions from microspecimens of vitrophyre (α95′s = 4.9° ‐ 12.4° ) and close correspondence of these directions with their site mean directions determined from standard samples suggest that the magnetic components of these units have not been disturbed by movements at low temperatures. Microspecimens from devitrified portions of the cooling units are of significantly lower coercivity than vitrophyre material and yield directions either more scattered about their site mean (α95 greater than 15° ) or uniformly displaced in one direction (up to 35°) away from the mean. Devitrification may be as significant a contributor to the scatter of microspecimen directions as are postblocking temperature randomizing or systematic microtectonic movements. Vitrophyres near the base and extremities of ash‐flow tuff cooling units are probably the most accurate indicators of the paleofield in each cooling unit.