Hard rocks can have long memories

One of the best ways to understand the geological history of our 4500 million year old planet is to study rocks formed under a wide variety of geological conditions. Geologists, equipped with their vast experience and advanced analytical instruments, can identify and interrogate those rocks that best preserve evidence of past geological events. One such instrument is the sensitive high resolution ion microprobe (SHRIMP), a large specialized mass spectrometer that measures the ages of rocks, their precursors and major thermal events by firing a 10,000 volt ion beam at crystals as small as 0.05 mm diameter and measuring the isotopic abundances of the lead, uranium and thorium that are released.

The reconstruction of the continents that existed in the past is an important part of understanding the dynamic evolution of earth. The ancient supercontinent of Gondwana once consisted of what are now the smaller continents of South America, Africa, Madagascar, southern India, Sri Lanka, Antarctica and Australia. Determining the timing of the geological events involving rock formation and modification (deformation, metamorphism etc.) in these continental fragments is vital in piecing together the evolution of the earth's crust during any period of geological time. Most rocks 'forget' their history if exposed to extreme geological conditions, but there are some rare cases where particular rocks derived from the earth's lower crust have preserved, in their distinctive mineralogy, convincing evidence of the very high temperatures that can be present at depth.
The rocks of the central Highland Complex in Sri Lanka, and some parts of Antarctica and southern India, have been subject to some of the highest peak temperatures of crustal metamorphism known, over 1100°C. At such temperatures most rocks would turn into molten magma, but in the November issue of Geology, Sajeev and others report rocks from near Kandy (Sri Lanka) that not only survived the high temperatures, but contain crystals of zircon in which a uranium-lead isotopic record of their provenance and thermal history have survived. Such survival is contrary to all predictions from experimental studies of the rate that lead should be lost from zircon by thermal diffusion.

From a study of the metamorphic minerals and thermodynamic modelling, and SHRIMP uranium-lead isotopic analyses of zircon and monazite (cerium phosphate), the authors have shown that the rocks near Kandy were originally sediments derived from sources ranging in age from 2500 to 830 million years. The sediments were heated to over 1100°C at a depth of about 25 km about 570 million years ago, and then rapidly lifted towards the surface, while still hot, about 550 million years ago. These Sri Lankan rocks were probably trapped and buried in the violent collision between the two halves of the Gondwana supercontinent about 600 million years ago, superheated by basalt magmas rising from the earth's interior, then forced to the near surface again as the tectonic pressures relaxed. The preservation of the isotopic record of these events is remarkable, and still remains to be fully explained.