Developments in analytical techniques for chemostratigraphy, chronostratigraphy, and geochemical fingerprinting studies: Current status and future trends

Abstract

Chemostratigraphy utilizes the elemental, isotopic, and mineralogical compositions of sediments and rocks to understand the conditions of deposition and diagenesis, variations in paleo-redox conditions, climate, provenance, tectonic setting, and reservoir characteristics. This utility is based on the occurrences and stratigraphic variations of concentrations of certain elements, isotopes, minerals, and some organic compounds in sedimentary rocks. Significant progress has been made in the elemental, isotopic, and mineralogical determination techniques over the last couple of decades and this development is critically helpful to the applications of chemostratigraphy in widely varying fields including, but not limited to stratigraphy, depositional and diagenetic modelling, petroleum exploration, reservoir characterization, forensics, etc. Currently, highly precise data on about 60 elements in the range Li–U can be obtained very rapidly on a routine basis in different types of sediment samples such as drill cuttings, sediment cores, or hand specimens using various analytical techniques such as inductively coupled plasma mass spectrometry (ICP-MS), high resolution ICP-MS (HR-ICP-MS), and Mattauch–Herzog geometry ICP-MS (MH-ICP-MS). Elemental data as ratios of certain elements or proxy values in profile form with depth, isotope values, and certain ratios are used for understanding various geological processes in the earth's history. Mineralogical studies using techniques such as x-ray diffraction spectrometry (XRD) and Raman spectroscopy have proven to be very powerful for identifying different sedimentary successions belonging to different ages. There have been notable advances in the measurement of isotopic concentrations and isotopic ratios both in stable isotopes and radiogenic isotopes. Certain analytical techniques such as laser-induced breakdown spectroscopy (LIBS) are able to distinguish different rock types, their geographic sources, and the geological processes associated with their formation in a field setting in real-time. In addition to the instrumental developments, substantial progress is made in other critical areas such as sample preparation, sample decomposition, analytical protocols, development of international certified geochemical reference materials (CRMs), and quality issues in general. Until recently, the general approach in elemental and mineralogical chemostratigraphic studies was to follow time taking process of collecting the sediment/drill core samples in the field and bringing them to the laboratory to generate the analytical data. This is very expensive, time-consuming, and difficult for covering large areas. But, rapid advances in the area of analytical instrumentation saw some instruments such as x-ray fluorescence spectrometer (XRF), LIBS, visible and near-infrared spectrophotometer, and Raman spectrometer dramatically shrunk in size and have become portable, and changed this scenario completely and increased on-site applications in chemostratigraphy studies. In this review, these developments in analytical instrumentation in general together with some key applications in chemostratigraphy, chronostratigraphy, and geochemical fingerprinting have been summarized.