Secular Variations in Seawater Chemistry Over the Past 600 MY: A Study of Fluid Inclusions in Marine Evaporites Using Three Analytical Techniques

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9725740 Lowenstein The proposed study will apply three state-of-the-art techniques, Environmental SEM X-Ray EDS (ESEM X-Ray EDS), UV laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAMP-ICP-MS), and fluid inclusion extraction-ion chromatography (Extraction-IC) to chemically analyze fluid inclusions in Phanerozoic evaporites in an attempt to answer the important but unresolved question of whether global seawater has changed chemical composition over geologic time. To date, the analysis of fluids trapped in large inclusions (102-103 microns) in halite crystals from ancient evaporites thought to be of marine origin has been the only direct method of obtaining clues to the major ion chemistry of ancient seawater (see Holser, 1963; Das et al., 1990; Horita et al., 1991, 1996; among other). However, the origin of the halite crystals carrying these large inclusions remains uncertain but they may be the result of recrystallization or other secondary grain growth processes (e.g. Das et al., 1990, p.323; Land et al., 1995). The fluid extraction techniques used in these studies could not be applied to the small inclusions in halite chevron and cumulate crystals of undoubtedly primary origin. Two new analytical techniques to be used in the proposed study overcome his drawback (ESEM X-Ray EDS and LAMP-ICP-MS); they allow direct chemical analysis of fluids in primary inclusions as small as 20 microns. For comparison, bulk extraction methods (Extraction IC) will be used to analyze large fluid inclusions (200 microns) on splits of the same samples analyzed with the Environmental SEM and by laser ablation-mass spectrometry. The purposes of the bulk extraction analysis are to crosscheck whether large fluid inclusions required for the direct extraction procedure are representative of the primary seawater parent brines present in smaller fluid inclusions along crystal growth bands, and to obtain high precision data on brine chemistry, including minor elements (such as Br). The central objective of the proposed research is the recovery of a "global" seawater signal for each geologic period sampled. In order to do this, small fluid inclusions in primary chevron and cumulate halite crystals from spatially separated but coeval marine evaporites will be analyzed. We will use the water evaporation computer program of Harvie and Weare (1980) as a forward modeling tool to backtrack from the brine analyses to the parent "seawater" compositions. We have chosen to analyze halites in evaporite deposits from four periods, the Devonian, the Permian, the Cretaceous, and the Miocene. Globally, potash evaporites of Permian and Miocene age contain abundant MgSO4 salts, a mineral assemblage not predicted to form from the evaporation of modern seawater and one that has perplexed geologists for over a century. Our results from these two groups of evaporites with such different primary mineralogies deposited in sedimentary basins on different continents during four different geologic periods should provide an important initial test of whether the major ion chemistry of seawater has changed over Phanerozoic time.