Controls on the Chlorine Isotope Composition of Sedimentary Rocks During Prograde Metamorphism, with Implications for Mantle Chloride Sources

University Of New Mexico, Albuquerque NM

Investigators

Abstract

Intellectual Merit: The chlorine isotope composition of Moon, meteorites and the major Cl reservoirs of the Earth have all been found to be similar (d37Cl near 0 permil). Upon detailed examination, however, significant Cl isotope variations are seen in different contaminated mantle-derived basalt types and within a single outcrop from metasomatized mantle peridotites. Because Cl is strongly partitioned into fluid and melt phases and is not expected to undergo any isotopic fractionation at high T/P conditions, the Cl isotope composition of these isotopically anomalous samples should record the sources of metasomatizing fluids. The unique chemical properties of Cl lead to an isotopic fingerprint that is different and complementary to other geochemical systems. Subducted sediments are thought to return significant Cl to the mantle, and several authors have proposed that non-zero d37Cl values of mantle samples are a consequence of this process. Remarkably, published Cl isotope data for sediments and metasedimentary rocks range from -4 to +7.5 permil. However, given the current lack of studies focused on systematic changes in d37Cl values as a function of prograde metamorphism - and the paucity of analyses of metasedimentary rocks - previously proposed fractionation mechanisms and actual d37Cl values of metasedimentary rocks in deep levels of subduction zones are completely speculative. It is proposed to measure d37Cl values in several prograde metamorphic sequences in order to identify characteristic changes that may occur, and identify likely fractionation mechanisms responsible for such changes. Two well-characterized prograde metasedimentary sequences are targeted for this study: the Triassic-Jurassic pelitic metamorphic sequence of the Central Alps and the turbidites of the Devonian Littleton Formation in New Hampshire. Both sequences can be followed intermittently up metamorphic grade ? from diagenetic zone to amphibolite grade in the Central Alps, and chlorite through sillimanite grade in New Hampshire. Two lithologies will be sampled in the Central Alps, the Triassic Keuper red beds and a Liassic (lower Jurassic) black shale, and a thick turbidite sequence will be studied from the New Hampshire locality. Two mutually exclusive hypotheses for chlorine isotope behavior during diagenesis and metamorphism of common clastic sedimentary rock types will be tested. The first is that fractionation during devolatilization is minimal, and that the d37Cl value of metasedimentary rocks at all metamorphic grades is inherited from the protolith. This result is supported by theoretical and some limited experimental work. A second hypothesis can equally be argued, namely that fractionation does occur during dehydration reactions, with 35Cl preferentially incorporated in the fluid phase. This hypothesis is supported by measurements of extremely low-d37Cl pore fluids in unmetamorphosed sediments and heavy isotopic values found in some metasediments. While an analysis of a single sample will provide little information to support or reject either of these two diametrically opposed hypotheses, systematic variations in a prograde sequence can be used to construct definitive mechanisms of Cl isotope fractionation during metamorphism. The results of this study will have direct applicability to understanding the variations in mantle samples that have been contaminated by subducted materials and will provide a necessary boundary condition for the Cl isotope system in general. Broader Impacts: The work proposed here will contribute to ongoing technique development for chlorine isotope analysis, and will greatly strengthen the interpretive framework for future Cl isotope studies. One graduate student will be trained in all aspects of Cl isotope geochemistry and in the integration and interpretation of field, petrologic, and geochemical data. In addition, at least one undergraduate student will be hired a year to help with sample preparation and formation of a senior honors thesis. UNM is recognized by the U.S. Department of Education as a 'High Hispanic Enrollment' institution (~33% Hispanic on the main campus). Effort will be made to ensure that some or all of the students involved in the project are from under-represented minorities (Hispanic or Native American).