Research Objectives

Garnet maintains a unique position in metamorphic petrology as an extraordinarily useful mineral to reconstruct the pressure-temperature-time (P-T-t) paths of metamorphic rocks and thus the evolution and development of earth’s crust throughout deep time. Garnet is ubiquitous in metamorphic rocks (particularly to mineral assemblages useful for thermobarometry), can be used as a geochronometer, and has a unique ability to preserve chemical zoning representing the history of dynamic changes experienced during peak metamorphic conditions, exhumation, and uplift (e.g. Ferry and Spear, 1978; Spear and Selverstone, 1983; Caddick et al., 2010). Historically, growth and diffusional zoning of major elements has been a standard analysis in garnet (e.g. Loomis and Nimick, 1982; Chakraborty and Ganguly, 1992; Carlson, 2006), but since growth zoning is commonly altered by volume diffusion, geochemical information relating to the conditions and mechanisms of garnet growth during prograde metamorphism can be completely or partially destroyed, especially during ultrahigh-pressure metamorphism (e.g. Caddick et al., 2010). Trace elements have lower diffusion coefficients compared to major elements, and these elements in conjunction with the major elements offer a more promising record of complicated metamorphic processes such as the growth and dissolution of important accessory minerals like zircon, monazite, and xenotime (e.g., Pyle and Spear, 1999; Anczkiewicz et al., 2012). Trace elements in garnet have been previously mapped using electron probe microanalysis (EPMA) or scanning electron microscopy (SEM), but newer techniques have emerged that make laser ablation-inductively coupled-mass spectrometry (LA-ICP-MS) useful for 2D mapping of a wide isotopic range (⁷Li to ²³⁸U) with excellent detection limits (e.g., Lockington et al., 2014; George et al., 2015; Raimondo et al, 2017). Additionally, large suites of isotopes can be collected concurrently, providing more geochemical information per sample than EPMA or SEM analysis (Raimondo et al., 2017).

For this project, I developed a method for creating linescans and high-resolution trace element maps of a large ultrahigh-pressure garnet porphyroblast with diamond inclusions collected from migmatitic pelitic gneiss in the central Rhodope Mountains, Bulgaria. I obtained 3 rim-core-rim linescans and 2 sets of 10 maps with different pixel sizes from the garnet. I analyzed the following elements: ¹⁴⁰Ce, ²³²Th, ⁹⁰Zr, ⁸⁹Y, ¹⁵³Eu, ¹⁷²Yb, ¹⁷⁵Lu, ²³⁸U, ⁴⁷Ti, and ⁵²Cr. I selected one LREE, a few HREEs, and some other trace elements as a strategically representative suite that can be analyzed in one run. I hope to use ¹⁴⁰Ce and ²³²Th to study the relationship between garnet and monazite. I analyzed ⁹⁰Zr, ⁸⁹Y, ¹⁷²Yb, ¹⁷⁵Lu, and ²³⁸U to study the relationship between garnet and zircon. I was interested in ¹⁵³Eu to possibly learn something about the garnet and the partial melting event. I analyzed ⁴⁷Ti because titanium has been demonstrated to preserve zonation lost at extreme temperature conditions due to intracrystalline diffusional relaxation (Ague and Axler, 2016).

I conducted this analysis with the hope that these trace element maps will provide previously unknown insight into the crystallization and dissolution history of the minerals in this ultrahigh-pressure metapelitic assemblage. Currently, there are conflicting models regarding tectonic setting and timing of ultrahigh-pressure metamorphism in the Rhodope Mountains in the literature (Petrik et al., 2016). Previous geochronological work yielded widely variable ages between c. 200 Ma and c. 90-70 Ma for the timing of ultrahigh-pressure metamorphism (Nagel et al., 2011; Collings et al., 2013), and some zircon U-Pb ages of c. 160-150 Ma are interpreted controversially as either evidence of an ultrahigh-pressure metamorphic event or as related to a later overprinting granulite facies/partial melting event (e.g. Liati, 2005; Wawrzenitz et al., 2015). There are at least two conflicting tectonic models for the timing of ultrahigh-pressure metamorphism in the Rhodope Mountains. I plan to interpret our new trace element garnet transects and maps in the context of pre-existing geochronologic and petrochronologic data, including monazite data collected by Petrik et al. (2016) to extract additional information about the net-transfer mineral reactions occurring during subduction and to provide further constraints on the P-T-t paths of garnets from the central Rhodope mountains and the resulting tectonic setting of formation.

Discussion of Significance

Elemental mapping of garnet is one of the most useful tools of the metamorphic petrologist to understand the complicated history of metamorphic rocks. The major elements can provide important information regarding the growth history of garnet and other metamorphic minerals. However, growth zoning of major elements is commonly altered by volume diffusion, meaning important context of peak metamorphic conditions and mineral evolution throughout metamorphism can be completely or partially erased (Caddick et al., 2010). Trace elements have some of the highest potential of any analyte to fill in the gaps of the metamorphic history of a given bulk rock assemblage that are missing from the major element analysis due to their relatively high partition coefficients. Though the usefulness of trace element mapping has been recognized by metamorphic petrologists for quite some time (e.g. Hickmott et al. 1987; Bea et al., 1994; Chernoff and Carlson, 1999), new analytical techniques are now allowing these theories to be tested (e.g. Raimondo et al., 2017). Additionally, the interpretation of these maps in the context of metamorphism remains enigmatic, and additional garnet maps from a wider range of metamorphic conditions is needed for a better understanding of the mobility and diffusional patterns of various trace elements during different metamorphic and geochemical conditions.

In these samples, I am most interested in yttrium and the heavy rare earth elements (REEs). Yttrium zoning in garnet is related to the breakdown of important accessory minerals, including xenotime, during garnet growth (Lanzirotti, 1995; Pyle and Spear, 1999). Though the interpretation of REE zoning in garnets requires further study, the heavy REEs show a higher potential for significant results because they partition preferentially into garnet compared to the light REEs (Pyle and Spear, 1999; Anczkiewicz et al., 2012). With this in mind, I am still interested in a suite of major and trace element analyses from this ultrahigh-pressure assemblage due in large part to a relative lack of zonation on previously constructed major element maps (Petrik et al., 2016).

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Previous LA-ICP-MS Method Development on Garnet Mapping