Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece
Introduction
The progressive graphitization of carbonaceous material (CM) with increasing temperature forms the basis of a metamorphic thermometer for metasedimentary rocks [1], [2], [3]. Sedimentary rocks generally contain trace amounts of initially poorly ordered CM, which transforms into well-ordered graphite with increasing metamorphic grade [4], [5], [6], [7]. Laser Raman spectroscopy is a tool to directly measure the degree of ordering of CM [6], [8], [9], [10]. Raman analysis is quick and applicable to both rock chips and standard petrographic thin sections.
Beyssac et al. [1] were the first to formulate an empirical metamorphic thermometer using Raman spectroscopy of CM (RSCM). They demonstrated that CM crystallinity is strongly correlated with peak metamorphic temperature but not with metamorphic pressure. The thermometer is based on an observed linear relation between metamorphic temperature and the R2 parameter, which is the ratio of the peak areas for the disordered and ordered bands as measured in the CM Raman spectra. Their RCSM thermometer works best for samples with metamorphic temperatures between 330 and 650 °C, a range over which R2 progressively decreases from about 0.7 to less than 0.05. However, R2 varies little outside of this temperature range and measurements at the limits of this R2 range cannot be confidently assigned a temperature. Yui et al. [7] showed that other aspects of the Raman spectra do change systematically for metamorphic temperatures less than 330 °C. This observation suggests that the RSCM thermometer could be extended to work over a larger temperature range.
Beyssac et al. [1] showed that the degree of graphitization is unaffected by retrograde metamorphic events. Therefore, the metamorphic transformation from organic carbon to graphite is largely an irreversible process and estimated temperatures should approximate peak metamorphic conditions. In detail, the situation is likely more complicated. Graphitization is a kinetically controlled process, and it takes millions of years to heat a rock up to metamorphic conditions and a similar amount of time to cool down. Our understanding of other similar kinetic processes suggests that reaction rate probably increases in a highly nonlinear fashion with increasing temperature (e.g., [11], [12]). Thus the degree of transformation is probably strongly weighted to the duration of time at peak temperature, a conclusion supported by experiments [13]. The RSCM thermometer is empirically calibrated using samples with known “peak temperatures” as estimated using metamorphic petrology. As a result, the RSCM temperature estimates are probably best called “metamorphic temperatures” in that they are representative of the peak temperature estimates that we might otherwise obtain from metamorphic thermometry.
In this paper, we introduce and calibrate a modified version of the RSCM thermometer using Raman spectra from CM in samples from the Olympic Mountains in Washington State. Apatite and zircon fission-track samples from the Olympic subduction wedge show various degrees of thermal resetting and therefore constrain metamorphic temperatures achieved during Miocene accretion [14]. The modified RSCM thermometer provides reliable temperature estimates between 100 and 700 °C. We demonstrate the reliability of the thermometer through application to a metamorphic sequence in New Zealand. We then use the modified thermometer to study tectonic exhumation of the Hellenic subduction wedge exposed on the Island of Crete, Greece [15], [16], [17], [18], [19].
Section snippets
Data acquisition and treatment
Laser Raman measurements of CM were made using standard petrographic thin sections for samples from Crete and New Zealand or using polished rock sections for the Olympics samples. Raman measurements of graphitic CM varies with mineral orientation [20], but the effects of this anisotropy are reduced by measuring the CM particles along their edges in oriented thin sections or rock chips [1]. Sections were generally cut normal to the macroscopic foliation (if present) and parallel to any
Calibration samples from Olympic mountains
Eleven samples from the Olympic Mountains in Washington State are used to calibrate the RSCM thermometer for very-low-grade metamorphic conditions. The Olympics mark the forearc high of the Cascadia subduction zone and expose a sequence of siliciclastic sediments that were deformed and metamorphosed over the last 20 m.y. [14], [22], [23]. Focused erosion on the center of the uplift has caused a “bulls-eye” map pattern with metamorphic grade increasing towards the center of the range. The
Revised calibration of the RSCM thermometer
Beyssac et al. [1] based their RSCM thermometer on the observed linear relationship between metamorphic temperature and R2. This relationship breaks down, however, below 330 °C (Fig. 3A). In fact, an R2 value of 0.7 to 0.8 can only be taken as evidence that the metamorphic temperature was < 330 °C. Our Olympic samples have metamorphic temperatures that range from 115 to 250 °C (Table 1), and R2 shows no variation, remaining steady at ∼ 0.75.
Yui et al. [7] measured RSCM for a metamorphic sequence
Otago subduction complex, South Island, New Zealand
We test the modified RSCM thermometer with samples from a metamorphic sequence of sandstones and mudstones exposed in the Otago high, a broad antiform about 150 km across and which trends roughly east–west across the South Island of New Zealand [41]. The Otago region marks the forearc high of a Mesozoic subduction wedge that formed along the eastern margin of Gondwana [42], [43], [44]. The sediments were deeply accreted, metamorphosed, and then exhumed with the deepest rocks exposed in the core
Conclusions
The modified RSCM thermometer is a promising method that can be readily applied to obtain estimates of metamorphic temperature. Beyssac et al. [1] established that the thermometer is relatively insensitive to pressures and thermal resetting and does provide reliable estimates of peak metamorphic temperatures. Our work here confirms the reliability of the thermometer and extends the applicable range to samples with metamorphic temperatures as low as 100 °C. The new calibration gives reasonable
Acknowledgements
Shun-Ichiro Karato, Zhenting Jiang, Phil Skemer, and Elizabeth Wong kindly provided assistance with the Raman spectroscopy equipment at Yale University. We thank Jay Ague, Eric Essene, Douwe van Hinsbergen, Stuart Thomson, and Elizabeth Wong for discussions; Karl Wegmann for help with sample collection; Olivier Beyssac and Gerd Rantitsch for critical reviews; and Ken Farley for editorial handling. In particular, we are grateful for comments by Gerd Rantitsch and Douwe van Hinsbergen that
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