Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece

Abstract

We present new Raman spectra data of carbonaceous material (CM) to extend the range of the Raman spectra of CM thermometer (RSCM) to temperatures as low as 100 °C. Previous work has demonstrated that Raman spectroscopy is an excellent tool to describe the degree of graphitization of CM, a process that is independent of pressure but strongly dependent on metamorphic temperature. A linear relationship between temperature and the Raman parameter R2 (derived from the area of the defect band relative to the ordered graphite band) forms the basis of a previous thermometer. Because R2 shows little variability in low-temperature samples, 330 °C serves as a lower limit on the existing thermometer. Herein, we present Raman spectra from a suite of low-temperature (100 to 300 °C) samples from the Olympics Mountains and describe other aspects of the Raman spectra of CM that vary over this range. In particular, the Raman parameter R1 (the ratio of heights of the disordered peak to ordered peak) varies regularly between 100 and 350 °C. These data, together with published results from higher-temperature rocks, are used to calibrate a modified RSCM thermometer, applicable from 100 to 700 °C. Application to low-grade metasediments in the Otago region in the South Island of New Zealand gives temperatures consistent with previous estimates, demonstrating the reliability of the modified RSCM thermometer.

We apply the modified RSCM thermometer to 53 samples from Crete to evaluate the role of the Cretan detachment fault in exhuming Miocene high pressure/low-temperature metamorphic rocks exposed there. The metamorphic rocks below the detachment (the Plattenkalk and Phyllite–Quartzite units) give metamorphic temperatures that range from 250 to 400 °C, consistent with previous petrologic estimates. We also demonstrate that the Tripolitza unit, which lies directly above the detachment, gives an average metamorphic temperature of about 260 °C. The modest break in metamorphic temperature in central Crete indicates that the Cretan detachment accounts for only 5 to 7 km of exhumation of the underlying HP–LT metamorphic rocks, which were initially accreted at ∼ 35 km. We argue that the bulk of the exhumation (∼ 28 km out of 35 km total) occurred by pervasive brittle stretching and erosion of structural units above the detachment.

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].