Discrimination of Clinozoisite–Epidote Series by Raman Spectroscopy: An application to Bengal Fan Turbidites (IODP Expedition 354)

Abstract

Epidote group minerals are one of the three most abundant kinds of heavy minerals in orogenic sediments, the other two being amphibole and garnet. They resist diagenesis better than amphibole and resist weathering in soils better than garnet. Their chemical composition and optical properties vary markedly and systematically with temperature and pressure conditions during growth. Useful information on the metamorphic grade of source rocks can thus be obtained by provenance analysis. In this study, we combine optical, SEM–EDS, and Raman analyses of nine standard crystals of epidote group minerals collected from different rock units exposed in the European Alps and Apennines and develop a Raman library for efficient discrimination of epidote, clinozoisite, zoisite, and allanite by establishing clear user-oriented relationships among optical properties, chemical composition, and Raman fingerprint. This new library allows us to distinguish and reliably determine, directly from their Raman spectrum, the chemical compositions of epidote group minerals during routine heavy mineral analyses of sand/sandstone and silt/siltstone samples down to the size of a few microns. The validity of the approach is illustrated by its application to 41 Bengal Fan turbidites collected from five cores during IODP Expedition 354 and ranging in grain size from medium sand to fine silt.

1. Introduction

Varietal studies of single detrital mineral species integrated using bulk petrography and heavy mineral data place fundamental constrains on the provenance analysis of modern sediments and ancient sedimentary rocks [1,2,3]. Raman spectroscopy is a powerful tool to accurately characterize mineralogical species and to improve our understanding of the sedimentary processes that modify provenance signals during sediment transfer from source to sink [4,5]. Raman spectroscopy is an efficient and non-destructive technique which has an excellent spatial resolution (down to a few µm; e.g., [6,7]) and does not require specific sample preparation [8,9,10], potentially making it suitable to replace other techniques such as QemScan and MLA [11,12,13], or other analytical tools (e.g., XRD). Notwithstanding its potential, Raman spectroscopy has been mainly used in a semi-quantitative way in the Earth Sciences until recently, and only on a limited number of selected minerals. Raman spectroscopy studies aimed at identifying the chemical composition of specific detrital minerals in relationship with the lithology of their source rocks have been dedicated to garnet [10], amphibole [14], pyroxene [15], titanite [16], and rutile [17]. Microprobe, X-ray, Raman, and infrared spectroscopy analyses have been applied for the chemical characterization of different epidote group minerals [18,19,20,21] and to identify their source rocks [22,23,24], but a quantitative study on their Raman fingerprints has not so far been carried out.

This study contributes to the filling of the existing knowledge gap concerning the identification of epidote group minerals in sediments, with the main aim of assessing the relationship between their composition and their parent rocks. Here we will focus on detrital epidote, clinozoisite, zoisite, and allanite, which are the most common detrital species derived from igneous and metamorphic rocks [23,25,26,27,28]. Zoisite, an orthorhombic polymorph of clinozoisite, is here considered as part of the epidote group (although it does not strictly belong to it [29]) because its optical properties are similar to those of the epidote–clinozoisite series and because it is an important provenance tracer for high-pressure source rocks. Epidote group minerals are ubiquitous in orogenic sediments [30] and, being moderately durable during both weathering and diagenesis, are widespread in sediments of Cenozoic age [31,32,33,34].

In this study, we combine optical, SEM–EDS, and Raman analyses of nine standard crystals collected from different rock units exposed in the European Alps and northern Apennines and develop a new Raman library for epidote group minerals by establishing clear user-oriented relationships among optical properties (e.g., pleochroism and birefringence), chemical composition (e.g., relative amounts of Fe and Al), and Raman fingerprint (i.e., position of principal and secondary peaks). The Raman reference spectra allow us to routinely classify the different minerals found in sand- and silt-sized sediments and to reliably determine their chemical composition from their Raman signal directly on grain mounts or from thin sections, with no need to resort to techniques involving more complex preparation procedures. The validity of this approach is illustrated by its application to Bengal Fan turbidites. A precise assessment of the relative amount of diverse detrital epidote group minerals provides information on the metamorphic grade of source rocks, complementary to that obtained by classical petrographic and heavy mineral analyses and useful to reconstruct the erosional evolution of the Himalayan orogen through the last 20 Ma.

2. Epidote Group Minerals

Epidote group minerals are monoclinic sorosilicates described with the general formula A₂M₃[T₂O₇] [TO₄] (O,F) (OH,O), and their topology is consistent with space group P2₁/m). The key cation sites −A = Ca, Sr, Pb, REE³⁺, M = Al, Fe³⁺, Mn³⁺, Mg, Fe²⁺, Mn²⁺, Cr, V, and T = Si− determine their root name [29]. The chemical composition of epidote group minerals is highly variable and reflects crystallization conditions (e.g., temperature, pressure, oxygen fugacity, and bulk rock composition), thus reflecting the characteristics and metamorphic grade of source rocks [26,35,36].

Epidote {Ca₂Al₂Fe³⁺[Si₂O₇] [SiO₄] O(OH)} is typical of greenschist facies metamorphic rocks, where it is associated with albite, actinolite, and chlorite, but it also occurs in intermediate to mafic amphibolite facies rocks associated with plagioclase and hornblende. Epidote forms from a variety of protoliths, including mixed siliciclastic–carbonate sediments and mafic to intermediate magmatic rocks [22,27,36], under a variety of temperature gradients, and from medium to high, and even ultrahigh, pressure regimes [26,37]. Commonly associated with glaucophane in blueschist facies metamorphic rocks [38], it may also be the product of the metasomatic alteration of pyroxene, amphibole, and plagioclase in magmatic rocks [39].

Clinozoisite {Ca₂Al₃[Si₂O₇] [SiO₄] O(OH)} is characteristic of low- to medium-grade metamorphic rocks (i.e., greenschist and epidote–amphibolite facies), and commonly occurs in association with actinolite, but it also occurs in blueschists facies mafic rocks. Zoisite {Ca₂Al₃[Si₂O₇] [SiO₄] O(OH)} is typical of regional metamorphism, being a relatively common constituent of epidote–amphibolite facies rocks derived from calcareous shales and sandstones as well as amphibolites derived from mafic igneous rocks, as in eclogites and blueschists [38]. It also occurs in calc–silicate granulites and calcite–zoisite micaschists [1]. Allanite {Ca₂(REE)³⁺Al₂Fe²⁺[Si₂O₇] [SiO₄] O(OH)} incorporates light rare earth elements (LREE) as well as Th [40,41,42,43], making it a mineral that is potentially economically valuable and frequently slightly radioactive. Allanite is relatively common in granite, granodiorite, monzonite, and syenite, and it may occur also in pegmatite and limestone skarn [38,44].

Some chemical properties of epidote group minerals can be deduced from optical properties (e.g., optical sign; pleochroism, birefringence) determined during routine heavy mineral counting. Pleochroism is related to the degree of Al, Fe³⁺ substitution, and it is strong in iron-rich deep yellowish green epidote grains and weak in iron-poor, colorless to pale yellowish green clinozoisite [29]. However, optical properties depend on crystallographic orientation and are insufficient to allow a precise assessment of the geochemical composition of detrital epidote group minerals [1].

Zoisite is commonly colorless, with greyish white or anomalous indigo-blue and yellow interference colors and parallel extinction. Allanite has strong greenish brown to reddish brown pleochroism and second-order interference colors, which may make it similar to reddish brown hornblende or dark brown dravitic tourmaline [45].

2.1. The Epidote–Clinozoisite Series

Epidote and clinozoisite are the two end members of the epidote–clinozoisite series. Solid solutions are distinguished by the ratio between iron and aluminum (X_Fe = Fe³⁺/(Fe³⁺ + Al)) and are optically negative if Fe-rich (epidote) and optically positive if Al-rich (clinozoisite). The X_Fe value tends to decrease with increasing metamorphic grade [26,27], and the appearance of clinozoisite marks the transition from the very low to the low metamorphic grade [46]. The X_Fe ranges in epidote from 13.6 to 53.3 (Fe₂O₃ 7.6–23.4; FeO ≤ 0.8; Al₂O₃ 13.1–30.9) and in clinozoisite from 0.2 to 13.1 (Fe₂O₃ 0.6–6.4; FeO 1.1–1.8; Al₂O₃ 27.8–30.7; [38]). In zoisite, X_Fe does not exceed 0.15, and it is mostly ≤ 0.074 (Fe₂O₃ 0.04–3.97; FeO ≤ 0.4; Al₂O₃ 32.0–33.5; [38,47]. The change from optically negative epidote to optically positive clinozoisite has been considered to occur at Ep 30% (6 wt% Fe₂O₃; [38,48]), at Ep 35% [47], or at Ep 40% [29].

2.2. Raman Analysis of Epidote Group Minerals

Raman spectroscopy has rarely been applied to estimate the chemical composition of epidote group minerals, in particular by considering the diagnostic peaks in the low-frequency region of the spectral range (i.e., 150–1200 cm⁻¹; [20,49,50]). Nagashima et al. [51] took into account the OH-stretching peak in the high-frequency region (3200–3600 cm⁻¹) and observed that stretching modes shift toward higher wavenumbers with increasing Fe content. In the low-frequency region, the Raman spectrum of epidote is characterized by an abundance of intense peaks at about 400 cm⁻¹ to 420 cm⁻¹, 550 cm⁻¹, −910 cm⁻¹, and 1080 cm⁻¹, and by a lack of peaks between 600 cm⁻¹ and 820 cm⁻¹ [49], which facilitates distinction from other silicates [5]. Clinozoisite displays diagnostic peaks at about 980 cm⁻¹ and 1090 cm⁻¹ originating from Si–O bond-stretching modes, and a peak at 570 cm⁻¹ associated with a Si–O–Si bending mode [51]. The zoisite spectrum is distinguished by an intense peak at 490 cm⁻¹ related to a Si–O bending mode, and by other peaks at about 680 cm⁻¹, 870 cm⁻¹, 1070 cm⁻¹, and 1091 cm⁻¹ [50].

3. Materials and Methods

For this study, the following mineral standards were used: six in the epidote–clinozoisite series (E1, E2, E3, E4 and C1, C2 [52,53,54,55]), one zoisite (Z [5]), and two allanites-(Ce) (A1 and A2 [55,56]). These standards were collected from different localities in the European Alps and Apennines (Supplementary File S1, Table S1) and selected to cover a wide range of compositions and metamorphic grade of source rocks and to define the limit between epidote and clinozoisite based on Raman spectroscopy data. Mineral standards were crushed in an agate mortar. The fragments were mounted on a Huntsman Araldite^® DBF resin stub, cut, and polished to perform optical, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy analyses on different fragments of the very same crystal. Several points were analyzed for each standard, first by Raman spectroscopy and then by SEM–EDS on the same spot.

The correlation of Raman spectra and SEM–EDS analyses allowed us to discriminate among epidote group minerals in the grain mounts of 41 Bengal Fan turbidite samples collected from five cores drilled during IODP Expedition 354 [57]. After separation by centrifuging in Na-polytungstate (density 2.90 g/cm³), following [58,59], heavy mineral analyses were carried out on the 15–500 μm (for sand) or >5 μm (for silt) size classes obtained by wet sieving. About 50 Raman spectra of detrital epidote group minerals were collected on each heavy mineral slide. Full information on core locations, sample age and grain size, heavy mineral data, and Raman peaks in the OH region for each of the five studied Bengal Fan cores is provided in Supplementary File S2 [60,61]. Provenance analysis based on integrated petrographic and mineralogical data is illustrated in [62].

3.1. SEM–EDS Mineral Chemistry

Mineral standards were coated with graphite and analyzed for major elements at the PMiB (University of Milano-Bicocca, Milano, Italy) with a Tescan VEGA^TM TS Univac 5136 XM scanning electron microscope coupled with an EDAX Genesis^TM 4000 XMS Imaging 60 SEM–EDS under an electron beam with a high voltage of 20 kV, 250-nm spot size, and current absorption 190 pA ± 1 pA, measured in a platinum Faraday cup. The quantification of main and trace elements (Si, Al Ca, Fe, Mg, Mn, and REE) was calibrated according to Astimex Scientific standards. Because of crystal zoning, compositional variability turned out to be stronger in the epidote standard E3. All SEM–EDS analyses of mineral standards are provided in Table S2.

3.2. Raman Spectroscopy

Optical techniques represent the fundamental and classic approach to investigating the mineralogical composition of sediments [1,63]. A first discrimination among epidote group minerals can be obtained based on relief, pleochroism, anomalous birefringence colors, angle between optical axes, and optical sign (Figure 1). This task, however, becomes challenging in silt or with grains not showing their prismatic face {100} on the slide. Raman spectroscopy does not only offer a quick method to solve these problems, but represents a very efficient tool to obtain quantitative compositional data.

Raman spectra for the nine mineral standards were collected at the Laboratory for Provenance Studies (University of Milano-Bicocca, Milano, Italy) by a high-resolution Renishaw inVia^TM Reflex confocal Raman microscope, equipped with a Leica DM2500 polarizing microscope with objectives up to 100× and motorized x–y stage. The system was calibrated using the 520.7 cm⁻¹ Raman peak of silicon. Spectra were obtained using the 50× LWD (long working distance) objective and a 532-nm line, solid-state laser with a power of 10 mW at the sample controlled by density filters to avoid heating effects. The acquisition time was 60 s and the spectral resolution was 1 cm⁻¹.

The minimum lateral resolution of the laser was 2 μm and the depth resolution was a few µm. We considered the low wavenumber range (140–1200 cm⁻¹) and in particular the high wavenumber range (3000–3800 cm⁻¹), which turned out to be crucial for detecting the OH-stretching signal. The Renishaw WIRE software package was used for baseline subtraction and to perform the spectral deconvolution using Gauss-Lorentzian functions and determine the position of the Raman peaks.

Between two and eleven spectra were collected for each mineral standard (Figure 2), and photos with the analyzed Raman spots were taken to carry out SEM–EDS analyses in exactly the same spots (Figure 3). The diagnostic peaks of the epidote–clinozoisite standards are provided in Table S3.

4. SEM–EDS and Raman Analyses of Mineral Standards

Epidote contains up to 14.0 wt% FeO_t and clinozoisite up to 29.3 wt% Al₂O₃. Zoisite contains only 2.2–2.7 wt% FeO_t, but up to 31.7 wt% Al₂O₃. In allanite, FeO_t ranges from 4.4 wt% to 13.5 wt%, and Al₂O₃ from 15.9 wt% to 20.1 wt%, with up to 20 wt% accounted for by La₂O₃, Ce₂O₃, Pr₂O₃, Nd₂O₃, UO₂, and ThO₂.

The Raman spectra of mineral standards display many peaks in the low-frequency region (140–1200 cm⁻¹) and one intense peak related to the OH-stretching vibration in the high-frequency region (3000–3600 cm⁻¹). The epidote–clinozoisite series is characterized by intense peaks at about 400 cm⁻¹ to 420 cm⁻¹, 570 cm⁻¹, 910 cm⁻¹, and 1080 cm⁻¹, a lack of peaks between 600 cm⁻¹ and 820 cm⁻¹ (Figure 3A), and a high wavenumber peak between 3350 cm⁻¹ and 3390 cm⁻¹ (Figure 3B). Peaks up to 200–250 cm⁻¹ are related to translations of cations or anionic groups, peaks between 275 cm⁻¹ and 300 cm⁻¹ are related to Ca-O-stretching vibrations, peaks in the ranges 300–400 cm⁻¹ and 500–550 cm⁻¹ are related to Al-O- or Fe-O-stretching vibrations, peaks in the ranges 430–450 cm⁻¹ and 565–580 cm⁻¹ are related to Si–O_b–Si bending, the peak near 600 cm⁻¹ is related to Si-O_b stretching, and the peaks between 830 and 1100 cm⁻¹ are related to Si-O_nb stretching [64]. Zoisite is identified by a major peak at 490 cm⁻¹ related to the Si–O-bending mode [50], by secondary peaks at about 680 cm⁻¹, 870 cm⁻¹, 1070 cm⁻¹, and 1091 cm⁻¹, and by the high wavenumber peak at about 3150 cm⁻¹ (Figure 4A). Allanite is singled out by one sharp major peak at 689 cm⁻¹ and displays several wide overlapping peaks in the high-frequency region (Figure 4B).

4.1. How to Calculate X_Fe in the Epidote–Clinozoisite Series

The progressive substitution of Fe²⁺ with Al³⁺ determines a progressive red shift of the main Raman peaks in the low-frequency region (Figure 3A) and a blue shift in the high-frequency region (Figure 3B). Such a progressive shift can be fitted by a linear interpolation of the X_Fe values obtained by SEM–EDS with the three strongest Raman peaks in the low-frequency region (υ₁: 565–573 cm⁻¹, υ₂: 980–987 cm⁻¹, υ₃: 1085–1096 cm⁻¹) and the main peak in the high-frequency region (υ₄: 3386–3360 cm⁻¹).

By using the three low-wavenumber peaks, which are all well separated from the other peaks, three well defined equations are obtained (Figure 5):

X_Fe = −0.022301 υ₁ + 12.915 (R² = 0.90)

X_Fe = −0.025055 υ₂ + 24.868 (R² = 0.84)

X_Fe = −0.017607 υ₃ + 19.434 (R² = 0.86)

The OH-vibration mode at high wavenumber is a couplet including a higher peak and a shoulder that progressively disappears with increasing X_Fe. The equation obtained by the deconvolution of the overlapping peak and shoulder using a combination of Lorentzian and Gaussian functions is:

X_Fe = 0.007112 υ₄–23.752 (R² = 0.88, Figure S1).

The MATLAB^TM program “epidotefull” was developed to estimate X_Fe by combining the compositional information provided by the four Raman peaks, considered as input. The average of the X_Fe values obtained by the four equations, weighted by their R² value, is taken as the best estimate of the X_Fe of the mineral. The calculations are based on all four main Raman peaks (υ₁, υ₂, υ₃, and υ₄), and the result is very sensitive to the exact value of the coefficient of the wavenumber, so it is recommended to use all the shown digits for the calculations. The presence of minor or trace amounts of Mg, Mn, Cr, and V can cause small shifts in the Raman spectrum, but their impact is not likely to cause a misidentification of epidote/clinozoisite. Such small chemical variations can indeed influence the calculation of the X_Fe, but this effect it is taken into account in the uncertainty associated with the method and is reduced by combining different peaks. A detailed description of the program “epidotefull” is provided as Supplementary File S3.

4.2. Distinguishing Epidote from Clinozoisite

Whereas zoisite and allanite are readily identified by their diagnostic peaks at about 490 cm⁻¹ and 689 cm⁻¹, respectively, discrimination within the epidote–clinozoisite series requires additional care. The boundary between optically negative epidote and optically positive clinozoisite is only broadly defined in the Ep₃₀ (X_Fe 0.20) to Ep₄₀ (X_Fe 0.27) range. In this study, we adopted the Ep₃₀ value recommended by [38], which separates the optically negative standards E1, E2, E3, and E4 from the optically positive standards C1 and C2. Based on the combined SEM–EDS and Raman analyses, the boundary between epidote and clinozoisite is thus established at FeO_tot 8 wt%, X_Fe 0.20, υ₁ 570 cm⁻¹, υ₂ 984 cm⁻¹, υ₃ 1092 cm⁻¹, and υ₄ 3369 cm⁻¹ (

Table 1. Total iron content determined by SEM–EDS, X_Fe values, and position of Raman peaks ν1, ν2, ν3, and ν4 for the six epidote and clinozoisite standards.

Sample	FeOt	XFe	v1 (cm−1)	v2 (cm−1)	v3 (cm−1)	v4 (cm−1)
E1	13.5–14.0	0.31–0.32	565	979–980	1085–1086	3385–3386
E2	11.7–11.9	0.27–0.29	566–567	981–982	1088	3379–3381
E3	10.6–11.9	0.23–0.28	567–568	981–983	1088–1091	3372–3378
E4	9.2–10.0	0.20–0.24	568–569	982–984	1091–1092	3369–3371
C1	6.3–7.9	0.15–0.16	570–572	985–986	1092–1093	3362–3366
C2	4.5–8.0	0.10–0.20	571–573	985–987	1094–1096	3361–3368

).

5. Epidote Group Minerals in the Bengal Fan

Bengal Fan turbidites collected from cores U1454, U1453, U1452, U1449, and U1451 during IODP Expedition 354 and ranging from fine silt to medium sand (9 µm to 300 µm; 6.9 ϕ to 1.7 ϕ) have provided an excellent opportunity to test the usefulness in provenance analysis of the Raman approach illustrated above. Epidote group minerals, including mainly Fe-rich epidote with subordinate clinozoisite and rare zoisite and allanite, represent 14% ± 4% of the rich to moderately rich transparent heavy mineral (tHM) suites of the Quaternary turbidites and 24% ± 3% of the moderately poor tHM suites of the Zanclean–Tortonian strata, reaching up to 30% of the poor tHM suite of the lowermost Miocene sample (

Table 2. Average composition of detrital epidote group minerals in turbidites of different ages from the five studied Bengal Fan cores, as determined using the new Raman library (mean in bold, standard deviation in italics); n°, number of analyzed grains; tHM, transparent heavy minerals; EpGM, epidote group minerals; Ep, epidote; Czo, clinozoisite; Zo, zoisite; Aln, allanite; RF Ep, epidote-bearing rock fragments.

Core Site	Depth (m)	Age	n°	tHM%	% EpGM	%Ep	%Czo	%Zo	%Aln	%Ep RF	XFe
U1454B	30-160	Quaternary	203	6.3	16	65	28	1	3	3	0.24
				3.6	6	7	12	2	4	4	0.06
U1453A	20-190	Quaternary	177	3.8	12	66	26	7	1	0	0.24
				1.0	1	14	19	10	2	0	0.06
U1452B	50-190	Quaternary	160	4.8	15	64	30	2	2	2	0.24
				1.6	1	12	10	3	2	4	0.07
U1449A	8-208	Quaternary	212	3.5	17	68	31	1	1	0	0.24
				0.9	2	9	6	2	1	0	0.06
U1451A	25-117	Quaternary	167	4.0	12	63	33	3	1	0	0.24
				0.6	4	9	6	3	1	0	0.06
U1451A	161	Pliocene	49	1.7	25	57	41	2	0	0	0.21
											0.05
U1451AB	200-658	Late Miocene	340	1.7	24	66	31	2	1	0	0.24
				0.8	3	4	6	1	2	0	0.07
U1451B	969	Early Miocene	36	0.7	18	61	36	3	0	0	0.23
											0.08
U1451B	1027	Early Miocene	62	0.6	30	63	26	3	8	0	0.26
											0.07

). Such a relative increase in relatively durable epidote group minerals in older strata containing depleted tHM suites is ascribed to selective dissolution of amphibole and other less chemically resistant minerals during diagenesis [62]. Relative to the bulk sample, the content of epidote group minerals remains approximately constant (0.6% ± 0.3%) in strata buried less than 500 m and as old as 8 Ma (Late Miocene), but progressively decreases in older and more deeply buried strata from core U1451, where epidote group minerals also start to be selectively dissolved (yet at a much lower rate than amphibole).

Quaternary samples from the five studied Bengal Fan cores show no significant difference in the relative abundance of epidote group minerals (Figure 6). Fe-rich epidote represents 65% ± 9%, and clinozoisite represents 30% ± 10%, with minor zoisite (mostly < 10% but locally up to 28%) and allanite (mostly < 5% but locally up to 12%). Older samples from core U1451 do not show marked differences, clinozoisite reaching its maximum (41%) in a Pliocene turbidite and allanite reaching its maximum (8%) in the oldest, lowermost Miocene turbidite.

The Raman-determined X_Fe values of detrital epidote and clinozoisite are also similar in all Quaternary samples, showing a mode between 0.25 and 0.3 (i.e., close to the epidote/clinozoisite boundary; Figure 7). The proportion between epidote and clinozoisite and also X_Fe values do not change notably in older strata, suggesting that the provenance signal carried by epidote group minerals is not markedly affected by selective diagenetic dissolution of Fe-rich vs. Fe-poor grains.

6. Conclusions

Epidote group minerals, being one of the most abundant heavy minerals in sediments, resisting diagenesis better than amphibole and resisting weathering in soils better than garnet, are among the most useful mineral tracers in varietal studies. For this reason, we developed a new Raman library to objectively distinguish among epidote, clinozoisite, zoisite, and allanite grains by coupling optical observations, SEM–EDS chemical analyses, and Raman spectra obtained for nine standard crystals of epidote group minerals collected from different rock units exposed in the European Alps and northern Apennines. We show how accurate information on the chemical composition of epidote group minerals can be obtained by Raman spectroscopy directly on grain mounts or from thin sections in a few seconds and with no additional sample preparation. Iron content is determined by considering the position of the intense diagnostic Raman peaks in the low-frequency (vibrational modes ν1 at ca 570 cm⁻¹, ν2 at ca 980 cm⁻¹, and ν3 at ca 1090 cm⁻¹) and high-frequency OH region (ν4 at ca 3370 cm ⁻¹). This approach was tested on Bengal Fan turbidites where the relative abundance of epidote, clinozoisite, zoisite, and allanite was identified in sediment ranging in size from medium sand to fine silt (9 µm). Quaternary Bengal Fan turbidites contain 65% ± 9% of Fe-rich epidote, 30% ± 10% clinozoisite, and minor zoisite and allanite. Epidote group minerals remain chiefly unaffected by diagenetic dissolution in strata buried less than 500 m and as old as 8 Ma. In older and more deeply buried strata, epidote and clinozoisite start to be dissolved, but their ratio remains constant, suggesting that it can be profitably used as a fingerprint in provenance analysis.