Biogenic apatite in carbonate concretions with and without fossils investigated in situ by micro-Raman spectroscopy

Abstract

Micro-Raman spectra of concretions with and without fossils were measured in a nondestructive manner. The band position and full width at half maximum height (FWHM) of ν₁-PO₄^3- of apatite in the concretions were analyzed to investigate the origin of apatite. The analyzed concretions were derived from the Kita-ama Formation of the Izumi Group, Japan. The micro-Raman analysis showed that the apatites in the concretions were divided into two groups: Group W (wide FWHM group) and Group N (narrow FWHM group). The apatite belonging to Group W is suggested to be biogenic apatite originating from the soft body tissues of organisms because the Sr content is high and the FWHM is similar to that of apatite in bones and teeth of present-day animals. The other apatite belonging to Group N is considered affected by the diagenetic process because of its narrow FWHM and F substitution. These features of both groups were observed regardless of the presence of fossils or absence of fossils in the concretions. This Raman spectroscopic study suggests that the apatite at the time of concretion formation belonged to Group W but was changed to Group N by the substitution of F during the diagenesis process.

Figure 1

(A) Raman spectra in the 1150–700 cm⁻¹ and 700–100 cm⁻¹ regions of concretion with fossils (UNO). The ν₁-PO₄³⁻ band positions of phosphate (970–950 cm⁻¹ region) in the top three spectra (a: green, b: orange and c: red spectra) and those in the bottom three spectra (d: blue, e: gray and f: purple spectra) are different. Abbreviations are used as follows: calcite (Cal), apatite (Ap), anatase (Ant) and quartz (Qz). (B) Enlarged spectra in the 980–930 cm⁻¹ ν₁-PO₄³⁻ region. The apatite in the concretions is divided into two groups: Group N (narrow FWHM group; a′: green, b′: orange and c′: red spectra) with a higher band position and a narrow FWHM of ν₁-PO₄³⁻ and Group W (wide FWHM group; d′: blue, e′: gray and f′: purple spectra) with a lower band position and a wide FWHM.

Figure 2

(A) Cross- section of the concretion with fossils (UNO). (B) XRF P X-ray intensity in the concretion. Phosphorus is especially concentrated in the fossils. (C) Cross-section of the concretion without fossils (DOS). The concretion was too small to fit the holder, and thus, it was adjusted with the Teflon plate. (D) XRF P X-ray intensity in the concretion. Phosphorus is widely distributed.

Figure 3

Major elements in the concretion (TRES). The horizontal axis indicates the sampling point from 0 to 2.0 cm. White circles (〇) show horizontal sampling points, and black circles (●) show vertical sampling points. The rectangle on the right in each figure represents the matrix. The data measured by WDXRF were unified to a total of 100%. Abbreviations are used as follows: core (C), mantle-horizontal (MH), front-horizontal (FH), mantle-vertical (MV) and front-vertical (FV).

Figure 4

REE pattern of concretion (TRES). The REE data are normalized to Post Archean average Australian sedimentary rocks (PAAS).

Figure 5

(A) ν₁-PO₄³⁻ band position (cm⁻¹) versus ν₁-PO₄³⁻ FWHM (cm⁻¹) of synthetic apatite samples and concretion samples (present study). The synthetic apatite sample data are obtained from Thomas et al.. The gray arrows indicate the amount of each element in the apatites. (B) ν₁-PO₄³⁻ band position (cm⁻¹) versus ν₁-PO₄³⁻ FWHM (cm⁻¹) of teeth samples from fossil, modern animal bones and teeth, geologic apatites and concretion samples (present study). Magmatic hydroxylapatite and chlorapatite had the same ν₁-PO₄³⁻ band position and FWHM. The apatite sample data are obtained from Thomas et al., with the exception of the concretion samples. The blue dotted arrow indicates the direction of progression of the diagenetic process. The gray arrows indicate the amount of each element in apatites.

Figure 6

${\left(\text{La}/\text{Yb}\right)}_{\text{N}}$ versus ${\left(\text{La}/\text{Sm}\right)}_{\text{N}}$ ratios in the concretion (TRES) and fresh and seawater^,. N index indicates normalization to PAAS.