Internal Structure and Inclusions: Constraints on the Origin of the Tancheng Alluvial Diamonds from the North China Craton

Abstract

The internal growth patterns and surface micromorphology of diamonds provide a record of their multi-stage evolution, from initial formation within the mantle to their eventual ascent to the Earth’s surface via deeply derived kimberlite magmas. In this study, gemological microscopic examination, Diamond View^TM, Raman spectroscopy, and electron probe analysis were employed to analyze the surface features, internal patterns, and inclusions of the Tancheng alluvial diamonds in Shandong Province, China. The results show that surface features of octahedra with triangular and sharp edges, thick steps with irregular contours or rounded edges, and thin triangular or serrated layers are developed on diamonds during deep-mantle storage, as well as during the growth process of diamonds, when they are not subjected to intense dissolution. The rounding of octahedral and cubic diamond edges and their transformation into tetrahedral (THH) shapes are attributed to resorption in kimberlitic magma. These characteristics indicate that the Tancheng diamonds were commonly resorbed by carbonate–silicate melts during mantle storage. Abnormal birefringence phenomena, including irregular extinction patterns, petaloid and radial extinction patterns, and banded birefringence, were formed during the diamond growth stage. In contrast, fine grid extinction patterns and composite superimposed extinction patterns are related to later plastic deformation. The studied diamonds mainly contain P-type inclusions of olivine and graphite, with a minority of E-type inclusions, including coesite and omphacite. The pressure of entrapment of olivine inclusions within the Tancheng diamonds ranges from 4.3 to 5.9 GPa, which is consistent with that of coesite inclusions, which yield pressure ranging from 5.2 to 5.5 GPa, and a temperature range of 1083–1264 °C. Overall, the evidence suggests that Tancheng diamonds probably originated from hybrid mantle sources metasomatized by the subduction of ancient oceanic lithosphere.

1. Introduction

Diamonds preserve a wealth of information about the composition of deep-earth materials, as well as the physicochemical conditions and tectonic environments in which they formed [1,2]. These evolutionary stages can be effectively deciphered through the surface textures of diamonds, internal growth patterns, spectroscopic features, and characteristic inclusions [3,4,5]. Inclusions in diamonds, such as olivine, garnet, coesite, and omphacite, provide critical insights into the temperature and pressure conditions of the diamond’s mantle source region [6,7,8].

This study investigates alluvial diamonds from the Tancheng deposit in Shandong Province, which is widely recognized as one of most significant deposits celebrated for yielding large-sized diamonds in China. For example, five extra-large diamonds weighing over 90 carats have been discovered, including the Linyi Star Diamond (338.6 ct), the largest natural diamond in China, and the famous Jinji Diamond (218.25 ct). However, the origin of these alluvial diamonds remains debated; particularly, there is considerable controversy over whether they originated from the diamondiferous kimberlite cluster in Mengyin. The majority of previous studies proposed that the diamonds are sourced from the Mengyin kimberlite primary deposit [9], evidenced from the geomorphic and geological features of the diamond-bearing strata in the Tancheng deposit in conjunction with the surface morphology of diamonds. Meanwhile, some research argued that the Tancheng diamonds exhibit distinct characteristics indicative of a proximal source, suggesting that not all the Tancheng diamonds originated from the Mengyin kimberlite cluster. This conclusion is based on a comprehensive comparison of features such as the diamond crystal color, grain size, lattice perfection, crystal morphology, surface morphology, and inclusions [10,11,12].

In addition to the Quaternary Tancheng diamond placer, multiple diamond-bearing strata have been discovered in the southern Shandong region, including the Lower Paleozoic Liguan Formation, the Upper Paleozoic Benxi Formation, the Mesozoic Santai Formation, and the Cenozoic Guanzhuang Group. During the deposition of the Liguan Formation, seawater flooded the region from the southeast to the northwest, forming a narrow, elongated sea trough extending in a north–northeast direction. This trough spanned areas including Zaozhuang, Cangshan, Linyi, Yinan, and Changle, roughly along the western side of the current Tan-Lu Fault. This environment facilitated barrier-free coastal deposition, with sediment sourced from the ancient uplift area to the west [13]. Furthermore, comparative studies of the diamond-bearing Baiyan conglomerate and Jurassic strata, combined with the sporozoan assemblage and basic material components, suggest that the possible existence of other primary diamond deposits in the southern Shandong region [14]. Thus, the question of whether the Tancheng alluvial diamonds originate from the Mengyin kimberlite belt is still controversial, and it is one of the key factors limiting breakthroughs in primary diamond deposit exploration in Tancheng.

One of the crucial approaches to resolving this issue is to study the crystallographic and inclusion characteristics of the Tancheng diamonds to trace their formation pressure, temperature and geological environment. This paper investigates these characteristics to explore the genesis of Tancheng diamonds and conducts comparative studies with the temperature-pressure features of Mengyin kimberlite-type diamonds. The purpose of the study is to identify the similarities and differences in the nature of the mantle source regions between the two study areas. This research is not only significant for tracing the provenance of Tancheng alluvial diamonds but also has important implications for guiding future mineral exploration and prospecting in Shandong Province and surrounding areas.

2. Geological Setting

The Tancheng alluvial diamond deposit is situated within the Tancheng fault depression basin, located in the central section of the Tan-Lu fault zone at the eastern edge of the western Shandong uplift block of the North China craton. The Tancheng–Lujiang fault zone, known as the “Yishu fault zone” within Shandong Province, consists of four main faults trending north–northeast. These faults form a fault zone approximately 20 to 60 km wide, presenting a structural pattern of “two grabens flanking a horst” [15]. The Tancheng basin has a topography that is higher in the east and lower in the west, comprising three geographical areas: the east, the middle, and the west. The eastern part mainly consists of Malingshan, characterized by flattened low hills. The central part includes Meibu, Yuquan, and Liugou, located on the second terrace of the Yi River. The western part encompasses Lizhuang and Tancheng, featuring the first terrace of the Yi River, as well as the riverbed and floodplain. These three regions are aligned in a north–northeast direction, with the eastern part forming a divide between the two major rivers, the Yi and Shu Rivers (Figure 1).

The diamond-bearing strata are primarily found in the Early Pleistocene Xiaobuling and Yuquan Formations in the central and western regions [13]. The Xiaobuling Formation, characterized by diamond-bearing sandy conglomerates, was formed under a cold climatic environment during the Early Pleistocene. It consists mainly of sandy conglomerate layers and sand-bearing conglomerate layers with well-developed oblique bedding and cross-bedding texture. The formation often contains lens-shaped sand layers or sandy clay layers, with gravel that is predominantly spherical or elliptical, ranging in diameter from 2 to 10 cm and constituting about 40% of the content. The gravel composition includes andesitic volcanic rock, metamorphic granite, limestone, sandstone, quartzite, and heavy minerals such as epidote, limonite, magnetite, and amphibole. The Yuquan Formation is composed of brownish-red or yellowish-brown conglomerate layers, with gravel consisting mainly of vein quartz, quartzite, or quartz sandstone. The gravel appears rounded or subrounded, with diameters ranging from 3 to 5 cm, and is mixed in size with no discernible bedding [16]. The heavy mineral assemblage is similar to that of the Xiaobuling Formation but with significantly lower content. The sandy conglomerate layers in the Yuquan Formation are richer in diamonds and have formed industrial deposits in the area stretching from Yuquan to Chenjiabu in the Tancheng region [10].

3. Materials and Methods

The studied samples include nine Chenbu diamonds and four Xiaobuling diamonds from the Tancheng alluvial deposit, weighing between 0.05 and 0.58 carats. Each sample was cleaned with 75% alcohol to remove surface contaminants.

Sample preparation was conducted by Zhengzhou Jingzuan Precision Industry Co., Ltd. in Henan Province. The diamond samples were cut and polished into double-sided, parallel-oriented slices with a thickness of approximately 0.7–0.9 mm. The slice surfaces underwent fine polishing and were cleaned with absolute ethanol before analysis.

Microscopic observation was performed at the Key Laboratory for Diamond Mineralization Mechanism and Exploration, Shandong Provincial Bureau of Geology and Mineral Resources. A GI-M6S91 gemological microscope (Nanjing Baoguang, Nanjing, China) was used, with the diamonds placed on a glass slide and illuminated using a top-mounted white light source. The microscope offered magnifications ranging from 20× to 55×, allowing for clear observation of the diamond characteristics.

Laser Raman spectroscopy tests were carried out using a inVia Reflex laser Raman spectrometer (Renishaw, London, the United Kingdom). The excitation source was a solid-state laser with a wavelength of 532 nm and an energy output of 50 mW. A 50× objective lens was used, and the scanning range was set between 100 and 2000 cm⁻¹. Raman shifts were calibrated using a silicon standard (520.5 ± 0.2 cm⁻¹). Each sample was scanned three times to obtain internal inclusion characteristics and Raman spectrum data. The inclusions were analyzed in situ within the diamonds to maximize the preservation of the inclusion minerals, ensuring the accuracy of the Raman frequency shift data acquired during testing.

The Diamond View^TM test was conducted at the Gemological Experimentation Teaching Center of China University of Geosciences (Beijing, China). The equipment used for the experiments was the Diamond View^TM diamond ultraviolet fluorescence instrument, produced by the UK-based company II DGR. The instrument settings were as follows: integration time of 0.30 s, power at 50%, gain at 2.72 dB, aperture at 5%, and field stop at 47%.

Panchromatic cathodoluminescence (CL) images were acquired using a Sunny-CL Cathodoluminescence Imaging System attached to a Mira field-emission scanning electron microscope (Tescan, Brno, Czech Republic), Goldenscope Technology Co., Ltd. (Beijing, China). The operating conditions included an acceleration voltage of 10–15 kV, a beam current of 1–3 nA, and a working distance of 20–30 mm.

A Fourier transform infrared (FTIR) spectroscopy test was conducted at the Fourier Transform Infrared Spectroscopy Laboratory of the Institute of Geology, Chinese Academy of Geological Sciences. The FTIR instrument is a HYPERION2000 microscope (Bruker, Billerica, MA, USA) with a liquid nitrogen-cooled Vertex 70V (Bruker, Billerica, MA, USA) spectrometer. The spectra were obtained using the software OPUS 6.5; FTIR absorption spectra ranging from 4000 cm⁻¹ to 600 cm⁻¹ were collected with a resolution of 2 cm⁻¹, and background spectra of 32 scans were recorded before the analytical session.

Electron probe microanalysis was completed at the Key Laboratory of Gold Mineralization Process and Resource Utilization of the Ministry of Natural Resources, Shandong Institute of Geological Sciences. The instrument used was a JXA8230 (JEOL, Tokyo, Japan), with an acceleration voltage of 15 kV, an electron beam current of 1.0 × 10⁻⁸ A, and a beam spot diameter of 5 to 10 μm. A series of standard samples from the Canadian company Astimex were employed in the analysis.

4. Results

4.1. Microscopic Examination

Thirteen alluvial diamonds studied in this paper are primarily colorless, with a few exhibiting light brown and light yellow colors. They exhibit a high degree of crystal integrity, with crystal morphologies being predominantly octahedral, rhombohedral dodecahedral (

Table 1. Characteristics of the studied diamonds from the Tancheng deposit, China.

Sample No.	Weight (ct)	Dimensions (mm)	Color	Crystal Form	Surface Features	Location
SDTC-05	0.09445	2.4 × 2.2 × 1.9	Light brown	Rhombic dodecahedron	Erosion ditch, imbricated image, with halo and slip lines	Xiaobuling
SDTC-13	0.30265	3.9 × 3.3 × 2.1	Colorless	Rhombic dodecahedron	Imbricated image, clear and obvious crystal edges and angles	Xiaobuling
SDTC-22	0.2093	4.0 × 2.9 × 1.7	Colorless	Rhombic dodecahedron	Clear and obvious crystal edges and angles, erosion ditch, imbricated image, halo and slip lines, and drop-like hills	Xiaobuling
SDTC-23	0.11915	2.7 × 2.7 × 1.6	Colorless	Rhombic dodecahedron and octahedron polyhedron	Clear and obvious crystal edges and angles, halo and slip lines, and erosion ditch	Xiaobuling
SDTC-47	0.3133	3.1 × 3.0 × 3.0	Colorless	Octahedral	Etch pits	Chenbu
SDTC-49	0.58485	4.3 × 4.3 × 4.1	Light yellow	Rhombic dodecahedron and octahedron polyhedron	Growth hills, erosion ditch, and halo and slip lines	Chenbu
SDTC-52	0.1177	2.7 × 2.5 × 1.9	Light yellow	Rhombic dodecahedron	Etch pits, halo and slip lines, and drop-like hills	Chenbu
SDTC-62	0.08	2.3 × 2.2 × 1.5	Light brown	Rhombic dodecahedron	Halo and slip lines, etch pits, and plastic deformation slip lines	Chenbu
SDTC-67	0.2939	4.5 × 3.3 × 2.4	Colorless	Rhombic dodecahedron and octahedron polyhedron	Etch pits, growth steps, growth hills, and plastic deformation slip lines	Xiaobuling
SDTC-68	0.0667	2.4 × 2.0 × 1.4	Colorless	Rhombic dodecahedral polyhedron	Halo and slip lines, erosion ditch, imbricated image, and drop-like hills	Xiaobuling
SDTC-69	0.08525	2.7 × 2.5 × 1.4	Colorless	Stepped octahedral polyhedron	Growth hills and growth steps	Xiaobuling
SDTC-74	0.0472	2.0 × 2.0 × 1.4	Colorless	Rhombic dodecahedron	Imbricated image, erosion ditch, and halo and slip lines	Xiaobuling
SDTC-81	0.083745	3.0 × 2.4 × 1.3	Colorless	Irregular fragment	Erosion ditch, growth hills, and halo and slip lines	Xiaobuling

), or combinations of both. The diamonds display a variety of surface morphological features, including growth hills, etch pits, and growth steps, which are more commonly observed in octahedral diamonds (Figure 2(e1,f1); Figure 3(d1)). Erosion ditches, plastic deformation slip lines, imbricated images, halos, slip lines, and drop-like hills are more prevalent in rhombohedral dodecahedral and their aggregated forms of diamonds (Figure 2(a1,c1,f1,g1); Figure 3(c1,e1)).

Diamonds, as isotropic minerals of the cubic system, should exhibit complete extinction under crossed-polarized light under ideal conditions. However, diamonds often display optical anomalies. Studies suggest that the phenomenon of anomalous birefringence in diamond crystals is caused by different internal stresses [17]. In this study, observations were made on 13 diamond thin sections from the Tancheng area, revealing several types of anomalous birefringence:

• Variable extinction patterns: The samples exhibited high anomalous interference colors without a fixed birefringence pattern. When the samples were rotated, the extinction bands moved randomly (Figure 3(c2,e2)).
• Petal-like and radial extinction patterns: Due to the presence of inclusions and cracks in the samples, they showed petal-like and radial extinction patterns under cross-polarized light (Figure 2(b2,c2); Figure 3(f2)).
• Banded birefringence patterns: A set of banded birefringence patterns was observed under cross-polarized light (Figure 2(d2)).
• Fine grid extinction patterns: For samples like SDTC-67, two sets of straight extinction bands were visible under cross-polarized light, with one set clear and the other weak. The upper left part of the sample displayed a special fine grid-like extinction, also known as “tatami”-like extinction (Figure 3(b2)).
• Composite superimposed extinction patterns: Samples like SDTC-13 and SDTC-23 did not show a single extinction pattern; instead, they often exhibited combinations of different patterns superimposed on each other (Figure 2(b2,e2,f2)).

4.2. Diamond View^TM

Diamond View^TM (DV) allows for the acquisition of ultraviolet fluorescence images of diamonds, enabling the observation of their growth structures [18]. In the DV luminescence images of the Tancheng diamonds, three main fluorescence types of fluorescence can be distinguished based on color:

• Monochromatic blue fluorescence [19]: Most diamonds exhibit varying shades of single blue fluorescence. For example, sample SDTC-49 shows a relatively uniform blue fluorescence (Figure 2(f3)); sample SDTC-47 has a gradient of blue fluorescence from deep to light, with a strong-to-weak luminosity feature from the center to the edge (Figure 2(e3)).
• Uniform yellow-green fluorescence [20]: Some diamonds display a relatively uniform yellow-green fluorescence (Figure 3(a3,b3)).
• Alternating blue and yellow-green fluorescence [19]: Some diamonds exhibit varying shades of intermingled blue and yellow-green fluorescence. For instance, in sample SDTC-13, the center shows an uneven mixture of blue and yellow-green fluorescence, with the former slightly weaker on the side of the inclusion and the latter stronger; the second growth ring is relatively stable, with even blue fluorescence. Towards the outer growth, yellow-green fluorescence gathers (Figure 2(b3)). In sample SDTC-23, the central inclusion shows blue fluorescence, with uneven growth in one direction and elongated growth in the other, resulting in a diamond with a euhedral inclusion structure; the second growth ring is relatively stable with uniform yellow-green fluorescence (Figure 2(c3)). Sample SDTC-74 has a mostly uniform yellow-green fluorescence in the center, with a hexagonal ring growth pattern in the second growth layer, leading to a gathering of blue fluorescence and a later growth layer showing blue and yellow-green fluorescence intermixed (Figure 3(b3)). The diamond’s crack morphology aligns with the locations of strong fluorescence, and the cracks developed within the diamond affect the observation of fluorescence intensity (Figure 3(a3,f3)).

4.3. CL Images

Diamonds can sometimes exhibit fluorescence under long-wave ultraviolet light, and the fluorescence color is closely related to the state of defects associated with nitrogen (N) atoms [21,22]. Research suggests that the formation of diamonds has a multi-phase and multi-stage characteristic [23,24]. Based on the internal growth structure, the Tancheng diamonds can be divided into the following three types:

• Diamonds with simple growth zones: These diamonds exhibit uniform cathodoluminescence (CL) characteristics or simple growth zones with discontinuous features (Figure 2(f4,g4); Figure 3(a4)).
• Diamonds with complex growth zones: These diamonds display various blue luminescent areas of different shades, as well as intermingled blue and yellow-green luminescent areas. The growth zones exhibit multiple centers of development and are densely layered (Figure 2(b4,e4)).
• Diamonds with special growth structures like “onyx-like” or circular concentric zones: These diamonds have onyx-like structures, circular concentric zones, and other special growth structures. The onyx-like structure and circular concentric zone structure are formed by dense layers of alternating blue and yellow-green luminescent areas and dark non-luminescent areas (Figure 3(d4,f4)).

4.4. Raman Spectroscopy

Raman spectroscopy reveals that the studied Tancheng diamonds host diverse mineral inclusions, including olivine, omphacite, coesite, calcite, garnet, and graphite. Olivine-type magnesium olivine inclusions are more frequent and numerous than eclogite-type diamond inclusions [11]. A single Tancheng diamond can contain multi-phase inclusion assemblages; for example, sample SDTC-013 contains coesite, omphacite, calcite, and graphite (Figure 4,

Table 2. Raman characteristic peak data of mineral inclusions within the Tancheng diamonds.

Sample Number	Raman Peak Positions (cm−1)	Identified Mineral
SDTC-13	138.0, 152.4, 214.6, 274.3, 330.3, 430.8, 471.2, 531.0	Coesite [7,25]
	137.2, 152.1, 211.6, 272.1, 328.3, 428.5, 469.4, 528.2	Coesite
	139.7, 188.5, 213.6, 272.3, 329.8, 429.1, 471.9, 529.9	Coesite
	134.7, 212.2, 271.9, 326.4, 428.3, 468.9, 528.0	Coesite
	1587.2	Graphite [25,26]
	224.6, 354.0, 396.0, 679.2, 1018.7	Omphacite [25]
	155.1, 278.7, 712.6, 1085.2	Calcite [25,27]
SDTC-022	354.6, 544.4, 555.7, 638.1, 855.0, 912.4, 1043.1	Garnet [25,28]
	143.3, 347.4, 513.2, 677.1, 712.6, 1014.0	Omphacite
	1587.1	Graphite [25,26]
SDTC-047	825.1, 857.2, 920.9, 966.1	Olivine [25]
	1592.4	Graphite
SDTC-069	822.6, 853.7, 916.7, 957.4	Olivine
	1593.0	Graphite
SDTC-074	823.6, 853.9, 877.9, 918.3	Olivine

), while sample SDTC-022 contains an omphacite, garnet, and graphite E-type diamond inclusion combination. The inclusions display distinct morphologies and characteristics. Omphacite is colorless and transparent, appearing as elongated prisms with well-defined crystal faces, measuring 40–80 μm in length. Graphite occurs as sheet-like or cloud-like aggregates, with some graphite inclusions filling near fractures. Calcite inclusions appear as irregularly shaped white and transparent minerals with a diameter of 20 μm.

Olivine inclusions in the Tancheng diamonds are forsterite, frequently occurring as multiple inclusions within single diamonds (Figure 5). These inclusions are colorless, transparent, and exhibit short prismatic or spherical morphologies, with diameters ranging from 30 to 200 μm. It is not uncommon for a single diamond to contain multiple forsterite inclusions. Coesite inclusions are numerous and vary in size from a few μm to tens of μm. They are colorless, transparent, and display diverse shapes, including needle-like, dumbbell-shaped, spherical, and elongated prismatic forms, and are located at the center of the diamond’s growth (Figure 4).

4.5. FTIR Spectra Features

The type of studied diamonds can be identified by typical characteristic peaks of the FTIR spectra in reflection mode. The infrared spectra of different samples within 1500–2700 cm⁻¹ fully exhibit the intrinsic peak of diamond (Figure 6), and the FTIR mapping of SDTC-13 has an obvious absorption peak at the C center (1286 cm⁻¹), which means it only has N pairs. Sample SDTC-68 exhibits a typical IaAB pattern and contains H atoms (VN3H, 3107 cm⁻¹), and H is the impurity with the most content in diamonds except N.

The Tancheng diamonds mostly exhibit complex growth structures. The nitrogen (N) content in the nucleation stage of Tancheng diamonds is higher or lower than that in other growth stages, and the variation in N impurity content across different growth stages does not follow a unidirectional pattern. This suggests complex changes in the fluid composition and growth environment during diamond formation.

The content and aggregation state of N in diamonds can retrieve primary geological information of the interior of our planet. In this study, the measurement points at different positions (core–mantle–rim) of diamonds from the Tancheng deposit were selected. By calculating the nitrogen A-center and B-center contents in diamonds using FTIR and then estimating the transformation rate of these centers with temperature and time based on mathematical relationships, it is possible to estimate the temperature of the diamond’s residence in the mantle. The contents of N (A, B center) in the samples are listed in

Table 3. N content and formation temperature of diamonds in Tancheng calculated using software QUIDDIT 3.0. [NA], [NB], and [Nt] mean concentrations of nitrogen in A, B center, and total nitrogen content, respectively.

Sample Name	[NA]/ppm	[NB]/ppm	[Nt]/ppm	T/°C
SD-TC-005	0	0	0	nan
SD-TC-013	14	0	14	nan
SD-TC-019	25	265	290	1216
SD-TC-022	0	0	0	nan
SD-TC-023	309	350	658	1138
SD-TC-047	1075	242	1318	1083
SD-TC-049	312	145	458	1125
SD-TC-052	183	326	510	1155
SD-TC-062	0	0	0	nan
SD-TC-063	0	225	225	nan
SD-TC-067	0	0	0	nan
SD-TC-068	32	36	69	1195
SD-TC-069	10	119	129	1242
SD-TC-074	0	0	0	nan
SD-TC-081	5	105	109	1264
SD-TC-085	0	36	36	nan

. Using the Python 3.11.5 software package QUIDDIT 3.0 and the infrared spectra of the Tancheng diamond samples, the mantle storage temperature of these diamonds can be calculated. The error in diamond temperature calculations using QUIDDIT 3.0 is primarily influenced by the sample quality and model parameters. Under ideal conditions, the accuracy of temperature calculation can reach ±5 °C. In cases of low aggregation (temperature ranges from 1165 to 1173 °C ), even with highly precise aggregation state measurements, the temperature constraints remain relatively loose (±8 °C). Conversely, in highly aggregated samples, when the aggregation degree is extremely high (temperature ranges from 1259 to 1303 °C), the temperature constraints are significantly reduced (±44 °C) [29]. During the calculation of nitrogen content, the storage time for the Tancheng diamonds is set at 1.8 Ga, the temperature range is estimated at 1083–1264 °C, and the uncertainty deviation value is better than ±44 °C.

4.6. Geochemical Contents

Coesite inclusions have a high SiO₂ content of 99.32–99.86% and a low Al₂O₃ content of 0.01 wt%, MnO (less than 0.01 to 0.06%), K₂O (0.02–0.03%), CaO (0.01–0.02%), FeO (0.01–0.10%), Na₂O (0.03–0.04%), MgO (0.01–0.02%), and total contents (99.53–100.01%). Garnet inclusions have a high Al₂O₃ content of 22.06–22.47%, FeO content of 20.6–20.68%, and MgO content of 9.99–10.08%. Other representative compositions are detailed in

Table 4. Electron probe microanalysis results of inclusions within diamond from the Tancheng deposit, China (wt%).

Sample	Inclusion	SiO2	TiO2	Al2O3	FeO	MnO	MgO	CaO	Na2O	K2O	Cr2O3	NiO	Total
SDTC-013	coesite	99.36	<0.01	<0.01	0.08	0.06	0.01	0.01	0.04	0.02	<0.01	<0.01	99.58
SDTC-013	coesite	99.32	<0.01	<0.01	0.10	0.03	0.02	0.01	0.03	0.03	<0.01	<0.01	99.53
SDTC-013	omphacite	55.11	0.46	8.68	7.88	0.05	8.27	13.44	4.65	0.71	0.12	0.03	99.40
SDTC-013	omphacite	55.18	0.58	8.53	7.85	<0.01	8.74	13.83	4.23	0.48	0.07	0.11	99.59
SDTC-013	omphacite	55.39	0.62	8.70	7.93	0.02	8.86	13.90	4.20	0.40	0.06	0.07	100.14
SDTC-013	omphacite	55.01	0.55	8.45	8.09	0.04	8.71	13.87	4.23	0.39	0.04	0.04	99.41
SDTC-022	garnet	40.30	0.28	22.47	20.65	0.43	10.08	6.36	0.15	0.05	0.03	0.06	100.85
SDTC-022	garnet	40.40	0.36	22.06	20.68	0.51	9.99	6.30	0.11	0.04	0.04	0.02	100.50
SDTC-022	omphacite	54.99	0.51	8.76	8.41	0.08	8.82	13.98	4.19	0.37	0.04	0.04	100.17
SDTC-022	omphacite	54.61	0.52	8.83	8.51	0.13	9.10	13.93	4.32	0.33	0.00	0.04	100.32
SDTC-022	omphacite	54.69	0.58	8.75	8.51	0.07	8.98	13.77	4.36	0.34	0.04	0.06	100.13
SDTC-022	coesite	99.86	0.01	<0.01	0.01	<0.01	0.02	0.02	0.04	0.02	0.03	<0.01	100.01

.

5. Discussion

5.1. Formation Pressure of Diamond

During the formation of diamonds, they can entrap substances from their surrounding environment, which are preserved as inclusions within the diamond. These inclusions contain crucial information about the diamond’s formation environment [30]. Izraeli et al. [6] developed a mineral inclusion Raman elastic thermo-pressure scale using olivine inclusions in diamonds, which allows for the estimation of the internal pressure of the olivine inclusions. Similarly, Nestola et al. [31] and Howell et al. [32] utilized olivine inclusions in diamonds to obtain pressure information. Qiu et al. [33] and Zhang et al. [20] used micro-Raman spectroscopy to infer the temperature of the mantle during the formation of alluvial diamonds from the Hunan Yuanshui deposit and kimberlite-type diamonds from the Mengyin deposit.

The relationship between shifts in Raman spectra and internal stresses and source pressures of inclusions in diamonds can be used to calculate the formation pressure of a diamond. Coesite is an ultrahigh-pressure phase of quartz, and the transition from quartz to coesite occurs at 3 to 4 GPa [34]. The equilibrium temperature of diamond-bearing eclogite is lower than the formation temperature of eclogite-type diamond inclusions, while the formation temperature of eclogite-type diamond inclusions is higher than that of olivine-type diamond inclusions. It is speculated that the eclogite-type diamonds formed in deeper parts of the mantle [35].

Sobolev et al. [7] studied the Raman peak shift data of the coesite inclusions in natural samples from Venezuela, finding a relationship between the Raman shift and pressure of 2.9 (±0.1) cm⁻¹ per GPa. The confining pressure of the inclusion was 3.62 (±0.18) GPa, and the initial pressure of the diamond was 5.5 GPa. In this study, we employed Sobolev’s calculation method to estimate the internal pressure range of the coesite inclusion in the Tancheng diamonds, which is Δν = 7.0–10.0 cm⁻¹. This estimation yields an internal pressure range of 2.41 to 3.45 GPa for the coesite inclusions and an initial pressure range of 5.2 to 5.5 GPa for the diamond mantle source.

Izraeli et al. [6] discovered that olivine inclusions trapped within diamonds can retain up to 0.13 to 0.65 GPa of residual stress. Using a single, independent olivine inclusion in a diamond as a standard, the characteristic spectrum peak at 826 cm⁻¹ was measured. Following the testing scale of Wang et al. [36] (where 1 GPa of pressure corresponds to a 3.09 cm⁻¹ shift), a Raman pressure scale was constructed based on the shift of the olivine’s 826 cm⁻¹ peak within the diamond [20]. This scale allows for the determination of the internal pressure of the olivine inclusion. The formation pressure of the diamond is then calculated using the following formula: P₀ = (3.259 × 10⁻⁴ P_i + 3.285 × 10⁻³) T₀ + 0.9246 P_i + 0.319, where T₀ is the temperature of the diamond’s source region, P₀ is the pressure of the diamond’s source region, and P_i is the internal pressure of the olivine inclusion in the diamond [37]. Forsterite inclusions within the studied Tancheng diamonds at 826 cm⁻¹ peak shift range approximately from 0.9 cm⁻¹ to 3.4 cm⁻¹ (

Table 5. Estimated formation pressure of forsterite inclusions within Tancheng diamonds, based on Raman spectroscopic analysis.

Sample Number	Raman Peak Positions (cm−1)	826 cm−1 Raman Drift	Pi	P0/GPa
			GPa	T0l
SDTC-007	822.7, 855.1, 916.5, 972.5	3.3	1.07	5.59
SDTC-019	825.0, 857.4, 967.8	1.0	0.32	4.74
SDTC-047	825.1, 857.2, 920.9, 966.1	0.9	0.29	4.25
SDTC-063	824.1, 856.3	1.9	0.61	5.00
SDTC-069	822.6, 853.7, 916.7, 957.4	3.4	1.10	5.86
SDTC-074	823.6, 853.9, 877.9, 918.3	2.4	0.78	5.21
SDTC-085	824.4, 856.4, 962.2	1.6	0.52	4.87

). The mantle storage temperature range of Tancheng diamonds in the mantle estimated through infrared spectra is from 1083 to 1264 °C. Among them, the mantle storage temperature of diamonds with olivine inclusions are 1216 °C, 1083 °C, and 1242 °C, with an average temperature of 1180 °C. Therefore, the growth temperature T₀ for the Tancheng diamonds is estimated at the mantle storage temperature with the infrared spectra of the Tancheng diamonds. Consequently, the pressure of entrapment of olivine inclusions within the Tancheng diamonds is estimated at 4.3–5.9 GPa, and the results are very close to the pressure range of inclusions hosted in the kimberlite-type diamonds from the North China Craton obtained in previous studies [37,38]. Angel et al. [39] proposed that the calculation of the formation pressure of diamonds using olivine inclusions is subject to limitations in data consistency due to the distinct characteristics of different equation of state (EoS) models for mantle olivine and diamond inclusions. The uncertainty in the entrapment pressure of diamond inclusions is generally 1 GPa. It means that the pressure/depth of entrapment of inclusions is not equal to the source pressure/depth of diamonds implying that some diamonds in Shandong Province probably form at depths up to 7 Ga. This inference holds significant implications for our study of the formation depth of diamonds in the North China Craton and is consistent with our recent discovery of some Type IIa diamonds in Changmazhuang and Xiyu belts [40], suggesting that ultra-deep diamonds may exist in Mengyin deposit.

5.2. Formation Environments

Based on the temperature and pressure derived from the inclusions in the diamonds, we infer that the Tancheng alluvial diamonds formed at a temperature of 1083–1264 °C and pressure of 4.3–5.9 GPa. As diamonds can be persisted in a metastable state for hundreds of millions or even billions of years, the mineral inclusions enclosed within diamonds maintain their original chemical equilibrium state, being unaffected by subsequent changes in the host diamond’s environmental conditions (although potential self-rebalancing of inclusions is not excluded), and are highly resistant to reaction with the surrounding magmas or subsequent alteration [41].

The crystallographic characteristics of Tancheng diamonds show significant diversity, reflecting variations in their formation environments. Studies indicate that cubic, octahedral, and twinned morphologies represent the original crystal forms of diamonds under natural conditions [42], while rhombohedral dodecahedral and octahedral–rhombohedral dodecahedral transitional forms indicate significant fluid etching stages [43]. The scarcity of cubic and octahedral diamond morphologies may result from either direct crystallization in the C-O-H system within the lithospheric upper mantle or processes involving a carbonate-C-O-H system replacement and carbonate–silicate metasomatism [44].

Fedortchouk et al. [45] proposed that diamond surface morphologies primarily develop during two stages: pre-kimberlite mantle storage and kimberlite magma etching. He identified four main types of pre-kimberlite absorption features: (1) deep pits ± knobs and plates; (2) octahedra with {111} face features; (3) ribbed dodecahedra; and (4) serrated/triangular laminae. The crystal morphology of Tancheng diamonds clearly exhibits features of melt etching from both mantle storage and kimberlite magma ascent phases [46]. The rounding of octahedral and cubic diamond edges and their transformation into tetrahedral (THH) shapes result from reactions with primary kimberlite magma, showing characteristic melt etching during magma ascent [47]. During kimberlite magma ascent, diamonds reacted with the magma, which could completely or partially cover the surface features formed during the mantle storage period [48]. Experiments confirm that the metasomatic action by C-O-H fluids does not damage diamonds; rather, these surface morphological features result from diamond dissolution caused by carbonate–silicate melt-driven metasomatism in the mantle environments [49].

The complex superimposed extinction patterns of abnormal birefringence reflect diamonds having undergone multiple stages of stress effects with different forms during and after the growth process [18]. Straight extinction patterns and the patterns with no fixed shape may result from combined effects of internal diamond stress and slice thickness [50]. Petaloid and radial extinction patterns relate to diamond growth and inclusions [51]. Fine grid extinction patterns and complex superimposed extinction patterns associate with slip lines caused by later plastic deformation [20]. The different fluorescence types observed in Diamond View further indicate varying growth environments of Tancheng diamonds. The appearance of different fluorescence colors represents different impurity elements or lattice defects in the diamonds. The mantle composition rich in nitrogen, which is related to diamond growth, results in a distribution of blue fluorescence throughout the diamond; green fluorescence indicates that the impurities captured in the diamond are mainly H3 centers, indicating a mantle fluid component dominated by hydrogen.

CL studies of diamonds from various mining regions around the world, including Argyle (Australia), Finsch (South Africa), Yakutian (Russia), and Point Lake (Canada), reveal that many single-crystal diamonds have central, transitional, and edge zones [52,53,54]. The internal growth formation process of Tancheng diamonds can be categorized into three types: (1) Diamonds with simple growth zones: These diamonds exhibit uniform CL characteristics, showing either no growth zones or relatively straight and uniform zones, indicating stable fluid composition during growth [55]. The crystals display a highly regular octahedral morphology with minimal surface etching and no evidence of plastic deformation, with the inclusion mineral composition remaining unchanged. (2) Diamonds with multiple growth stages and complex growth zones: These diamonds display varying shades of blue luminescent areas and intermingled blue and yellow-green luminescent areas, indicating complex growth zones developing at multiple centers and presenting a dense layered distribution. The boundaries of the growth zones are clear and relatively straight, suggesting a stable growth process without plastic deformation or etching. The discontinuous growth is attributed to changes in fluid composition and growth environment [20,51]. (3) Diamonds with special growth structures: These diamonds, characterized by “onyx-like” or circular concentric zones, exhibit dissolution between growth zones during formation [56], reflecting the presence and participation of melt/fluid during the growth process. These special growth structures are formed under specific growth conditions, developing from multiple centers. Early multiple centers may form more complex morphologies, which subsequent crystallization processes build upon [57,58]. The “onyx-like” growth structure is also influenced by high-temperature deformation of the diamond [49,50]. This structure results from a combination of factors, including early formation of multiple centers, local uneven etching, mixed growth mechanisms, and changing crystallization conditions [59].

5.3. Genesis of the Tancheng Diamonds

The abundances and types of mineral inclusions in peridotite-type diamonds can represent the composition of their parent rock at the time of formation [60]. In the case of Tancheng diamonds, the parent rock is predominantly lithospheric mantle, which is mainly composed of olivine rock.

The Tancheng diamonds frequently contain multiple magnesium olivine inclusions, often accompanied by graphite crystals attached to the olivine [61]. The presence of graphite suggests formation after forsterite, possibly during the diamond crystallization process when the diamond/graphite phase transition line was present, indicating a significant change in the diamond growth environment [12]. CL and DV image analysis reveals that these inclusions are located near the center of the diamond, with variations in the fluorescence color and brightness of the growth zones, indicating their formation during diamond crystallization.

Eclogitic (E-type) inclusions in Tancheng diamonds, including omphacite, coesite, and garnet, are primarily found in only two diamonds. The garnet inclusions in Tancheng diamonds typically have a low Cr₂O₃ content less than 1 wt%, contrasting with mantle olivine-type diamonds, which typically have a high Cr₂O₃ content greater than 1 wt% [62]. The calculated molecular formula (Fe_1.295Mg_1.121Ca_0.509Mn_0.030Ti_0.018)₃ (Al_1.967)₂[SiO₄]₃ classifies these garnets as G5 group magnesium–aluminum–iron garnet, which belongs to Group A eclogite (with Na₂O > 0.07 wt%), reflecting the characteristics of formation under high-pressure conditions [10].

Coesite inclusions in Tancheng diamonds exhibit diverse morphologies, ranging from elongated columnar crystals with relatively straight edges and complete crystal shapes to bell-shaped, spherical, and irregular crystals with multiple curved surfaces, indicating modification by etching. As a high-pressure polymorph of SiO₂, coesite can exist stably within the diamond stability field [20]. Schulze et al. [63] studied the δ¹⁸O values of coesite inclusions in diamonds from Venezuela’s Gauniamo area, obtaining a range of 10.2 to 16.9 wt%. This wide range of values suggests that the eclogite-type diamond protolith has a subducted origin. The presence of coesite inclusions and other high-pressure metamorphic minerals in the Hunan diamond placer confirms that continental crustal material cycled to the deep lithospheric mantle may have played a significant role in the formation of Hunan diamonds [32]. Studies have shown that coesite and other ultrahigh-pressure minerals can be subducted with low-density crustal rocks to the deep upper mantle and then rapidly return to the surface [64,65]. This implies that more crustal material cycles to the mantle, transforming the lithospheric mantle [66]. Therefore, the source of Tancheng eclogite-type inclusion diamonds may be related to the circulation or subduction of oceanic crust.

The mantle storage temperature of diamonds from the three kimberlite belts (Changmazhuang, Xiyu, and Poli) in the Mengyin deposit, calculated using the same method, range from 1118 to 1251 °C for Changmazhuang diamonds, 1091 to 1167 °C for Xiyu diamonds, and 1132 to 1172 °C for Poli diamoWe found that there are only 63 references in total, but there are mentions of 64, 65, and 66 here. Please modify the reference numbers.nds [51]. These temperature ranges are closely aligned with the formation temperature range of 1083–1264 °C for Tancheng diamonds. Chi et al. [38] calculated the source pressure of olivine inclusions in Mengyin diamonds using 10 Raman shift data, resulting in a range of 4.70–5.46 GPa. The result is consistent with the source pressure range (4.3–5.9 GPa) calculated for Tancheng diamonds.

Overall, Tancheng alluvial diamonds share many similarities or even identical characteristics with the Mengyin kimberlite-type diamonds in terms of crystal morphology, surface texture, and internal growth features, suggesting that some Tancheng diamonds may originate from the Mengyin kimberlite clusters. However, there are still significant differences in terms of diamond grain size, lattice perfection, and integrity between the two regions [11,12]. Some Tancheng diamonds exhibit characteristics suggesting proximity to their source, which cannot be ruled out as evidence of the presence of a different ore-bearing source rock than the Mengyin kimberlite primary deposit. Additionally, the area has multiple diamond-bearing reservoir horizons, with diamond occurrences in the Precambrian Liuguan Formation conglomerates, accompanied by indicators such as garnet, chromium spinel, and chromium diopside [14,16]. This indicates the presence of a Precambrian, different ore-bearing source rock from the Mengyin kimberlite primary deposit. Research on the material source of Tancheng diamonds has not yet reached a unified understanding, and further in-depth study is required to resolve these issues.

6. Conclusions

This study further confirms the diverse and complex growth process of Tancheng alluvial diamonds, marked by post-formation etching and stress effects. These diamonds exhibit distinct features indicative of both mantle storage and magmatic resorption stages, indicating the influence of carbonate–silicate–fluid interaction in the lithospheric mantle.

Based on inclusions of Raman spectra and FTIR analysis, we estimate that these diamonds formed under a temperature of 1083–1264 °C and pressure of 4.3–5.9 GPa.

Most inclusions in Tancheng diamonds are olivine and graphite, with a minority of coesite originating from eclogite. The eclogite-type inclusions indicate a process of ancient oceanic crust subduction into deep mantle. The inclusions from Tancheng can provide crucial data on the temperature, pressure, and chemical composition of the mantle, facilitating a deeper understanding of the geological process in deep earth. Diamonds with abundant coesite and other ultrahigh-pressure inclusions may serve as a natural probe for studying subduction process in the North China Craton

The kimberlite-type diamonds in Mengyin deposit and the Tancheng alluvial diamonds share many similarities, even identical characteristics, suggesting that some Tancheng alluvial diamonds may originate from the Mengyin kimberlite deposit. However, considering the differences in diamond characteristics and the main provenance of diamond-bearing layers between the two regions, the existence of a different diamond-bearing source, distinct from the Mengyin kimberlite primary deposit, cannot be ruled out.