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![(A) Location of various alluvial zircon deposits in Cambodia and Vietnam within SouthEast Asia [12,24]. (B) Distribution of Neogene and Quaternary basalts with their K-Ar and Ar-Ar ages (after [4,6]). Reproduced with permission from all authors. Green circles indicate the sampling localities.](publication/330819558/figure/fig1/AS:721722542149632@1549083471809/A-Location-of-various-alluvial-zircon-deposits-in-Cambodia-and-Vietnam-within-SouthEast.png)
(A) Location of various alluvial zircon deposits in Cambodia and Vietnam within SouthEast Asia [12,24]. (B) Distribution of Neogene and Quaternary basalts with their K-Ar and Ar-Ar ages (after [4,6]). Reproduced with permission from all authors. Green circles indicate the sampling localities.
Source publication
Gem-quality zircon megacrysts occur in placer deposits in the Central Highlands, Vietnam, and have euhedral to anhedral crystal shapes with dimensions of ~3 cm in length. These zircons have primary inclusions of calcite, olivine, and corundum. Secondary quartz, baddeleyite, hematite, and CO2 fluid inclusions were found in close proximity to cracks...
Contexts in source publication
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... Asia was formed by an amalgamation of several crustal blocks, including South China, Indochina, Siumasu, Inthanon, the West Burma block, and the Trans Vietnam Orogenic Belt (TVOB) ( Figure 1A) [24]. The TVOB was first proposed by Osanai et al. [24] as a zone of Permo- Triassic metamorphic rocks in Vietnam, which were formed by continent-continent collision between South China and Indochina blocks. ...
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... northern extension of these shear zones reaches the Yuan Nan Province through Ailaoshan in China. The Central Highlands lies entirely within the Indochina block and are located in the southern part of the TVOB, partly extending into eastern Vietnam and Laos (e.g., Hutchison [25]) ( Figure 1A). ...
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... alkaline and subalkaline basalts are exposed over a vast region of Thailand, Laos, and Vietnam [26]. In southern and central Vietnam, this province stretches across an area of approximately 23,000 km² with a thickness up to several hundred meters [3] ( Figure 1A). The basalt plateau is accompanied by pull-apart structures composed of short extensional rifts bounded by strike-slip faults [27]. ...
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... Pleiku (4.30-0.80 Ma), Xuan Loc (0.83-0.44 Ma), and the Re Island centers (0.80-0 Ma) [27] (Figure 1B). Hoang et al. [28] proposed that tholeiitic basalts are the most common basalt type in the region and build up much more volumetric mass in comparison to the alkali basalts and the rarely occurring nephelinites which erupted from small volcanoes. ...
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... of these Cenozoic (Neogene-Quaternary) basalts formed placer deposits that represent a major source of gem-quality corundum, zircon, olivine, garnet, pyroxene, and plagioclase [4,14,26]. In South-Central Vietnam, gem-quality zircons have been found in alluvial deposits from six provinces, including Kontum, Gia Lai, Dak Lak, Dak Nong, Lam Dong, and Binh Thuan [12] ( Figure 1A,B). [12,24]. ...
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... Striation most prominent PL emission of Dy 3+ ( 4 F9/2 → 6 H13/2) was used as a plotted spectral parameter (see Figure 6A; and Figure 1 in Lenz et al. [29]). Note that the Dy sub-level bands of Vietnamese and Cambodian (Rata) zircon samples were characterized by exceptionally narrow band-widths (sublevel at 581 nm had a band-width of 11.0 cm −1 ; see Figure 6A), which indicated a high degree of crystallinity and the absence of structural radiation-damage that might be caused by decay of radioactive U and Th. ...
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... Pleiku (4.3-0.8 Ma), and the younger basalt eruptions exposed at Xuan Loc (0.83-0.44 Ma)( Figure 1B) [4]. Zircon megacrysts from North-East Cambodia not only occur geographically close to the megacrysts found in the present study, but also share a similar U-Pb age connected to the younger period of volcanic activity in the region (0.98 ± 0.04 Ma, Cong et al. [10]; 0.88 ± 0.22 Ma to 1.56 ± 0.21 Ma, Piilonen et al. [22]). ...
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Citations
... Dry Cottonwood Creek zircon inclusion data from this study are compared to zircon megacrysts from alkali basalt (Sinh et al., 2019) and the Stillwater anorthosite ; these data are chondrite-normalized according to Anders & Grevesse (1989) and presented as a range (gray field) and median (dashed lines). Sinh et al. (2019) analyzed La-Lu from zircon megacrysts in sapphire-bearing alkali basalt from Vietnam; for simplicity, data from two younger zircons (∼1 Ma) were excluded from the median and range. ...
... Dry Cottonwood Creek zircon inclusion data from this study are compared to zircon megacrysts from alkali basalt (Sinh et al., 2019) and the Stillwater anorthosite ; these data are chondrite-normalized according to Anders & Grevesse (1989) and presented as a range (gray field) and median (dashed lines). Sinh et al. (2019) analyzed La-Lu from zircon megacrysts in sapphire-bearing alkali basalt from Vietnam; for simplicity, data from two younger zircons (∼1 Ma) were excluded from the median and range. The anorthosite reference spectrum was selected from Wall et al. (2018; sample ST11-08, supplementary material); sample ST11-08 are zircons from the groundmass of Zone I anorthosite in the Beartooth Mountains, MT, USA, which occurs as part of the Stillwater Complex in the middle-banded series. ...
... The darker gray reference spectrum in Fig. 8 shows that zircons from the Stillwater anorthosite in Montana are similar to zircon inclusions at Dry Cottonwood Creek; while this reference represents only one locality of anorthosites, the presence of negative Eu-anomalies is seen among almost all REE datasets from anorthositic zircon, though the degree of the anomaly and overall REE concentrations vary. Middle and rim domains from the Dry Cottonwood Creek zircon inclusion have a small to neutral Euanomaly, like the reference signature seen in zircon megacrysts from sapphire-bearing alkali basalt at Dat Long, Vietnam (Sinh et al., 2019). Homogeneous δ 18 O values from Dry Cottonwood Creek zircon inclusions (5.9 ± 0.6 2 s; Fig. 7) are consistent with both mantle melts (1200 • C mantle-like zircon = 4.7 to 5.9 ; Valley et al., 1998) or anorthosite (zircon = 5.5 to 7.3 at 1200 • C, calculated from Taylor, 1969 andTurnier et al., 2020). ...
Montana hosts the largest sapphire deposits in the US, but the genesis of and connection among the various secondary and primary sapphire occurrences remains cryptic. In situ SIMS measurements of oxygen isotopes in sapphires and zircon inclusions in sapphires provide an opportunity to study the isotope and trace element geochemistry in order to understand sapphire-forming protoliths (i.e., crustal setting and alteration). Sapphire from Montana was transported as xenocrysts in carrier (host) magmas that resorbed sapphire exteriors during transport. The timing and nature of sapphire genesis is elucidated by SIMS measurements of trace elements and U-Pb from discrete zones in zircon inclusions with rims that are interpreted to be syngenetic with host sapphire.
Montana sapphires exhibit a large range of δ18O values, from -3 to +12‰ VSMOW. However, all but two anomalous crystals fall in the range of 0 to 8‰. There is significant crystal-to-crystal variability yet averages at most deposits are consistent with high-temperature equilibration with the mantle (δ18O(Crn) = 4.4 to 5.7‰), with the exception of the commercial sapphire deposits at Rock Creek that average 2.7‰. Ruby analyses are limited, but typically have lower δ18O values compared to sapphires from the same detrital localities. Homogeneity within individual crystals (avg. 2s = ±0.2‰) indicates the absence of isotopically distinct fluid or melt during crystallization. But intercrystalline δ18O ranges by up to 3‰ at a single locality, suggesting sapphire variability at a deposit reflects heterogeneity in the original protolith. Oxygen isotope fractionations between zircon rims and surrounding sapphire suggest comagmatic zircon inclusions and corundum equilibrated at high temperature. No correlation is seen for the degree of radiation damage and alteration of δ18O(Zrc) when zircon inclusions are surrounded and armored by sapphire.
U-Pb ages and trace elements were measured in a small subset of syngenetic zircon inclusions in Dry Cottonwood Creek sapphires, revealing a Proterozoic (1778 ± 9 Ma) age for the protolith of sapphires at this locality and a likely polygenetic history. Previous work has suggested formation of these sapphires through partial melting of anorthosites and several anorthosites occur locally and match the age of zircon inclusion cores—the Boehls Butte anorthosite (~180 km NW of Rock Creek) and the Bitterroot anorthosite (~55 km W of Rock Creek) could correlate with Al-rich protoliths at depth. Proterozoic U-Pb ages of zircon from the Boehls Butte anorthosite (1787 ± 2 Ma) match well with the age of zircon inclusion cores in Dry Cottonwood Creek sapphires and suggest genesis in these or similar protoliths. Zircon rims with Tera-Wasserburg lower intercept ages of 110 ± 9 Ma are consistent with previous observations of a xenocrystic relationship to the ~50 Ma Eocene volcanic rocks. Corundum that formed over 50 Ma prior to being scavenged by Eocene magmas likely originated by the anatexis of Precambrian anorthosites and possibly other aluminum-rich rocks at depth.
... Zircon là một trong những khoáng vật cổ xưa nhất của trái đất, đồng thời cũng là một loại đá quý quan trọng và có giá trị cao, đã thu hút sự quan tâm của các nhà khoa học và nghiên cứu trên thế giới. Đá quý zircon được phát hiện tại nhiều quốc gia như Thái lan, Úc, Mexico, Lào, Campuchia và Việt Nam [1][2][3][4] . Nguồn gốc của zircon liên quan đến basalt đã thu hút nhiều nỗ lực nghiên cứu, các phương pháp như tổ hợp nguyên đất hiếm (REE) và Ce đã được sử dụng để xác định nguồn gốc thành tạo 5 . ...
Bài báo trình bày việc nghiên cứu đá quý zircon tìm thấy ở Đông Nam Bộ, Việt Nam, liên quan basalt kiềm Kainozoi và tích tụ tại các sườn đồi hoặc lòng suối dưới dạng sa khoáng. Zircon trong khu vực này chủ yếu có màu nâu, cam, xám và không màu, trong suốt, và mang ánh kim cương. Ngoài ra, chúng hầu hết có hiệu ứng đổi màu dưới ánh sáng khác nhau và thường có hình dạng của các hình trụ 4 mặt chóp, đặc trưng cho kiểu tinh thể kết tinh hệ bốn phương. Các đặc điểm ngọc học tiêu chuẩn như chiết suất, tỷ trọng, phổ hấp thu, đặc điểm quang học, và phát quang dưới đèn cực tím (sóng dài và sóng ngắn) đều chứng minh rằng zircon nghiên cứu thuộc nhóm khoáng vật zircon. Đáng chú ý, chiết suất không đo được bằng khúc xạ kế và tỷ trọng cao (từ 4,66 đến 4,71) chỉ ra đây là loại zircon cao. Phân tích nhiễu xạ tia X xác định cấu trúc và thành phần cho kết quả mẫu nghiên cứu kết tinh hệ bốn phương, công thức hóa học ZrSiO 4. Phổ Raman đặc trưng với các đỉnh 226, 352 438, 972 và 1004 cm −1 , và phổ FTIR có các đỉnh 448, 607, 898 cm −1. Đặc điểm của hai phổ này chứng tỏ zircon bị phóng xạ hóa. Sự so sánh giữa hàm lượng HfO 2 và nhiệt kế Ti cho thấy zircon Đông Nam Bộ có nguồn gốc từ Syenit nóng chảy, với nhiệt độ kết tinh từ 829,78 đến 869,04 • C. Hệ tầng Xuân Lộc có diện phân bố khoảng 1.000 km 2 với độ sâu thay đổi từ 50 đến 250 m, vỏ phong hóa dày từ 1 đến hơn 10 m, là nguồn chứa tiềm năng zircon có thể khai thác sử dụng cho đá quý trang sức. Từ khoá: zircon, basalt, ĐNB, đá quý GIỚI THIỆU Zircon là một trong những khoáng vật cổ xưa nhất của trái đất, đồng thời cũng là một loại đá quý quan trọng và có giá trị cao, đã thu hút sự quan tâm của các nhà khoa học và nghiên cứu trên thế giới. Đá quý zircon được phát hiện tại nhiều quốc gia như Thái lan, Úc, Mexico, Lào, Campuchia và Việt Nam 1-4. Nguồn gốc của zircon liên quan đến basalt đã thu hút nhiều nỗ lực nghiên cứu, các phương pháp như tổ hợp nguyên đất hiếm (REE) và Ce đã được sử dụng để xác định nguồn gốc thành tạo 5. Nghiên cứu gần đây của C. Siégel và cộng sự (2018) đã sử dụng tổ hợp các phương pháp nhiệt kế và hàm lượng đồng vị oxygen 18 để xác định nguồn gốc và điều kiện thành tạo của zircon 6. Tuy nhiên, những phương pháp này cũng gặp hạn chế, đặc biệt là đối với các kết quả thấp bất thường về nhiệt độ. Ở Việt Nam, đá quý zircon cũng được tìm thấy ở nhiều nơi như Tây Nguyên, Bình Thuận (Đá Bàn) 7-9. Zircon ở những vùng này đều liên quan đến basalt, xuất hiện dưới dạng xenocryst của basalt, được phong hóa và tập trung tại sườn đồi, các lòng suối. Bài báo này mô tả đặc điểm khoáng vật và chất lượng đá quý của zircon liên quan đến basalt Đông Nam Bộ (ĐNB). Sử dụng các phương pháp định lượng để xác định nguồn gốc và điều kiện thành tạo ban đầu của zircon, đồng thời đưa ra đánh giá về những hạn chế của các phương pháp này và đề xuất hướng phát triển nghiên cứu đặc điểm zircon đá quý trong tương lai. ĐẶC ĐIỂM ĐỊA CHẤT TẠI VÙNG KHẢO SÁT Ở Việt Nam, có 2 sự kiện kiến tạo sơ cấp quan trọng góp phần định hình địa chất của vùng khảo sát. Sự kiện thứ nhất diễn ra trong giai đoạn kỷ Trias (250-240 triệu năm) với quá trình tạo núi Indosinia. Sự kiện này là kết quả sự va chạm lục địa của hai khối Đông Dương và khối Hoa Nam, làm cho phía tây bắc của khối Đông Dương nổi cao về các khối lục địa tiếp giáp có nguồn gốc từ Gondwana 10,11. Sự kiện kiến tạo thứ hai (sự kiện Hymalya) liên quan đến va chạm của hai địa mảng Ấn Độ và Âu−Á xảy ra trong Pa-leogen (50−55 triệu năm) 12,13. Sự kiện này đóng vai trò quan trọng trong quá trình kiến tạo khu vực, bởi nó gây ra sự phun trào trên diện rộng của các basalt trên một khu vực rộng lớn của các lưu vực ven biển phía đông và đông nam châu Á, cũng như tây Thái Bình Dương 14. Các đá basalt này có cấu trúc tách rời nhau và được giới hạn bởi các đứt gãy trượt bằng và đứt gãy thuận, đã hình thành hầu hết các trung tâm basalt ở đông Đông Dương trong đó có vùng ĐNB, Việt Nam 15. Trích dẫn bài báo này: Năng L N, Trí N T. Đặc điểm đá quý Zircon liên quan Basalt vùng Đông Nam
... Megacrystic zircons (which are typically centimeter-sized) are found within intraplate alkali basalt deposits around the world; their derivatives are formed as a result of secondary processes, especially those seen in the countries along the western Pacific continental margins [6][7][8][9]. These basalts commonly contain abundant mantle xenoliths, along with a variety of high-pressure megacrysts such as corundum, pyroxene, garnet and spinel [10][11][12][13], offering scientists an opportunity to elucidate the composition and evolutionary history of the deep lithosphere. Some species of megacrysts (e.g., corundum and zircon) have also been commercially exploited for several centuries to supply the global market with gemstones [14]. ...
... The partial melting of metasomatized peridotites would produce melts with carbonatitic compositions at a low degree, and carbonated silicate melts with further melting. Some recent studies have invoked such a model (partial melting of a carbonatitic-metasomatized mantle to a variable degree) to illustrate the generation of 'carbonatitic signature' zircon megacrysts, which are associated with alkali basalts found in Vietnam and Cambodia [10,14]. However, it appears difficult to simply reconcile the formation of zircon megacrysts from Huadian with crystallization from melts produced by the partial melting of a carbonatiticinfluenced mantle to a variable degree. ...
Megacrystic zircons have recently been found in alluvial deposits in the Huadian area of northeastern China. However, studies have rarely been conducted on these zircons. In this article, we present systematic in situ trace element, U-Pb age and Hf isotope data regarding these Huadian zircons, with the aim of investigating their source characteristics and provenance. The studied zircons, with a diameter of 0.7–1.2 cm, are dominantly irregular in shape and have a rounded termination, with a color ranging from near-colorless to reddish brown to maroon. The zircons show oscillatory zoning in CL images, with a Th/U value of 0.18–1.27, which is consistent with the typical features of magmatic zircons. The positive εHf(t) value of Huadian zircons (4.8–9.2) further indicates the presence of precipitation from the mantle-derived melt, with limited contamination of the crustal components. The obtained weighted mean 206Pb/238U age for these zircons is 17.9 ± 0.12 Ma, which is slightly older than the eruptional ages of the associated alkali basalts (from 17.6 ± 1.09 Ma to 17.8 ± 0.69 Ma), implying a short residence time in the mantle before entrainment. Huadian zircons incorporate a wide range of trace elements, including ΣREE (117–2790 ppm), Hf (4902–11856 ppm) and Y (145–3645 ppm) contents, generating mixed-source protolith assignments. As is suggested by the moderate variations seen in the Hf isotopes, we propose that the source melts of Huadian zircons are complex in nature, which is likely the result of the chemical heterogeneity of the upper mantle.
... Zircon inclusions and their host sapphires from basaltic terrains, mainly in Southeast Asia, were reported to have crystallized in the lower crust (Guo et al., 1996a;Izokh et al., 2010;Khamloet et al., 2014;Sutherland et al., 2002;2015b;Vu et al., 2020) and in the lithospheric mantle (Vuong et al., 2019). In this study, Zr/Hf ratios with an average of 41 are close to those of the continental crust zircon (36 -45) than the mantle zircon (60 -68). ...
... K-Ar age dating was also performed at the Far East Geological Institute, Far East Branch, RAS, Vladivostok, following the procedure given in Ignat'ev et al., (2010) [43]. Some of the K-Ar age samples were reanalyzed using Ar-Ar and zircon U-Pb age dating [44] to verify the accuracy of the K-Ar analysis. The accuracy of the K-Ar method is (1σ) ± 0.1-0.2 for ages < 1 Ma, and about (1 σ) ± 0.3-0.4 for ages > 5-7.5 Ma. ...
The spreading of the East Vietnam Sea (EVS, also known as Bien Dong, or the South China Sea), leading to the occurrence of syn-spreading (33-16 Ma) and post-spreading (< 16 to present) volcanism. Syn-spreading magma making up thick layers of tholeiitic basalt with a geochemical composition close to the refractory and depleted mid-ocean ridge basalt (MORB) is mainly distributed inside the EVS basin. The post-spreading magma is widely distributed inside the basin and extended to South and SE China, Hainan island, Southern Laos (Bolaven), Khorat Plateau (Thailand), and Vietnam, showing the typical intraplate geochemistry. Basaltic samples were collected at many places in Indochina countries, Vietnam’s coastal and continental shelf areas, to analyze for eruption age, petrographical, geochemical, and isotopic composition to understand the similarities and differences in the mantle sources between regions. The results reveal that basalts from some areas show geochemical features suggesting they were derived subsequently by spinel peridotite and garnet peridotite melting, forming high-Si, low-Mg, and low-Ti tholeiitic basalt to low-Si, high-Mg, and high-Ti alkaline basalt with the trace element enrichment increasing over time. Other basalts have geochemical and isotopic characteristics unchanged over a long period. The post-spreading basalt’s radiogenic Sr-Nd-Hf-Pb isotopic compositions show different regional basalts distribute in the various fields regardless of eruption age, suggesting that their mantle source feature is space-dependent. The post-EVS spreading basalts expose the regional heterogeneity, reflecting the mixture of at least three components, including a depleted mantle (DM) represented by the syn-EVS spreading source, similar to the DUPAL-bearing Indian MORB source; an enriched mantle type 1 (EM1), and type 2 (EM2). The DM may interact and acquire either EM1 or EM2 in the sub-continental lithospheric mantle; as a result, different eruption at different area acquires distinct isotopic signature, reflecting the heterogeneous nature of the subcontinental lithospheric mantle. The study proposes a suitable mantle dynamic model that explains the EVS spreading kinematics and induced volcanism following the India - Eurasian collision from the Eocene based on the research outcomes.
... The diffusion experiments were conducted using ∼3 ×3 ×3 mm cubes of oriented zircon, cut from two different stocks of natural crystals. The majority of the experiments used gem-quality zircon megacrysts originating from the central highlands of Vietnam (Huong et al., 2016;Sinh et al., 2019). These zircons (VZ zircons) contain sparse populations of apatite, hematite and ilmenite inclusions, which were easily avoided when the samples were sectioned. ...
Ti-in-zircon thermometry zircon diffusion diffusion anisotropy Ti-in-zircon thermometry has become a widely used tool to determine zircon crystallization temperatures, in part due to reports of extremely sluggish Ti diffusion perpendicular to the crystallographic c-axis in this mineral. We have conducted Ti-in-zircon diffusion experiments, focusing on diffusion parallel to the c-axis, at 1 atm pressure between 1100 and 1540 • C, with oxygen fugacities equivalent to air and the Ni-NiO buffer. There is no resolvable dependence of Ti diffusion in zircon upon silica or zirconia activity, or upon oxygen fugacity. The diffusion coefficient of Ti in zircon is found to be a weak function of its own concentration, spanning less than 0.5 log units across any profile induced below 1300 • C. Ti diffusion in zircon, parallel to the c-axis at 1 atm pressure, is well described using: log 10 D Ti = 1.34(±1.44) − 555425(±44820) J mol −1 2.303 RT (K) m 2 s −1 where R is the gas constant in J/(mol·K). In conjunction with diffusion coefficients for Ti in zircon perpendicular to the c-axis reported by Cherniak and Watson (2007), strong diffusion anisotropy for Ti in zircon is observed. Diffusion parallel to the c-axis is ∼4-5 orders of magnitude faster than diffusion perpendicular to the c-axis within the experimentally constrained temperature range shared between these two studies (1540-1350 • C). This difference increases if the data are extrapolated to lower temperatures and reaches ∼7.5-11 orders of magnitude between 950-600 • C, a typical range for zircon crystallization. Diffusion of Ti in natural zircons will predominantly occur parallel to the c-axis, and the Ti-in-zircon thermometer appears susceptible to diffusive modification under some crustal conditions. Temperatures calculated using this system should therefore be evaluated on a case-by-case basis, particularly when considering high-T, slowly cooled, reheated and/or small zircons.
... Zircon megacrysts occur for instance in late Cenozoic intraplate basalts, which are widespread in East and Southeast Asia (Fig. 1). These alkaline lavas contain various mantle xenoliths and megacrysts of olivine, pyroxene, garnet, and zircon (e.g., Cong et al., 2016;Piilonen et al., 2018;Sinh et al., 2019). In northeastern Cambodia, and specifically in its Ratanakiri province, such zircon megacrysts occur in basalts of Cenozoic age as xenocrystic fragments and crystals and are sought after as raw materials for gem production using heat treatment techniques (e.g., Zeug et al., 2018). ...
... Zircon megacrysts from the RVP are considered to have crystallized from carbonatite-like melts generated by low degrees of partial melting of metasomatized sub-continental lithospheric mantle (Piilonen et al., 2018). The geochemical signatures and primary inclusions of gem-quality zircons in the Central Highland of Vietnam also suggest a genesis from carbonatitedominant melts as a result of partial melting of a metasomatized lithospheric mantle source (Sinh et al., 2019). In the broader ZIP, zircon megacrysts from southeast New Zealand are interpreted to have formed by the reaction between low-degree melts derived from preexisting mantle metasomes and the depleted mantle lithosphere prior to eruption and transport to the surface (van der Meer et al., 2019). ...
... Although precise chronological constrains on the host basalt of zircon megacryst are unavailable, it has been suggested that zircons and host magma are coeval (Cong et al., 2016;Sinh et al., 2019). However, rounding and channels in the zircon megacryst indicate resorption and reaction with the host basaltic melts, which implies considerable disequilibrium between zircon megacrysts and the host alkaline basaltic magma. ...
Lithium in zircon has attracted considerable attention for identifying magma sources and as a diffusion chronometer, despite the exact diffusion mechanisms remaining controversial. Zircon megacrysts sourced from Quaternary intraplate basaltic volcanoes in the Ratanakiri Volcanic Province of northeastern Cambodia were analyzed here for Li isotopic and elemental concentration profiles to constrain diffusion behavior of Li in zircon megacrysts entrained in high temperature melts. This is complemented by high-spatial resolution U-Pb geochronology, O-Hf isotope compositions and trace element abundances to refine the long-term pre-eruptive history of these zircon megacrysts. Ratanakiri zircon megacrysts yield a Tera–Wasserburg regression U-Pb age of 0.98 ± 0.02 Ma (n = 615 spot analyses) that is indistinguishable from eruptions ages reported for the host basalts, although zircon was unstable in the enclosing magma as evidenced by marginal corrosion and abundant tubular channels within the crystals. Extreme Li-isotopic variations (within -30 - +96 % on the δ⁷LiLVEC scale) of the zircon megacrysts require kinetic Li fractionation due to diffusive mobilization, which is further supported by offsets in intracrystalline domain boundaries with different Li and Y abundances detected in scanning ion images. Complex abundance and isotopic ratio patterns for Li in zircon megacrysts point to multi-mode Li diffusion that is influenced by variable trace element inventories within individual crystals, which correlate with minor differences in the O-Hf isotopic compositions. The overall tendency of decreasing Li abundances along with increasing δ⁷Li values toward crystal rims indicates diffusion-driven equilibration between zircon and Li-depleted basaltic melt. Diffusion modeling of Li and δ⁷Li on domain-boundaries within crystals show pre-eruptive heating timescales of about 18 days, which is consistent with the magma transfer duration estimated from zircon megacryst settling rates calculations, implying the zircon megacrysts should be transported from their source region to the surface by ascending alkali basaltic magma within few weeks. Nearly one to two orders of magnitude longer Li diffusion modeling timescales were computed for crystal rim domains. This apparent faster diffusion of Li is attributed to H± loss into a volatile phase during magma eruption, providing an additional driving force for Li± diffusion, which might become more pronounced at the crystal boundary where zircon is exposed to strong chemical gradients in volatile elements due to decompression and concomitant degassing. Lithium isotope compositions for the innermost domains of zircon megacrysts are relatively heavy with δ⁷Li values of +12 - +22%. These values are considered to be less influenced by Li-loss after entrainment by the host basalts than rim compositions. These values in combination with O (δ¹⁸O= +3.5 - +5.3%) and Hf (εHf(0.98 Ma)= +6.5 - +7.9) isotopic compositions as well as inclusions and internal structures suggest that zircon megacrysts were originally derived from partial melting of a metasomatized lithospheric mantle source altered by fluid released from dissolved carbonates during subduction. Lithium isotopes in Ratanakiri zircon megacrysts provide novel insights into using Li isotopes as a tool to constrain the durations of magma transfer from the mantle to the surface, although Li diffusion mechanisms in zircon need to be further scrutinized.
... Recently, Vu et al. [9] reported spinel and other unidentified oxide inclusions in sapphires from many deposits in Southern Vietnam which more details and further investigation are reported herein this manuscript. Besides, the crystallization of sapphire and related zircon from Southern Vietnam may occur in the lithospheric mantle which is related to carbonatite-dominant melts as a result of partial melting of a metasomatized lithospheric mantle source, at over 900 • C [10]. ...
... On the other hand, alkali basalt, olivine tholeiite, and basanite appear to have erupted along the conjugate strike-slip faults [18]. It should be noted that sapphire and zircon occurrences mainly discovered in Quaternary and Upper-Pleistocene alluvial deposits derived from the alkali basalts [8][9][10]21]. They would originate in the deep-seated formations before being transported as megacryst onto the Earth's surface by alkali basaltic magmas. ...
... [18]. It should be noted that sapphire and zircon occurrences mainly discovered in Quaternary and Upper-Pleistocene alluvial deposits derived from the alkali basalts [8][9][10]21]. They would originate in the deep-seated formations before being transported as megacryst onto the Earth's surface by alkali basaltic magmas. ...
Sapphires from alluvial deposits associated with Cenozoic basalts in Southern Vietnam were collected for investigation of mineral inclusions. In this report, primary iron oxide inclusions were focused on, with detailed mineral chemistry using a Raman spectroscope and electron probe micro-analyzer. Consequently, a variety of iron oxide inclusions were recognized as wüstite, hercynite, and ilmenite. Ilmenite falling within an ilmenite–hematite series ranged in composition between Il24-30He36-38Mt35-40 and Il49-54He34-40Mt7-10, classified as titanomagnetite and titanohematite, respectively. Wüstite with non-stoichiometry, (Fe2+0.3-0.9)(Ti3+<0.179Al3+≤0.6Cr3+<0.1Fe3+≤0.46)☐≤0.23O, was associated with hercynite inclusions, clearly indicating cogenetic sapphire formation. Wüstite and sapphire appear to have been formed from the breakdown reaction of hercynite (hercynite = sapphire+wüstite) within a reduction magma chamber. Titanohematite and titanomagnetite series might have crystallized during iron–titanium reequilibration via subsolidus exsolution under a slightly oxidized cooling process.
... Similar mantle melt-derived zircon megacrysts brought to the surface by Cenozoic alkaline basalts have been described from other localities, most importantly from the so called zircon megacryst Indo-Pacific zone (ZIP) -a 12,000 km long belt along the western Pacific continental margins in eastern Australasia, Asia and Russia ( Fig. 1; Belousova et al., 2002;Garnier et al., 2005;Qiu et al., 2005;Zaw et al., 2006;Graham et al., 2008;Yu et al., 2010;Chen et al., 2011;Sutherland et al., 2015aSutherland et al., ,b, 2016Cong et al., 2016;Piilonen et al., 2018;Zeug et al., 2018;Sinh et al., 2019;van der Meer et al., 2019). Similar zircon megacrysts are also known from Central Europe (Visonà et al., 2007;Paquette & Mergoil-Daniel, 2009;Siebel et al., 2009;Schmitt et al., 2017). ...
... The observed chemical composition in sample PA are highly indicative of magmatic zircon with REE patterns comparable, for instance, with zircon xenocrysts from alkali basalts in Ratanakiri Volcanic Province, Cambodia (Zeug et al., 2018) within the zircon Indo-Pacific zone (Piilonen et al., 2018;Zeug et al., 2018) but also with zircon megacrysts from mafic alkaline lavas in the Venetian Volcanic Province, Italy (Visonà et al., 2007). The lack of a Eu anomaly is consistent with growth from a feldspar free source, such as nonfractionated or mantle-derived rocks, with the low U, Th, and Nd concentrations typical of Si-poor melts (Sinh et al., 2019). Accordingly, the crystal morphology, CL patterns, and trace element geochemistry are interpreted to indicate growth within a primary, mantle-derived alkaline melt. ...
... Thirdly, CL textures at least in the case of BA-1, reveal evidence of dissolution and regrowth in a magmatic environment, which could point to a more protracted history in a high temperature environment. Sinh et al. (2019) also found spatially invariant U-Pb ages and similar CL textures in comparable mantle-derived zircon grains from placer deposits in the central highlands of Vietnam and interpreted those data as evidence of multiple resorption and re-growth processes for zircon in deep sub-crustal carbonatitic magma chambers at temperatures above the U-Pb closure temperature. Hence, if this interpretation is applied to the zircon crystals in this study, then the U-Pb system would record cooling ages. ...
U-Pb geochronology of Quaternary zircon crystals is challenging due to their low levels of radiogenic-Pb, the importance of accurate common-Pb correction, and the potential need for secular disequilibrium correction. (U-Th)/He dating offers an independent benchmark for the quality of U-Pb data and their reduction, including the various corrections, as the closure temperature of the He system in zircon is <500 °C, whereas for Pb it is >900 °C. Hence, U-Pb ages must predate (U-Th)/He ages, or at least be equal to them in the case of rapid cooling. Analyses of southeast Asian mantle-derived zircon megacrysts from Tram Poung (TR), Tok Phrom (TO), Bang Kacha (BA), and Pailin (PA) provide new information about their pre-eruptive and eruptive history. SIMS U-Pb zircon ages of 6.7 ± 3.4 Ma (TR), 2.45 ± 0.23 Ma (TO), 1.44 ± 0.13 Ma (BA), and 0.97 ± 0.41 Ma (PA), correspond to (U-Th)/He ages of 5.30 ± 0.52 Ma (TR), 2.42 ± 0.10 Ma (TO), 1.05 ± 0.05 Ma (BA), and 0.91 ± 0.02 Ma (PA), respectively. The youngest of these U-Pb ages (PA) is corroborated by LA-ICPMS mapping of a grain that yields an age of 1.017 ± 0.067 Ma. The grain mapping reveals no evidence for Pb diffusion and the calculated Pb diffusion temperatures for these megacrysts of ∼1110–1250 °C are significantly greater than Ti-in-zircon apparent temperatures of ∼659 °C. These observations may imply that the U-Pb ages reflect magmatic crystallization and hence, the close similarity of the U-Pb and (U-Th)/He dates observed in all samples documents high zircon megacryst growth rates of ∼1 × 10⁻¹² (TO) to ∼2 × 10⁻¹⁴ cm/s (TR).
... Zircon is another important mineral geologically [27,28], which can be of gem-quality. Studies on gem-quality zircons from two occurrences are also included in this Special Issue [29,30]. Data by Piilonen et al. [29] on zircon xenocrysts from alkali basalts in Ratanakiri Province (Cambodia) suggest that their genesis involved zirconium-saturated, aluminium-undersaturated, carbonatitic-influenced, lowdegree partial melting (<1%) of peridotitic mantle at ca. 60 km beneath the Indochina terrane. ...
... Data by Piilonen et al. [29] on zircon xenocrysts from alkali basalts in Ratanakiri Province (Cambodia) suggest that their genesis involved zirconium-saturated, aluminium-undersaturated, carbonatitic-influenced, lowdegree partial melting (<1%) of peridotitic mantle at ca. 60 km beneath the Indochina terrane. Data by Bui Thi Sinh et al. [30] on zircon crystals (up to 3 cm long) from placer deposits in the Central Highlands of Vietnam suggest a genesis from carbonatite-dominant melts as a result of partial melting of a metasomatized lithospheric mantle source as well as resorption and re-growth processes. ...
Gems are materials used for adornment or decoration that must satisfy several criteria where they must be aesthetic and visually appealing; relatively rare; hard and tough enough to resist "normal" wear; and able to withstand corrosion by skin contact and cosmetics [1]. Gems have been used since antiquity thus, gemology, the science dealing with gems, is positioned between academia and industry. As an applied science, in gemology, the instruments used should be non-or micro-destructive and their cost should be reasonable (both in terms of equipment and time consumption [2,3]). Gemology can also contribute to the development of pure science and in some cases, destructive techniques may have to be used [1-3]. This special issue presents recent advances on the study of various types of gems based on a variety of research (e.g., geology, trace element geochemistry, inclusion studies, geochronology, spectroscopy). It includes 20 articles by around 100 researchers from over 30 different institutions situated in 20 countries from around the globe. These articles will hopefully contribute to our better understanding of the formation of gems. Pegmatites are known to be a source of several (>50) gem-quality minerals [4,5]; and in the present Special Issue, three works related to gems and pegmatites are published [6-8]. The first work by Strmić Palinkaš et al. [6] is concerned with a detailed analysis of the economically important gem-bearing Boqueirão granitic pegmatite situated in the Borborema Pegmatitic Province (BPP) from Rio Grande do Norte in Northeast Brazil. The Boqueirao granitic pegmatite is classified as a member of the Lithium-Caesium-Tantalum (LCT) pegmatite family and was emplaced during a late stage of magmatic activity in the late Cambrian. The second work by Huong et al. [7] presents a comparison of the trace element geochemistry of danburite, a gem mineral commonly formed within transition zones of metacarbonates and pegmatites as a late magmatic accessory phase from Mexico, Tanzania, and Vietnam. Differences in the rare earth elements (REE) concentrations of danburite from the different localities were observed and these show that trace element variations reflect different degrees of involvement of metacarbonates and pegmatites among different locations. The third work by Diella et al. [8] is a study of the first gem-quality multicoloured tourmalines found in the Alps hosted in LCT pegmatites of the Adamello massif, Italy. Tourmaline is considered as an important recorder of its geological formation [9,10] and the results of the study may contribute to understanding the evolution of the pegmatites in this massif. Giuliani et al. [11] and Karampelas et al. [12] present studies on emeralds; the bluish-green to green to yellowish-green variety of beryl coloured by chromium and sometimes vanadium. The first group of authors in their review proposed an enhanced classification for emerald deposits based on the geological environment (magmatic or metamorphic), host-rock types (mafic-ultramafic rocks, Minerals 2019, 9, 778 2 of 5 sedimentary rocks, and granitoids), degree of metamorphism, styles of mineralization (veins, pods, metasomatites, shear zone) as well as the type of gem-forming fluids and their temperature, pressure, and composition [11]. Karampelas et al. [12] present an applied study that provides a chemical and spectroscopic analysis of gem-quality emeralds from the most important sources (i.e., Afghanistan (Panjsher Valley), Brazil (Itabira), Colombia (Coscuez), Ethiopia (Shakisso), Madagascar (Mananjary), Russia (Ural mountains), Zambia (Kafubu) and Zimbabwe (Sandawana)). Their study demonstrates how these different analyses can collectively be used to distinguish them from one another (i.e., geographic gem determination). Rubies and sapphires are coloured gem varieties of corundum and can be found in various places around the globe. Ruby is the red variety and sapphire is the blue variety, while all other coloured corundums are called fancy sapphires and need a colour prefix (e.g., pink sapphire, yellow sapphire). A comparative study of ruby chemistry and inclusions between Myanmar and eastern Australia is presented by Sutherland et al. [13] and clearly shows that although having formed in different parts of the world, at different times and under different tectonic settings, unusual Ga-rich rubies occur in both regions, indicating primary generation involving magmatic processes. Sorokina et al. [14] focus on the genesis of gem-quality sapphires from the Ilmen Mountains (South Urals, Russia) found in situ within ultramafites. These sapphires were formed together with a spinel-chlorite-muscovite rock during the metasomatic alteration of orthopyroxenites at a temperature around 700-750 °C and pressure of about 1.8-3.5 kbar. Despite their metasomatic genesis, Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses of these blue sapphires showed that they clearly fall into the range of "metamorphic" sapphires (e.g., Ga/Mg < 2.7). Filina et al. [15] presented data on another occurrence of sapphires in the same region of the South Urals occurring in anorthosites (kyshtymites). Syngenetic zircon inclusions of magmatic origin were found in these sapphires; however, a few of the measured elements using LA-ICP-MS show that some of these sapphires also fall into the range for "metamorphic" sapphires (e.g., Ga/Mg < 0.8). Formation of anorthosites (kyshtymites) is still debatable and two possible scenarios (magmatic and metamorphic-metasomatic) are proposed. Voudouris et al. [16] give an overview of gem corundum deposits from Greece where they occur within clearly diverse geological settings. For instance, pink sapphires to rubies from Paranesti (Drama area; found in boudinaged lenses of Al-rich metapyroxenites alternating with amphibolites and gneisses), pink to purple to blue sapphires from Gorgona (Xanthi area; occurring within marble layers alternating with amphibolites), and sapphires from the central part of Naxos Island (associated with desilicated granite pegmatites intruding ultamafic lithologies, a.k.a. plumasites) can be classified as metamorphic in origin. On the other hand, blue sapphires from the southern part of the Naxos and Ikaria Islands (both occurring in fissures within metabauxites hosted in marbles) display atypical magmatic signatures, indicating a likely hydrothermal origin. In these four papers [13-16], it is evident that some ratios used to separate gem corundum from different geological environments (e.g., Ga/Mg ratio [17]) should be applied with caution. They possibly need revision using solely data obtained from gem-quality samples, in addition, the data should be grouped carefully according to colour. Corundum oxygen isotope values have been used to determine the likely geological origin of gem-quality corundum as well as to constrain the likely geological environment of samples collected from secondary alluvial deposits [18-22]. There are two contributions on the use of in situ oxygen isotope analysis on gem corundum from a primary and secondary occurrence to help better understand their origin [23,24]. The first by Wang et al. [23] is on fingerprinting rubies from Paranesti in northern Greece and importantly suggests that this method can be used to distinguish between two similar occurrences only 500 metres apart. The second paper by Graham et al. [24] is on the use of in situ oxygen isotopes to help in determining the genesis and evolution of alluvial sapphires from the Orosmayo region (Jujuy Province, NW Argentina) and importantly shows that there is a wide, although systematic, range in oxygen isotope values, which can be explained by differing degrees of interaction between mantle-derived magmas, lower crustal felsic magmas, and, most likely, both mantle-and crustal-derived metasomatic fluids. Pearls are biogenic gems and are of historic and present-day importance [1,25]. A study of a large number of natural and cultured pearls found in various bivalves from saltwater and freshwater Minerals 2019, 9, 778 3 of 5 environments using LA-ICP-MS and X-ray luminescence is given by Karampelas et al. [26]. LA-ICP-MS can be used to accurately separate freshwater from saltwater samples using manganese, barium, sodium, magnesium, and strontium ratios, and in some cases, even to identify their host bivalve species. Additionally, X-ray luminescence reactions of the studied samples have confirmed a correlation between yellow-green intensity and manganese content in aragonite. It is suggested that orange luminescence, observed in a few freshwater samples under X-rays, is due to a different coordination of Mn 2+ in vaterite as compared to aragonite. Zircon is another important mineral geologically [27,28], which can be of gem-quality. Studies on gem-quality zircons from two occurrences are also included in this Special Issue [29,30]. Data by Piilonen et al. [29] on zircon xenocrysts from alkali basalts in Ratanakiri Province (Cambodia) suggest that their genesis involved zirconium-saturated, aluminium-undersaturated, carbonatitic-influenced, low-degree partial melting (<1%) of peridotitic mantle at ca. 60 km beneath the Indochina terrane. Data by Bui Thi Sinh et al. [30] on zircon crystals (up to 3 cm long) from placer deposits in the Central Highlands of Vietnam suggest a genesis from carbonatite-dominant melts as a result of partial melting of a metasomatized lithospheric mantle source as well as resorption and re-growth processes. The paper by Dill [31] is an excellent review of gem placer deposits, outlining their processes of formation, controls on deposition, and concludes with a new classification system. Štubňa et al. [32] describe some relatively small (< 0.2 ct) and rare gem-quality demantoid (i.e., yellowish-green to green coloured andradite garnet) from serpentinized harzburgites situated in Dobšiná, Slovakia. Curtis et al. [33] in their work reviewed the current opal classification including additional data on samples from new localities. Classification of opal-A, opal-CT, and opal-C as well as transitional types can use XRD and infrared spectroscopy with the aid of Raman spectroscopy and nuclear magnetic resonance (NMR). Klemme et al. [34] focus on the genesis of prase (green-coloured quartz) and amethyst from Serifos Island (Cyclades, Greece). The stable oxygen and hydrogen isotopic composition of both quartz varieties suggest a mixing of magmatic and meteoric (and/or marine) fluids. Large (up to several centimetres) and vividly coloured Mn-rich minerals (kyanite, green andalusite, garnet-grossular, and spessartine-and red-epidote) of "near" gem-quality from Thassos Island (Rhodope, Greece) are described by Tarantola et al. [35]. They also show that the orange colour of kyanite from Thassos is due to Mn 3+ ; this is the second reported occurrence (after Loliondo, Tanzania) of such kyanite. Voudouris et al. [36] offer an overview of collector and gem-quality mineral occurrences from Greece, relating them to various geological environments such as regional metamorphic-metasomatic, alpine-type fissures, plutonic-subvolcanic intrusions and pegmatites, zones of contact metamorphism, and peripheral volcanic rocks. This Special Issue is a good example of the growing number of scientists working and collaborating on various gem-related topics around the world. We hope that this issue will shed light on various aspects of gemology, enhance scientific debate, and attract more scientists from various disciplines to become involved in this field of research.
