The conditions of formation of sapphire and zircon in the areas of alkali basaltoid volcanism in central vietnam

Koptyuga 3, Novosibirsk, 630090 Russia b Geological Institute of the Vietnamese Academy of Sciences and Technologies, Hanoi, Vietnam Received 5 February 2009; received in revised form 2

The conditions of formation of sapphire and zircon in the areas

of alkali-basaltoid volcanism in Central Vietnam A.E Izokh a,* , S.Z Smirnov a, V.V Egorova a, Tran Tuan Anh b, S.V Kovyazin a,

Ngo Thi Phuong b, V.V Kalinina a

a

V.S Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, prosp Akad Koptyuga 3, Novosibirsk, 630090 Russia

b

Geological Institute of the Vietnamese Academy of Sciences and Technologies, Hanoi, Vietnam

Received 5 February 2009; received in revised form 27 July 2009; accepted 14 August 2009

Abstract

Study of the chemical composition of clinopyroxene and garnet megacrysts from the Dak Nong sapphire deposit and model calculations have shown that megacrysts originated from the crystallization of alkali basaltoid magma in a deep-seated intermediate chamber at 14–15 kbar, which is close to the Moho depth (50 km) in this part of southeastern Asia The chamber was a source of heat and CO2 fluids for the generation of crustal syenitic melts producing sapphires and zircons The formation conditions of sapphires and zircons are significantly different The presence of jadeite inclusions in placer zircons points to high pressures during their crystallization, which is confirmed by the ubiquitous decrepitation of CO2-rich melt inclusions Sapphires crystallized from iron-rich syenitic melt in the shallower Earth’s crust horizons with the participation of CO2 and carbonate–H2O–CO2 fluids The subsequent eruptions of alkali basalts favored the transportation of garnet and pyroxene megacrysts as well as sapphire and zircon xenocrysts to the surface It is shown that sapphire deposits can be produced only during multistage basaltic volcanism with deep-seated intermediate chambers in the regions with thick continental crust The widespread megacryst mineral assemblage (clinopyroxene, garnet, sanidine, ilmenite) and the presence of placer zircon megacrysts can be used as indicators for sapphire prospecting

© 2010, V.S Sobolev IGM, Siberian Branch of the RAS Published by Elsevier B.V All rights reserved

Keywords: basaltic volcanism; continental Earth’s crust; sapphire; zircon; Vietnam

Introduction

Placers of gem sapphire and zircon are widespread in

Southeastern Asia, Australia, and Russian Far East (Primor’e),

where they are related to multistage Cenozoic basaltic

mag-matism These placers form the West Pacific belt (Sutherland

et al., 2004; Vysotskii and Barkar, 2006) Zircon and

corun-dum megacrysts were also revealed in Cenozoic basalts in

Central and Southeastern Mongolia (Agafonov et al., 2006)

In Central Vietnam, sapphire placers are explored in the Gia

Lai (Pleiku), Dak Lac (Dak Nong), Bin Phuoc, Bin Tuan

(Hong Liem), Lam Dong, and other provinces (Fig 1) Zircon

and garnet placers are exploited in the Dong Nai (Gia Kiem)

province There are also noncommercial sapphire occurrences

in this area In the placers confined to volcanic areas zircon

dominates over sapphire, whereas in the distant placers

corundum is predominant Alkali basalts in these regions favor

transport of clinopyroxene, sanidine, garnet, and titanomag-netite megacrysts to the surface, which evidences the existence

of deep-seated magma chambers In the Gia Kiem Village region (Dong Nai province), clinopyroxene, garnet, sanidine, and titanomagnetite megacrysts and large (up to 3 cm) zircon crystals were found in the cinder cones of alkali basalts We also discovered zircon placers and occasional sapphire grains

in the Pleiku region In the placer confined to the cinder cone

of Nui Boong Volcano, sapphire and zircon are associated with clinopyroxene, sanidine, and ilmenite megacrysts The occurrence of sapphires and zircons exclusively in alkali basalts and their assemblage with minerals of the deep-seated parageneses permit them to be considered indica-tors of petrogenesis in deep-level zones of the Earth’s crust

or the upper mantle Thus, complex mineralogical and ther-mobarometrical studies of sapphires, zircons, and minerals of the megacryst assemblage confined to the same volcanic area will help to reconstruct the petrogenetic conditions

In this work we present results of the study of sapphires, zircons, and clinopyroxene and garnet megacrysts from the

* Corresponding author.

E-mail address: izokh@uiggm.nsc.ru (A.E Izokh)

doi:10.1016/j.rgg.2010.0 001

1068-7971/$ - see front matter D 2010, V S Sabolev IGM, Siberian Branch of the RAS Published by Elsevier B.V All rights reserved.

6

Dak Nong placer (Dak Lac province) The placer is localized

in eluvial laterites developed after the alkali-basalt flow of the

Dak Nong Volcano (Garnier et al., 2005) The heavy

concen-trates from this placer contain not only dark blue, green, and

yellow sapphires but also corroded zircon grains up to 0.5 cm

in size Among the typical megacryst minerals associated with

alkali basaltoids, the authors found clinopyroxene, garnet,

sanidine, and ilmenite Other abundant minerals are garnet,

olivine, clino- and orthopyroxenes, and spinel, which

origi-nated from lherzolite xenoliths Also, occasional grains of

tourmalines of the schörl-dravite and uvite-schörl-dravite

series and unusual Y-containing garnet were discovered The

main goal of our study was to elucidate the formation

conditions of garnet, clinopyroxene, sapphire, and zircon

megacrysts and to construct a model relating mantle basaltoid

magmatism to the formation of zircon and sapphire

paragene-ses

The state of the art of the problem and object of study

The Neogene–Quaternary intraplate basaltoid magmatism covered vast areas in Eastern and Southeastern Asia—from Northern Mongolia and Tuva to Southern Vietnam,—where basalts compose large fields with volcanic centers of different preservation degrees In Central and Southern Vietnam, basal-tic plateaus are often more than 100 km across and up to several hundred meters in thickness, with the total area of basaltic flows being more than 23,000 km2 (Hoang and Flower, 1998) (Fig 1) Tholeiitic basalts are predominant in the basaltic sequences, and alkaline rocks are subordinate Two stages of volcanic activity were recognized The first stage included eruptions of quartz-normative and olivine tholeiites with subordinate alkali basalts, and the second, eruptions of olivine tholeiites, alkali basalts, basanites, and, more seldom, nephelinites (Hoang and Flower, 1998) It is at the second stage that megacrysts of pyroxenes, garnet, amphiboles, K-Na-feldspars, phlogopite, garnet and spinel lherzolites, websterites, and pyroxenites were let out to the surface There are also sapphire and zircon placers in the reported areas Most of the existing models relate the formation of sapphire and zircon to the crystallization of alkali-basaltic magma in deep-seated intermediate chambers There are several groups

of models for sapphire and zircon genesis (Vysotskii and Barkar, 2006) Some researchers relate this genesis to frac-tional crystallization of alkali basalts in deep-seated chambers (Hong-sen et al., 2002; Irving, 1986; Kievlenko et al., 1982) They suggest that the highly differentiated melt from which sapphire crystallized corresponded in composition to phonolite (nepheline syenite) or trachytes (syenite) In this case, corun-dums should be considered phenocrysts despite the signs of their corrosion or the formation of spinel and titanomagnetite rims (Chen et al., 2006)

Other researchers suggest that the sapphire and zircon are xenogenic and that alkali basalts served as transporters of xenocrysts of these minerals, which formed from different crustal or mantle sources Study of mineral inclusions in corundums and zircons showed that the minerals crystallized from alkaline-salic or salic melts in the lower or middle Earth’s crust (Aspen et al., 1990; Guo et al., 1996a; Smirnov

et al., 2006; Vysotskii et al., 2002) In some cases, the metamorphic or metasomatic origin of sapphires is substanti-ated (Graham et al., 2004; Sutherland and Schwarz, 2001; Sutherland et al., 2002) Some researchers admit that corun-dums result from the contamination of basaltic magma with aluminous (including boxites) or carbonate rocks (Levinson and Cook, 1994)

Several xenoliths of corundum-bearing rocks in alkali basalts were discovered For example, corundum-anorthoclase rock was found in Australia (Stephenson, 1976), and xenolith

of alkali-feldspathic rock bearing corundum, zircon, magnetite, ilmenorutile, Y-titanoniobates, biotite, and apatite was re-vealed in alkali basalts in Scotland (Aspen et al., 1990; Upton

et al., 1999) Similar xenoliths are known in China In all these cases, corundum is in paragenesis typical of syenites

Fig 1 Distribution of Neogene (1) and Quaternary (2) basalts and sapphire

placers (3) in Central Vietnam Sapphire and zircon placers: 1, Dak Nong; 2, Gia

Kiem; 3, Hong Liem.

Thus, the above review shows that the genesis of sapphire

and zircon is related to alkali-basaltoid magmatism, but there

were only few complex studies of minerals throwing light on

the evolution of basaltic-magma chambers and sapphire and

zircon parageneses in a particular volcanic center, and the

results of these studies are contradictory The Dak Nong placer

in the Dak Lac province (Central Vietnam) seems to be the

most appropriate for such studies Its eluvial nature and the

coexistence of minerals of megacryst assemblage (garnet,

clinopyroxene, K-Na-feldspar), sapphire, and zircon permit the

reconstruction of the history and depth of their formation along with the degree of the influence of basaltic melts on the formation of crustal parageneses containing sapphire and zircon

Methods of investigation

The compositions of mineral phases and glasses of melt inclusions were studied on a Camebax Micro electron

micro-Table 1 Contents of the major (wt.%) and trace (ppm) elements in clinopyroxene and garnet megacrysts from alkali basalts from the Dak Nong volcanic plateau, Central Vietnam

Note Major elements were determined by electron microprobe analysis, and trace elements, by ICP MS Mg# = Mg ⋅ 100/(Mg + Fe) at.%; b.d.l., below detection limit; dash, not determined.

probe at the Analytical Center of the Sobolev Institute of

Geology and Mineralogy (SIGM) at accelerating voltage

20 kV Analysis of mineral phases and inclusions was

per-formed at the probe current of 50 and 20–30 nA, respectively

Calibration was made using natural minerals and synthetic

compounds whose compositions were earlier determined by

different methods

The trace-element composition of clinopyroxene and

sap-phire megacrysts and zircon inclusions in sapsap-phire was

determined by LA ICP MS at the Analytical Center of the

SIGM Analysis of each grain was made in parallel with

analysis of standard glass samples (NIST 612 and NIST 614)

The analytical error (standard deviation) was no more than

25% for element contents of <1 ppm and 10% for >1 ppm

The trace-element composition of placer zircons was

deter-mined by secondary ion mass-spectroscopy (SIMS) (ion microprobe) Analyses were made on a CAMECA IMS-4f ion microprobe at the Institute of Microelectronics and Informatics RAS, Yaroslavl’ To obtain secondary ions, a primary O2− beam with an energy of ~14.5 keV was used Secondary ions were collected from the field of view 25 µm across To suppress the interfering molecular and cluster ions and reduce the matrix effect, the method of energy filtration was applied The absolute concentration of each element was calculated from the measured intensities of positive single-atomic secon-dary ions of elements normalized to the intensity of seconsecon-dary

30Si+ ions The concentrations of SiO2 at all points were determined independently by electron microprobe analysis To suppress the contribution of interfering molecular ions to the intensities of analytical signals of 153Eu+ and 174Yb, we used

Fig 2 Compositional variations of clinopyroxene megacrysts from alkali basalts from the Dak Nong volcanic plateau, Vietnam 1, clinopyroxene megacrysts;

2, clinopyroxenes from lherzolite xenoliths.

Fig 3 REE (a) and trace-element (b) patterns of clinopyroxenes of the megacryst assemblage from alkali basalts from the Dak Nong volcanic plateau, Vietnam.

the subtraction procedure The accuracy of determination of

concentrations is 5–10 rel.% for >1 ppm and 15–20 rel.% for

1–0.1 ppm

To identify mineral inclusions and daughter phases of fluid

and melt inclusions, we also applied Raman spectroscopy The

Raman spectra were recorded on a X-Y Dilor OMARS

multichannel spectrometer with a Peltier cooled CCD detector

Gas Ar laser with line 514 nm was used for exitation

Microthermometric studies of melt inclusions were carried

out on a Sobolev–Slutsky heating stage at 500 ºC to 1300 ºC

Results

Clinopyroxenes of the megacryst assemblage from the

Dak Nong placer compositionally correspond to high-alumina

augites (En42-54, Fs12-20, Wo34-44), whose Mg# varies from

74 to 81 at.% They contain 7.7–8.6 wt.% Al2O3, 0.7–

1.54 wt.% TiO2, and 1.42–1.76 wt.% Na2O; the contents of

these oxides increase as Mg# decreases (Table 1, Fig 2)

The content of Cr2O3 in all clinopyroxenes is below its

detection limit Compared to clinopyroxenes from lherzolite

and websterite xenoliths (Mg# = 90–92) transported with the

same basalts, the studied ones have lower Mg# and Cr2O3

contents and higher TiO2 content Their REE contents are 2–6

chondrite units (Fig 3) The chondrite-normalized REE

pat-terns of the minerals show their enrichment in MREE

((Ce/Sm)n = 0.68–0.81), have a negative slope in the HREE

field ((Sm/Yb)n = 2–2.6), and lack a Eu anomaly

Garnets of the megacryst assemblage correspond in

com-position to pyrope-almandine (Prp59-66, Alm20-27, Grs13-15)

and show Mg# = 69–76 at.% (Table 1, Fig 4) In contrast to

garnets of lherzolite xenoliths, which contain up to 0.25 wt.%

Cr2O3, the studied megacrysts lack chromium but have high

TiO2 contents (0.48–0.69 wt.%) Similarly to the

clinopy-roxenes, the compositions of pyrope and the host basalts are

intimately related The Vietnamese garnets are more

magne-sian (Mg# = 69–76 at.%) than the Mongolian ones (Mg# =

61–63 at.%)

The total content of LREE in the garnets is 0.1–40

chondrite units The REE patterns are strongly fractionated

and show a LREE depletion and strong HREE enrichment

((Nd/Yb)n = 0.04–0.07)) (Fig 5) The multielemental patterns show Sr and Ba anomalies The HREE content reaches 40 chondrite units

Zircon- and sapphire-bearing assemblages Sapphires are

moderately or poorly shaped crystals and crystal clastics Most

of them are dark blue and blue-green Sometimes, pink sapphires also occur in the placer The mineral color is unevenly distributed in growth zones and sectors (Fig 6) Most crystals have a weakly colored or colorless core and an intensely colored outer zone Sapphyres from the Dak Nong placer are poorer in Ga than those from other basalt placers and lack Cr (Table 2)

Zircon is the only mineral from the Dak Nong placer that

was found both as placer crystals and as inclusions in sapphire, which suggests their genetic relationship Cathodoluminescent studies showed that all placer zircons have an oscillatory

polygonal zoning (Fig 7, a) In some of them, the zoning is

superposed by a curved pattern called a convolute zoning

(Fig 7, b).

Most of the placer zircons contain 0.6–1.3 wt.% HfO2 and minor U and Th The Th/U ratio is 0.3–1.6 (Table 3), which

is close to those in magmatic zircons (Hoskin and Schaltegger, 2003)

Fig 4 Compositional variations of garnet megacrysts from alkali basalts from

the Dak Nong volcanic plateau, Vietnam 1, garnet megacrysts; 2, garnets from

lherzolite xenoliths.

Fig 5 REE (a) and trace-element (b) patterns of garnets of the megacryst assemblage from alkali basalts from the Dak Nong volcanic plateau, Vietnam.

These zircons are characterized by a significant domination

of HREE over LREE The REE patterns show a positive Ce

and a negative Eu anomalies (Table 3, Fig 8, a) The total

REE content varies from 957 to 5509 chondrite units The

placer zircons are characterized by great variations in the depth

of Eu anomaly (Eu/Eu* = 0.05–0.66) In most of the placer

zircons Eu/Eu* = 0.5–0.6 The REE contents and patterns of

the core and peripheral zones of crystals are almost identical,

but the cores have somewhat higher REE contents than the

peripheral ones (Fig 8, a).

Inclusions of mineral phases and mineral-forming

me-dia in sapphire and zircon Sapphires and zircons contain

inclusions of syngenetic minerals (Fig 9) and mineral-forming

media—melts and fluids (Fig 10) Sapphires also contain

crystalline inclusions resulted from corundum solid solution

breakdown (topotactic inclusions)

Mineral inclusions Topotactic inclusions in sapphire are

tabular crystals of hematite, pseudobrookite, and

högbomite-like mineral (Fig 9, a, Table 4) regularly oriented relative to

corundum A specific feature of hematite inclusions is high

contents of Al2O3 and TiO2 In the inclusion-rich zones, the

content of Al2O3 in hematite reaches 17–18 mol.%, whereas

the content of Fe2O3 in corundum is ~3 mol.% (Tables 2

and 4)

Inclusions of syngenetic minerals in sapphire are zircon

(Fig 9, b, c), plagioclase, and ferrocolumbite crystals

Plagio-clase trapped by sapphire during its growth is oligoPlagio-clase (An12 mol.%) Ferrocolumbite has a low MnO (~3.6 wt.%) and an increased ZrO2 (~0.8 wt.%) contents (Table 4)

Zircon occurs in sapphire as prismatic crystals (Fig 9, c, d).

Compared with placer zircons, the zircon inclusions have higher concentrations of HfO2 (~2.5 wt.%), Th, and U (Table 3, runs 13–15) The Th/U ratio in them is 0.8–2.3, which is higher than that of placer zircons but falls into the range of magmatic-zircon values (Hoskin and Schaltegger, 2003) The total REE content in the inclusions is 1230–3487 chondrite units, which corresponds to the range of REE concentrations in placer zircons The REE patterns of the

inclusions are similar to those of placer zircons (Fig 8, a) but

show a narrower range of Eu/Eu* variations (0.31–0.46) The REE contents in the zircon inclusions from the Dak Nong placer sapphire are close to the lowest REE contents in the zircon inclusions from other deposits in Southeastern Asia

(Fig 8, b).

Syngenetic minerals in the placer zircon are rare inclusions

of baddeleyite and tourmaline, most likely, of foititic com-position Electron microprobe analysis revealed a jadeite ((Na0.91K0.04)0.95(Al1Fe0.05)1.05[(Si1.91Al0.1)2.01O6]) inclusion

Fig 6 The internal structure of sapphires from the Dak Nong deposit: a, irregular distribution of color in dark blue sapphire (core is colorless); b, sector distribution

of color and inclusions (the latter are concentrated in the crystal core and form a six-ray star pattern).

Table 2 Compositions of sapphires from the Dak Nong placer, wt%

X

_

_

_

Note X

_

, average content of element (parenthesized is the number of grains); SD, standard deviation; FeO, all iron as Fe 2+ The Ga content in these sapphire varieties measured by LA ICP MS is 33–43 ppm.

Fig 8 Chondrite C1-normalized REE pattern of zircons from inclusions in sapphire (solid lines) and from placer zircons a, Comparison of the REE compositions of zircon inclusions in sapphire and placer zircons from the Dak Nong placer: 1, REE of zircon inclusions; 2, composition range of placer zircons; 3, composition range

of the cores of placer zircon crystals; 4, composition range of the margins of placer zircon crystals; b, comparison of the REE compositions of zircon inclusions in the Dak Nong placer sapphires (1) and zircon inclusions in sapphires from Thailand, Laos, and China (2), after Guo et al (1996b) and Sutherland et al (1998);

c, comparison of the REE compositions of zircon inclusions in sapphires (1) and the Dak Nong placer zircons (2) with the composition of zircons from

corundum-containing syenites (3), after Hinton and Upton (1991) Chondrite composition is given after Sun and McDonough (1989).

Fig 7 The growth (a) and convolute (b) zonings of placer zircon, revealed by cathodoluminescence.

in zircon This differs the placer zircon from the sapphire,

which contains inclusions of acid plagioclase, and points to

the higher pressures of zircon crystallization

Inclusions of mineral-forming media in sapphire and

zircon Fluid and melt inclusions in sapphire and zircon

crystals from the Dak Nong placer are shown in Fig 10 The

sapphire crystals contain scarce primary melt inclusions (MIs)

(Fig 10, a) Most of the MIs are secondary and are localized

along the healed cracks (Fig 10, b) Their silicate phase is

either glass or crystallized aggregate Sometimes, iron oxide

crystals (probably, magnetite) are present Both the primary

and secondary inclusions in the sapphires show wide variations

in the volume fractions of fluid segregation (Fig 10, a, b).

This suggests the heterogeneous trapping of coexisting fluid

and silicate phases

The fluid inclusions (FIs) accompanying the secondary MIs

in the sapphire (Fig 10, b) are mainly of carbon dioxide

composition One of the sapphire crystals contains primary FIs containing an aqueous solution and a bubble consisting of liquid and gas CO2 (Fig 10, c, d) One of these FIs contains

an aggregate of crystalline phases Raman spectroscopic analyses showed the presence of carbonates among these phases (weak but distinct line at 1086 cm–1) The studied sapphires lack such phases as individual crystalline inclusions, which permits them to be considered daughter phases Most of primary MIs in the zircon contain glass and a fluid

segregation (Fig 10, e–h) There are also subordinate partly

crystallized inclusions containing crystals of silicate phases compositionally corresponding to amphibole, albite, and K-feldspar Many inclusions contain rather large crystals of

Table 3 Compositions of placer zircons and zircon inclusions in sapphires from the Dak Nong placer

Compo-nent

DLS195-6-12 DLS195-6-13 DLS195-6-25 DLS195-6-26 DLS195-6-31 DLS195-6-32 DLS192 DN01 DN01

Electron microprobe analysis (wt.%)

SiO 2 32.17 32.12 32.00 32.10 32.21 32.30 32.20 32.09 32.19 32.00 32.05 32.23 31.93 32.27 32.34 HfO 2 0.93 1.85 0.83 0.78 0.99 0.90 0.85 0.83 1.35 0.89 0.84 0.80 2.60 2.58 2.44 ZrO 2 67.33 66.82 67.41 67.23 67.46 67.50 67.28 67.30 66.83 67.29 66.78 67.24 65.63 64.39 63.96 ThO 2 b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l 0.25 0.69 0.76

UO 2 b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l 0.06 0.30 0.93 Total 100.43 100.79 100.24 100.11 100.66 100.70 100.33 100.22 100.37 100.18 99.67 100.27 100.23 100.24 100.43

Y 28.28 1109.00 289.24 200.51 670.40 181.61 90.02 355.83 74.89 249.47 278.51 490.06 880.81 181.12 182.93

Nb 24.00 80.71 15.53 11.68 14.79 8.92 9.94 10.60 8.28 6.58 5.17 6.54 337.57 37.31 –

La 0.02 0.09 0.02 b.d.l 0.00 0.01 0.04 b.d.l 0.03 0.00 b.d.l b.d.l 0.30 0.05 16.84

Ce 0.45 7.39 0.96 0.79 1.65 0.66 0.50 1.17 0.43 0.87 0.65 1.54 4.10 0.74 20.69

Nd 0.02 0.73 0.12 0.09 0.40 0.07 0.06 0.17 0.03 0.18 0.07 0.15 0.45 0.03 7.32

Sm 0.06 2.27 0.46 0.35 1.18 0.29 0.18 0.57 0.10 0.49 0.38 0.68 1.20 0.24 1.65

Eu 0.01 1.23 0.27 0.18 0.84 0.14 0.07 0.37 0.05 0.35 0.24 0.48 0.46 0.14 0.42

Gd 0.21 14.35 3.09 2.18 8.39 1.84 0.95 4.00 0.57 3.48 2.75 5.29 8.75 2.32 4.30

Dy 1.93 93.12 22.05 14.30 53.53 13.71 6.77 28.03 4.98 20.69 20.28 35.41 71.98 18.02 18.24

Er 5.24 179.73 52.19 35.37 113.69 32.26 16.06 60.02 14.08 45.38 52.78 91.43 161.22 36.11 27.70

Yb 12.44 272.23 99.06 74.54 200.60 65.51 30.00 101.07 33.58 85.46 107.32 174.05 367.71 69.40 39.33

Lu 1.89 32.85 14.22 11.84 27.80 9.74 4.41 14.55 4.89 12.48 17.52 26.75 – 11.41 6.00

Th 3.12 837.92 44.47 9.73 128.49 17.89 6.43 45.56 5.73 60.06 15.82 61.21 1092.71 – –

U 11.41 508.22 88.15 26.93 196.16 44.74 18.73 83.76 17.02 70.01 30.33 82.50 1252.73 – – Th/U 0.27 1.65 0.50 0.36 0.66 0.40 0.34 0.54 0.34 0.86 0.52 0.74 0.87 2.33 0.82

Note 1–12, placer zircons (odd, crystal cores; even, crystal margins); 13–15, zircon inclusions in sapphires b.d.l., Below detection limit; dash, not determined.

complex Fe–Cu oxide (cuprospinel?) The fluid segregations

in the vitreous inclusions are either heterogeneous (gas +

liquid) or homogeneous (liquid) bubbles The Raman spectra

of these segregations show that they consist of carbon dioxide

with a nitrogen impurity The main difference between the

mineral-forming media in the zircon and in the sapphire is the

absence of FIs cogenetic with the MIs But most of the MIs

are surrounded by a swarm of finer inclusions localized in

radial fissures (Fig 10, g, h) This arrangement suggests that

the inclusions were decrepitated, probably, during the

trans-portation of zircon crystals to the surface

Thermometry of melt inclusions Crystallized aggregate

of MIs in sapphires starts melting at 685 ºC, and the last

crystal dissolves in the melt at 740–780 ºC The glass of

vitreous inclusions softens at 725–860 ºC When it becomes

a liquid, the bubble becomes mobile and starts moving within

the vacuoles This argues for the extremely low viscosity of

the melt Since the substance of MIs might be heterogeneous

at the moment of trapping, their homogenization temperature

might not reflect the trapping temperature For this reason, we determined the trapping temperatures, using homogenization temperatures of inclusions with the finest bubbles, in which a gas bubble is mainly the fluid phase of MIs Thus, we succeeded in estimating the temperature of MI trapping, which

is ~930–1100 ºC The primary and secondary MIs in sapphire are characterized by similar features of behavior, close temperatures of phase transitions, and close minimum homog-enization temperatures This suggests that the secondary and primary MIs were trapped at similar temperatures and might

be portions of the same sapphire-producing melt, which were trapped in different time

Since the MIs in zircons are unsealed, we could not obtain reliable data on their trapping temperatures The thermometric experiments showed that the glass of MIs in zircon passes into

a liquid (without crystallization) at 800–900 ºC (the bubble becomes mobile), but the homogenization temperature was not reached even after the 3–4 h exposure at 1300 ºC

Fig 9 Mineral inclusions in the Dak Nong placer sapphire a, Topotactic inclusions of Al-hematite and other Fe-Ti oxides; b, inclusion of zircon-columbite intergrowth; c, inclusion of intergrown prismatic zircon crystals.

Fig 10 Inclusions of mineral-forming media in the Dak Nong placer sapphires and zircons a, b, Melt inclusions in sapphire: a, primary vitreous inclusion with abnormal gas phase, b, series of secondary vitreous melt and fluid (low-density CO2) inclusions; c, d, primary fluid inclusions in sapphire, with heterogeneous CO2 segregation (liquid (lCO 2 ) and gas (gCO 2)) and daughter carbonate crystals; e–h, vitreous melt inclusions in placer zircon (CO2 , high-density CO 2 bubble; gl, glass;

cr, unsealing cracks); oc, opaque crystal.