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Origin and distribution of corundum from an intraplate alkali basaltic province in Thailand : evidence from field and inclusion studies

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Limtrakun, P (2003) Origin and distribution of corundum from an intraplate alkali basaltic province in Thailand : evidence from field and inclusion studies. PhD thesis, University of Tasmania.

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Abstract

It has long been recognised that gem-quality corundums (sapphires and rubies) are
commonly found in close proximity to intraplate alkali basalts. On this basis, a general
consensus has developed that the gem-quality corundums were the products of weathering of
"nearby" alkali basalts. However, despite this association, gem-quality corundums are rarely
found hosted within fresh basalts.
Through the detail study of their mineralogy and inclusion compositions, this study derives
new evidence on the origin of sapphires. All sapphires examined in this thesis were
recovered from alluvial placer deposits in the Denchai gem fields of Phrae Province,
Northern Thailand and are interpreted to have been derived from the late Cenozoic alkali
Denchai basalts. The basalts have olivine + clinopyroxene + plagioclase phenocryst and
microphenocryst assemblages, are alkali in character and have compositions varying from
basanites, basalts, basaltic trachyandesites to trachybasalts. They have abundant mantlederived
xenoliths, mostly spinel-lherzolites, and rare crustal xenoliths. The equilibration P-T
estimates of spinel-lherzolite xenoliths are 1030°C at 8 to 20 kbars. The basalts themselves
show evidence of fractionation at mantle depths (> 10 kbars) and no evidence of lowpressure
crystal fractionation prior to eruption. Isotopic data suggest that the basalts have not
experienced crustal contamination and that mixing of three mantle components was
involved; (1) an "A" component (Depleted Mantle, HIMU and a component lying
somewhere between DM and HIMU line), (2) "A"-EMl mixing component and (3) EM2-
enriched subcontinental lithospheric mantle. Most Denchai sapphires are blue in colour, having very low Cr contents and high Ti/Ga
ratios. Their oxygen isotope compositions (+4.7 to +8.4%0) suggest a mixed source
composition (crust and mantle) for sapphire crystallisation. Four types of inclusions were
identified in the sapphires; COi-rich inclusions (Type-I), polyphase (V+L+S) inclusions
(Type-II), silicate-melt inclusions (Type-III) and mineral inclusions. Type-I, COi-rich fluid
inclusions have wide ranges of C02 density, up to about 0.87 g/cm3
• Type-II inclusions
contain -60 wt% NaCl equivalent, H20 and anhydrite daughter crystals. Glass compositions
of Type-III are silica-rich (-61 wt% Si02) 15-30 wt% Ah03, -10 wt% K10 + Na20 and high
volatiles (S, Cl and H20). Critical assessment of the data suggests that the Denchai sapphires
originated at pressure of in the range of 6-15 kbars, at a temperature between 700 and 900°C. The source composition had low Si activity, high NaCl and was highly oxidised (hematite
stable).
New evidence from this study suggests that the Denchai sapphires crystallised from melts
that formed in the continental lithosphere. The crystallisation of these sapphires requires
hybrid melts dominated by a mantle 8 18 0 signature, with high salinity, j02, C0 2 and low in
Ca, Mg, Fe contents. It is also concluded that the role of the associated alkali basalts is to
provide (I) a heat source, (2) possibly to act as a low aSi0 2 buffer that reacts with the
contaminant, and (3) to provide an effective method ofrapid transport to the surface.
This inclusion-based study emphasises the large variability of source compositions that can
account for the genesis of corundum. Many sapphire provinces have a strong association with
Nb-Ta oxides and have mantle 8180. These features suggest carbonatites are a very important
component. However the Thai sapphire provinces lack these feature. The Denchai sapphires
were derived from highly oxidised Ca-poor parental melts which have variable 8180. Partial
melts of seafloor weathered MORB basalts are the most likely source for this composition.

Item Type: Thesis (PhD)
Keywords: Corundum, Precious stones
Copyright Holders: The Author
Copyright Information:

Copyright 2003 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2003. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:45
Last Modified: 11 Mar 2016 05:54
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