Exceptional REE-enrichment in apatite during the low pressure fractional crystallisation of alkali olivine basalt; an example from the British Tertiary Igneous Province

R J Preston, M J Hole, J Still

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3 Citations (Scopus)

Abstract

The Cnoc Rhaonastil dolerite boss on Islay, NW Scotland represents a body of alkali-olivine basalt magma which differentiated at low pressure and in situ, from dolerite through teschenite to minor nepheline-syenite. The syenites occur as isolated pods and pegmatitic schlieren within the leucodolerite, and have an exotic mineralogy including Zr-aegirine, Zr-arfvedsonite, Ca-catapleiite, zirconolite and aenigmatite. Fluor-apatite occurs as an accessory phase in the dolerite, but becomes more abundant within the teschenite and syenites. Total REE contents within apatites in the dolerites are typically low (Sigma REE = 0.57-3.21 wt.% oxide), the highest REE contents occurring in irregular, deuterically altered rims and internal patches. The REE-enriched rims also have slightly elevated SiO2 contents at 0.81-0.95 wt.%, suggesting that the substitution scheme Ca2+ + P5+ double left right arrow REE3+ + Si4+ was operating. These apatites have up to 0.08 wt.% Cl and 3.7 wt.% F, with most being almost pure end-member fluor-apatite. The majority of the teschenite apatites show the least REE-enrichment (Sigma REE = 0.27-0.45 wt.%), coupled with low Na (<0.12 wt.%) and low SiO2 (<0.39 wt.%) contents. However, within the syenites two distinct populations of apatite exist. The first, most common, variety consists of unzoned, low-REE apatites (max. 3.1 wt.% Sigma REE, again in irregular rims and patches), whereas the second variety is often complexly zoned, and has variably enriched zones up to a maximum Sigma REE content of 42 wt.%; this is by far the most REE-enriched natural fluor-apatite so far reported from the British Isles. The REE-enriched zones are often less than 3 mum wide, and have Na content up to 5.4 wt.% Na2O, implying that the substitution scheme Na++REE(3+)double left right arrow 2Ca(2+) dominated over the more typical scheme involving Si4+ which operated in the dolerites and teschenite. Other zones are either variably enriched in Y (up 2.1 wt.% Y2O3) or Th (up to 0.85 wt.% ThO2). However, there is no correlation between Y and REE contents, suggesting that crystallographic factors were involved in apatite Y and REE partitioning. The REE-rich apatites have very low Cl content (<0.04 wt.%), but high F concentrations (<2.8 wt.%). It is believed that these strongly enriched apatites crystallised under disequilibrium conditions from isolated, variably REE-enriched domains, within the fluid-rich residual syenitic magma. These domains may have been generated by the prior crystallisation of monazite, Ca-catapleiite or zirconolite, which can be found as small inclusions within albite and interstitial analcime. The dynamic process of slumping of the denser teschenite back into the leucodolerite crystal mush is believed to have played an important role in the release of deuteric fluids and the concentration of residual magmas.

Original languageEnglish
Pages (from-to)273-285
Number of pages13
JournalTransactions of the Royal Society of Edinburgh. Earth Sciences
Volume90
Publication statusPublished - 2000

Keywords

  • alkali olivine basalt
  • apatite
  • fractional crystallisation
  • REE-enrichment
  • RARE-EARTH ELEMENTS
  • ELECTRON-MICROPROBE ANALYSIS
  • NORTH QOROQ CENTER
  • PARTITION-COEFFICIENTS
  • STILLWATER COMPLEX
  • SOUTH GREENLAND
  • SILICATE MELTS
  • MINERALS
  • LIQUID
  • RICH

Cite this

@article{7774fc99345549a3a5807c1b53427650,
title = "Exceptional REE-enrichment in apatite during the low pressure fractional crystallisation of alkali olivine basalt; an example from the British Tertiary Igneous Province",
abstract = "The Cnoc Rhaonastil dolerite boss on Islay, NW Scotland represents a body of alkali-olivine basalt magma which differentiated at low pressure and in situ, from dolerite through teschenite to minor nepheline-syenite. The syenites occur as isolated pods and pegmatitic schlieren within the leucodolerite, and have an exotic mineralogy including Zr-aegirine, Zr-arfvedsonite, Ca-catapleiite, zirconolite and aenigmatite. Fluor-apatite occurs as an accessory phase in the dolerite, but becomes more abundant within the teschenite and syenites. Total REE contents within apatites in the dolerites are typically low (Sigma REE = 0.57-3.21 wt.{\%} oxide), the highest REE contents occurring in irregular, deuterically altered rims and internal patches. The REE-enriched rims also have slightly elevated SiO2 contents at 0.81-0.95 wt.{\%}, suggesting that the substitution scheme Ca2+ + P5+ double left right arrow REE3+ + Si4+ was operating. These apatites have up to 0.08 wt.{\%} Cl and 3.7 wt.{\%} F, with most being almost pure end-member fluor-apatite. The majority of the teschenite apatites show the least REE-enrichment (Sigma REE = 0.27-0.45 wt.{\%}), coupled with low Na (<0.12 wt.{\%}) and low SiO2 (<0.39 wt.{\%}) contents. However, within the syenites two distinct populations of apatite exist. The first, most common, variety consists of unzoned, low-REE apatites (max. 3.1 wt.{\%} Sigma REE, again in irregular rims and patches), whereas the second variety is often complexly zoned, and has variably enriched zones up to a maximum Sigma REE content of 42 wt.{\%}; this is by far the most REE-enriched natural fluor-apatite so far reported from the British Isles. The REE-enriched zones are often less than 3 mum wide, and have Na content up to 5.4 wt.{\%} Na2O, implying that the substitution scheme Na++REE(3+)double left right arrow 2Ca(2+) dominated over the more typical scheme involving Si4+ which operated in the dolerites and teschenite. Other zones are either variably enriched in Y (up 2.1 wt.{\%} Y2O3) or Th (up to 0.85 wt.{\%} ThO2). However, there is no correlation between Y and REE contents, suggesting that crystallographic factors were involved in apatite Y and REE partitioning. The REE-rich apatites have very low Cl content (<0.04 wt.{\%}), but high F concentrations (<2.8 wt.{\%}). It is believed that these strongly enriched apatites crystallised under disequilibrium conditions from isolated, variably REE-enriched domains, within the fluid-rich residual syenitic magma. These domains may have been generated by the prior crystallisation of monazite, Ca-catapleiite or zirconolite, which can be found as small inclusions within albite and interstitial analcime. The dynamic process of slumping of the denser teschenite back into the leucodolerite crystal mush is believed to have played an important role in the release of deuteric fluids and the concentration of residual magmas.",
keywords = "alkali olivine basalt, apatite, fractional crystallisation, REE-enrichment, RARE-EARTH ELEMENTS, ELECTRON-MICROPROBE ANALYSIS, NORTH QOROQ CENTER, PARTITION-COEFFICIENTS, STILLWATER COMPLEX, SOUTH GREENLAND, SILICATE MELTS, MINERALS, LIQUID, RICH",
author = "Preston, {R J} and Hole, {M J} and J Still",
year = "2000",
language = "English",
volume = "90",
pages = "273--285",
journal = "Transactions of the Royal Society of Edinburgh. Earth Sciences",
issn = "0263-5933",
publisher = "Royal Society Of Edinburgh",

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TY - JOUR

T1 - Exceptional REE-enrichment in apatite during the low pressure fractional crystallisation of alkali olivine basalt; an example from the British Tertiary Igneous Province

AU - Preston, R J

AU - Hole, M J

AU - Still, J

PY - 2000

Y1 - 2000

N2 - The Cnoc Rhaonastil dolerite boss on Islay, NW Scotland represents a body of alkali-olivine basalt magma which differentiated at low pressure and in situ, from dolerite through teschenite to minor nepheline-syenite. The syenites occur as isolated pods and pegmatitic schlieren within the leucodolerite, and have an exotic mineralogy including Zr-aegirine, Zr-arfvedsonite, Ca-catapleiite, zirconolite and aenigmatite. Fluor-apatite occurs as an accessory phase in the dolerite, but becomes more abundant within the teschenite and syenites. Total REE contents within apatites in the dolerites are typically low (Sigma REE = 0.57-3.21 wt.% oxide), the highest REE contents occurring in irregular, deuterically altered rims and internal patches. The REE-enriched rims also have slightly elevated SiO2 contents at 0.81-0.95 wt.%, suggesting that the substitution scheme Ca2+ + P5+ double left right arrow REE3+ + Si4+ was operating. These apatites have up to 0.08 wt.% Cl and 3.7 wt.% F, with most being almost pure end-member fluor-apatite. The majority of the teschenite apatites show the least REE-enrichment (Sigma REE = 0.27-0.45 wt.%), coupled with low Na (<0.12 wt.%) and low SiO2 (<0.39 wt.%) contents. However, within the syenites two distinct populations of apatite exist. The first, most common, variety consists of unzoned, low-REE apatites (max. 3.1 wt.% Sigma REE, again in irregular rims and patches), whereas the second variety is often complexly zoned, and has variably enriched zones up to a maximum Sigma REE content of 42 wt.%; this is by far the most REE-enriched natural fluor-apatite so far reported from the British Isles. The REE-enriched zones are often less than 3 mum wide, and have Na content up to 5.4 wt.% Na2O, implying that the substitution scheme Na++REE(3+)double left right arrow 2Ca(2+) dominated over the more typical scheme involving Si4+ which operated in the dolerites and teschenite. Other zones are either variably enriched in Y (up 2.1 wt.% Y2O3) or Th (up to 0.85 wt.% ThO2). However, there is no correlation between Y and REE contents, suggesting that crystallographic factors were involved in apatite Y and REE partitioning. The REE-rich apatites have very low Cl content (<0.04 wt.%), but high F concentrations (<2.8 wt.%). It is believed that these strongly enriched apatites crystallised under disequilibrium conditions from isolated, variably REE-enriched domains, within the fluid-rich residual syenitic magma. These domains may have been generated by the prior crystallisation of monazite, Ca-catapleiite or zirconolite, which can be found as small inclusions within albite and interstitial analcime. The dynamic process of slumping of the denser teschenite back into the leucodolerite crystal mush is believed to have played an important role in the release of deuteric fluids and the concentration of residual magmas.

AB - The Cnoc Rhaonastil dolerite boss on Islay, NW Scotland represents a body of alkali-olivine basalt magma which differentiated at low pressure and in situ, from dolerite through teschenite to minor nepheline-syenite. The syenites occur as isolated pods and pegmatitic schlieren within the leucodolerite, and have an exotic mineralogy including Zr-aegirine, Zr-arfvedsonite, Ca-catapleiite, zirconolite and aenigmatite. Fluor-apatite occurs as an accessory phase in the dolerite, but becomes more abundant within the teschenite and syenites. Total REE contents within apatites in the dolerites are typically low (Sigma REE = 0.57-3.21 wt.% oxide), the highest REE contents occurring in irregular, deuterically altered rims and internal patches. The REE-enriched rims also have slightly elevated SiO2 contents at 0.81-0.95 wt.%, suggesting that the substitution scheme Ca2+ + P5+ double left right arrow REE3+ + Si4+ was operating. These apatites have up to 0.08 wt.% Cl and 3.7 wt.% F, with most being almost pure end-member fluor-apatite. The majority of the teschenite apatites show the least REE-enrichment (Sigma REE = 0.27-0.45 wt.%), coupled with low Na (<0.12 wt.%) and low SiO2 (<0.39 wt.%) contents. However, within the syenites two distinct populations of apatite exist. The first, most common, variety consists of unzoned, low-REE apatites (max. 3.1 wt.% Sigma REE, again in irregular rims and patches), whereas the second variety is often complexly zoned, and has variably enriched zones up to a maximum Sigma REE content of 42 wt.%; this is by far the most REE-enriched natural fluor-apatite so far reported from the British Isles. The REE-enriched zones are often less than 3 mum wide, and have Na content up to 5.4 wt.% Na2O, implying that the substitution scheme Na++REE(3+)double left right arrow 2Ca(2+) dominated over the more typical scheme involving Si4+ which operated in the dolerites and teschenite. Other zones are either variably enriched in Y (up 2.1 wt.% Y2O3) or Th (up to 0.85 wt.% ThO2). However, there is no correlation between Y and REE contents, suggesting that crystallographic factors were involved in apatite Y and REE partitioning. The REE-rich apatites have very low Cl content (<0.04 wt.%), but high F concentrations (<2.8 wt.%). It is believed that these strongly enriched apatites crystallised under disequilibrium conditions from isolated, variably REE-enriched domains, within the fluid-rich residual syenitic magma. These domains may have been generated by the prior crystallisation of monazite, Ca-catapleiite or zirconolite, which can be found as small inclusions within albite and interstitial analcime. The dynamic process of slumping of the denser teschenite back into the leucodolerite crystal mush is believed to have played an important role in the release of deuteric fluids and the concentration of residual magmas.

KW - alkali olivine basalt

KW - apatite

KW - fractional crystallisation

KW - REE-enrichment

KW - RARE-EARTH ELEMENTS

KW - ELECTRON-MICROPROBE ANALYSIS

KW - NORTH QOROQ CENTER

KW - PARTITION-COEFFICIENTS

KW - STILLWATER COMPLEX

KW - SOUTH GREENLAND

KW - SILICATE MELTS

KW - MINERALS

KW - LIQUID

KW - RICH

M3 - Article

VL - 90

SP - 273

EP - 285

JO - Transactions of the Royal Society of Edinburgh. Earth Sciences

JF - Transactions of the Royal Society of Edinburgh. Earth Sciences

SN - 0263-5933

ER -