Celestial climate driver: a perspective from four billion years of the carbon cycle
Geoscience Canada, March, 2005 by Jan Veizer
Table 1: Petrographic and mineralogical characteristics of alkali
and tholeiitic basalts.
Alkali Basalts Tholeiitic Basalts
Contain normative or modal Nepheline absent.
nepheline
Biotite may be present in the Biotite absent.
matrix.
Interstitial alkali feldspar No modal alkali feldspar.
present but difficult to identify.
In rare cases there is modal No modal analcite.
analcite.
Low-Ca-pyroxene (pigeonite or Contain a low-Ca-pyroxene
orthopyroxene) absent. (pigeonite or orthopyroxene) in
the norm or mode.
Quartz absent. Quartz may be present in the
matrix.
Olivine occurs in most samples, Olivine phenocrysts less common,
phenocrysts zoned with Fe-rich generally unzoned, and may have a
rims. low-Capyroxene reaction rim.
Olivine missing from matrix.
Augite phenocrysts more strongly Augite phenocrysts generally
pleochroic having purplish-brown, pale-brown and zoning is not as
Tirich rims. apparent as in alkali basalts.
Phenocryst sequence = olivine, Phenocryst sequence = olivine,
augite, plagioclase such that plagioclase, augite locally
plagioclase phenocrysts are less yielding ophitic and subophitic
common than in tholeiites. textures typical of most ter-
restrial basalts.
Coarser grained with mafic Finer grained, intergranular
pegmatites common in segregation textures compared to alkaline
veins even in thin flows. flows of comparable thickness.
Ultramafic xenoliths common. Ultramafic xenoliths extremely
rare.
Notes: Modified from Hughes, 1982, p. 297.
Table 2: Example analyses of major silicate minerals in alkali and
tholeiitic basalts.
Mineral Olivine Olivine Augite
Series Tholeiite Alkaline Tholeiite
Island Iceland Gr.Com. Iceland
Sample TH-4 MA-1 GR-22
Si[O.sub.2] 39.11 37.54 51.81
Ti[O.sub.2] 0.85
[A1.sub.2][O.sub.3] 2.02
[Cr.sub.2][O.sub.3] 0.15
FeO 15.97 24.33 8.78
CaO 0.33 0.20 19.71
MgO 43.93 36.99 14.85
MnO 0.23 0.33 0.16
NiO 0.17
[K.sub.2]O
[Na.sub.2]O 0.30
Sum 99.57 99.56 98.63
Si 0.993 0.993 1.947
Ti 0.024
Al 0.089
Cr 0.004
Fe 0.339 0.538 0.276
Ca 0.009 0.006 0.793
Mg 1.662 1.459 0.832
Mn 0.005 0.007 0.005
Ni 0.004
K
Na 0.022
Total 3.007 3.007 3.993
Fo% 83.1 73.1
Ca% 41.7
Mg% 43.8
Fe% 14.5
Ca%
K%
Na%
Mineral Augite Opx Opx
Series Alkaline Tholeiite Tholeiite
Island Gr.Com. Haw ML Haw K
Sample 13 184-1 KP26S1
Si[O.sub.2] 50.84 54.92 53.8
Ti[O.sub.2] 0.88 0.34 0.66
[A1.sub.2][O.sub.3] 3.99 1.92 1.02
[Cr.sub.2][O.sub.3] 0.44 0.05
FeO 5.02 10.79 16.7
CaO 21.63 2.32 2.37
MgO 15.7 29.1 25.4
MnO 0.10 0.22 0.30
NiO
[K.sub.2]O 0.02
[Na.sub.2]O 0.34 0.03 0.05
Sum 98.96 99.64 100.35
Si 1.886 1.952 1.955
Ti 0.025 0.009 0.018
Al 0.174 0.080 0.044
Cr 0.013 0.001
Fe 0.156 0.321 0.507
Ca 0.859 0.088 0.092
Mg 0.868 1.542 1.376
Mn 0.003 0.007 0.009
Ni
K 0.001
Na 0.024 0.002 0.004
Total 4.009 4.000 4.006
Fo%
Ca% 45.6 4.5 4.7
Mg% 46.1 79.0 69.6
Fe% 8.3 16.4 25.7
Ca%
K%
Na%
Mineral Plag Plag Nepheline
Series Tholeiite Alkaline Alkaline
Island Haw ML Gr. Com Tubuai
Sample 183-14 MA-17 TU 4
Si[O.sub.2] 49.9 58.03 44.11
Ti[O.sub.2]
[A1.sub.2][O.sub.3] 31.4 26.44 33.75
[Cr.sub.2][O.sub.3]
FeO 0.70 0.32 0.91
CaO 15.1 8.98 0.04
MgO 0.06
MnO
NiO
[K.sub.2]O 0.07 0.49 4.82
[Na.sub.2]O 2.63 5.89 16.88
Sum 99.8 100.21 100.51
Si 9.142 10.386 8.396
Ti
Al 6.780 5.577 7.571
Cr
Fe 0.107 0.048 0.145
Ca 2.964 1.722 0.008
Mg 0.016
Mn
Ni
K 0.016 0.112 1.170
Na 0.934 2.044 6.229
Total 19.943 19.904 23.518
Fo%
Ca%
Mg%
Fe%
Ca% 75.7 44.4
K% 0.4 2.9
Na% 23.9 52.7
Notes: Major-element oxides in wt.% with total Fe as FeO. Formulas for
olivine, augite, orthopyroxene (Opx), plagioclase (Plag) and nepheline
calculated on the basis of 4, 6, 6, 32 and 32 oxygen, respectively.
Mole proportions of endmember components are given at the bottom of
each column (except for nepheline).
Island abbreviations: Gr. Com = Grande Comore; Haw ML = Hawaii,
Mauna Loa; Haw K = Hawaii, Kilauea.
Reference citations appear in supplementary materials online at
http://wwwgac.ca/JOURNALS/geocan.html: Sample TH 4 & GR-22 = Makopaa,
1978; MA-1 & MA-17 = Spaeth et al., 1996; 13 = Strong, 1972; 184-1 &
183-14 = Garcia et al. 1995; KP-26 Scoria 1 = Wright and Helz, 1996;
TU 4 = Brousse and Maury, 1980.
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