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.