Geoscience Reference
In-Depth Information
Goudie (1973), from a sample of 300 bulk chemical
analyses, showed that calcretes on average comprised
79.28 % calcium carbonate (42.62 % CaO), 12.30 % sil-
ica, 3.05 % MgO, 2.03 % Fe
2
O
3
and 2.12 % Al
2
O
3
. These
figures, however, mask considerable chemical variability.
This is in part due to the diverse ways in which calcretes
can form, but is also related to the increase in carbonate
content as a calcrete profile develops. Hutton and Dixon
(1981) have shown from studies in South Australia that
pedogenic hardpan calcretes are the most calcareous and
powder calcretes the least, and that CaO levels typically
decline down-profile with a parallel rise in the propor-
tion of MgO (see also McQueen, Hill and Foster, 1999).
In contrast, CaO contents of thick groundwater calcretes
from southeast Spain have been shown to be remarkably
homogenous (Nash and Smith, 1998).
Much of the recent work on pedogenic calcrete chem-
istry has focused upon understanding the range of factors
that influence C and O stable isotope compositions. C
and O isotopes have been used, for example, to deter-
mine the palaeotemperature and range of C
3
versus C
4
vegetation types present during calcrete formation and
the partial pressure of atmospheric CO
2
during diagenesis
(e.g. Andrews
et al.
, 1998; Cerling, 1999; Deutz
et al.
,
2001; Dhir
et al.
, 2010). However, as Kelly, Black and
Rowan (2000) and Deutz, Montanez and Monger (2002)
discuss, considerable care in interpretation is needed. The
chemical signatures of pedogenic calcretes are invariably
time-averaged as a result of calcite dissolution, recrys-
tallisation and crystal overgrowth during profile develop-
ment. This severely limits the temporal resolution of O and
C stable isotope interpretations; conclusions should only
be drawn where a calcrete is isotopically heterogenous
(Wright, 2007). Strontium isotopes have been used suc-
cessfully to identify the sources of carbonates within indi-
vidual calcrete profiles. For example, Chiquet
et al.
(1999)
used
87
Sr/
86
Sr ratios to demonstrate that local weathered
granite contributed at most 33 % (and as little as 3 %) of
the Sr within pedogenic calcretes in central Spain, with
atmospheric inputs of Ca and Sr from dust dominating.
Similar results have been produced in New Mexico, where
87
Sr/
86
Sr analyses suggest that atmospheric contributions
to soil carbonate comprise at least 94 % and more likely
98 % of the total (Capo and Chadwick, 1999).
The carbonate mineralogy of calcretes is dominated by
low-Mg calcite (Wright and Tucker, 1991) with variable
amounts of dolomite present. In Australia, dolomite abun-
dances are highest within the fine carbonate powders at the
base of pedogenic calcrete profiles and decrease upwards
with increasing induration (Hutton and Dixon, 1981). If
abundances of diagenetic dolomite are particularly high,
The diagenetic processes leading to dolocrete formation
can be determined by both the mineralogy of the host ma-
terials (Hay and Reeder, 1978; Hay and Wiggins, 1980;
Hutton and Dixon, 1981) and the introduction of foreign
ions (El Aref, Abdel Wahab and Ahmed, 1985). Signifi-
cant proportions of high-Mg calcite have been reported
in calcretes from Australia (Hutton and Dixon, 1981;
McQueen, Hill and Foster, 1999) and the Kalahari (Watts,
1980). The latter study showed that calcrete profiles ex-
hibit significant variations in the percentage of high-Mg
calcite and dolomite. These variations in turn appear to
be related to the occurrence of authigenic silica and sili-
cates, notably the clay minerals palygorskite and sepiolite.
Aragonite may also occur in some calcretes (Watts, 1980;
Milnes and Hutton, 1983).
Quartz is the most important non-carbonate mineral in
most calcretes, both in bulk samples and within the clay-
size fraction. Silica may be present in other forms, includ-
ing opal and chalcedony, as a result of mineral replacement
or emplacement during diagenesis (e.g. Brown, 1956;
Reeves, 1970; Arakel
et al.
, 1989; Nash and McLaren,
2003). Minor noncarbonate minerals include glauconite,
grossularite, gypsum, haematite, magnetite, muscovite,
rutile, tourmaline and zircon (Aristarain, 1971; Goudie,
1973). The other main components of calcretes are the
clays palygorskite, sepiolite, illite, kaolinite, montmoril-
lonite and chlorite (e.g. Aristarain, 1971; Bachman and
Machette, 1977; Hay and Wiggins, 1980; Watts, 1980;
Milnes and Hutton, 1983). In some calcretes, clays may be
detrital (Beattie and Haldane, 1958; Shadfan and Dixon,
1984) or related to waterlogging (Hodge, Turchenek and
Oades, 1984). In many instances, however, clay min-
erals are authigenic, produced by chemical interactions
between high- and low-Mg calcite, silica and dolomite
during diagenesis. There is strong evidence, for exam-
ple, that sepiolite and palygorskite (attapulgite) are au-
thigenic under alkaline conditions (Hassouba and Shaw,
1980; Watts, 1980; Mackenzie, Wilson and Mashhady,
1984; Singer, 1984). The types of authigenic clay min-
eral within a calcrete may also reflect the provenance of
cementing agents. Relatively high levels of palygorskite
and sepiolite in calcretes from the Murray Basin of South
Australia are suggested to indicate that calcrete formation
(and clay neoformation) occurred in an environment with
abundant available Mg (Hutton and Dixon, 1981).
8.5.4
Mode of origin
The carbonate within a calcrete can come from a vari-
ety of sources (Figure 8.10). These include solid and
