Page 1 of 47
Lawson, M., Sitgreaves, J., Ras
bury, T., Wooton, K., Esch, W.,
Marcon, V., Henares, S.,
1
Konstantinou, A., Kneller, E., G
ombosi, D., Torres, V., Silva,
A., Alevato, R., Wren, M., Becker,
2
S., and Eiler, J., 2022, New age and l
ake chemistry constraints
on the Aptian pre-salt carbonates
3
of the central South A
tlantic: GSA Bulletin,
https://doi.org/10.1130/B36378.1
.
4
5
Supplemental Material
6
7
Text.
Isotopic box modeling of evaporation of Santos Basin pre-salt
Lake
8
Table S1.
Sensitivity range for major and
rare earth elemental analysis
by laser ablation for development of
9
maps.
10
Table S2.
Laser Ablation Instruments and Parameters Used in this Study.
11
Table S3.
Summary of U-Pb ages from laser
ablation (LA) inductively coup
led mass spectrometry and isotope
12
dilution (ID) for the sample
s analyzed in this study.
13
Table S4.
Laser Ablation Inductively Coupled Plasma Mass Spectrometry U-
Pb data compilation from
14
samples obtained from wells 1–4.
15
Table S5.
Isotope Dilution U-Pb data compilation for wells 1, 3 and 4.
16
Table S6.
Laser Ablation data compiled on
the Barstow reference material
during the course of analysis
17
Brazilian samples in this study.
18
Figure S1.
Modeled values of δ
18
O for lake water that evolves over time as a result of contribu
tions from two
19
rivers in to the pre-salt lakes and a relative constant evapora
tion at a humidity of 50%. This represents the best
20
fit conditions to the calculated δ
18
O
VSMOW
of 2 – 5‰ from the carbonate samples that yield depositional
21
temperatures below 40 °C.
22
Figure S2.
Isochron age derived from repeated laser ablation analysis of
the Barstow secondary reference
23
material used to monitor analy
tical performance and reproducibi
lity of sample analysis.
24
Figure S3.
Maps of laser ablation location
s for U-Pb analysis and support
ing laser ablation elemental maps for
25
Well 1 – 5115.6 m.
26
Figure S4.
Terra-Wasserburg isochron plot for all laser ablation data acq
uired on sample Well 1 – 5115.6 m.
27
Figure S5.
Terra-Wasserburg isochron plot for the two distinct families o
f laser ablation data acquired on
28
sample Well 1 – 5115.6 m from dolomite and quartz.
29
Figure S6.
Terra-Wasserburg isochron plot from isotope dilution data acqu
ired on sample Well 1 – 5115.6 m.
30
Figure S7.
Maps of laser ablation location
s for U-Pb analysis and support
ing laser ablation elemental maps for
31
Well 1 – 5136.9 m.
32
Figure S8.
Terra-Wasserburg isochron plot from laser ablation data acquir
ed on sample Well 1 – 5136.9 m.
33
Figure S9.
Maps of laser ablation location
s for U-Pb analysis and support
ing laser ablation elemental maps for
34
Well 1 – 5385m.
35
Figure S10.
Terra-Wasserburg isochron plot fromlaser ablation data acquire
d on sample Well 1 – 5385 m.
36
Figure S11.
Maps of laser ablation locations
for U-Pb analysis and support
ing laser ablation elemental maps
37
for Well 2 – 5325.5 m.
38
Figure S12.
Terra-Wasserburg isoc
hron plot from laser ablation data acquir
ed on sample Well 2 – 5325.5 m
39
Figure S13.
Maps of laser ablation locations
for U-Pb analysis and support
ing laser ablation elemental maps
40
for Well 2 – 5591 m.
41
Figure S14.
Terra-Wasserburg isoc
hron plot from laser ablation data acquir
ed on sample Well 2 – 5591m.
42
Page 2 of 47
Figure S15.
Maps of laser ablation locations
for U-Pb analysis and support
ing laser ablation elemental maps
43
for Well 2 – 5722 m.
44
Figure S16.
Maps of laser ablation locations
for U-Pb analysis and support
ing laser ablation elemental maps
45
for Well 3 – 5420 m.
46
Figure S17.
Cathodoluminescence image of thin section from Well 3 - 5420m.
47
Figure S18.
Terra-Wasserburg isoc
hron plot from laser ablation data acquir
ed on sample Well 3 – 5420m.
48
Figure S19.
Terra-Wasserburg isochron plot from isotope dilution data acqu
ired on sample Well 3 – 5420m.
49
Figure S20.
Maps of laser ablation locations
for U-Pb analysis and support
ing laser ablation elemental maps
50
for Well 3 – 5549.3 m.
51
Figure S21.
Terra-Wasserburg isoc
hron plot from laser ablation data acquir
ed on sample Well 3 – 5549.3 m.
52
Figure S22.
Maps of laser ablation locations
for U-Pb analysis and support
ing laser ablation elemental maps
53
for Well 3 – 5708 m.
54
Figure S23.
Terra-Wasserburg isoc
hron plot from laser ablation data acquir
ed on sample Well 3 – 5708 m.
55
Figure S24.
Terra-Wasserburg isochron plot from isotope dilution data acqu
ired on sample Well 3 – 5708 m.
56
Figure S25.
Maps of laser ablation locations
for U-Pb analysis and support
ing laser ablation elemental maps
57
for Well 4 – 5370.5 m.
58
Figure S26.
Terra-Wasserburg isoc
hron plot from laser ablation data acquir
ed on sample Well 4 – 5370.5 m.
59
Figure S27.
Terra-Wasserburg isochron plot from isotope dilution data acqu
ired on sample Well 4 – 5370.5 m.
60
Figure S28.
Maps of laser ablation locations
for U-Pb analysis and support
ing laser ablation elemental maps
61
for Well 4 – 5444.4 m.
62
Figure S29.
Terra-Wasserburg isoc
hron plot from laser ablation data acquir
ed on sample Well 4 – 5444.4 m.
63
Figure S30.
Maps of laser ablation locations
for U-Pb analysis and support
ing laser ablation elemental maps
64
for Well 4 – 5546 m.
65
Figure S31.
Terra-Wasserburg isoc
hron plot from laser ablation data acquir
ed on sample Well 4 – 5546 m.
66
Figure S32.
δ
13
C vs Δ
47
clumped isotope temperatures obtained on the carbonate samples
analyzed as part of
67
this study.
68
Figure S33.
δ
18
O vs Δ
47
clumped isotope temperatures obtained on the carbonate samples
analyzed as part of
69
this study.
70
Figure S34.
δ
13
C vs δ
18
O obtained on the carbonate sampl
es analyzed as part of this st
udy. The symbols are
71
colored by the Δ
47
clumped isotope temperature obtained from the same sample.
72
73
74
Page 3 of 47
ISOTOPIC BOX MODELING OF EVAPO
RATION OF SANTOS BASIN PRE-SALT
75
LAKE
76
77
In order to understand the paleoenvironmental conditions of the
South Atlantic lacustrine
78
basin, we developed a box model of
fresh water inflow and evapo
ration with two sources of river
79
water δ
18
O
VSMOW
to try to replicate the calculated values of δ
18
O
VSMOW
for the pre-salt lake and
80
consider the time variable c
ontributions in the north and south
of this vast lake system. The box
81
model estimates the geometry of t
he Brazilian salt basins as a
semi-elliptical cap with length of
82
2500 km, width 600 km and maximum water-filled depth of 3.5 km.
A semi-elliptical cap
83
geometry ensures that the su
rface-area to volume ratio of the b
asin is changing (increasing) with
84
depth, which is consistent with t
he geometry of basins. This ge
ometry also ensures that at any
85
given combination of freshwater
flux and evaporation rate, an “
equilibrium” lake level will exist.
86
The depth of the basin was estim
ated using subsidence analysis
at the regional scale. Given the
87
position of the observed occurrences
of shallow-water lacustrin
e carbonates the basin was
88
modeled as initially air-fi
lled with ~1.4 km air-filled compone
nt and freshwater lakes as deep as
89
1.5 km. Two fresh-water inputs
were modeled at 425 billion m
3
/year (river A) and increasing
90
volume from 112 to 225 billion m
3
/year (river B). Cyclic variability in the river flux with an
91
amplitude of ~10% and a peri
od of 23 k.y. was imposed to replic
ate the effect of Milankovic
92
cyclicity. The evaporation rate wa
s defined at 90cm/yr and the
dynamic inflow-evaporation was
93
calculated at 1000 time steps of
1000 years each (a total of 1
m.y).
94
The δ
18
O
VSMOW
isotope evolution of the modeled l
ake was calculated at each t
ime step
95
using the Rayleigh equation i
n (Cappa et al., 2003) and the a
eff
values reported for 50% humidity
96
in their paper. The extent of e
vaporation at each time step (f)
was estimated using the modeled
97
total inflow of river water, the
volume of the lake from the pr
evious time step and the
98
evaporation volume [f = evaporated
volume/(total inflow of rive
r water + lake volume at
99
previous timestep)]. For the model that best fits the δ
18
O
VSMOW
data of the samples in this study
100
(from 2 to 5 ‰), we assume that th
e evaporation fractions at ea
ch time step ranges from 0.40 to
101
0.56 (Supplemental Fig. 1).
102
Page 4 of 47
103
Supplemental Figure 1
. Modeled values of δ
18
O for lake water that evolves over time as a result
104
of contributions from two rivers
in to the pre-salt lakes and a
relative constant evaporation at a
105
humidity of 50%. This represents the best fit conditions to the
calculated δ
18
O
VSMOW
of 2 – 5‰
106
from the carbonate samples that
yield depositional temperatures
below 40 °C
107
108
Waters from “River A” and “Rive
r B” were both defined to have a
δ
18
O
VSMOW
value of
109
−9‰, consistent with precipitati
on at tropical latitudes. These
values, together with the river flux
110
values outlined above replicate the calculated δ
18
O
VSMOW
from the pre-salt l
acustrine carbonates
111
of the South Atlantic basins. This model can be made available
upon request to the authors.
112
113
114
Page 5 of 47
RESULTS
115
116
U-Pb Age Dating
117
118
We document here all images of the samples that were analyzed b
y laser ablation ICPMS
119
in the FIRST lab (Facility for
Isotope Research and Student Tra
ining at Stony Brook University).
120
Instrument- Agilent 7500cx coupled
to a New Wave 213UV laser sy
stem. Spot size 120 microns.
121
For maps, the rate was 15 micrcons per second. For spots, the d
well time was 30 seconds. The
122
data were reduced in Iolite4 a
nd NIST glass standard 612 (G_NIS
T612) was used for the
123
element reference material. The absolute numbers are likely not
accurate because of the
124
difference in the laser interac
tion with the surfaces, but the
relative differences are meaningful.
125
WC-1 was used as the carbonate r
eference material for the U/Pb
reduction. All of the maps are
126
plotted at the same scales and the minimum and maximum for each
element and ratio is shown in
127
Supplemental Table S1.
128
We used the calcite reference mate
rial (RM) WC-1 (Roberts et al
., 2017) as a primary
129
RM. The average
238
U/
206
Pb for this RM is 20.83 and the average
207
Pb/
206
Pb for this RM is 0.18.
130
These values are slightly diffe
rent than reported in Roberts et
al. (2017), but they are measured
131
with isotope dilution of a number
of aliquots and they have bee
n vetted against published calcite
132
results from the FIRST laborat
ory. We monitored for reproducibi
lity of the WC-1 values in Iolite
133
and only accepted runs where the di
spersion about the mean was
controlled. We used an inhouse
134
secondary RM called Barstow to mon
itor performance with the U/P
b analysis. This sample is
135
~15 Ma. Isotope dilution is in pr
ogress, but the published resu
lts from similar samples from Cole
136
et al., (2005) range from 14.8 t
o 16.3 Ma. Batches of data wher
e the Barstow RM did not return a
137
reasonable age were excluded. O
nly one data set was discarded u
sing this criteria. All of the
138
Barstow data run during the analyt
ical sessions relevant to thi
s project is presented in
139
Supplemental Figure 2, and all da
ta is provided in Supplemental
Table S6 for documentation.
140
141
Page 6 of 47
142
Supplemental Figure 2.
Isochron age derived from repeat
ed laser ablation analysis of
the
143
Barstow secondary reference mate
rial used to monitor analytical
performance and reproducibility
144
of sample analysis.
145
146
A subset of 19 samples, selecte
d based on their stratigraphic p
ositions and diagenetic
147
histories, were analyzed by LA
ICPMS to determine if they had f
avorable U/Pb ratios. Multiple
148
maps were made across samples with complex fabrics to obtain in
formation on all phases
149
represented by the sample. Bas
ed on this initial screening, 11
samples from the Barra Velha and
150
4 samples from the Itapema Formations were selected for more co
mprehensive LA ICPMS spot
151
analyses in an effort to define iso
chron ages for the samples (
Supplemental Table S3). To
152
determine the best isochron age
for each sample we plotted the
isotope measurements in
153
238
U/
206
Pb versus
207
Pb/
206
Pb space (i.e., a Tera-Wasserbur
g plot (Tera and Wasserburg, 19
72)).
154
The data were regressed first us
ing the ‘model 1’ approach in I
soPlotR (Vermeesch, 2018) which
155
is an implementation of an error we
ighted least squares approac
h. This assumes the spread in the
156
data is accounted for entirely by the analytical uncertainty. W
hen we determined a sample was
157
overdispersed, as defined by a P
-value of <0.05; we reduced the
data using a ‘model 3′ approach
158
which includes the scatter in the
ages. Note a ‘model 3′ age do
es not necessarily produce a
159
geologically meaningful age as di
spersion can result from mixin
g of multiple age components in
160
Page 7 of 47
a given sample. While the supplemental figures cite a 1σ uncert
ainty on the isochron ages, we
161
discuss these isochron ages in t
he context of a broader 2σ unce
rtainty.
162
163
Supplemental Figure 3.
Maps of laser ablation locatio
ns for U-Pb analysis and support
ing laser
164
ablation elemental maps for Well 1 – 5115.6 m
165
166
Well 1 - 5115.6m: Map 1
167
Page 8 of 47
168
Well 1 – 5115.6: Map 2
169
170
Well 1 - 5115.6m: Map 3
171
Page 9 of 47
172
The sample from well 1 at 5115.6 m depth (Supplemental Figures
3 – 6) was evaluated
173
because it is a good target for understanding the diagenetic hi
story. The brown carbonate
174
observed in the sample correlate
s with regions of high Mg and h
igh Sr while high Si regions
175
(quartz) have higher
238
U/
206
Pb. In this sample, the
238
U/
206
Pb is favorable in the quartz mineral
176
phase (supplemental figure 3). Not
e that the high Si regions ar
e not regions of high Mg, so we
177
are confident that t
he high Si areas are quartz and not stevens
ite. It is also worth noting the high
178
affinity for boron in quartz while R
EEs are higher in the brown
carbonate. For this sample, we
179
obtained a bulk laser-abl
ation isochron age of 103.91 ± 13.98 M
a (Supplemental Fig. 4).
180
181
Page 10 of 47
Supplemental Figure 4.
Terra-Wasserburg isochron plot
for all laser
ablation data acq
uired on
182
sample Well 1 – 5115.6 m.
183
184
From laser ablation we get a ni
ce separation between the spots
on the brown carbonate
185
(brown and red spots in supplemen
tal figure 5) and the white qu
artz (blue spots in supplemental
186
figure 5). The carbonate age exceeds expectations for a reasona
ble depositional age for this
187
relatively young pre-salt
stratigraphy, which w
e interpret to s
uggest the potential for some U
188
loss, while the quartz age sugge
sts an event around 70 Ma. This
sample also records the highest
189
precipitation temperature based on c
lumped isotope analyses (Ta
ble 1), consistent with a
190
significant diagenetic overprint
to the depositional fabric of
this sample. We consider it possible
191
that further analysis of these
distinct diagenetic events may y
ield a more resolvable age
192
constraint on these diagenetic phases.
193
194
Supplemental Figure 5.
Terra-Wasserburg isochron plot f
or the two distinct families o
f laser
195
ablation data acquired on sampl
e Well 1 – 5115.6 m from dolomit
e and quartz.
196
197
With isotope dilution we obtained w
hat we interpret to represen
t mixed age (see
198
supplemental figure 6) than what was determined by higher resol
ution laser ablation. This was
199
also associated with a less radi
ogenic Pb isotopic signature, s
uggesting that the larger sampling
200
Page 11 of 47
size tends to favor the less ra
diogenic, higher Pb concentratio
ns. Perhaps smaller sample sizes
201
targeting areas around the most
radiogenic LA spots could help
to get a better spread and allow
202
us to get a depositional age from
the carbonate phase in this s
ample. Nonetheless, this highlights
203
both the challenges and opportuni
ties associated with U-Pb dati
ng to constrain both depositional
204
and diagenetic ages in these pr
e-salt samples given an appropri
ate level of sample preservation
205
206
Supplemental Figure 6.
Terra-Wasserburg isochron plot
from isotope d
ilution data acqu
ired on
207
sample Well 1 – 5115.6 m.
208
209
210
Page 12 of 47
Supplemental Figure 7.
Maps of laser ablation locatio
ns for U-Pb analysis and support
ing laser
211
ablation elemental maps for Well 1 – 5136.9 m
212
213
Well 1 - 5136.9m: Map 1
214
215
Well 1 - 5136.9m: Map 2
216
Page 13 of 47
217
In the sample evaluated from
well 1 at a depth of 5136.9m, the
lighter calcite has more
218
favorable
238
U/
206
Pb than the darker stylolites
(see supplemental figure 7, map 2
). Note the
219
higher concentrations of Sr and F
e in the lighter calcite versu
s the styolites. Analyses of the
220
lighter calcite show dispersion a
round an isochron (supplementa
l figure 8), with scatter likely
221
caused by mixing with U and Pb from the altered stylolitic calc
ite. This sample yields a strong
222
laser ablation isoch
ron age of 114.46 ± 4.72 Ma.
223
224
Page 14 of 47
225
Supplemental Figure 8.
Terra-Wasserburg isochron plot f
rom laser ablation data acquir
ed on
226
sample Well 1 – 5136.9 m.
227
228
229
Page 15 of 47
Supplemental Figure 9.
Maps of laser ablation locatio
ns for U-Pb analysis and support
ing laser
230
ablation elemental maps for Well 1 – 5385 m.
231
232
Well 1 - 5385m: Map 1
233
234
Well 1 - 5385m: Map 2
235
Page 16 of 47
236
This sample has elevated U, Th, and Pb and higher
238
U/
206
Pb in areas with elevated Mg
237
and Si, which we interpret to be
stevensite. Using all of the l
aser ablation spots gives a very
238
young isochron age of 58.85 ± 10.74 Ma that we interpret to be
a diagenetic age with a large
239
uncertainty. However, there are
higher U/Pb spots with correlat
ed radiogenic Pb that plot well
240
below this isochron suggesting th
at the slope should be signifi
cantly higher. More data focusing
241
on regions with the highest U/Pb c
ould give a depositional age.
242
243
244
Supplemental Figure 10.
Terra-Wasserburg isochron plot f
rom laser ablation data acquir
ed on
245
sample Well 1 – 5385 m.
246
Page 17 of 47
Supplemental Figure 11.
Maps of laser ablation locati
ons for U-Pb analysis and support
ing
247
laser ablation elemental
maps for Well 2 – 5325.5 m.
248
249
Well 2 - 5325.5m, top slab (fr
agmented piece): Map 1
250
Page 18 of 47
251
Well 2 - 5325.5m, bottom sla
b (silicious piece): Map 1
252
253
Well 2 - 5325.5m, bottom sla
b (silicious piece): Map 2
254
Page 19 of 47
255
This sample has very low U concentrations and therefore low
238
U/
206
Pb in the calcite.
256
Note the higher Sr, Mg, Mn, and Fe
in the calcite versus the ne
ar zero levels of all of these
257
elements in the high Si regions (
supplemental figure 11, map 3)
. The
238
U/
206
Pb in the silicified
258
region is slightly more elevate
d than in the calcite, but is no
t favorable for dating. We obtained a
259
laser ablation isochron age
of 129.99 ± 120.32 Ma for this samp
le that we consider
260
unrepresentative.
261
Page 20 of 47
262
Supplemental Figure 12.
Terra-Wasserburg isochron plot f
rom laser ablation data acquir
ed on
263
sample Well 2 – 5325.5 m.
264
265
266