JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 12
PAGES 2581^2596
2013
doi:10.1093/petrology/egt058
40
Ar/39Ar Geochronology of Subaerial Ascension
Island and a Re-evaluation of the Temporal
Progression of Basaltic to Rhyolitic Volcanism
BRIAN R. JICHA1*, BRAD S. SINGER1 AND MICHAEL J. VALENTINE2
1
DEPARTMENT OF GEOSCIENCE, UNIVERSITY OF WISCONSIN^MADISON, MADISON, WI 53706, USA
2
DEPARTMENT OF GEOLOGY, UNIVERSITY OF PUGET SOUND, TACOMA, WA 98416, USA
RECEIVED DECEMBER 18, 2012; ACCEPTED SEPTEMBER 20, 2013
40
Ar/39Ar geochronology of basaltic to rhyolitic lavas, domes, and
pyroclastic deposits from Ascension Island indicates that the maximum age of subaerially exposed samples is 1094 ka. Thirty-eight
40
Ar/39Ar ages, coupled with new and existing geochemical data,
constrain the eruptive histories of the four distinct mafic magma
types (high Zr/Nb, low Zr/Nb, intermediate Zr/Nb, and Dark
Slope Crater) and document temporal variations in magma sources.
Lavas from the eastern felsic complex, previously assumed to be as
old as or slightly younger than the 602^1094 ka Middleton Ridge
complex, are as young as 52 ka. Basaltic to benmoreitic scoria
cones and associated flows of the intermediate Zr/Nb magma type
have been inferred to be the most recent eruptive products, yet their
eruptive histories extend back to 705 ka. These intermediate Zr/
Nb magmas are likely to be parental to the abundant trachytic to
rhyolitic lavas and domes, which contradicts previous interpretations that call upon a high Zr/Nb parental basalt. Two distinct
fractionation trends are observed in the trace element variations of
Ascension trachytes and rhyolites. 40Ar/39Ar ages of the samples
defining the two trends suggest that ilmenite fractionation dominated the Nb budget and thus controlled Zr/Nb ratios in early
(4931ka) Ascension evolved magmas, whereas zircon^titanite
fractionation was predominant in younger felsic magmas.The eruptive sequence and compositions of the subaerial lavas and domes at
Ascension Island are unique in comparison with other ocean island
volcanoes because of its on-axis location and eruptions of highSiO2 trachyte and rhyolite.
Ascension Island;
volcanism; ocean island
KEY WORDS:
40
Ar/39Ar geochronology; alkaline
*Corresponding author. E-mail: [email protected]
I N T RO D U C T I O N
Our understanding of the petrological and geochemical
evolution of intra-oceanic volcanic systems has benefited
from studies of linear island chains (Hawaii^Emperor,
Louisville, French Polynesia, Marquesas) and isolated
islands or island clusters (Gala¤pagos, Socorro, Canaries)
(e.g. Geist et al., 1988; Bohrson et al., 1996; Caroff et al.,
1999; Carracedo et al., 2001, 2007; Harpp & White, 2001;
Koppers & Staudigel, 2005; Weis et al., 2011). Numerous
intra-oceanic volcanoes have been extensively sampled because their geochemical signatures provide information
about specific mantle melting processes or reservoirs.
Geochemical and isotopic data for samples from these volcanoes can be combined with precise geochronology to
document how and when the sources or magmatic processes change. However, the temporal evolution of most
intra-oceanic volcanoes beyond the historical realm is
commonly lacking. There are several notable exceptions
that rival the growing database of well-dated arc volcanoes
(e.g. Jicha et al., 2012a). For example, Carracedo et al.
(2007) published a set of 54 14C and K/Ar ages that provide
precise age control on the recent (51Ma) eruptive history
and differentiation processes at Tiede volcano in the
Canary Islands. On the island of Hawaii, the 5500 ka
eruptive histories of Mauna Loa and Mauna Kea are constrained via numerous 40Ar/39Ar ages (Sharp & Renne,
2005; Jicha et al., 2012b).
The Mid-Atlantic Ridge and the adjacent seamounts
located between the Ascension and Bode Verde Fracture
ß The Author 2013. Published by Oxford University Press. All
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JOURNAL OF PETROLOGY
VOLUME 54
Zones, and Ascension Island itself (Fig. 1), have been extensively studied because they exhibit anomalous crustal
thicknesses and geochemical compositions that have been
interpreted to reflect the presence of either small, enriched
heterogeneities in the upper mantle or the presence of a
weak, diffuse mantle plume (Hoernle et al., 2011). The numerous volcanic islands along the Mid-Atlantic Ridge are
also of interest because large landslides capable of producing tsunamis are common on the edifices that make up
these islands (Mitchell, 2003). Obtaining geological and
geochronological constraints on ocean islands may shed
light on the morphological processes affecting their structure. We have obtained 38 new 40Ar/39Ar ages and 54
whole-rock major and trace element analyses for
Ascension Island volcanic rocks in an effort to gain a
better understanding of the temporal progression of mafic
and silicic volcanism and thus evaluate the changes, if
any, in magma sources through time. Sample locations
are indicated in Fig. 1 and listed in the Appendix.
GEOLOGIC A L S ET T I NG OF
ASCENSION ISLA N D
Ascension Island (7856’S, 1482’W) lies 90 km west of the
Mid-Atlantic Ridge and 50 km south of the Ascension
Fracture Zone in the South Atlantic Ocean (Fig. 1). Geodynamic models suggest that submarine Ascension lavas
erupted at or near the Mid-Atlantic Ridge axis at about
6^7 Ma, 3200 m below sea level (Klingelho«fer et al.,
2001; Bruguier et al., 2003). The 3800 km3 edifice rises
4 km to 859 m above sea level, and covers 2000 km2 of
ocean floor (Brozena, 1986). The subaerial sector of the volcano covers 98 km2, and accounts for only 1% of its
total volume (Harris, 1983).
The first geological account of Ascension Island was
given by Darwin (1845). Subsequent work by Daly (1925),
Atkins et al. (1964), Harris (1983), Nielson & Sibbett (1996),
Weaver et al. (1996), and Kar et al. (1998) provide more
comprehensive descriptions of the geology of the island
and the chemistry and petrology of the erupted products.
Lava flows and domes make up 57% of the island’s total
areal extent, with the remainder consisting of pyroclastic
deposits and scoria (Harris, 1983). Knowledge of the subsurface geology has been obtained from seven shallow
core holes (up to 533 m) and a deep (3126 m) geothermal
exploration well (Nielson & Stiger, 1996). This study
focuses on the subaerial portion of the island.
The central sector of the island is dominated by a felsic
complex that includes Green Mountain, the highest peak
(859 m), and Middleton Ridge. Green Mountain is primarily composed of pyroclastic deposits, both scoria and
pumice, whereas Middleton Ridge is made up of trachytic
flows and domes and a small volume of rhyolite and
basalt (Fig. 1). An eastern felsic complex is made up of
NUMBER 12
DECEMBER 2013
numerous trachytic to rhyolitic domes and flows including
those of Devil’s Cauldron, White Hill, Little White Hill,
and South East Head (Fig. 1). The southern, western, and
northern parts of the island have considerably less relief
and comprise mafic lava flows that are punctuated by
numerous scoria cones (Weaver et al., 1996).
Ascension Island volcanic rocks define a transitional
to mildly alkaline olivine basalt^hawaiite^mugearite^
benmoreite^trachyte^rhyolite series that is characterized
by a lack of erupted lavas with compositions between 58
and 63 wt % SiO2, also known as a ‘Daly Gap’. Ascension
was among the first ocean islands where this gap was documented (Daly, 1910, 1925), although a few pumice samples
have major element compositions that fall within the gap.
Trachyte and rhyolite make up 14% of the surface area
of the island (Nielson & Sibbett, 1996). Trace element geochemistry and least-squares mass-balance modeling
suggest that the trachyte and rhyolite originated by fractionation of high Zr/Nb basalt, the most prevalent of
the four mafic magma types (high Zr/Nb, intermediate
Zr/Nb, low Zr/Nb, Dark Slope Crater hawaiiite) observed
on the island (Weaver et al., 1996; Kar et al., 1998).
Previous K^Ar dating of whole-rock samples by Harris
et al. (1982) indicated a subaerial eruptive history from 1·5
to 0·1Ma; however, the younger 5400 ka record of volcanism is only broadly constrained by five imprecise K^Ar
ages that span the period 350 60 ka to 120 120 ka.
Subsequent studies by Nielson & Sibbett (1996) and Kar
et al. (1998) suggest that felsic volcanism prevailed from
1Ma to 0·56 Ma, followed by eruptions of more mafic
lavas until 0·12 Ma. Before this study, 21K^Ar and four
40
Ar/39Ar ages had been determined for Ascension Island
lavas. There is no record of historical activity on
Ascension, but numerous large-volume hawaiitic flows
from Sisters Peak appear to be only hundreds to a few
thousand years old (Atkins et al., 1964) (Fig. 1).
A N A LY T I C A L M E T H O D S
40
Ar/39Ar incremental heating experiments were undertaken on groundmass, sanidine, and plagioclase. Groundmass separates were prepared following Jicha et al. (2012b).
Sanidine was isolated from the bulk sample via magnetic
sorting and density separation using methylene iodide.
Three trachytic lavas, which lacked abundant sanidine,
had groundmass that was not suitable for dating, and thus
plagioclase separates were prepared. Mineral separates
were weighed and then irradiated at the Oregon State University TRIGA reactor in the Cadmium-Lined In-Core
Irradiation Tube (CLICIT) in four batches during the
period 2001^2011. Fish Canyon Tuff sanidine (FCs) was
used as the neutron fluence monitor for all irradiations.
All age data presented here are calculated relative to
28·201Ma for FCs (Kuiper et al., 2008); the decay constants
used are those of Min et al. (2000).
2582
Fig. 1. Simplified geological map of Ascension Island showing sample locations. Map modified from Nielson & Sibbett (1996). Inset (bottom right) shows the location of Ascension Island.
JICHA et al.
PROGRESSION OF VOLCANISM, ASCENSION
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JOURNAL OF PETROLOGY
VOLUME 54
At the University of Wisconsin Rare Gas Geochronology
Laboratory, 20^50 mg of purified sanidine and plagioclase
from trachytic and rhyolitic lavas were placed in a
3 mm 20 mm copper trough and incrementally heated
using a defocused 25 W CO2 laser. Each step included a
scan across the trough at 150 mm s1 at a given laser
power, followed by an additional 7 min for gas cleanup.
The gas was cleaned during and after the heating period
with two SAES C50 getters. Blanks were analyzed after
every second step. For mafic samples lacking sanidine or a
sufficient amount of plagioclase, furnace incremental heating experiments were performed on 100^180 mg of groundmass following the methods of Jicha et al. (2012b). All
argon isotope analyses were carried out using a MAP 21550, and the data were reduced using ArArCalc software
version 2.5 (http://earthref.org/ArArCALC/). The age
uncertainties reported in Table 1 and Fig. 2 reflect analytical contributions only at the 2s level. The complete dataset is reported in Supplementary Data Table SD 1
(supplementary data are available for downloading at
http://www.petrology.oxfordjournals.org).
Major and trace element whole-rock analyses were performed using X-ray fluorescence (XRF) and inductively
coupled plasma mass spectrometry (ICP-MS) techniques
at the GeoAnalytical Laboratory at Washington State
University (WSU; http://www.sees.wsu.edu/Geolab/index.
html). Major elements analyzed using XRF are precise to
greater than 0·2% (see http://sees.wsu.edu/Geolab/note/
xrfprecision.html). Total alkali^silica (TAS) and Harker
diagrams show major element variations for the rocks analyzed in this study and previous work (Fig. 3). Analytical
precision for most of the trace elements by ICP-MS is typically 3% (R. Conrey, WSU, personal communication).
Throughout this study, we focus on the Zr/Nb ratio,
which has an uncertainty of approximately 4% given
the precision to which each element can be measured.
Moreover, Zr and Nb are also measured by XRF to a
precision of 3·9 ppm and 1·6 ppm, respectively, which
translates to a Zr/Nb ratio with an uncertainty of better than 3·5% for most samples. The precision of the
XRF data is based on hundreds of replicate sample measurements across a wide range of concentrations (R.
Conrey, WSU, personal communication). All major and
trace element data are reported as Supplementary Data
Table SD2.
Paleomagnetic samples were collected to quantify the
paleosecular behavior of the Earth’s magnetic field as recorded during the past 2 Myr at this unique low-latitude
site in the Southern Hemisphere. A diamond-bit, gasolinepowered drill was used to collect 6^14 cores with 1inch
(2.5 cm) diameter and 1^4 inch (2.5^10 cm) length from
each rock unit. All units sampled for paleomagnetic analysis are lava flows or igneous intrusions, and each was
sampled over several meters of each unit’s thickness, as
NUMBER 12
DECEMBER 2013
well as lateral extent, to best characterize the magnetic
signal recorded by the unit. Cores were oriented using
both sun and magnetic compasses, and site locations were
determined using global positioning system (GPS) and
plotting on a topographic map of the island (Directorate
General of Military Survey, 1992). Paleomagnetic analyses
were conducted at the University of Puget Sound
Paleomagnetics Laboratory. The remanent magnetism of
the samples was measured using a Molspin Minispin spinner magnetometer. Alternating field (AF) demagnetization was carried out using a Molspin shielded AF
demagnetizer with dual-axis tumbler, and thermal demagnetization with an ASC TD48 thermal specimen demagnetizer. Stepwise AF demagnetization studies were
performed in 9^11 steps per sample from 0 to 99·9 mT.
Thermal demagnetization was performed in 15 steps from
room temperature to 6808C for each sample. A minimum
of one sample per site was thermally demagnetized, with
two or more samples for most sites. Demagnetization results for both demagnetization methods were plotted on
vector end-point diagrams (Zijderveld, 1967), and characteristic magnetic directions for each sample were determined using line-fitting techniques (Kirschvink, 1980). A
minimum of four demagnetization steps and in most cases
six or more were used for each determination. The maximum angle of deviation (MAD) for acceptable samples
was 58, with most MAD values being less than 38. The
data are reported as Supplementary Data Table SD3.
E RU P T I V E C H RO N O L O G Y
Results from 55 incremental heating experiments on 38
lavas and pyroclastic rocks are summarized in Table 1.
Representative age spectra and isochrons and complete
analyses for each experiment can be found in Fig. 2 and
Supplementary Data Electronic Appendix A, respectively.
All experiments give age spectra with plateaus comprising
483% of the gas released. Multiple experiments were conducted on 11 samples and yield indistinguishable results.
Samples that define statistically acceptable plateaux have
isochrons with trapped 40Ar/36Ar ratios that are indistinguishable from the atmospheric 40Ar/36Ar ratio of 295·5.
Thus, we consider the plateau ages to give the best estimate
of the time elapsed since eruption (Table 1). We also used
the magnetic directions of the samples as an independent
constraint on the eruptive age. All samples with ages
5780 ka have normal polarity, consistent with eruption
during the Brunhes normal chron, whereas all the samples
with ages4780 ka are reversed (Table 1). Results were variable among the multiple samples taken from sites AI-35,
-23, -64, -14, and -10, and no definitive polarity could be
determined for these sites. Samples from site AI-32 are
very weakly magnetized and also yield directions with a
fair amount of scatter.
2584
JICHA et al.
PROGRESSION OF VOLCANISM, ASCENSION
Table 1: Summary of 40Ar/ 39Ar incremental heating experiments on Ascension Island lavas and domes
Sample
Rock type
Material
K/Ca
40
Ar/36Ari 2s
Isochron 2s
total
MSWD
39
Ar%
age (ka)
Plateau
2s
Polarity
N
age (ka)
AI-26
mugearite
gmass
0·41
295·9
0·8
27
19
0·15
85·5
38
9
AI-23
rhyolite
san
20·46
295·7
1·7
52
3
0·57
100·0
52
3
AI-25
rhyolite
san
20·73
296·1
6·6
53
5
0·81
95·2
54
2
N
AI-51
hawaiite
gmass
0·26
295·4
1·7
142
28
0·14
100·0
140
19
N
AI-53
trachyte
plag
0·20
296·8
2·7
163
44
0·69
99·4
169
43
N
AI-64
hawaiite
gmass
0·38
295·0
1·5
314
55
0·68
100·0
298
22
AI-14
hawaiite
gmass
0·18
267·7
1·3
278
50
0·81
97·2
323
18
AI-24
basalt
gmass
0·14
297·0
3·9
326
14
0·29
100·0
329
11
N
AI-27
basalt
gmass
0·24
294·7
2·4
336
11
0·18
100·0
333
8
N
AI-28
basalt
gmass
0·27
295·0
1·8
342
11
0·43
100·0
340
7
N
AI-60
hawaiite
gmass
0·16
296·8
1·7
306
109
0·63
100·0
386
33
N
AI-66
basalt
gmass
0·14
296·2
3·6
389
49
0·34
95·3
395
34
N
AI-31
benmoreite
gmass
0·43
295·1
1·1
470
30
0·45
98·7
461
18
N
AI-59
mugearite
gmass
0·52
296·0
1·1
509
20
1·02
100·0
514
14
N
AI-02
mugearite
gmass
0·36
295·7
5·9
517
22
0·16
100·0
517
11
N
AI-56
hawaiite
gmass
0·26
296·9
1·6
518
26
0·63
100·0
534
18
N
AI-11
basalt
gmass
0·20
292·7
5·2
591
30
0·04
100·0
577
12
N
AI-04
mugearite
gmass
0·36
295·5
1·9
584
33
0·24
100·0
582
9
N
AI-54
DS hawaiite
gmass
0·38
294·8
2·8
593
23
0·37
100·0
589
14
N
AI-15
trachyte
san
17·87
444·8
482·8
575
68
0·48
100·0
602
7
N
AI-47
trachyte
san
24·69
308·1
68·0
599
20
0·54
100·0
603
6
N
AI-68
trachyte
san
8·38
290·6
10·3
609
9
1·41
100·0
606
6
N
AI-16
trachyte
san
23·28
291·7
18·4
608
5
0·50
100·0
607
3
N
AI-16
trachyte
gmass
33·08
295·6
2·6
607
3
1·11
99·4
607
3
N
AI-46
trachyte
plag
0·12
296·0
1·7
634
46
0·31
100·0
637
45
N
AI-63
trachyte
san
6·74
290·4
21·0
655
13
0·65
100·0
652
5
N
AI-09
trachyte
gmass
2·14
295·4
1·9
690
3
0·11
83·6
690
2
N
AI-20
basalt
gmass
0·17
296·3
0·9
643
79
0·36
91·2
705
29
N
AI-35
rhyolite
san
22·69
295·5
1·4
720
33
0·55
98·5
719
13
AI-55
basalt
gmass
0·24
296·0
1·1
744
40
0·38
100·0
758
27
AI-10
mugearite
gmass
0·49
295·5
3·3
791
13
0·38
100·0
791
6
AI-01
trachyte
san
18·03
294·8
19·1
823
11
0·75
100·0
823
6
R
AI-06
trachyte
san
16·88
290·9
12·0
833
13
0·89
100·0
829
7
R
AI-34
benmoreite
gmass
1·03
295·7
1·8
903
12
0·02
98·8
903
6
R
AI-43
rhyolite
san
25·47
296·5
8·2
928
30
0·24
96·8
931
14
R
AI-12
trachyte
san
24·75
298·8
21·3
943
8
0·30
100·0
943
7
R
AI-42
benmoreite
gmass
0·72
296·8
4·0
963
32
0·48
100·0
972
11
R
AI-32
rhyolite
plag
0·28
293·4
7·8
1000
43
0·32
100·0
994
37
AI-44
rhyolite
san
3·31
296·3
7·9
1093
16
0·65
100·0
1094
12
N
R
Ages calculated relative to 28·201 Ma Fish Canyon sanidine (Kuiper et al., 2008) using decay constant of Min et al. (2000).
gmass, groundmass; san, sanidine; plag, plagioclase; DS hawaiite, Dark Slope Crater hawaiite; N, normal polarity; R,
reversed polarity. Samples for which no polarity is listed either showed scatter between samples or were weakly
magnetized.
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NUMBER 12
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Fig. 2. 40Ar/39Ar age plateau and isochron diagrams for three Ascension lavas showing the various ages (2s errors) and spectra obtained
from incremental heating experiments. Arrows indicate steps used to determine plateau age.
1094^719 ka Middleton Ridge
Based on field relationships and a limited number of K^Ar
and 40Ar/39Ar ages, Kar et al. (1998) suggested that the
oldest exposed rocks on Ascension Island are 1Myr old
and consist of trachytic and rhyolitic activity from the
Middleton Ridge eruptive center. Our new age determinations are consistent with this interpretation. Sanidine
from the Middleton Ridge rhyolite (AI-44) yields an
40
Ar/39Ar plateau age of 1094 12 ka, the oldest material
we have found on the island. The 40Ar/39Ar age is slightly
older than published K^Ar ages, which range
from 940 380 to 990 40 ka (Nielson & Sibbert, 1996).
Table 2 provides a comparison of the published age data
for Ascension with those in this study. It should be noted
that our sample of the Middleton Ridge Rhyolite flow
from map unit ‘Mr’ of Nielson & Sibbett (1996) is actually
a trachyte with 68·1wt % SiO2 and unusually high P2O5
(1·47 wt %) (Figs 1 and 3). The Middleton Ridge rhyolite
is overlain by a series of interbedded trachytic pyroclastic
rocks and lavas and a minor amount of mafic material.
Trachytic (AI-12), rhyolitic (AI-32), and benmoreitic (AI42) lavas from within the Middleton Ridge complex yield
40
Ar/39Ar ages ranging from 994 37 to 943 7 ka
(Fig. 1). Several of these units have been cut by ENE^
WSW-trending, high-angle, normal faults. A benmoreite
dike (AI-34) that intruded one of the faults was dated at
903 6 ka (Table 1) and plagioclase from a rhyolitic
dike (AI-43) gave an age of 931 14 ka (Tables 1 and 2).
2586
JICHA et al.
PROGRESSION OF VOLCANISM, ASCENSION
Fig. 3. Major element variation diagrams for Ascension Island volcanic rocks. (a) Total alkali vs silica (TAS) diagram and classification after
Le Bas et al. (1986); (b) wt % MgO vs wt % SiO2; (c) wt % TiO2 vs wt % SiO2. The low total alkali contents of several rhyolites may be the
result of Na loss (Weaver et al., 1996). Data are from this study and from published studies by Daly (1925), Harris (1983), Nielson & Sibbett
(1996), Weaver et al. (1996), Kar et al. (1998) and Paulick et al. (2010).
The youngest dated unit from Middleton Ridge is a rhyolite (AI-35), which gave an age of 719 13 ka.
829^652 ka trachytic volcanism and the first
mafic lavas
Numerous small trachytic flows and domes crop out to the
north and west of Middleton Ridge, dated from 829 to
652 ka. Trachytes from Daly’s Crags (AI-01), Bear’s Back
(AI-06) and Thistle Hill (AI-09) in the northern half of
the island (Fig. 1) give 40Ar/39Ar plateau ages ranging
from 829 7 ka to 690 2 ka. 40Ar/39Ar ages for the
Bear’s Back trachyte and Daly’s Crags fall within the
range of the whole-rock K^Ar ages of Harris et al. (1982)
and Nielson & Sibbert (1996), but are more precise.
Sanidine from a trachyte dome (AI-63) that intruded into
mafic flows near Devil’s Riding School 1km west of
Middleton Ridge gives a 40Ar/39Ar age of 652 5 ka.
Pumice that filled a basin atop the dome yielded a
40
Ar/39Ar age of 610 20 ka (Kar et al., 1998). The
829^652 ka period, which is dominated by trachytic volcanism, also includes the first subaerially exposed mafic
lavas. A mugearite (AI-10) just north of Cross Hill near
Long Beach gave a 40Ar/39Ar age of 791 6 ka. Two basalts, one east of Cross Hill (AI-20) and another just west
of Devil’s Riding School, yield ages of 758 27 ka and
705 29 ka, respectively.
637^602 ka Green Mountain and Ragged
Hill trachytes
The final stage of the 500 kyr period of silicic volcanism
consists of trachytic flows and domes on the SW slope of
Green Mountain and at Ragged Hill (Fig. 1). Plagioclase
from the dome NE of Mountain Red Hill (AI-46) gave a
plateau age of 637 45 ka, whereas sanidine from two
trachytic lavas on Green Mountain (AI-47 and AI-16)
gave nearly identical ages of 603 6 and 607 3 ka
(Tables 1 and 2). The 40Ar/39Ar ages are consistent with
previous K^Ar ages for these trachytes from Green
Mountain, which ranged from 560 60 to 650 40 ka
(Table 2). Emplacement of the trachyte at Ragged Hill
was contemporaneous with the lavas on the flank of
Green Mountain as indicated by the 40Ar/39Ar age of
606 6 ka from sample AI-68 (Fig. 1, Table 1).
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Table 2: Comparison of age data on Ascension Island lavas and domes
Unit
Middleton Ridge rhyolite
Middleton Ridge trachyte
Material
K–Ar
K–Ar
40
40
Harris et al. (1982)
N & S (1996)
Kar et al. (1998)
This study
Sample
Age (ka)
Age (ka) 2s
Age (ka)
Age (ka) 2s
number
1094
12
AI-44
943
7
AI-12
2s
Ar/39Ar
Ar/39Ar
2s
sanidine
glass
990
40
whole-rock
940
380
820
40
sanidine
890
groundmass
40
Dike at Middleton Ridge
feldspar
920
60
931
14
AI-43
Benmoreitic dike, Middleton Ridge
groundmass
800
60
903
6
AI-34
Bear’s Back trachyte
whole-rock
610
560
829
7
AI-06
740
340
823
6
AI-01
sanidine
Daly’s Crags trachyte
whole-rock
Thistle Hill trachyte
whole-rock
1000
600
800
1400
sanidine
groundmass
Devil’s Riding School trachyte pumice sanidine
Green Mountain trachyte
sanidine
Hawaiite, 1000 yards SW of
whole-rock
Castle Hill
Basalt, east side of Cricket Valley
200
140
350
60
660
40
650
40
560
60
groundmass
whole-rock
groundmass
Weather Post trachyte
sanidine
Crystal Bay flow
whole-rock
670
glass
60
120
120
20
690
2
AI-09
652
5
AI-63
607
3
AI-16
395
34
AI-66
329
11
AI-24
54
2
AI-25
20
no
40
Ar*
AI-48
N & S (1996), Nielson & Sibbett (1996).
589^298 ka mafic volcanism
Weaver et al. (1996) and Kar et al. (1998) showed that
Ascension Island mafic rocks have highly variable trace
element compositions. The basalt and hawaiite compositions have been subdivided into four mafic suites following
the nomenclature of Weaver et al. (1996). Three of the
basalt types are discriminated based on differences in
their Zr/Nb ratios, which are large enough to be resolved
given the precision to which the ratio can be determined
(4%; see Analytical Methods for details).
In the southwestern part of the island, the low Zr/Nb
hawaiite flows and scoria have Zr/Nb ratios of 4·1^4·4.
The high Zr/Nb flows are mostly restricted to the southern
part of the island and have Zr/Nb ratios of 5·7^6·3.
Intermediate Zr/Nb (4·8^5·4) basalts and hawaiites are
scattered throughout the island and are interpreted to be
the result of mixing of the low and high Zr/Nb magmas
or their sources (Weaver et al., 1996). The fourth magma
type is rare and is related to eruptions from the Dark
Slope Crater vent (Fig. 1). Hawaiite and mugearite lavas
from Dark Slope Crater are similar in composition to the
intermediate Zr/Nb basalts, but are distinguished by
higher Sr concentrations.
Published K^Ar data from mafic lavas on Ascension are
restricted to a few high Zr/Nb lavas and are imprecise, ranging from 470 200 ka to 120 120 ka (Harris et al., 1982).
40
Ar/39Ar experiments were conducted on groundmass
from basaltic to hawaiitic lavas from all four magma
types. The oldest lava erupted during the 300 kyr period
of predominantly mafic volcanism (Fig. 4) is a hawaiite
from the eastern flank of Dark Slope Crater (AI-54) that
gave a 40Ar/39Ar age of 589 14 ka, which is indistinguishable from a 40Ar/39Ar age of 599 11ka obtained for a
Dark Slope Crater hawaiite from shallow core hole GH3
(M. Heizler, personal communication). The majority of
the 589^298 ka mafic lavas erupted on the southern half
of the island. However, it should be noted that we did not
collect many samples of mafic lava from northeastern
Ascension (i.e. Broken Tooth) or along the eastern shoreline. The high Zr/Nb basalt is the most common basalt
2588
JICHA et al.
PROGRESSION OF VOLCANISM, ASCENSION
Fig. 4. (a) SiO2 (wt %) vs age (ka) and (b) Zr/Nb vs age (ka).
40
Ar/39Ar ages from this study are preferred over the previously published K^Ar and 40Ar/39Ar ages. The 610 ka age for the Riding
School rhyolite pumice is from Kar et al. (1998). Dotted rectangle
shows the range of Zr/Nb ratios reported for older (41Ma)
Ascension Island lavas in the geothermal drill hole (Kar et al., 1998;
Paulick et al., 2010). Symbols as in Fig. 3.
type on southern Ascension, but samples were also obtained from a low Zr/Nb flow located SW of South
Gannett Hill (AI-56, 534 18 ka) and an intermediate
Zr/Nb basalt (AI-64, 298 22 ka).
emplaced. A small outcrop of rhyolite south of Weather
Post atop the eastern flank of Cricket Valley (AI-23) gave
a 40Ar/39Ar plateau age of 52 3 ka (Fig. 1). Kar et al.
(1998) reported a 40Ar/39Ar age of 670 20 ka for a trachyte from the Weather Post dome. Because no sample locations or analytical details were provided, it remains
unclear exactly where this sample was collected or why
the age is significantly older than our 40Ar/39Ar age. Nielson & Sibbert (1996) obtained a K^Ar age of 60 20 ka
for glass from a trachyte at Weather Post but did not publish the age because they believed it to be erroneous. We attempted 40Ar/39Ar experiments on several geomorphically
younger rhyolites, such as the White Hill dome (AI-58),
but they did not yield detectable radiogenic 40Ar because
they are probably late Holocene in age. The new
40
Ar/39Ar age determinations indicate that the eastern
eruptive complex is significantly younger than the Middleton Ridge silicic center.
Numerous late Pleistocene to Holocene basalts occupy
the low-lying areas between the older domes and lavas.
A mugearite basalt near Long Beach (AI-26) gave a
40
Ar/39Ar age of 38 9 ka. The Crystal Bay basalt (AI48), a high Zr/Nb basalt, did yield detectable radiogenic
40
Ar* and is therefore probably late Holocene in age.
Several large-volume, intermediate Zr/Nb hawaiites from
Sisters Peak (including AI-61) appear to be only hundreds
to a few thousand years old (Atkins et al., 1964). Overall,
the 38 new 40Ar/39Ar ages do not dramatically revise the
eruptive history of Ascension Island, but the precision of
these ages is significantly better than those of the previous
studies and thereby allows us to assess the progression of
mafic volcanism on the island and evaluate changes in
magma composition through time.
4169 ka volcanism: eastern felsic complex
and Holocene mafic lavas
DISCUSSION
Eruptive sequence of the four mafic
magma types
The eastern felsic complex is believed to be younger than
Middleton Ridge, but, up to this point, there have been insufficient age data to support this hypothesis. The complex
is composed of trachytic to rhyolitic domes, mafic lavas
and scoria, and two explosion craters, Cricket Valley and
Devil’s Cauldron (Fig. 1). After the mafic lavas and pyroclastic rocks built up the walls of Cricket Valley, a voluminous trachyte lava erupted from the Weather Post^Devil’s
Cauldron area and flowed towards North East Point
(Fig. 1; Nielson & Sibbert, 1996). Plagioclase from this
trachyte (AI-53) gave a 40Ar/39Ar age of 169 43 ka. This
is the first radioisotopic age available for the Devil’s
Cauldron flow, as previous attempts yielded no detectable
radiogenic argon (Nielson & Sibbert,1996). Approximately
100 kyr after the eruption of the Devil’s Cauldron flow,
the 54 2 ka Weather Post rhyolite dome (AI-25) was
Prior to this study, there were insufficient age data to define
the time of eruption of the different basalt types. Weaver
et al. (1996) used field relationships and limited K^Ar
data to suggest that the eruptive phases of the different
mafic magma types were non-overlapping. They suggested
that the high Zr/Nb basaltic volcanism was contemporaneous with, and preceded the felsic volcanism. Subsequently,
there was a localized eruption of Dark Slope Crater
hawaiite followed by eruption of the low Zr/Nb magma.
The intermediate Zr/Nb basaltic to benmoreitic flows are
interpreted to be recent eruptions and thus younger than
the other three mafic magma types (Weaver et al., 1996).
The new 40Ar/39Ar ages are not entirely consistent with
the proposed age relationships. The 534 18 ka low Zr/Nb
hawaiite from the southwestern part of the island is
younger than the 589 14 ka Dark Slope Crater hawaiite
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JOURNAL OF PETROLOGY
VOLUME 54
(Table 1), which is in accord with the Weaver et al. (1996)
age model. The intermediate Zr/Nb lavas are among the
most recent eruptive products on the island with ages as
young as 38 9 ka. However, the eruptive history of the
intermediate Zr/Nb lavas extends back to 705 ka, thereby
making it the second oldest mafic magma type erupted.
The high Zr/Nb magmas are the oldest mafic magma
type. They also appear to have been contemporaneous
with eruptions of the other three mafic magma types
from 600 to 330 ka (Fig. 4) and have erupted recently,
which contradicts the suggestion of Weaver et al. (1996)
that the four mafic phases were non-overlapping.
Sources and genesis of Ascension mafic
magmas
Previous studies of Ascension Island volcanism favored a
plume origin for the mafic magmas (Burke & Wilson,
1976; Morgan & Chen, 1993). P- and S-wave tomography
data show a low-velocity anomaly indicative of an upper
mantle plume beneath the Ascension region down to
650 km, where it merges with an upper mantle plume beneath St. Helena (Montelli et al., 2006). The continuous
lower mantle plume visible in the P-wave velocity model
bends eastward and appears to connect with the African
superplume. In contrast, it has been suggested, based on
Pb isotope variations in mid-ocean ridge basalts from 7 to
128S, that a mantle plume is not present beneath Ascension, but exists beneath Circe seamount, located 550 km
east of Ascension on the eastern side of the Mid-Atlantic
Ridge (Hanan et al., 1986). A wide-angle seismic profile
crossing the island reveals a crustal thickness of 12^13 km
and little evidence of a hotspot origin (Klingelho«fer et al.,
2001).
Despite the disagreement regarding the origin of
Ascension, only Weaver et al. (1996) and Paulick et al.
(2010) have attempted to use the geochemical and isotopic
data to evaluate the petrogenesis of the different
Ascension mafic magmas (high Zr/Nb basalt, low Zr/Nb
hawaiite, intermediate Zr/Nb basalt and hawaiite, and
Dark Slope Crater hawaiite). Weaver et al. (1996) suggested
that the low and high Zr/Nb basalts were not produced
by different degrees of melting of a common source because of the significant contrast in isotopic compositions
between the two suites of basalts (see Kar et al., 1998,
fig. 6). We expand upon this comparison of the mafic
magmas and explore the genetic relationship, if any, between the high Zr/Nb and intermediate Zr/Nb basalts,
which have similar Sr, Nd, and Pb isotope compositions
(Kar et al., 1998).
Most of the trace element compositions and ratios
appear to suggest that the intermediate Zr/Nb basalts are
related to the high Zr/Nb basalts by fractional crystallization. For example, the rare earth element patterns of the
two basalt types are subparallel, numerous trace element
ratios are indistinguishable (e.g. Zr/Hf, Lu/Hf, Sm/Nd),
NUMBER 12
DECEMBER 2013
and several incompatible trace element abundances
(Th, U, Ba, Hf) are higher in the intermediate Zr/Nb basalts, suggesting that they are differentiates of the high
Zr/Nb basalt (Fig. 5). Upon closer investigation, a genetic
relationship between the intermediate and high Zr/Nb
basalts becomes less apparent when comparing certain
high-field strength element (HFSE) ratios and abundances. Both the intermediate and high Zr/Nb basalts
have typical ocean island basalt (OIB) signatures such as
positive Nb and Ta anomalies (Weaver et al., 1986).
However, Ti, Ta, and Nb abundances are lower in the
high Zr/Nb basalts (Figs 3 and 6), thereby resulting in
more muted positive Nb and Ta anomalies. Differences in
the abundances of these HFSE in ocean island basalts are
often attributed to pelagic sediment contamination of the
magma source region, mantle source mineralogy, or
degree of mantle melting. A pelagic sediment component
is absent in the Ascension magmas despite the fact that it
is present in other southern Atlantic Ocean island volcanoes; for example, Tristan da Cunha and Gough (Weaver
et al., 1986). Therefore, the Ti, Ta, and Nb abundances in
the Ascension basalts are probably controlled by the
mantle mineralogy and/or the degree of melting. A larger
per cent mantle melt could result in the lower Ta abundances observed in the high Zr/Nb basalt suite (Fig. 6).
Alternatively, rutile, amphibole and, to a lesser extent,
clinopyroxene are capable of controlling the Ti, Nb, and
Ta budgets during mantle melting. Numerous experimental studies of HFSE partitioning have identified rutile as
the major host for Nb and Ta (Jenner et al., 1994; Brenan
et al., 1995; Foley et al., 2000). Mu«nker et al. (2004) demonstrated that Nb/La and Zr/Sm ratios in Kamchatka^western Aleutian lavas correlate positively if rutile is in
control during magma genesis, whereas the presence of
low-Mg amphibole would cause these ratios to be inversely
correlated. In Fig. 7, Ascension high Zr/Nb basalts have
positively correlated Nb/La and Zr/Sm ratios, which
trend to lower values than those of the intermediate
Zr/Nb basalts. These observations suggest that rutile may
be present during the mantle melting that produced the
high Zr/Nb basalts. Thus, the difference between the
HFSE abundances in the high vs intermediate Zr/Nb basalts could be explained by residual rutile during melting,
origin from a distinctly different mantle source, or varying
degrees of melting of a common source. A detailed interrogation of these petrogenetic models is beyond the scope of
this paper.
The sources of the subaerial high and intermediate
Zr/Nb basalts differ from the mantle domain that generated
the 41 Ma submarine Ascension edifice. Data from shallow
boreholes and the deep exploration well indicate that submarine Ascension basalts have higher Zr/Nb and Lu/Hf
ratios, as well as lower La/Yb ratios than the subaerial
flows (Kar et al., 1998; Paulick et al., 2010) (Fig. 4). Two
2590
JICHA et al.
PROGRESSION OF VOLCANISM, ASCENSION
Fig. 6. V ppm vs Ta ppm for all Ascension Island lavas. Symbols as in
Fig. 3.
Fig. 5. (a) Primitive mantle normalized rare earth element patterns
for Ascension basalts to bemoreites. The concentrations for each
mafic suite represent an average composition for the entire suite.
Data are normalized to the primitive mantle values of McDonough
& Sun (1995). (b) Zr ppm vs Hf ppm for all Ascension lavas from
this study. Symbols as in Fig. 3.
recent trace element and multi-isotope investigations of onand off-axis volcanism in the vicinity of Ascension agree
that the mantle domain during submarine volcanism had
a particular elevated eHf signature and was not involved
in the genesis of the magmas that produced the subaerial
edifice (Paulick et al., 2010; Hoernle et al., 2011). The submarine Ascension seamount probably formed at or near the
Mid-Atlantic Ridge at 5^6 Ma (Klingelho«fer et al., 2001;
Paulick et al., 2010). As a result of westward drift of the lithosphere, the Ascension submarine edifice eventually became
disconnected from the magma source along the ridge axis
and eruptions ceased (Paulick et al., 2010). After a hiatus of
unknown duration, magmatism was reinitiated when enriched melts entered beneath Ascension, resulting in the formation of the subaerial edifice and eruption of numerous
trachytic to rhyolitic lavas and domes.
Contrasting fractionation trends in old vs
young rhyolites
Even though the origin of Ascension basalts (on-axis vs
plume-related) has been debated, there is general
Fig. 7. Nb/La vs Zr/Sm for all Ascension basalts and hawaiites with
51wt % SiO2, after Mu«nker et al. (2004). Symbols as in Fig. 3.
agreement regarding the origin of the evolved lavas.
Benmoreites have been modeled to result from crystal fractionation of the intermediate Zr/Nb magma type along a
simple liquid line of descent (Kar et al., 1998), whereas the
trachytes and rhyolites are assumed to have been generated
by crystal fractionation of high Zr/Nb basalts (Harris,
1983; Weaver et al., 1996; Kar et al., 1998). This genetic association is based primarily on the limited age information
available prior to this study, which suggested that (1) the
high Zr/Nb magmas were contemporaneous with, and
probably preceded, the felsic volcanism, and (2) intermediate basalts to benmoreites are the most recent eruptive
products and are unlikely to represent magmas parental
to the earlier felsic volcanism (Kar et al., 1998). However,
our new age information indicates that the intermediate
Zr/Nb basalts are also contemporaneous with the
1094^602 ka period of felsic volcanism and several benmoreites are older than the oldest dated high Zr/Nb
2591
JOURNAL OF PETROLOGY
VOLUME 54
basalt (Table 2, Fig. 4). This observation, coupled with the
linear trends exhibited in both major and trace element
variations between the intermediate Zr/Nb basalts and
trachytes, suggests that the felsic magmas may have been
derived from intermediate Zr/Nb basalts. If the trachytes
and rhyolites were indeed differentiates of high Zr/Nb
basalt as previously suggested (Weaver et al., 1996; Kar
et al., 1998), it is unusual that no intermediate by-products
(i.e. mugearites, benmoreites) along this fractionation
trend have erupted, thereby resulting in a very large compositional gap between the erupted magmas (48^51
to463 wt % SiO2).
Based on Zr/Nb vs SiO2 variations, Kar et al. (1998)
identified two fractionation trends among the evolved
Ascension magmas, one with a moderately increasing Zr/
Nb and the other with rapidly increasing Zr/Nb with
SiO2. We also observe two distinct fractionation trends in
plots of Zr/Nb vs Nb and Zr/Nb vs Zr (Fig. 8), and attribute the sharp increase in Zr/Nb ratios to the crystallization of ilmenite, which has a high partition coefficient for
Nb (DNb 50) compared with Zr (DZr 1) (Ewart &
Griffin, 1994; Stimac & Hickmott, 1994). Removal of a cumulate assemblage with low Zr/Nb (55) was postulated as
an alternative means by which fractionating liquids could
be driven to higher Zr/Nb (Kar et al., 1998). The decreasing
trend in Zr/Nb ratios in several trachytes and rhyolites is
probably due to zircon^titanite fractionation (Fig. 8).
Most of the fractionating assemblage in the trachytes
and rhyolites (K-feldspar plagioclase clinopyroxene titanomagnetite), with the exception of the trace phases,
does not significantly differ from the assemblage in the
benmoreites. Therefore, in the absence of ilmenite or
zircon^titanite fractionation, one would expect there to
continue to be a positive correlation on the Zr/Nb variation diagrams in Fig. 8.
The dated lavas that form the zircon^titanite fractionation trend include rhyolites from the Weather Post region
(AI-23 and AI-25), White Hill Dome (AI-58), the youngest
rhyolite (719 ka) from Middleton Ridge (AI-35), and the
823 ka trachyte from Daly’s Crags (AI-01) (Fig. 1). With
the exception of the 652 ka trachyte dome (AI-63), all the
trachytes and rhyolites that form the ilmenite fractionation
trend are older than 931ka. Although not conclusive, the
ages of the samples in the two trends suggest that ilmenite
fractionation dominated the Nb budget and thus controlled the Zr/Nb ratios in older (4931ka) Ascension
evolved magmas, whereas zircon^titanite fractionation
was predominant in younger magmas.
Despite the change in fractionating assemblage, highSiO2 dacite (66^69 wt % SiO2) and rhyolite were erupted
for almost 500 kyr, which is uncommon for an ocean
island volcano in terms of the compositions erupted and
the duration of evolved lava eruptions. It remains unclear
what mechanisms promoted this prolonged pulse of felsic
NUMBER 12
DECEMBER 2013
Fig. 8. (a) Zr/Nb vs Nb (ppm) and (b) Zr/Nb vs Zr (ppm) for all
Ascension lavas. The two distinct trachyte to rhyolite fractionation
trends should be noted. Symbols and data sources are the same as
those in Fig. 3.
volcanism. Basaltic input and associated heat output to the
surrounding rock upon cooling has occurred repeatedly
for at least 6 Myr during the construction of the 3800 km3
submarine Ascension edifice, which is an unusually long
period of magma and heat flux at a given location. The sustained magma flux at Ascension has probably produced
the anomalously high subsurface temperatures. The
Ascension #1 geothermal well had a bottomhole temperature of 246·88C at a depth of 3048 m (Nielson & Stiger,
1996), implying a gradient of 4758C km1çthree times
greater than the standard geothermal gradient of
258C km1 and on par with the proposed upper crustal
geotherm for large silicic magma complexes such as the
Altiplano^Puna (de Silva & Gosnold, 2007). The elevated
subsurface temperatures and thermal maturation of the
subvolcanic crust may have facilitated the production of
51·1Ma evolved magmas.
Comparison with other ocean island
volcanoes
In this section, we compare the 1094 ka subaerial eruptive
history and compositional evolution of Ascension Island
with those of other well-characterized Pacific and Atlantic
Ocean island volcanoes.
2592
JICHA et al.
PROGRESSION OF VOLCANISM, ASCENSION
Most intraplate ocean island volcanoes are interpreted
to form as a result of decompression melting of a mantle
plume at shallow depths (Morgan, 1971). Linear volcanic
island chains form as melts ascend through moving oceanic lithosphere for periods of tens of millions of years. In
the prototypical Hawaiian^Emperor chain, the islands of
Kaua’i and Ni’ihau preserve eruptive histories spanning
4·5^5·5 Myr (Garcia et al., 2010), which is similar to the
inferred age for the submarine þ subaerial Ascension edifice. These volcanoes follow a progression from shield to
post-shield to rejuvenated volcanism. Rejuvenated lavas at
Hawaiian volcanoes closely resemble subaerial Ascension
lavas in that they represent a tiny fraction (1%) of the
total eruptive volume and are geochemically and isotopically distinct from the underlying shield lavas (e.g. Clague
& Frey, 1982; Garcia et al., 2010). However, Hawaiian rejuvenated lavas typically range from alkali basalt to foidite,
but the Ascension lavas are not silica-undersaturated.
In addition to the wide range in erupted mafic lava compositions observed at ocean island volcanoes, evolved
lavas (trachytes, phonolites) are also prevalent, whereas
lavas of intermediate composition are typically lacking
(i.e. there is a Daly Gap). Ua Pou in the Marquesas archipelago, French Polynesia, is known for its exceptional
abundance of phonolites, which cover 65% of the surface
of the island. It has an eruptive sequence similar to that of
Ascension in that it has erupted olivine tholeiite at 4 Ma,
phonolites from 2·90 to 2·86 Ma, basanites and tephriphonolites from 2·76 to 2·38 Ma, and coeval basanites and
phonolites from 2·60 to 2·35 Ma (Legendre et al., 2005).
The cyclic nature of mafic and evolved eruptions at Ua
Pou and Ascension is a common feature of ocean islands
as noted decades ago by Le Maitre (1959). However, the
preferred petrogenetic process responsible for both the
Daly Gap and the abundant evolved lavas at Ua Pou is
extensive remelting at depth of basanitic materials
(Legendre et al., 2005), not simple crystal fractionation as
we advocate for Ascension.
Several other southern Atlantic Ocean island volcanoes
also have evolved similarly to Ascension, but they also
have notable differences. On Tristan da Cunha, basanite is
the dominant rock type with few exposures of tephriphonolite and phonolite (Le Roux et al., 1990). The parental
mafic magmas are derived from a heterogeneous mantle
source and follow two distinct fractionation trends, which
is also observed at Ascension. However, the basanites are
related to melts derived from a mantle plume and the
evolved lavas are nepheline normative.
The magmatic evolution of Gough Island is perhaps the
most analogous to that of Ascension. K^Ar ages suggest
that the subaerial eruptive history spans 1Myr and includes a progression from alkali basalt to trachyte (up to
63 wt % SiO2) from 1000 to 500 ka, followed by a re-emergence of basalt at 130 ka (Le Roux, 1985). Alkali basalt
suites have contrasting trace element ratios, which is similar to Ascension. Trachytes have been modeled to result
from 60% fractional crystallization of basalt (Le Roux,
1985; Harris et al., 2000). However, there are a limited
number of erupted compositions between 56 and 61wt %
SiO2 and there are no erupted compositions with 464 wt
% SiO2 on Gough. In summary, the eruptive sequence
and compositions of the subaerial lavas and domes at
Ascension Island are unique in comparison with other
ocean island volcanoes because of its on-axis location and
eruptions of high-SiO2 trachyte and rhyolite.
CONC LUSIONS
40
Ar/39Ar geochronology of mafic lavas on Ascension
Island has revealed that eruptions of high Zr/Nb magma
were contemporaneous with those of the other three mafic
magma types from 600 to 330 ka. Intermediate Zr/Nb
magmas are not restricted to the most recent eruptions,
and are sourced from a mantle domain that has a different
mineralogy from that of the high Zr/Nb basalts.
Ascension trachytes and rhyolites probably formed via
fractional crystallization of the intermediate Zr/Nb
magmas, which contradicts previous interpretations. Two
distinct fractionation trends are observed in the trachytes
and rhyolites. Ilmenite fractionation dominated the HFSE
distribution in older (4931ka) evolved magmas, whereas
zircon^titanite fractionation controlled the trace element
budget in younger felsic magmas. We suggest that the prolonged period of basaltic magmatism during construction
of the submarine edifice, coupled with recent thermochemical upwelling, may have provided the appropriate
subvolcanic conditions necessary for differentiation of alkaline basalt to high-SiO2 trachyte and rhyolite, compositions not typically seen at most ocean island volcanoes.
AC K N O W L E D G E M E N T S
We thank the Amanda Miller and Alexandra Macho for
sample preparation. The paper was greatly improved by
the constructive comments of Jim Gill and two anonymous
reviewers.
FU N DI NG
This project has been supported, in part, by US NSF
grants including EAR-0337667, OCE-0825659, and EAR1144494.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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R EF ER ENC ES
Atkins, F. B., Baker, P. E., Bell, J. D. & Smith, D. G. W. (1964). Oxford
expedition to Ascension Island. Nature 204, 722^724.
Bohrson, W. A., Reid, M. R., Grunder, A. L., Heizler, M. T.,
Harrison, T. M. & Lee, J. (1996). Prolonged history of silicic peralkaline volcanism in the eastern Pacific Ocean. Journal of
Geophysical Research 101, 11457^11474.
Brenan, J. M., Shaw, H. F., Ryerson, F. J. & Phinney, D. L. (1995).
Mineral^aqueous fluid partitioning of trace elements at 9008C
and 2·0 GPa: constraints on the trace element geochemistry of
mantle and deep crustal fluids. Cosmochimica et Cosmochimica Acta
59, 3331^3350.
Brozena, J. M. (1986). Temporal and spatial variability of seafloor
spreading processes in the northern South Atlantic. Journal of
Geophysical Research 91, 497^510.
Bruguier, N. J., Minshull, T. A. & Brozena, J. M. (2003). Morphology
and tectonics of the Mid-Atlantic Ridge, 78^128S. Journal of
Geophysical Research 108, doi:10.1029/2001JB001172.
Burke, K. C. & Wilson, J. T. (1976). Hot spots on the Earth’s surface.
Scientific American 235, 46^57.
Caroff, M., Guillou, H., Lamiaux, M., Maury, R. C., Guille, G. &
Cotton, J. (1999). Assimilation of ocean crust by hawaiitic and
mugearitic magmas: an example from Eiao, Marquesas. Lithos 46,
235^258.
Carracedo, J. C., Rodr|¤ guez Badiola, E., Guillou, H., De La Nuez, J.
& Pe¤rez Torrado, F. J. (2001). Geology and volcanology of La
Palma and El Hierro (Canary Islands). Estudios Geolo¤ gicos 57,
175^273.
Carracedo, J. C., Rodr|¤ guez Badiola, E., Guillou, H., Paterne, M.,
Scaillet, S., Pe¤rez Torrado, F. J., Paris, R., Fra-Paleo, U. &
Hansen, A. (2007). Eruptive and structural history of Teide
Volcano and rift zones of Tenerife, Canary Islands. Geological
Society of America Bulletin 119, 1027^1051.
Clague, D. A. & Frey, F. A. (1982). Petrology and trace element
geochemistry of the Honolulu Volcanics, Oahu; implications for
the oceanic mantle below Hawaii. Journal of Petrology 23, 447^504.
Daly, R. A. (1910). Origin of alkaline rocks. Geological Society of America
Bulletin 21, 87^118.
Daly, R. A. (1925). The geology of Ascension Island. Proceedings of the
American Academy of Arts and Sciences 60, 1^80.
Darwin, C. (1845). Journal of Researches into the Natural History and Geology
of the Various Countries Visited during the Voyage of HMS Beagle round the
World, 2nd edn. London: John Murray, 324 p.
de Silva, S. L. & Gosnold, W. D. (2007). Episodic construction of
batholiths: Insights from the spatiotemporal development of an ignimbrite flare-up. Journal of Volcanology and Geothermal Research 167,
320^335, doi:10.1016/j.jvolgeores.2007.07.015.
Directorate General of Military Survey (1992). Ascension Island
1:25,000 Map, Series G 892, Edition 4-GSGS, Ministry of Defence,
UK, 1 sheet.
Ewart, A. & Griffin, W. L. (1994). Application of proton-microprobe
data to trace-element partitioning in volcanic rocks. Chemical
Geology 117, 251^284, doi:10.1016/0009-2541(94)90131-7.
Foley, S. F., Barth, M. G. & Jenner, G. A. (2000). Rutile/melt partition
coefficients for trace elements and an assessment of the influence
of rutile on the trace element characteristics of subduction zone
magmas. Geochimica et Cosmochimica Acta 64, 933^938.
Garcia, M. O., Swinnard, L., Weis, D., Greene, A. R., Tagami, T.,
Sano, H. & Gandy, C. (2010). Petrology, geochemistry, and geochronology of Kaua’i lavas over 4·5 Myr: Implications for the
origin of rejuvenated volcanism and the evolution of the Hawaiian
plume. Journal of Petrology 51, 1507^1540.
NUMBER 12
DECEMBER 2013
Geist, D. J., White, W. M. & McBirney, A. R. (1988). Plume^asthenosphere mixing beneath the Gala¤pagos archipelago. Nature 333,
657^660.
Hanan, B. B., Kingsley, R. H. & Schilling, J.-G. (1986). Pb isotope evidence in the South Atlantic for migrating ridge^hotspot interactions. Nature 322, 137^144.
Harpp, K. S. & White, W. M. (2001). Tracing a mantle plume: Isotopic
and trace element variations of Gala¤pagos seamounts. Geochemistry,
Geophysics, Geosystems 2, paper number 2000GC000137.
Harris, C. (1983). The petrology of lavas and associated plutonic inclusions of Ascension Island. Journal of Petrology 24, 424^470.
Harris, C., Bell, J. D. & Atkins, F. B. (1982). Isotopic composition of
lead and strontium in lavas and coarse-grained blocks from
Ascension Island, South Atlantic. Earth and Planetary Science Letters
60, 79^85.
Harris, C., Smith, H. S. & Le Roex, A. P. (2000). Oxygen isotope
composition of phenocrysts from Tristan da Cunha and
Gough Island lavas: variation with fractional crystallization and
evidence for assimilation. Contributions to Mineralogy and Petrology
138, 164^175.
Hoernle, K., Hauff, F., Kokfelt, T. F., Haase, K., Garbe-Scho«nberg, D.
& Werner, R. (2011). On- and off-axis chemical heterogeneities
along the South Atlantic Mid-Ocean-Ridge (5^118S): Shallow or
deep recycling of ocean crust and/or intraplate volcanism? Earth
and Planetary Science Letters 306, 86^97.
Jenner, G. A., Foley, S. F., Jackson, S. E., Green, T. H., Fryer, B. J. &
Longerich, H. P. (1994). Determination of partition coefficients for
trace elements in high pressure^temperature experimental run
products by laser ablation microprobe-inductively coupled plasmamass spectrometry (LAM-ICP-MS). Geochimica et Cosmochimica et
Acta 58, 5099^5103.
Jicha, B. R., Coombs, M. L., Calvert, A. T. & Singer, B. S. (2012a).
Geology and 40Ar/39Ar geochronology of the medium- to high-K
Tanaga volcanic cluster, western Aleutians. Geological Society of
America Bulletin 124, 842^856.
Jicha, B. R., Rhodes, J. M., Singer, B. S. & Garcia, M. O. (2012b).
40
Ar/39Ar geochronology of submarine Mauna Loa volcano,
Hawaii. Journal of Geophysical Research 117, B09204, doi:10.1029/
2012JB009373.
Kar, A., Weaver, B., Davidson, J. & Colucci, M. (1998). Origin of differentiated volcanic and plutonic rocks from Ascension Island,
South Atlantic Ocean. Journal of Petrology 39, 1009^1024.
Kirschvink, J. L.. (1980). The least squares line and plane and the analysis of paleomagnetic data. Geophysical Journal of the Royal
Astronomical Society 62, 699^718.
Klingelho«fer, F., Minshull, T. A., Blackman, D. K., Harben, P. &
Childers, V. (2001). Crustal structure of Ascension Island from
wide-angle seismic data: implications for the formation of nearridge volcanic islands. Earth and Planetary Science Letters 190, 41^56.
Koppers, A. A. & Staudigel, H. (2005). Asynchronous bends in Pacific
seamount trails: A case for extensional volcanism? Science 307,
904^907.
Kuiper, K. F., Deino, A., Hilgen, F. J., Krijgsman, W., Renne, P. R. &
Wijbrans, J. R. (2008). Synchronizing rock clocks of Earth history.
Science 320, 500^505.
Le Bas, M. J., Le Maitre, R. W., Streckeisen, A. & Zanettin, B. (1986).
A chemical classification of volcanic rocks based on the total
alkali^silica diagram. Journal of Petrology 27, 745^750.
Legendre, C., Maury, R. C., Caroff, M., Guillou, H., Cotten, J.,
Chauvel, C., Bollinger, C., He¤mond, C., Guille, G., Blais, S.,
Rossi, P. & Savanier, D. (2005). Origin of exceptionally abundant
phonolites in Ua Pou island (Marquesas, French Polynesia): partial
2594
JICHA et al.
PROGRESSION OF VOLCANISM, ASCENSION
melting of basanites followed by crustal contamination. Journal of
Petrology 46, 1925^1962.
Le Maitre, R. W. (1959). The geology of Gough Island, South Atlantic.
Overseas Geology and Mineral Resources 7, 371^380.
Le Roex, A. P. (1985). Geochemistry, mineralogy, and magmatic evolution of the basaltic and trachytic lavas from Gough Island,
South Atlantic. Journal of Petrology 26, 149^186.
Le Roex, A. P., Cliff, R. A. & Adair, B. J. I. (1990). Tristan da Cunha,
South Atlantic: Geochemistry and petrogenesis of a basanite^
phonolite lavas series. Journal of Petrology 31, 779^812.
McDonough, W. F. & Sun, S.-S. (1995). Composition of the Earth.
Chemical Geology 120, 223^253, doi:10.1016/0009-2541(94)00140-4.
Min, K., Mundil, R., Renne, P. R. & Ludwig, K. R. (2000). A test for
systematic errors in 40Ar/39Ar geochronology through comparison
with U/Pb analysis of a 1·1-Ga rhyolite. Geochimica et Cosmochimica
Acta 64, 73^98.
Mitchell, N. C. (2003). Susceptibility of mid-ocean ridge volcanic islands and seamounts to large-scale landsliding. Journal of
Geophysical Research 108, 2397, doi:10.1029/2002JB001997.
Montelli, R., Nolet, G., Dahlen, F. A. & Masters, G. (2006). A catalogue of deep mantle plumes: new results from finite-frequency
tomography. Geochemistry, Geophysics, Geosystems 7, doi:10.1029/
2006GC001248.
Morgan, J. P. & Chen, Y. J. (1993). Dependence of ridge-axis morphology on magma supply and spreading rate. Nature 364, 706^708.
Morgan, W. J. (1971). Convection plumes in the lower mantle. Nature
230, 42^43.
Mu«nker, C., Wo«rner, G., Yogodzinski, G. & Churikova, T. (2004).
Behaviour of high field strength elements in subduction zones: constraints from Kamchatka^Aleutian arc lavas. Earth and Planetary
Science Letters 224, 275^293.
Nielson, D. L. & Sibbett, B. S. (1996). Geology of Ascension Island,
South Atlantic Ocean. Geothermics 25, 427^448.
Nielson, D. L. & Stiger, S. G. (1996). Drilling and evaluation of
Ascension #1, a geothermal exploration well on Ascension
Island, South Atlantic Ocean. Geothermics 25, 543^560.
Paulick, H., Mu«nker, C. & Schuth, S. (2010).The influence of small-scale
mantle heterogeneities on mid-ocean ridge volcanism: evidence from
the southern Mid-Atlantic Ridge (7830’S to 11830’S) and Ascension
Island. Earth and Planetary Science Letters 296, 299^310.
Sharp, W. D. & Renne, P. R. (2005). The 40Ar/39Ar dating of core recovered by the Hawaii Scientific Drilling Project (phase 2), Hilo,
Hawaii. Geochemistry, Geophysics, Geosystems 6, Q04G17, doi:10.1029/
2004GC000846.
Stimac, J. & Hickmott, D. (1994). Trace-element partition coefficients
for ilmenite, orthopyroxene and pyrrhotite in rhyolite determined
by micro-PIXE analysis. Chemical Geology 117, 313^330, doi:10.1016/
0009-2541(94)90134-1.
Weaver, B., Kar, A., Davidson, J. & Collucci, M. (1996). Geochemical
characteristics of volcanic rocks from Ascension Island, South
Atlantic Ocean. Geothermics 25, 449^470.
Weaver, B. L., Wood, D. A., Tarney, J. & Joron, J. L. (1986). Role of
subducted sediment in the genesis of ocean island basalts:
Geochemical evidence from South Atlantic Ocean islands. Geology
14, 275^278.
Weis, D., Garcia, M. O., Rhodes, J. M., Jellinek, M. & Scoates, J. S.
(2011). Role of the deep mantle in generating the compositional
asymmetry of the Hawaiian mantle plume. Nature Geoscience 4,
831^838.
Zijderveld, J. D. A. (1967). AC demagnetization of rocks: analysis of
results. In: Collinson, D. W., Creer, K. M. & Runcorn, S. K. (eds)
Methods in Palaeomagnetism. Elsevier. pp. 254^286.
A P P E N D I X : S A M P L E L O C AT I O N S A N D D E S C R I P T I O N S
Sample
Latitude
Longitude
Rock type
Location
AI-01
–7·918
–14·379
trachyte
Daly’s Crags
AI-02
–7·922
–14·345
mugearite
Bear’s Back
AI-04
–7·929
–14·348
mugearite
south of Bear’s Back
AI-06
–7·929
–14·348
trachyte
Bear’s Back
AI-09
–7·931
–14·365
trachyte
Thistle Hill
AI-10
–7·925
–14·406
mugearite
north side of Cross Hill
AI-11
–7·961
–14·363
high Zr/Nb basalt
basalt north of Grazing Valley
AI-12
–7·957
–14·365
trachyte
Middleton Ridge trachyte
AI-14
–7·924
–14·402
intermediate Zr/Nb hawaiite
Long Beach
AI-15
–7·957
–14·357
trachyte
SE of Middleton Ridge
AI-16
–7·957
–14·355
trachyte
Green Mountain trachyte
AI-20
–7·926
–14·392
intermediate Zr/Nb basalt
west of Sister’s Peak
AI-21
–7·928
–14·387
intermediate Zr/Nb basalt
west of Sister’s Peak
AI-23
–7·950
–14·327
rhyolite
south of Weather Post
AI-24
–7·949
–14·327
high Zr/Nb basalt
east side of Cricket Valley
AI-25
–7·920
–14·380
rhyolite
Weather Post
AI-26
–7·923
–14·406
mugearite
Long Beach
AI-27
–7·954
–14·326
high Zr/Nb basalt
SE of Cricket Valley
(continued)
2595
JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 12
DECEMBER 2013
Sample
Latitude
Longitude
Rock type
AI-28
–7·959
–14·335
high Zr/Nb basalt
Castle Hill
AI-31
–7·967
–14·349
benmoreite
north of Ragged Hill
AI-32
–7·957
–14·369
rhyolite
Valley of Dikes
AI-33
–7·955
–14·370
rhyolite
Middleton Ridge
AI-34
–7·957
–14·370
benmoreite
Benmoreitic dike, Middleton Ridge
AI-35
–7·952
–14·371
rhyolite
Middleton Ridge
AI-37
–7·910
–14·402
mugearite
north of Long Beach
AI-38
–7·910
–14·402
mugearite
north of Long Beach
AI-39
–7·911
–14·402
benmoreite
north of Long Beach
AI-40
–7·902
–14·375
benmoreite
north of Sister’s Peak
AI-41
–7·941
–14·368
intermediate Zr/Nb basalt
south of Traveler’s Hill
AI-42
–7·952
–14·361
benmoreite
Middleton Ridge
AI-43
–7·951
–14·368
rhyolite
dike at Middleton Ridge
AI-44
–7·952
–14·361
rhyolite
Middleton Ridge rhyolite
AI-45
–7·943
–14·409
mugearite
along Georgetown road
AI-46
–7·963
–14·354
trachyte
dome ENE of Mountain Red Hill
AI-47
–7·962
–14·353
trachyte
flow NE of Mountain Red Hill
AI-48
–7·955
–14·322
high Zr/Nb basalt
Crystal Bay flow
AI-50
–7·928
–14·345
mugearite
south of Bear’s Back
AI-51
–7·921
–14·340
intermediate Zr/Nb hawaiite
east of Bear’s Back
AI-52
–7·921
–14·340
intermediate Zr/Nb hawaiite
east of Bear’s Back
AI-53
–7·920
–14·338
trachyte
Ariane Station (Devil’s Cauldron flow)
AI-54
–7·961
–14·385
Dark Slope Crater hawaiite
Dark Slope Crater
AI-55
–7·959
–14·385
high Zr/Nb basalt
west side of Devil’s Riding School
AI-56
–7·979
–14·397
low Zr/Nb hawaiite
south of South Gannett Hill
AI-57
–7·981
–14·395
intermediate Zr/Nb basalt
south of South Gannett Hill
AI-58
–7·949
–14·317
rhyolite
White Hill
AI-59
–7·953
–14·319
mugearite
south of White Hill
AI-60
–7·925
–14·365
intermediate Zr/Nb hawaiite
Perfect Crater
AI-61
–7·930
–14·363
intermediate Zr/Nb hawaiite
Street Crater
AI-62
–7·936
–14·393
benmoreite
west of Lady Hill
AI-63
–7·959
–14·376
trachyte
Devil’s Riding School trachyte pumice
AI-64
–7·967
–14·382
intermediate Zr/Nb hawaiite
south of Devil’s Riding School
AI-65
–7·954
–14·367
benmoreite
Middleton Ridge
AI-66
–7·960
–14·339
high Zr/Nb basalt
1000 yards SW of Castle Hill
AI-68
–7·974
–14·351
trachyte
south of South East Crater
2596
Location