JOURNAL OF
PETROLOGY
Journal of Petrology, 2015, Vol. 56, No. 8, 1495–1518
doi: 10.1093/petrology/egv043
Advance Access Publication Date: 28 August 2015
Original Article
Olivine Major and Trace Element Compositions
in Southern Payenia Basalts, Argentina:
Evidence for Pyroxenite–Peridotite Melt
Mixing in a Back-arc Setting
Nina Søager1*, Maxim Portnyagin1, Kaj Hoernle1, Paul Martin Holm2,
Folkmar Hauff1 and Dieter Garbe-Schönberg3
1
GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany, 2Department of Geosciences and
Natural Resource Management, University of Copenhagen, Copenhagen DK-1350, Denmark and 3Institute of
Geosciences, Christian-Albrechts-Universität zu Kiel, 24118 Kiel, Germany
*Corresponding author. Present address: Department of Geosciences and Natural Resource
Management, University of Copenhagen, Copenhagen DK-1350, Denmark. Telephone: þ49 431 600 2572.
Fax: þ49 431 600 2924. E-mail: [email protected]
Received January 20, 2015; Accepted July 20, 2015
ABSTRACT
Olivine major and trace element compositions from 12 basalts from the southern Payenia volcanic
province in Argentina have been analyzed by electron microprobe and laser ablation inductively
coupled plasma mass spectrometry. The olivines have high Fe/Mn and low Ca/Fe and many fall at
the end of the global olivine array, indicating that they were formed from a pyroxene-rich source
distinct from typical mantle peridotite. The olivines with the highest Fe/Mn have higher Zn/Fe, Zn
and Co and lower Co/Fe than the olivines with lower Fe/Mn, also suggesting contributions from a
pyroxene-rich source. Together with whole-rock radiogenic isotopes and elemental concentrations,
the samples indicate mixing between two mantle sources: (1) a pyroxene-rich source with EM-1
ocean island basalt type trace element and isotope characteristics; (2) a peridotitic source with
more radiogenic Pb that was metasomatized by subduction-zone fluids and/or melts. The increasing contributions from the pyroxene-rich source in the southern Payenia basalts are correlated
with an increasing Fe-enrichment, which caused the olivines to have lower forsterite contents at a
given Ni content. Al-in-olivine crystallization temperatures measured on olivine–spinel pairs are between 1155 and 1243 C and indicate that the magmas formed at normal upper mantle (asthenospheric) temperatures of 1350 C. The pyroxene-rich material is interpreted to have been brought
up from the deeper parts of the upper mantle by vigorous asthenospheric upwelling caused by
break-off of the Nazca slab south of Payenia during the Pliocene and roll-back of the subducting
slab beneath Payenia. The pyroxene-rich mantle mixed with peridotitic metasomatized South
Atlantic mantle in the mantle wedge beneath Payenia.
Key words: back-arc basalts; mantle temperature; olivine; peridotite; pyroxenite
INTRODUCTION
The early Miocene to Recent Payenia volcanic province
in Argentina is one of many basaltic provinces in the
Patagonian back-arc, but it has experienced the most
intensive Quaternary volcanic activity continuing into
historical times. Lava compositions in Payenia range
from arc andesites to alkaline intraplate basalts with
minimal or no subduction-zone influence. The
Quaternary lavas are mainly alkaline basalts with varying slab inputs; the southern Payenia basalts have the
lowest slab influence and Enriched Mantle-1 (EM-1)
type trace element patterns (e.g. Stern et al., 1990;
C The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: [email protected]
V
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Bermudez et al., 1993; Kay et al., 2006a, 2013; Bertotto
et al., 2009; Dyhr et al., 2013a, 2013b; Jacques et al.,
2013; Søager & Holm, 2013; Søager et al., 2013). Søager
& Holm (2013) proposed that the southern Payenia basalts were asthenospheric melts primarily derived from a
pyroxenitic mantle source, though in some cases also
with contributions from peridotitic mantle. On the basis
of an electrical resistivity model, it was recently suggested that the volcanism in southern Payenia was
related to a plume-like structure in the upper mantle beneath the area (Burd et al., 2014).
The use of primitive olivine compositions to
decipher the nature of primary basaltic magmas and
their mantle sources has become increasingly popular,
because olivine is often the first phase to crystallize and
can therefore preserve features of the primitive
magmas. In studies of olivines from ocean island
basalts (OIB) and large igneous province (LIP) basalts,
the main aim has been to deduce the lithology of the
mantle sources; that is, whether the magmas were
produced by melting of peridotitic or some form of
pyroxenitic mantle. Many OIB and LIP magmas apparently contain large fractions of pyroxenite-derived melts
(e.g. Sobolev et al., 2005, 2007, 2008; Gurenko et al.,
2009, 2010, 2013; Barker et al., 2014; Mallik & Dasgupta,
2014).
Sobolev et al. (2007) proposed a model to estimate
the relative amounts of pyroxenite- and peridotitederived melt in the magmas of oceanic and asthenospheric intraplate basalts using the minor elements Mn,
Ni and Ca in olivine. Their model was based on the assumption that the pyroxenite melts were derived from
second-stage pyroxenite, a pyroxenite formed through
reaction of silica-rich eclogite melt with peridotite in the
upwelling asthenospheric mantle. The lower bulk partition coefficient (D) for Ni in pyroxenite relative to peridotite causes the melts to be enriched in Ni, whereas
the higher bulk D values for Ca and Mn cause the melts
and olivines to have low Ca and Mn and higher Fe/Mn
relative to peridotite melts, thereby allowing a distinction between these types of melt (Sobolev et al., 2005,
2007; Herzberg, 2011). Similarly, the lower bulk D values
for Zn and Co in pyroxenite relative to peridotite (Le
Roux et al., 2011; Davis et al., 2013) should cause the
pyroxenite melts to have higher contents of Zn and Co.
Recently, it has been proposed that variations in the
concentrations and ratios of these and other first row
transition elements (FRTE) can be used to distinguish
melts of different mantle lithologies (Qin & Humayun,
2008; Le Roux et al., 2010, 2011; Davis et al., 2013; Lee,
2014). Recent experiments by Mallik & Dasgupta (2012,
2013, 2014) have also shown that mixtures of eclogite
melt and peridotite produce melts with elevated Fe/Mn
in equilibrium with residues with lower olivine modes
than in peridotite, but not necessarily olivine-free.
Therefore, the term pyroxenite will subsequently be
used as defined by Le Maı̂tre et al. (2002) for pyroxenedominated rocks with <40% olivine, whereas peridotites have >40% olivine.
Journal of Petrology, 2015, Vol. 56, No. 8
Using the Sobolev et al. (2007) olivine model and
whole-rock isotope analyses, Gurenko et al. (2009, 2010,
2013) assigned different isotopic end-members for
mafic volcanic rocks from the Canary and Madeira
Islands to either peridotitic or pyroxenitic mantle lithologies. Another approach involves the comparison of
the whole-rock major element compositions of volcanic
rocks with experimentally produced melts using, for example, Fe/Mn ratios (e.g. Liu et al., 2008; Wang et al.,
2012) or CaO contents (Timm et al., 2009), assuming
that pyroxenite melts have lower CaO and higher Fe/Mn
than peridotite melts (Herzberg & Asimow, 2008;
Herzberg, 2011). In this study, we combine electron
microprobe (EMP) major and minor element and laser
ablation inductively coupled plasma mass spectrometry
(LA-ICP-MS) trace element compositions of olivines
with both radiogenic isotope and major and trace element whole-rock data to characterize the lithology of the
Payenia mantle source and to test the hypothesis that
the EM-1-type southern Payenia back-arc mantle source
is pyroxenitic. The LA-ICP-MS trace element compositions of the olivines are then used to check if the FRTE
reflect the variations expected for pyroxenite–peridotite
melt mixtures (Le Roux et al., 2010, 2011; Davis et al.,
2013). The new data add to the currently sparse trace
element data for magmatic olivines [see compilation by
Foley et al. (2013)].
Furthermore, we present magmatic temperatures
calculated with the Al-in-olivine geothermometer developed by Wan et al. (2008) and later modified by
Coogan et al. (2014) for a subset of the studied samples.
This geothermometer is based on the temperature dependence of the partitioning of Al2O3 between forsterite-rich olivine and chromium spinel, and has the
advantage that it is independent of the H2O content and
oxygen fugacity of the magmas and the crystallization
pressure at crustal and upper mantle depths (Wan et al.,
2008).
AGES AND REGIONAL GEOLOGY
The Payenia volcanic province is positioned in the backarc of the Southern Volcanic Zone (SVZ) of the Andes at
34–38 S and volcanism occurs up to 500 km away from
the Chile Trench (Fig. 1). The Nazca plate is currently
subducting beneath the region at a rate of 63 cm a–1
(Kendrick et al., 2003) and the slab dip is 30 in the
southern part. Tomography studies show that the slab
beneath Payenia is continuous down to the lower part
of the upper mantle and forms a smooth transition to
the Pampean flat slab segment to the north (Pesicek
et al., 2012). South of 38 S, however, the slab extends
to only 200 km depth owing to slab break-off in the
Pliocene (Pesicek et al., 2012). The crust in the area has
a thickness of 40 km and the total lithospheric thickness extends to 80 km (Gilbert et al., 2006; Tassara
et al., 2006; Alvarado et al., 2007; Mazzarini et al., 2008).
Quaternary volcanism in the Payenia province occurred
in seven volcanic fields: Llancanelo, Payún Matrú
Journal of Petrology, 2015, Vol. 56, No. 8
1497
Fig. 1. (a) Map of southernmost South America showing the Payenia province as the northernmost of the Cenozoic Patagonian basaltic back-arc provinces (in black). The volcanoes of the SVZ arc front are shown as yellow triangles. The sample location for the
Laguna Blanca sample is shown with a white circle. The blue lines are slab contours for the Payenia region based on the tomographic model of Pesicek et al. (2012) and the orange dashed line shows the inferred southern edge of the slab beneath Payenia.
The map is partly redrawn from Stern et al. (1990). (b) Map of the southern part of the Payenia province [marked with a rectangle in
(a)] with sample locations marked with white circles. The extent of the various Quaternary volcanic fields (VF) is outlined with black
lines. The green dashed lines show the approximate boundaries of the central depression (CD) foreland basin and the black dotted
line marks the Carbonilla fracture (CF). The area of the San Rafael Block (SRB) largely coincides with the Nevado VF. The dashed
blue lines outline the Payún Matrú (PM) east plateau flows erupted from the eastern Carbonilla fracture.
including Pampas Negras, Rı́o Colorado, Auca Mahuida
and Tromen in the south (termed the southern Payenia
group) and Nevado and the Northern Segment in the
north (northern Payenia group) (Fig. 1). The Payún
Matrú and Llancanelo volcanic fields in the western
Payenia province overlie Cenozoic sediments of the
Central Depression, which is a north–south-trending
foreland basin running along the foot of the Andean
Principal Cordillera (e.g. Yrigoyen, 1993; Llambı́as et al.,
2010). The Rı́o Colorado and Auca Mahuida volcanic
fields are located on Mesozoic to Paleogene sediments
of the Neuquén basin (e.g. Vergani et al., 1995). The
Nevado and Northern Segment volcanism occurs on
the San Rafael block, which is a basement block uplifted
during late Miocene compressional tectonics.
The volcanism of the Payenia province started in the
earliest Miocene during a period of extensional deformation, with eruption of the Matancilla and Fortunoso
basalts, which form a large plateau north, south and SE
of the Payún Matrú volcano (Kay & Copeland, 2006;
Dyhr et al., 2013a, 2013b). Lava remnants of similar age
and geochemistry to the Matancilla basalts are also
found in the Filo Morado ridge south of the Colorado
river (Kay & Copeland, 2006) and in the southern part of
1498
the San Rafael block (P. M. Holm, unpublished data),
indicating that this was a major volcanic event. During
the middle and late Miocene, shallowing of the slab dip
instigated compressional deformation, which led to the
uplift of the San Rafael block and caused eruption of arc
and back-arc lavas in large parts of the province (Kay
et al., 2004, 2006a, 2006b; Ramos & Kay, 2006). The
compressional regime continued into Pliocene to early
Pleistocene times (e.g. Rosello et al., 2002; Cobbold &
Rosello, 2003; Galland et al., 2007; Giambiagi et al.,
2008). It was followed by mild extension and relaxation
of the crust associated with a steepening of the slab
(Folguera et al., 2008, 2009; Ramos & Folguera, 2011).
Volcanism in southern Payenia resumed in the
Pliocene, possibly contemporaneous with slab breakoff just south of Payenia (Pesicek et al., 2012). In the
Payún Matrú volcanic field, extensive plateau basalts
were erupted from the eastern part of the Carbonilla
fracture (CF, Fig. 1 inset) during the Pliocene and
Pleistocene (Núñez, 1976; Melchor & Casadio, 1999;
Dyhr et al., 2013a). The Auca Mahuida volcanic field
was active between 18 and 09 Ma and produced a
basaltic shield volcano and pyroclastic cones (Rosello
et al., 2002; Kay et al., 2006a, 2013). In the Rı́o Colorado
region, volcanism predominantly formed scattered
monogenetic events between 15 and 03 Ma (Bertotto
et al., 2006, 2009; Kay et al., 2006b, 2013; Gudnason
et al., 2012). The latest activity took place in the Payún
Matrú volcanic field where the large basaltic to trachytic
Payún Matrú and Payún Liso volcanoes were built over
the Carbonilla fracture within the last 300 kyr (Germa
et al., 2010; Gudnason et al., 2012). The basaltic Pampas
Negras cone field and associated flow field were
formed over the western part of the Carbonilla fracture
zone between 300 ka and the present (González Dı́az,
1972).
South of Payenia, in the near back-arc of the SVZ, a
small volcanic field developed within the Loncopué
trough, an extensional basin formed by crustal attenuation during the Pliocene and Quaternary period
(Folguera et al., 2007; Rojas Vera et al., 2014). The volcanism at the southernmost end of the basin around
the Laguna Blanca lake (39 S) dominantly consists of
basaltic cinder cones and lava flows, presumably
formed during Late Pleistocene to Holocene times
(Varekamp et al., 2010; Rojas Vera et al., 2014).
PREVIOUS WORK
The vast majority of the Payenia volcanic rocks are alkali basalts and trachybasalts with moderate incompatible trace element enrichments (Bermudez et al., 1993;
Kay et al., 2006a, 2013; Bertotto et al., 2009; Dyhr et al.,
2013a, 2013b; Jacques et al., 2013; Søager et al., 2013).
Based on major and trace element and isotope variations in the Payenia basalts, it has been argued that
the basalts from the Payenia volcanic province originated from two mantle sources (Søager & Holm, 2013;
Søager et al., 2013, 2015). The first is a South Atlantic
Journal of Petrology, 2015, Vol. 56, No. 8
mid-ocean ridge basalt (MORB)-like peridotitic mantle
metasomatized by slab fluids and melts, proposed to be
the main source for the northern Payenia basalts and
the SVZ arc rocks (Jacques et al., 2013), but which probably also contributed to the southern Payenia basalts, in
particular in the Payún Matrú volcanic field. The second
is a pyroxenitic mantle component with an EM-1 OIBtype isotope and trace element composition termed the
Rı́o Colorado component. This component was found
to contribute to the southern Payenia basalts and to the
early Miocene Matancilla and Fortunoso basalts (Dyhr
et al., 2013a, 2013b; Kay et al., 2013). It was also proposed that the basalts from the Loncopué trough south
of Payenia could have been formed in part from the Rı́o
Colorado mantle component (Søager & Holm, 2013).
However, there is a strong slab fluid component in
these basalts, which makes it difficult to evaluate the
nature of the source mantle (Varekamp et al., 2010).
Isotopically, both the northern and southern Payenia
basalts are distinct from Pacific MORB crust, which is
subducting beneath the region (Bach et al., 1996;
Jacques et al., 2013). Therefore it is unlikely that the Rı́o
Colorado component is related to melting of the currently subducting slab (Søager & Holm, 2013).
Owing to the uniform geochemistry of the intraplate
lavas erupted in the southern Payenia region in both
the early Miocene and the Pliocene to Recent volcanism, it has been argued that the Rı́o Colorado component is an asthenospheric mantle component (Søager &
Holm, 2013). The Rı́o Colorado composition differs from
the composition of mantle xenoliths from the Rı́o
Colorado volcanic field (Bertotto, 2000; Conceição et al.,
2005; Bertotto et al., 2013), but these xenoliths from
only two localities may not be representative of the entire lithospheric mantle. Alternatively, the EM-1 signature in Payenia volcanic rocks may ultimately be
derived from the lithosphere (Jacques et al., 2013); for
example, from asthenosphere enriched by melts of
veins in detached local lithospheric mantle blocks
abraded mainly during the earliest Miocene (Kay et al.,
2013).
The basalts from the Rı́o Colorado volcanic field in
southern Payenia and the early Miocene Matancilla–
Fortunoso lavas were interpreted to show the least
subduction signature and therefore to best reflect the
composition of the OIB-type mantle source. However,
the Rı́o Colorado basalts fall into two compositionally
distinct groups that have overlapping isotopic compositions (Søager & Holm, 2013); these are termed the high
and low Nb/U groups. The low Nb/U basalts have
higher contents of the most incompatible elements,
higher Ba/Nb, Th/La, K/Ti and La/Sm than the high Nb/U
basalts, and lower Nb/U (<40), Ce/Pb and Ba/Th.
Nevertheless, the similar isotopic compositions and the
similarly high U/Pb (up to 037) and low La/Nb and Zr/
Nb of the two groups led to the conclusion that the trace
element characteristics of the low Nb/U group were not
caused by an input from the subducting slab or by crustal contamination. It was proposed that the low Nb/U
Journal of Petrology, 2015, Vol. 56, No. 8
basalts were formed at lower temperature from pyroxenitic mantle with less interaction between the melts and
the surrounding peridotite and that the pyroxenitic material could be part of either the upwelling asthenosphere or the lithospheric mantle.
ANALYTICAL METHODS
Sample selection
The majority of the samples selected for this study
come from the Rı́o Colorado volcanic field. They include
five tephra samples [Cerro Carne, Cerro Morado, Cerro
Chico, Older Rı́o Colorado (RC) cone and Pata Mora]
and one lava flow (Cerro Mendéz) from the cone complex just north of the town of Rı́ncon de los Sauces and
the fissure eruption south of the Pata Mora bridge. The
older RC cone is apparently one of the oldest cones
from this volcanic field. Three additional tephra samples were chosen: one from a small cone on the northern flank of the Auca Mahuida complex, one from the
Pampas Negras area on the west flank of the Payún
Matrú volcano and one from an outcrop on Road 46 in
the Laguna Blanca region. Additionally, a lava of unknown age (Las Chilcas, most probably early Miocene)
sampled SE of the Payún Matrú volcano and two lavas
from the early Miocene Matancilla and Fortunoso volcanism were included. The major and trace element
compositions of the Las Chilcas and Cerro Mendéz
lavas were published by Søager et al. (2013) and radiogenic isotope compositions for the Cerro Mendéz lava
by Søager & Holm (2013). Major and trace element analyses of the Matancilla and Fortunoso lavas were published by Dyhr et al. (2013a, 2013b).
Whole-rock analyses
Major and trace elements
The tephra samples were crushed manually in a mortar
and sieved. The 1–2 mm fractions were washed in distilled water in an ultrasonic bath and any fragments
containing zeolites or alteration were discarded. Finally,
the samples were powdered in an agate mill. Major
elements were analyzed at the University of Hamburg
by X-ray fluorescence on a MagixPro PW 2540 system
on fused glass discs. The volatile content was determined by the weight loss-on-ignition at 1000 C. (See results in Table 1.)
Forty trace elements were determined by ICP-MS
using an Agilent 7500cs at the University of Kiel.
Analytical procedures have been described by GarbeSchönberg (1993). The BIR-1 and BHVO-2 standards
were run as unknown and sample CL467 was run three
times to monitor the data quality and reproducibility.
The reproducibility of the three runs of CL467 was better than 11% for all elements except Sb and W
(Supplementary Data Electronic Appendix 1, available
at http://www.petrology.oxfordjournals.org). (See results in Table 1.)
1499
Isotopes
Sr–Nd–Pb isotope analyses were carried out at the
GEOMAR Helmholtz Centre for Ocean Research Kiel by
thermal ionization mass spectrometry (TIMS). Sample
chips (1–2 mm, 100–150 mg) were leached in 2N HCl at
70 C for 1–2 h and subsequently triple rinsed in 18 MX
water prior to a 48 h digestion in hot HF–HNO3. Ion chromatography followed established standard procedures
(Hoernle et al., 2008). Pb isotope analyses were performed on a Finnigan MAT 262 RPQ2þ whereas Nd and
Sr were determined on a Thermo Fisher TRITON TIMS
system in static mode. Nd and Sr ratios were mass fractionation corrected within run to 146Nd/144Nd ¼ 07219
and 86Sr/88Sr ¼ 01194 respectively. Sample data are
normalized to measured values for NBS987,
87
Sr/86Sr ¼ 0710250 6 0000011 (n ¼ 7; 2r external reproducibility), and La Jolla, 143Nd/144Nd ¼ 0511850 6
0000008 (n ¼ 9; 2r external reproducibility), which were
run along with the samples. Pb mass bias correction followed the 207Pb–204Pb double-spike (DS) technique of
Hoernle et al. (2011). The long-term (2012–present) DScorrected values for NBS981 are 206Pb/204Pb ¼
169416 6 00028, 207Pb/204Pb ¼ 154990 6 00028 and
208
Pb/204Pb ¼ 367246 6 00069 (n ¼ 84; 2r external reproducibility). Total chemistry blanks for Pb were below
15 pg and thus negligible. Replicate analysis of sample
123949 by means of separate digests and chemistry lies
within the external 2r errors of the standards. (See results in Table 2.)
Mineral grain analyses
Microprobe olivine and spinel
Major and minor element compositions of olivines and
spinel inclusions in olivines were analyzed using a
JEOL JXA 8200 EMP at GEOMAR, Kiel, Germany.
Analyses were run with an accelerating voltage of 15 kV
and current of 100 nA for olivine and 50 nA for spinel
with a focused beam (1 mm). Counting times are given
in Supplementary Data Electronic Appendix 2. For calibration and monitoring of data quality, natural and synthetic reference samples from the Smithsonian
Institution (Jarosewich et al., 1980) were run 2–3 times
each for every 50–60 sample measurements and at the
beginning and end of each analytical session. For the
olivines, all elements were normalized to the results of
the San Carlos olivine (USNM 11312/444; Sobolev et al.,
2007) and for the spinels, Mg, Cr and Al were normalized to chromite (117075) and Ti, Fe and Mn to ilmenite
(96189). Spinel analyses with SiO2 > 02 wt % were
assumed to be contaminated with entrapped olivine
and therefore discarded. The analyses are listed in
Supplementary Data Electronic Appendix 3 and the
average composition of the 10 most Fo-rich olivine
cores from each sample is given in Table 3.
For the calculation of magmatic temperatures with
the Al-in-olivine geothermometer of Wan et al. (2008)
and Coogan et al. (2014), olivine compositions were
measured with the EMP at GEOMAR at four points
1500
Journal of Petrology, 2015, Vol. 56, No. 8
Table 1: Whole-rock major and trace element compositions of basalts from the southern Payenia volcanic province
Sample:
Location:
wt %
SiO2
TiO2
Al2O3
FeOT
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
Mg#
ppm
Li
Sc
V
Cr
Co
Ni
Cu
Zn
Ga
Rb
Sr
Y
Zr
Nb
Mo
Sn
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Tl
Pb
Th
U
124570
Co
Chico
124564
Co
Carne
124567
Co
Morado
CL467
Pata
Mora
Cl393
Auca
Mahui
4868
206
1585
1091
016
651
876
372
149
053
027
9816
541
4582
190
1176
1137
016
1561
771
268
124
048
000
9865
731
683
174
175
158
430
105
470
108
218
210
669
219
167
314
180
159
0393
303
225
459
569
242
564
194
532
0816
451
0842
212
0298
183
0272
349
185
0118
216
215
0673
427
214
196
590
710
513
591
98
174
160
616
187
143
265
152
137
0334
265
198
410
518
221
514
173
480
0715
391
0718
181
0251
151
0228
295
150
0106
175
165
0530
Cl373
Pampas
Negras
CL476
Laguna
Blanca
4723
155
1330
1039
016
1292
759
332
180
046
152
9890
711
4784
196
1354
1122
016
1111
799
308
130
045
000
9916
662
678
166
155
548
653
438
499
92
172
362
630
174
172
286
215
134
1194
445
231
458
536
219
472
154
429
0652
354
0657
169
0244
150
0229
338
167
0237
371
341
0998
516
200
192
342
588
275
492
101
191
181
593
194
146
262
147
139
0397
270
190
392
493
213
501
173
479
0730
401
0739
186
0263
158
0240
305
151
0682
216
186
0545
124569
Older
RC cone
4590
201
1268
1166
017
1307
935
226
101
054
244
9923
689
4694
201
1589
1065
017
847
1002
304
123
039
000
9930
612
4906
183
1612
1044
018
861
834
226
150
050
433
9963
620
4885
175
1509
1069
020
801
993
297
084
033
185
9906
597
443
222
231
419
570
292
546
96
183
202
738
201
134
222
121
129
0670
270
185
398
532
240
578
197
545
0797
431
0778
191
0258
155
0228
297
124
0146
193
183
0687
616
306
285
259
426
91
512
90
209
237
622
232
161
182
134
146
0934
315
182
406
537
237
560
184
532
0822
463
0879
229
0322
202
0307
344
105
0177
359
234
0684
712
277
236
253
471
143
483
104
217
403
867
225
197
238
200
148
1695
452
268
568
730
306
653
205
608
0863
461
0840
214
0292
183
0260
418
141
0190
572
368
1025
558
208
188
272
492
163
415
104
204
146
629
208
138
131
119
130
0461
1881
146
327
422
190
482
148
480
0739
419
0786
201
0288
174
0263
298
075
0120
439
148
0460
124569
duplicate
552
207
186
270
492
161
414
105
204
146
625
208
139
131
119
132
0461
1878
148
331
429
193
482
148
482
0739
418
0788
202
0285
174
0267
297
076
0121
445
152
0470
Major elements are recalculated on a volatile-free basis. Mg# is calculated as 100 Mg/(Mg þ Fe2þ) with Fe2þ/RFe ¼ 09.
around each of the most primitive spinels (at 20 mm
distance) in six samples. Analyses were carried out with
a high-precision program at an accelerating voltage of
15 kV and current of 300 nA. The counting times for
minor elements and Fe were increased (see
Supplementary Data Electronic Appendix 2), in particular for Al (240 s on peak and 120 s on background), but
the analytical procedures were the same as described
for the routine olivine analyses above. An average of
the four data points was used together with the spinel
analyses to calculate the temperatures (Supplementary
Data Electronic Appendix 3).
LA-ICP-MS
Trace elements in olivines were determined by LA-ICPMS using a 193 nm Excimer laser with a large-volume
ablation cell (ETH Zürich, Switzerland) coupled with an
Agilent 7500s ICP-MS system at the University of Kiel
(CAU), Germany. Analyses were run with 60 mm pit size,
10 Hz pulse frequency and 85 J cm–2 fluence. The
Journal of Petrology, 2015, Vol. 56, No. 8
1501
Table 2: Whole-rock isotope analyses of the studied rocks
Sample
87
Sr/86Sr
123949
123949*
124564
124567
124569
124570
127040
127081
Cl373
Cl393
CL467
CL476
0703853
0703846
0703537
0703890
0704441
0703574
0703548
0703831
0703855
0703959
0703743
0703891
2SE
143
2
2
3
3
3
3
3
2
3
2
3
2
Nd/144Nd
0512819
0512815
0512874
0512822
0512759
0512870
0512895
0512816
0512815
0512818
0512853
0512812
2SE
206
3
3
3
3
3
2
3
3
2
3
3
2
Pb/204Pb
183111
183100
183313
184181
184160
183652
183390
183602
184280
184160
183679
185355
207
Pb/204Pb
2SE
16
12
9
7
16
14
16
9
15
11
23
22
155713
155712
155672
155852
155883
155740
155668
155773
155793
155791
155742
155882
2SE
208
15
11
8
7
15
12
14
8
13
10
20
21
Pb/204Pb
381494
381481
382044
383286
383666
382542
380960
382041
383114
383045
382534
383795
2SE
45
29
23
21
37
33
37
22
31
25
49
56
*Sample replicate.
The given 2SE values are the standard error of the mean on the last stated digits.
Table 3: Average compositions of the most Fo-rich olivine cores from each sample
Sample:
Location:
Latitude:
Longitude:
n:
127040
Fortunoso
–36215
–69423
10
127081
Matancilla
–36839
–69479
9
123949
Las Chilcas
–36751
–68950
12
124570
Co Chico
–37316
–69029
10
124564
Co Carne
–37303
–68981
10
EMP data
Fo
SiO2 (wt %)
FeO (wt %)
MgO (wt %)
Al (ppm)
Mn (ppm)
Ca (ppm)
Ni (ppm)
Total
812 (03)
392 (03)
179 (03)
433 (06)
282 (49)
1665 (109)
1509 (85)
1856 (246)
1012 (07)
840 (04)
396 (13)
154 (04)
452 (06)
229 (97)
1477 (154)
1150 (361)
2418 (436)
1009 (11)
821 (02)
394 (04)
170 (03)
438 (02)
259 (53)
1559 (69)
1446 (65)
2153 (160)
1010 (06)
806 (07)
392 (06)
184 (06)
429 (04)
241 (58)
1729 (85)
1529 (160)
1588 (156)
1012 (08)
862 (03)
401 (06)
133 (03)
468 (03)
278 (55)
1278 (120)
1177 (54)
2619 (383)
1010 (07)
205 (12)
25 (06)
251 (53)
180 (44)
1340 (161)
49 (04)
88 (12)
102 (08)
277 (17)
1834 (104)
218 (13)
2107 (155)
571 (040)
160 (12)
024 (004)
012 (002)
766 (98)
109 (005)
83 (5)
115 (007)
166 (18)
22 (03)
249 (100)
149 (106)
1164 (321)
45 (10)
64 (16)
92 (37)
289 (100)
1558 (156)
203 (11)
2690 (417)
557 (031)
132 (25)
021 (011)
010 (002)
822 (145)
096 (030)
81 (7)
111 (020)
176 (09)
22 (03)
266 (26)
183 (70)
1425 (166)
48 (04)
88 (9)
102 (11)
254 (15)
1606 (65)
207 (10)
2247 (134)
554 (045)
142 (9)
022 (004)
012 (002)
837 (66)
109 (006)
85 (4)
107 (006)
197 (12)
20 (02)
257 (142)
114 (107)
1518 (137)
51 (05)
84 (13)
95 (09)
240 (27)
1756 (125)
215 (15)
1707 (170)
577 (027)
141 (8)
021 (004)
011 (002)
681 (64)
107 (014)
83 (3)
098 (006)
139 (11)
19 (03)
293 (27)
99 (86)
1285 (197)
45 (06)
69 (8)
96 (16)
401 (50)
1355 (96)
181 (17)
2725 (537)
682 (069)
100 (9)
020 (004)
009 (001)
747 (111)
114 (006)
81 (8)
097 (009)
1242 (22)
LA-ICP-MS data (ppm)
FeO (wt %)
Li
Al
P
Ca
Sc
Ti
V
Cr
Mn
Co
Ni
Cu
Zn
Ga
Y
Ni FeO/MgO
100Ca/Fe
Fe/Mn
1000Zn/Fe
T ( C)
Isotope
57
7
27
31
43
45
49
51
52
55
59
60
63
67
71
89
(continued)
aerosol was transported with 15 l min–1 He and mixed
with 06 l min–1 Ar prior to introduction into the ICP. The
ICP-MS system was operated under standard conditions at 1500 W and optimized for low oxide formation
(typically ThO/Th 04%). The GLITTER software package (Access Macquarie Ltd.) was used for data
reduction of the time-resolved measurements, which
was done individually for each analysis. The blank signal was measured 20 s prior to each ablation and used
for calculation of the actual detection limits. The analyzed isotopes are listed in Table 3. 63Cu was measured
owing to an interference from 39Ar26Mg on 65Cu. 29Si
1502
Journal of Petrology, 2015, Vol. 56, No. 8
Table 3: Continued
Sample:
Location:
126230
Co Mendéz
124567
Co Morado
124569
Older RC co.
Cl467
Pata Mora
Latitude:
Longitude:
n:
–37329
–68958
10
–37298
–69003
10
–37294
–68996
10
EMP data
Fo
SiO2 (wt %)
FeO (wt %)
MgO (wt %)
Al (ppm)
Mn (ppm)
Ca (ppm)
Ni (ppm)
Total
877 (03)
405 (07)
119 (03)
477 (03)
216 (79)
1276 (71)
1141 (67)
2303 (271)
1007 (08)
863 (12)
403 (07)
132 (11)
467 (08)
264 (91)
1337 (179)
1331 (145)
2129 (407)
1009 (06)
122 (07)
19 (02)
276 (71)
63 (33)
1383 (182)
46 (05)
76 (31)
82 (11)
369 (57)
1351 (106)
172 (9)
2419 (196)
661 (034)
80 (5)
015 (006)
008 (002)
575 (61)
123 (007)
73 (4)
087 (007)
135 (17)
21 (02)
299 (65)
98 (95)
1555 (171)
46 (03)
79 (21)
95 (12)
398 (65)
1454 (168)
188 (19)
2397 (490)
705 (038)
97 (8)
019 (006)
010 (001)
603 (63)
130 (011)
77 (6)
093 (008)
1215 (11)
LA-ICP-MS data (ppm)
FeO (wt %)
Li
Al
P
Ca
Sc
Ti
V
Cr
Mn
Co
Ni
Cu
Zn
Ga
Y
Ni FeO/MgO
100Ca/Fe
Fe/Mn
1000Zn/Fe
T ( C)
–37235
–69092
10
Cl393
Auca
Mahuida
–37535
–68695
9
Cl373
Pampas
Negr.
–36377
–69392
9
Cl476
Laguna Blan.
845 (01)
404 (09)
149 (01)
455 (02)
251 (51)
1392 (101)
1152 (66)
2443 (402)
1015 (09)
828 (03)
398 (10)
164 (03)
444 (03)
214 (34)
1579 (92)
1304 (83)
1721 (244)
1013 (09)
855 (05)
402 (08)
140 (05)
462 (03)
234 (78)
1396 (107)
1286 (238)
2127 (332)
1011 (09)
857 (20)
399 (11)
138 (18)
464 (14)
252 (60)
1504 (226)
1385 (176)
1785 (683)
1008 (12)
850 (04)
401 (10)
144 (04)
459 (04)
203 (29)
1560 (65)
931 (42)
2036 (204)
1010 (08)
159 (12)
19 (04)
300 (29)
84 (66)
1249 (159)
48 (03)
63 (7)
106 (16)
351 (156)
1494 (128)
203 (14)
2647 (302)
636 (017)
116 (9)
022 (003)
010 (001)
801 (132)
099 (006)
83 (6)
100 (008)
1243 (17)
170 (18)
20 (05)
244 (48)
101 (55)
1465 (329)
53 (07)
82 (26)
83 (12)
241 (53)
1681 (164)
209 (20)
1906 (219)
617 (063)
123 (18)
019 (005)
011 (002)
636 (91)
102 (007)
81 (5)
097 (014)
135 (11)
20 (03)
261 (61)
113 (277)
1434 (385)
48 (05)
66 (15)
78 (14)
401 (57)
1447 (177)
188 (13)
2286 (287)
689 (027)
102 (5)
018 (006)
010 (002)
642 (100)
119 (022)
78 (5)
094 (004)
134 (22)
16 (03)
268 (56)
105 (119)
1413 (375)
61 (13)
71 (36)
79 (13)
212 (64)
1602 (260)
186 (18)
1793 (387)
582 (088)
94 (12)
020 (008)
009 (003)
533 (137)
129 (008)
72 (4)
088 (006)
1210 (21)
152 (10)
24 (03)
243 (29)
169 (47)
1007 (82)
46 (04)
76 (13)
63 (07)
191 (35)
1617 (132)
188 (14)
2204 (219)
552 (029)
105 (5)
016 (004)
008 (001)
640 (52)
083 (003)
72 (2)
093 (006)
1155 (16)
–39036
–70241
10
Numbers in parentheses are the 2r standard deviations on the sample averages. n is the number of olivine grains included in the
averages. Except for Zn, the listed ratios are calculated with EMP data only. The temperatures are calculated with the Al-in-olivine
equation from Coogan et al. (2014).
was used for internal standardization utilizing data from
EMP analyses. The SRM NIST 612 glass (Jochum et al.,
2011) and the MPI-DING standards BM90/21-G and
GOR128-G (Jochum et al., 2006) were reanalysed in triplicate and duplicate, respectively, for every 30 sample
acquisitions and at the beginning and end of each session. BM90/21-G and GOR128-G were used for calibration and conversion of the integrated count data to
concentrations. Sc concentrations were calculated from
the measured count rate on mass 45 and corrected for
interference with 29Si16Oþ. The production rate of silica
oxide
was
calculated
in
every
run
from
29 16 þ 29
Si O / Si ¼ 0022% as measured on synthetic nominally Sc-free quartz. The correction was 08 ppm for
most olivine data. Reference glasses ML3B-G and
BCR2G (Jochum et al., 2006) were analysed as unknown to check the analytical accuracy. 138Ba was
measured as a monitor for glass entrapment and excessive Cr and Cu concentrations were used as indicators for
entrapment of spinel and sulphide respectively. The relative standard deviations for six and 10 runs of the BM90/
21-G standard during the two sessions were below 9% for
most elements. The full dataset including standard measurements is given in Supplementary Data Electronic
Appendix 4 and the average compositions for each analyzed sample are given in Table 3. Plots of LA-ICP-MS vs
EMP analyses are given in Supplementary Data Fig. A.
Average LA-ICP-MS and EMP concentrations for the most
magnesian olivine from every sample agree within 10%
for Ni, Mn and FeO, and within 20% for Ca and Al. The
large 2r uncertainties for Al and Ca in sample 127081 in
both LA-ICP-MS and EMP data reflect natural heterogeneity of the olivine compositions.
RESULTS
Whole-rock analyses
The major element compositions of the analyzed rock
samples largely overlap with previously published data
from the southern Payenia region (e.g. Kay et al., 2006a,
2013; Bertotto et al., 2009; Dyhr et al., 2013a, 2013b;
Jacques et al., 2013; Søager et al., 2013) (Fig. 2). Four of
Journal of Petrology, 2015, Vol. 56, No. 8
Fig. 2. FeOT/MnO vs MgO for basalts from northern Payenia
(Nevado volcanic field and Northern Segment) and southern
Payenia (Rı́o Colorado, Payún Matrú, Llancanelo, Auca
Mahuida and Tromen volcanic fields). The grey field marks the
range of FeO/MnO found in peridotite-derived melts according
to Herzberg (2011). Pyroxenite melts often have elevated FeO/
MnO. Data from southern Payenia (S.P.) are from this study,
and from southern and northern Payenia from Søager et al.
(2013) and Holm et al. (in preparation).
the tephra samples, however, have very high MgO up
to 156 wt %, probably owing to accumulated olivine.
Whereas most Northern Payenia samples (Søager et al.,
2013; Holm et al., in preparation) have FeOT/MnO between 50 and 60 as expected for peridotite-derived
melts (Liu et al., 2008; Herzberg, 2011), nearly all samples from the southern Payenia volcanic fields have
FeOT/MnO > 60 similar to pyroxenite melts formed by
less than 70% melting (e.g. Pertermann &
Hirschmann, 2003; Keshav et al., 2004; Kogiso &
Hirschmann, 2006; Sobolev et al., 2007) (Fig. 2). In a
total alkalis–silica diagram (Le Maı̂tre et al., 1989) (not
shown), the samples classify as alkali basalts and trachybasalts, except for the samples from Laguna Blanca
and the older RC cone, which are subalkaline basalts
and have FeOT/MnO < 60. The Cerro Morado tephra has
high K2O/TiO2 (116) and relatively low CaO/Al2O3 (057)
like the low Nb/U basalts from the Rı́o Colorado region
(Søager & Holm, 2013), whereas the other tephra samples all have K2O/TiO2 < 085.
The primitive mantle normalized trace element patterns of the studied samples are shown in Fig. 3. The
majority of the samples have patterns similar to the
high Nb/U group of Søager & Holm (2013) (labelled
High Nb/U type), which have patterns characteristic for
EM-1 OIB (Willbold & Stracke, 2006; Søager & Holm,
2013) with positive Ba, K and Sr anomalies and negative
Pb anomalies. The samples from Las Chilcas and
Fortunoso, however, have considerably lower Cs
and Rb than the other samples. The Cerro Mendéz and
Cerro Morado samples have trace element patterns typical for the low Nb/U group (Søager & Holm, 2013)
(labelled Low Nb/U) with very small positive Ba and Pb
anomalies and higher concentrations of the most incompatible elements relative to the high Nb/U samples.
The samples from Laguna Blanca and Pampas Negras
1503
(termed transitional in Fig. 3) have lower Nb–Ta contents relative to Th and the fluid-mobile elements Cs,
Rb, Ba, U, K and Pb and slightly higher Ba/Nb than most
Rı́o Colorado basalts, indicating a small subduction
component input into their mantle sources. The tephra
from the older Rı́o Colorado cone has a distinctly different trace element pattern compared with the others,
with an extremely large positive Ba anomaly and positive Sr and Pb anomalies but otherwise relatively low
concentrations of the most incompatible elements and
low La/Sm (302).
The isotopic compositions (Table 2) of the studied
basalts, 87Sr/86Sr ¼ 070354–070396, 143Nd/144Nd ¼
051282–051290 and 206Pb/204Pb ¼ 18314–18547, generally overlap the literature data for the southern
Payenia region (e.g. Pasquarè et al., 2008; Dyhr et al.,
2013a, 2013b; Jacques et al., 2013; Kay et al., 2013;
Søager & Holm, 2013; Søager et al., 2015). However, the
older RC cone sample has lower 143Nd/144Nd ¼ 051276
and higher 87Sr/86Sr ¼ 070444 than any previously analyzed southern Payenia sample, suggesting crustal contamination, but the Pb isotope data are within the range
of the other samples.
Olivine
In Fig. 4, the compositions of olivine are shown for each
sample. Around 50 olivine cores from each sample
were analyzed to obtain a representative overview of
the spread in the olivine compositions. The olivine Fo
contents range from 71 to 90, with most ranging between 75 and 88. The fractionation trends outlined by
the samples in Fig. 4a are all shifted to lower Fo contents at a given Ni content relative to olivines from magmas that evolved from primary peridotite-derived
magmas (Fig. 4; Herzberg, 2011). Some of the most Forich olivines overlap the field of olivine compositions
that would crystallize from partial melts of stage 2 pyroxenites (Herzberg, 2011). The Fe/Mn ratios in Fig. 4b
for most olivine grains are higher than the 60–70 range
expected for olivines from peridotite-derived melts
(Herzberg, 2011) and are similar to the Fe/Mn values
found in Koolau olivines from Hawaii (Sobolev et al.,
2007). The CaO contents (most between 900 and
1800 ppm, Fig. 4c) are also similar to the concentrations
found in the Koolau olivines and lower than what is expected for olivines from peridotite melts. Except for a
few grains, the olivines have higher CaO contents than
found in olivines from mantle peridotites (e.g. Sobolev
et al., 2009; De Hoog et al., 2010), suggesting that they
are all magmatic in origin. At higher Fo contents, the
olivines delineate trends of weakly increasing CaO and
constant Ca/Fe with decreasing Fo, whereas at lower Fo
contents CaO content becomes roughly constant and
100Ca/Fe decreases. This indicates a fractionation path
dominated by olivine at high Fo, replaced by clinopyroxene at Fo contents below 825–84 varying from
sample to sample.
1504
Journal of Petrology, 2015, Vol. 56, No. 8
Fig. 3. Primitive mantle normalized trace element patterns of the studied samples. Normalization values from McDonough & Sun
(1995). The samples termed transitional are from Pampas Negras and Laguna Blanca. The Older RC cone is possibly lower crustally
contaminated. The low Nb/U samples are from Cerro Mendéz and Cerro Morado and the rest are all of high Nb/U type (see Søager
& Holm, 2013).
On an Fe/Mn versus Ni FeO/MgO plot (Fig. 5a), the
southern Payenia olivines (shown as the averages of
the 10 most Fo-rich grains in each sample with ratios
calculated using EMP data) plot at the high Fe/Mn end
of the global olivine array, indicating derivation from
melts enriched by a pyroxenite component. They overlap the within-plate magmatic rocks group (Sobolev
et al., 2007) in terms of their Fe/Mn and Ni FeO/MgO
(Fig. 5a), but most samples fall slightly below the global
trend. The lower Ni FeO/MgO is likely to reflect small
amounts of olivine fractionation, which would lower
Ni FeO/MgO in the magmas and olivines. On the Fe/
Mn versus Ca/Fe plot (Fig. 5b), the southern Payenia
olivines overlap within-plate magmatic rocks with the
lowest Ca/Fe, but there is no change in Ca/Fe with
increasing Fe/Mn for the southern and northern (from
Brandt & Holm, in preparation) Payenia olivines, in stark
contrast to the negative correlation of the global withinplate magmas and MORB array. Because the basalts
from this study have undergone some olivine fractionation, the Ni FeO/MgO ratio cannot be used to estimate the amount of pyroxenite melt in the magmas.
Therefore the Fe/Mn ratio in olivine was used as a
proxy, because this ratio is largely unchanged by olivine fractionation (e.g. Sobolev et al., 2007; Herzberg,
2011; Fig. 4b). The lowest Fe/Mn (72) is found in the
Pampas Negras and Laguna Blanca olivines, for which
the whole-rock compositions suggest a small metasomatic component (Fig. 3). The highest values are found in
the high Nb/U type samples (up to 85). The northern
Payenia basalts from Brandt & Holm (in preparation)
range from 60 to 68, overlapping the range expected for
peridotite melts.
Between nine and 12 of the most forsteritic grain
cores from each sample were analyzed by LA-ICP-MS
and an average of these was calculated (Fig. 6 and
Table 3). The Fe/Mn of the southern Payenia olivines defines crude positive correlations with Zn/Fe and Ga/Sc
and negative correlations with Co/Fe and Sc/Y. Also,
various trace element concentrations form crude positive correlations with Fe/Mn, such as Zn, Co, Ga and V
(Fig. 6e–h) and Y (not shown). Other elements such as
Sc, Ti, Al, Ca, P, Li, and Cu do not show any clear correlation with Fe/Mn (not shown).
Spinel compositions, oxygen fugacity and
crystallization temperatures
The spinels analyzed are all chromites with TiO2 <31 wt
% and Cr2O3 > 90 wt %; Mg/(Mg þ Fe2þ) is in the range
042–073, most Cr/(Cr þ Al) values in the range 018–
050 and Fe3þ/(Cr þ Al þ Fe3þ) values in the range
004–018. The spinels used to calculate the Al-in-olivine
temperatures all have TiO2 < 15 wt % and Fe3þ/
(Cr þ Al þ Fe3þ) < 016.
The oxygen fugacities (fO2) of the magmas have
been estimated using V concentrations in olivine and
whole-rock to be between –01 and þ14 DQFM (where
QFM is quartz–fayalite–magnetite; Fig. 7a and c) based
on Canil & Fedortchouk (2001). There is a negative correlation between the calculated fO2 and Fe/Mn among
the samples, except for the two low Nb/U basalts, which
have fO2 as low as the high Nb/U samples with highest
Fe/Mn.
The temperature estimates for all the olivine–spinel
pairs calculated with the Al-in-olivine method (Wan
et al., 2008; Coogan et al., 2014) delineate trends of
decreasing temperature with decreasing Fo and NiO
(Fig. 8), which follow the liquid lines of descent as expected for olivine fractionation [28 C/Fo-number as
calculated in PETROLOG (Danyushevsky & Plechov,
2011); thin arrows in Fig. 8a]. The average temperature
estimates for the four most forsterite-rich olivine grains
from each sample range between 1155 to 1243 C
(Table 3) and overlap with the upper range of
temperatures recorded by MORB (Coogan et al., 2014).
Journal of Petrology, 2015, Vol. 56, No. 8
1505
Fig. 4. (a–c) Ni, Fe/Mn and Ca vs Fo content for EMP olivine data. The white field in (a) shows the expected compositions of olivines
crystallized from magmas of stage 2 pyroxenites (secondary pyroxenites). The black fields in (a)–(c) show the compositions of olivines crystallized from primary peridotite-derived magmas and the grey fields show the olivine compositions in magmas that
evolved by olivine fractionation from primary peridotite-derived magmas. The arrows indicate the effect of olivine and/or clinopyroxene fractionation [all according to Herzberg (2011)]. The Koolau olivine fields are based on data from Sobolev et al. (2007). In (c),
the field for mantle olivines is based on data from Sobolev et al. (2009) and De Hoog et al. (2010). The error bars show 2SD of the
EMP measurements of San Carlos olivine.
An average temperature for the low Nb/U sample
126230 has not been calculated, because the olivines
in this sample indicate very inhomogeneous temperatures covering the temperature range of the other
Payenia samples, extending to temperatures as low as
1126 C.
Projecting the calculated crystallization temperatures
(up to 1260 C, Fig. 8) along the olivine liquidus slope
(5 C/100 MPa; Ford et al., 1983) from lower-crustal pressures set to 1 GPa up to a melt segregation pressure of
28 GPa (estimated for the Rı́o Colorado basalts; Søager
et al., 2013) yields a mantle temperature of 1350 C in
line with temperature estimates for the upper convecting mantle (e.g. Herzberg et al., 2007). In comparison,
the geotherm defined by the P–T estimates for lithospheric mantle xenoliths from the Rı́o Colorado region
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Journal of Petrology, 2015, Vol. 56, No. 8
Fig. 5. Ni FeO/MgO (a) and 100Ca/Fe vs Fe/Mn (b) in olivine (after Sobolev et al., 2007). The olivine compositions from this study
are averages of the 10 most Fo-rich olivine cores from each sample. Also shown are olivine compositions of northern Payenia and
three southern Payenia basalts from Brandt & Holm (in preparation). The fields show the expected compositions of olivines from
peridotite and secondary pyroxenite melts (Sobolev et al., 2007). The arrow in (a) shows the effect of olivine fractionation. The olivines from within-plate magmas (OIB and LIP) and MORB are from Coogan et al. (2014) and Sobolev et al. (2007). EMP data were
used for the plotted ratios (Table 3). The error bars show 2SD on the sample averages. For 100Ca/Fe, the error bars are often
smaller than the symbols.
(Bertotto et al., 2013) projects to only 1200 C at 25–
3 GPa.
DISCUSSION
Pyroxenite–peridotite melt mixing
The olivine compositions of the studied rocks strongly
suggest that their primary melts were derived by melting of pyroxenite in agreement with the elevated wholerock FeOT/MnO of the southern Payenia basalts (Fig. 2)
and their major element compositions in general
(Søager & Holm, 2013). This is also consistent with the
correlations of the olivine trace elements versus Fe/Mn
(Fig. 6). Le Roux et al. (2010, 2011) and Davis et al.
(2013) showed that up to 80% melts of eclogite will
have higher Zn/Fe and Ga/Sc and lower Co/Fe than peridotite melts and higher Zn and Co concentrations
owing to lower bulk D values for Zn and Co and lower
KDZn/Fe and KDGa/Sc in pyroxenite relative to peridotite.
The lower Co/Fe expected for pyroxenite melts is
related to the generally lower Co/Fe found in pyroxenitic rocks relative to mantle peridotite, which translates
into the melts. Although there is a considerable variability in the measured ratios for each sample (the 2r
shown), in particular for Ga/Sc, the higher Zn, Co, Zn/Fe
and Ga/Sc and lower Co/Fe of the samples with higher
Fe/Mn (Fig. 6a–c) support the evidence for pyroxenite
melts in the magmas. The MORB-like temperatures calculated for the Payenia samples show that the magmas
are derived from the convecting (asthenospheric) mantle. Therefore, the Rı́o Colorado source could include
recycled lithospheric material such as mixtures between recycled oceanic crust plus lower continental
crust (Willbold & Stracke, 2006, 2010; Søager et al.,
2013) or previously delaminated pyroxenitic vein material from the local sub-continental lithospheric mantle
(Jacques et al., 2013; Kay et al., 2013) upwelling in the
asthenosphere. However, the temperatures of the
Payenia samples are in the upper range of MORB
(Coogan et al., 2014) and thus do not indicate any significant thermal anomaly beneath the region, in agreement with the tomographic images of Pesicek et al.
(2012). Thus, a different upwelling mechanism than a
thermal plume is necessary to explain why dense Ferich pyroxenite is brought up beneath the lithosphere.
In comparison with the global array defined by
Sobolev et al. (2007) (Fig. 5), the olivines from northern
and southern Payenia apparently reflect the full spectrum of pyroxenite–peridotite melt mixing proportions.
The high Fe/Mn of the high Nb/U-type basalts from the
Rı́o Colorado region and the early Miocene plateau
flows suggest that these basalts represent the pure
intra-plate mantle source. Furthermore, the intermediate Fe/Mn of the Pampas Negras, Auca Mahuida and
Journal of Petrology, 2015, Vol. 56, No. 8
1507
Fig. 6. LA-ICP-MS olivine trace elements vs Fe/Mn in olivine. (a) 1000Zn/Fe; (b) 10000Co/Fe; (c) Ga/Sc; (d) Sc/Y; (e) Ga (ppm); (f) V
(ppm); (g) Co (ppm); (h) Zn (ppm). The error bars show 2r standard deviations on the sample averages. In (a) and (b), the EMP Fe
values are used for the ratios.
Llancanelo (from Brandt & Holm, in preparation) olivines (termed intermediate in Figs 7a and 9c) point to a
mixed peridotite–pyroxenite mantle source for this volcanism, as was also indicated by the whole-rock major
element compositions of the Payún Matrú and Auca
Mahuida basalts (Søager & Holm, 2013). The pyroxenite
is unlikely to have formed through interaction between
silica-rich hydrous melts from the subducting slab and
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Journal of Petrology, 2015, Vol. 56, No. 8
Fig. 8. (a) Calculated temperature (T) vs Fo in olivine; (b) T vs
NiO in olivine. Temperatures were calculated with the Al-inolivine thermometer using the equation of Coogan et al.
(2014). A data point for each olivine–spinel pair from this study
is shown with 2r standard deviation for the average olivine
composition around each spinel grain. The thin arrows show
olivine fractionation trends. The decrease in Fo shown by the
longer arrow in (a) corresponds to 10% olivine fractionation.
The wide arrow in (a) outlines the indicated trend for the low
Nb/U sample 126230 from Cerro Mendéz suggesting mixing
with a low-T mafic magma. The dataset for large igneous provinces (LIP) and MORB is from Coogan et al. (2014).
Fig. 7. (a) fO2 expressed as DQFM vs Fe/Mn in olivine; (b)
whole-rock (WR) V vs Fe/Mn in olivine; (c) DQFM vs Fo content
in olivine. The low Nb/U samples are excluded from the shown
correlations. The intermediate group includes the Pampas
Negras, Auca Mahuida and Llancanelo (from Brandt & Holm,
in preparation) olivines.
mantle wedge peridotite (Straub et al., 2008, 2011) because the slab component, as indicated by trace elements and isotopes (Fig. 10), decreases with increasing
Fe/Mn. Trace element ratios such as Nb/U, Ce/Pb, U/Pb
and Ba/Nb of the basalts with highest Fe/Mn overlap
those of EM-1 OIB (e.g. Willbold & Stracke, 2006) and
do not indicate any input from the slab.
Fe-enrichment in pyroxenite melts
The whole-rock elemental concentrations have been
fractionation corrected for equilibrium olivine
fractionation to MgO ¼ 10 wt % using PETROLOG
(Danyushevsky & Plechov, 2011) with f O2 at the QFM
buffer and the Fe-oxidation model of Kress &
Carmichael (1988). Corrected compositions are marked
with an asterisk in Fig. 9. MnO was assumed to have the
same compatibility as FeO to keep the Fe/Mn ratio unchanged. Incompatible trace elements in olivine were
treated as perfectly incompatible like Al2O3.
In Fig. 9b, the northern Payenia basalts have fairly
uniform FeOT* contents of 9–10 wt % as expected for
peridotite melts (e.g. Hirose & Kushiro, 1993; Walter,
Journal of Petrology, 2015, Vol. 56, No. 8
1509
Fig. 9. Major element whole-rock (WR) compositions vs Fe/Mn in olivine for northern and southern Payenia samples. (a) FeOT*/
MnO*; (b) FeOT*; (c) CaO* (* denotes olivine fractionation corrected compositions to MgO ¼ 10 wt %; see text). In (c), the high and
low Nb/U samples are outlined. The intermediate group includes the Pampas Negras, Auca Mahuida and Llancanelo (from Brandt
& Holm, in preparation) olivines. LB is the Laguna Blanca sample. The older RC cone (124569) whole-rock composition is presumably lower crustally contaminated and is therefore excluded from the correlations. Data from this study, Dyhr et al. (2013a, 2013b),
Søager et al. (2013), Brandt & Holm (in preparation) and Holm et al. (in preparation).
1998) whereas the FeOT* contents of the southern
Payenia basalts show a positive correlation with Fe/
Mnolivine and range to higher FeOT* (up to 126 wt %)
similar to many OIB (see, e.g. Dasgupta et al., 2010;
Mallik & Dasgupta, 2013). In contrast, although the
southern Payenia basalts generally have lower MnO*
and CaO* than the northern Payenia basalts, there is no
clear correlation between MnO* or CaO* and Fe/Mn
among the southern Payenia samples (Fig. 9c). This indicates that the Fe/Mn and Ca/Fe ratios of the olivines
are primarily controlled by the varying Fe-enrichment
of the melts, which is probably also partly responsible
for the lower maximum Fo contents in the samples with
highest Fe/Mn owing to the lower Mg-numbers of the
Fe-enriched pyroxenite melts. This agrees with the
study of Mallik & Dasgupta (2013), who found that pyroxenite melts that reacted with peridotite will be in equilibrium with lower Fo olivine (down to 85) than
normal peridotite melts. In addition, the generally lower
whole-rock MgO contents of the southern Payenia basalts relative to the northern Payenia basalts (Fig. 2),
even in samples from monogenetic eruptions through
similarly thick lithosphere, suggest that the primary
melts of the southern Payenia basalts also had lower
MgO. The excellent agreement between the whole-rock
FeOT*/MnO* and the olivine Fe/Mn (Fig. 9a) confirms
that the whole-rock FeOT*/MnO* of high Mg-number
basalts can on its own be used as a valuable indicator
for the fraction of pyroxenite melt in the Payenia
basalts.
Fe-enriched melts could have been formed by melting of Fe-rich eclogite with or without carbonate (Fig.
11) (Takahashi & Nakajima, 2002; Gerbode & Dasgupta,
2010), by mixtures of Fe-rich eclogite melts and peridotite (Mallik & Dasgupta, 2012, 2013, 2014), by reactive
crystallization of the eclogite melts with peridotite
(Takahashi & Nakajima, 2002), or by melting of silicapoor and bimineralic pyroxenites (e.g. Hirschmann
et al., 2003; Keshav et al., 2004; Dasgupta et al., 2006;
Kogiso & Hirschmann, 2006) or olivine websterite
(HK66, Hirose & Kushiro, 1993; Kogiso et al., 1998).
Some of these processes could produce melts similar
to the composition of the southern Payenia basalts.
However, owing to the low Ni contents of Si-poor and
bi-mineralic pyroxenites (typically < 100 ppm), they cannot produce melts with high Ni like the southern
Payenia basalts (Ni ¼ 300–700 ppm at equilibrium with
Fo88 olivine; Søager & Holm, 2013) because Ni is compatible in garnet and clinopyroxene [bulk DolNi ¼ 34 calculated using partition coefficients and modes from
Pertermann et al. (2004) and Le Roux et al. (2011)].
Because such melts are not highly reactive with peridotite, they would not be expected to increase their Ni
concentrations significantly in this way, and they are
therefore unlikely to be the source for the Payenia basalts. Similarly, pure eclogite melts are too low in MgO,
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Journal of Petrology, 2015, Vol. 56, No. 8
Fig. 10. Trace element and isotope whole-rock compositions vs Fe/Mn in olivine. (a) 206Pb/204Pb; (b) eNd; (c) Nb/U; (d) Ce/Pb; (e) Ba/
Th; (f) Pb* (ppm). The older RC cone (124569) whole-rock composition with Ba/Th ¼ 1271 is presumably lower crustally contaminated and is therefore excluded from the correlations. Sample 123949 may also have a small lower crustal melt component. Data
sources as in Fig. 9.
Mg# and Ni and, in the case of Si-saturated melts, too
high in SiO2 to be parental to the Payenia basalts (e.g.
Pertermann & Hirschmann, 2003; Sobolev et al., 2007;
Spandler et al., 2008; Gerbode & Dasgupta, 2010;
Kiseeva et al., 2012).
The higher Ni and Mg-numbers of the Payenia basalts indicate that either the pyroxenite melts reacted
with peridotite by partial reactive crystallization, in
which case the eclogite melts become alkaline upon reaction with the peridotite owing to precipitation of
orthopyroxene and dissolution of olivine (Yaxley &
Green, 1998; Takahashi & Nakajima, 2002; Lambert
et al., 2012; Mallik & Dasgupta, 2012), or that the eclogite or eclogite melts were mixed with peridotite.
Mechanical mixtures of basalt or eclogite melt with
peridotite can produce high-Mg# alkaline melts with
decreasing amounts of residual olivine the higher the
added eclogite melt/basalt component (Kogiso et al.,
1998; Yaxley, 2000; Mallik & Dasgupta, 2012, 2013,
2014). Such melts have higher Fe/Mn than peridotite
melts (Fig. 12) and would have higher Ni contents than
eclogite melts owing to the much higher Ni concentrations in the peridotite. In this case, the Fe/Mn and degree of Fe-enrichment of the mixed source controls the
Fe/Mn and Fe content of the melts. It is also clear from
Fig. 12 that carbonated melts have higher Fe/Mn than
dry melts formed from a very similar starting composition with similar Fe/Mn. In the case of a low-carbon/dry
eclogite, the melts of a mixed source at 1375 C and
25–3 GPa will have Fe/Mn according to the equation
FeO=MnO ¼ 0 71Xecl þ 48 01
where Xecl is the percentage of eclogite melt in the
source. The higher SiO2 contents of the southern
Payenia basalts relative to the carbonated melts of
Mallik & Dasgupta (2013, 2014) indicate very little or no
CO2 in the source [see the model of Mallik & Dasgupta
(2014)] and hence the above equation is reasonable to
apply to assess the amount of eclogite melt added to
the source. The FeO/MnO of the high Nb/U basalts thus
Journal of Petrology, 2015, Vol. 56, No. 8
1511
Fig. 12. Whole-rock FeO/MnO vs per cent eclogite melt added
to peridotite in the mixed experiments of Mallik & Dasgupta
(2012, 2013, 2014). The FeO/MnO ratios of the starting compositions are given in the legend. The regression line given is for
dry experiments with FeO/MnO ¼ 103 in the starting composition (Mallik & Dasgupta, 2012). The olivine-out line shows the
approximate limit for the presence of olivine in the residue
(Yaxley, 2000; Mallik & Dasgupta, 2012, 2013, 2014).
suggests 30–38% eclogite melt added to the source. If,
instead, the basalts were formed by partial reactive
crystallization of Fe-rich eclogite melts with peridotite,
the FeO/MnO of the melts would also depend on the degree of interaction with peridotite, decreasing with
increasing degree of interaction, and the parameterization would not apply.
Owing to the relatively reduced fO2 conditions calculated for the southern Payenia basalts (DQFM 0, Fe2þ/
FeOT ¼ 09), a high oxidation state of the source cannot
account for the high FeO of the pyroxenite end-member
basalts, as proposed by Mallik & Dasgupta (2014) to explain the high FeO of many OIB. We therefore suggest
that the southern Payenia basalts were derived through
melting of low-carbon, Fe-rich pyroxenite and that these
melts either mixed with or reacted with peridotite.
Temperatures
Fig. 11. (a) Whole-rock FeO/MnO vs FeO; (b) SiO2 vs FeO.
Experimental melts (25–35 GPa) are shown for comparison
with the high Nb/U southern Payenia basalts from this study.
The Si-saturated pyroxenite (pxt) fields include data from
Pertermann & Hirschmann (2003) and Sobolev et al. (2007) and
in (b) also from Takahashi & Nakajima (2002) and Spandler
et al. (2008). The Si-poor pyroxenite fields include data
from Hirschmann et al. (2003), Keshav et al. (2004), Dasgupta
et al. (2006) and Kogiso & Hirschmann (2006). Carbonated
peridotite melts are from Dasgupta et al. (2007). Peridotite
melts are from Hirose & Kushiro (1993), Walter (1998) and
Davis et al. (2013). Carbonated eclogite melts are from
Gerbode & Dasgupta (2010) and Kiseeva et al. (2012).
Carbonated eclogite melt–peridotite mix is from Mallik &
Dasgupta (2013, 2014). Dry eclogite melt–peridotite mix is from
Mallik & Dasgupta (2012). Olivine websterite HK66 is from
Hirose & Kushiro (1993). Low-Fe eclogite melt–peridotite layered experiment is from Mallik & Dasgupta (2012). High-Fe
eclogite–peridotite layered experiment is from Takahashi &
Nakajima (2002).
In contrast to the relatively high asthenospheric temperatures recorded by most of the Payenia olivines
(most above 1200 C), the temperatures calculated for
the Laguna Blanca olivines are lower, but still within the
range of MORB, suggesting that this magma also contained asthenospheric melt. In Fig. 8a, the samples are
offset towards lower forsterite contents relative to most
MORB and LIP basalts with similar temperatures. This
is most probably an effect of the high pyroxenite melt
contents of the Payenia magmas, which apparently resulted in lower Mg-numbers in the primary melts as discussed above (see, e.g. Herzberg, 2011). At relatively
constant maximum Ni in olivine (Fig. 4a), the single
sample trends are shifted to lower Fo the higher the Fe/
Mn. Therefore the olivine NiO contents may be a better
monitor of the degree of fractionation of the magmas.
When the temperatures are plotted against NiO in olivine (Fig. 8b), all the Payenia samples (except sample
126230 from Cerro Mendéz) can be interpreted to fall on
one common fractionation trend, suggesting that these
1512
magmas were derived from mantle with a similar temperature, as would also be expected if they had been
formed within the same upwelling asthenosphere.
The low Nb/U sample from Cerro Mendéz displays a
considerable heterogeneity in the calculated temperatures, also within single olivine grains (see
Supplementary Data Electronic Appendix 3). The range
of temperatures recorded by the olivines in this sample
indicates late-stage mixing between a magma with a
temperature similar to the other Payenia samples and a
lower temperature magma with higher Mg# (Fig. 8a).
Low Nb/U magmas
The olivines of the low Nb/U samples [Cerro Mendéz,
Cerro Morado and sample 126175 from Brandt & Holm
(in preparation)] have lower Fe/Mn than the high Nb/U
olivines, thus indicating a larger fraction of peridotite
melt component in these samples. However, in contrast
to the intermediate group in Figs 7a and 9c, the low Nb/
U-type basalts have similarly low fO2 to the high Nb/U
basalts and low CaO contents, which cannot be produced by melting of peridotite (see, e.g. Herzberg &
Asimow, 2008). These samples, however, lack the
Fe-enrichment and elevated Ni, Zn and Co for a given
Mg-number that is seen in the high Nb/U group. The
late-stage mixing with a low-T magma, indicated by the
temperatures calculated for the Cerro Mendéz sample,
is likely to result from mixing between an asthenospheric and a lithospheric mantle melt during the earliest stage of fractionation; for example, at the base of
the crust. The low-T melt could represent the low Nb/U
component, but the lithospheric component would
have to have a similar isotope signature to the asthenospheric component. This suggests that the enriched
lithospheric mantle components that melted were ultimately derived from Rı́o Colorado source melts and/or
fluids shortly before melting of the enriched lithospheric components; that is, during the same volcanic
period.
Oxidation state of the magmas
Using the equation of Maurel & Maurel (1982), the Fe2þ/
Fe3þ values of the measured spinels with the highest
Mg/(Mg þ Fe2þ), which are presumably the most primiP
tive, correspond to Fe2þ/ Fe values in the melts of
089–091 for the high and low Nb/U basalts and 088
for the transitional samples. A more oxidized nature of
the peridotitic end-member is suggested by the negative correlation between the calculated DQFM and Fe/
Mn (Fig. 7a) and the positive correlation between V in
olivine and Fe/Mn olivine (Fig. 6f), which is in contrast
to the negative correlation between whole-rock V and
Fe/Mn olivine (Fig. 7b). This contrast can be explained
with a lower DV(ol) in the peridotite-derived melts owing
to a higher fO2 (Lee et al., 2005; Mallmann & O’Neill,
2009, 2013), which could have been caused by mantle
metasomatization of the peridotite by slab fluids and
melts. The lack of correlation between DQFM and Fo-
Journal of Petrology, 2015, Vol. 56, No. 8
content in olivine (Fig. 7c) shows that the variation in
DQFM is not a fractionation feature, as this is expected
to generate a negative correlation owing to incorporation of Fe2þ into olivine. Lee et al. (2010) showed that
more oxidized mantle will produce more Fe-rich melts
with lower Zn/Fe owing to the lower compatibility of
Fe3þ. A lower fO2 in the high Fe/Mn samples is, however, unlikely to have been the main cause for their
higher Zn/Fe because they have lower Mn/Fe and higher
FeO contents than the lower Fe/Mn samples, contrary to
what would be expected. The relatively low fO2
DQFM ¼ 0 of the pyroxenite melt end-member, is similar to that of MORB (e.g. Cottrell & Kelley, 2011, 2013)
and confirms that the pyroxenitic mantle was
unmetasomatized.
Model for Payenia back-arc volcanism from
whole-rock and olivine compositions
The olivine compositions display a good coherence
with the whole-rock data both in Pb isotope and trace
element space (Fig. 10). The well-correlated trends between Fe/Mn and trace element ratios such as Nb/U and
Ba/Th, 206Pb/204Pb as well as various trace element
concentrations such as Pb* indicate a simple twocomponent system. There is no clear correlation
between Fe/Mn and 143Nd/144Nd (expressed as eNd in
Fig. 10b) or 87Sr/86Sr (not shown), which is probably primarily due to the small contrast in Nd–Sr isotopic composition between the back-arc and volcanic front
sources (Jacques et al., 2013; Søager et al., 2015). A few
of the whole-rock compositions may also be crustally
contaminated as in the case of sample 124569 from the
older Rı́o Colorado cone. Although the whole-rock composition of this sample indicates contamination, possibly by lower crustal material with high Ba/Th, the
most primitive olivines from this sample show no deviation from the trends of the other southern Payenia olivines (Figs 4–6). This suggests that the olivines
crystallized at an early stage before crustal assimilation
took place. Also, the sample from Las Chilcas (123949)
has slightly higher 87Sr/86Sr and Ba/Th and slightly
lower eNd than the other samples with similar Fe/Mn
and could have a small lower crustal component.
The correlations indicate that pyroxenite melts with
EM-1 OIB-type trace element patterns, unradiogenic Pb,
high Nb/U, Ce/Pb and Ba/Th, and low Pb* contents
(16 ppm) (and generally lower incompatible trace
element contents except Nb–Ta) mix with peridotite
melts with more radiogenic Pb, low Nb/U, Ce/Pb and
Ba/Th, and higher Pb* contents. The relatively high Ba/
Th of the pyroxenite component is characteristic of EM1 basalts and is also observed in Tristan and Gough
basalts (Willbold & Stracke, 2006), whereas the relatively low Ba/Th in many SVZ arc and back-arc rocks
has been explained by an input of upper continental
crustal material to the sub-arc mantle (Holm et al.,
2014). The very uniform compositions in plots of Fe/Mn
versus trace elements and Pb isotopes (Fig. 10) of the
Journal of Petrology, 2015, Vol. 56, No. 8
early Miocene basalts and the Pleistocene Rı́o Colorado
basalts with the highest Fe/Mn confirm the similarity between their mantle sources (Kay et al., 2013; Søager &
Holm, 2013). Therefore, the same type of homogeneous
pyroxenite material must have been upwelling at both
times.
The northern Payenia basalts representing the peridotite end-member have a composition similar to the
SVZ arc rocks in terms of isotopes and trace element
ratios (Jacques et al., 2013; Søager et al., 2013, 2015),
suggesting a common peridotite mantle source for the
transitional SVZ (TSVZ) and northern Payenia magmas
with a similar type of subduction-zone metasomatic imprint. This pre-metasomatic source was proposed by
Jacques et al. (2013) and Søager et al. (2013, 2015) to be
similar to the South Atlantic normal (N)-MORB source
mantle and probably represents the normal upper mantle in the area. There is no indication of mixing with an
unmetasomatized peridotite component with, for example, high Nb/U and Ce/Pb. Therefore, the ambient
peridotite mantle does not melt unless its solidus temperature is lowered by subduction-zone fluids or melts.
Pliocene slab break-off southwards from 38 S
(Pesicek et al., 2012) opened space for upwelling of
unmetasomatized material over the southern edge of the
slab beneath Payenia and would have caused deep mantle to upwell to fill the void left by the sinking slab. Also,
the gradual downwarping (steepening) of the slab beneath Payenia from south to north ended the shallow
subduction phase at this time (Gudnason et al., 2012)
and would have sucked deeper mantle from the back-arc
region into the mantle wedge from south and east directions and would have enhanced the mantle upwelling.
This in-flowing new mantle would in that case fill out the
increasing mantle wedge space beneath southern
Payenia, replacing the metasomatized mantle (Fig. 13).
The much larger slab component and lower Fe/Mn in
the Nevado basalts relative to the Payún Matrú and
Llancanelo basalts erupted closer to the volcanic front
could indicate that the upwelling of pyroxenite was
focused in the southern Payenia area and later also
Llancanelo. This is supported by the resistivity model of
Burd et al. (2014), which shows a deeply rooted plumelike structure related to the presence of melts beneath
the Rı́o Colorado–Auca Mahuida region and another
shallower anomaly beneath the Payún Matrú and
Tromen volcanoes, indicating that the pyroxenitic melts
are not evenly distributed in the mantle but mainly are
present in a limited zone. The metasomatized peridotite
generating the Nevado volcanic field volcanism could
partly have been the mantle wedge material beneath
Payenia from the period of shallow subduction that
melted and mixed with the upwelling asthenosphere
(Fig. 13). The weak slab component in both older and
younger Payún Matrú basalts is most probably caused
by a small continuous slab input owing to the closer
proximity to the arc front (410 km from the trench)
relative to the Rı́o Colorado volcanic field (490 km
from trench).
1513
The plume-like structure identified by Burd et al.
(2014) seems to come up beneath southern Payenia on
top of the slab. This suggests that the ‘plume’ comes
from the Atlantic side of the slab, but a role for Pacific
sub-slab mantle cannot be ruled out as sub-slab mantle
is expected to flow through a slab window (e.g.
Guillaume et al., 2010). The imaged plume-like structure
could be caused by deep melting of upwelling pyroxenitic material, which can melt at high pressures if the
pyroxenite is alkali-rich (e.g. Kogiso et al., 2004;
Spandler et al., 2008), and the reaction of these melts
with the peridotite. Tappe et al. (2013) suggested that
vigorous mantle upwellings through or from the mantle
Transition Zone could cause fertilization of the upwelling peridotite by melts of ancient recycled oceanic crust
(with negative DeHf like the Rı́o Colorado component;
Jacques et al., 2013; Søager et al., 2015) stored in the
Transition Zone. Such a scenario would explain why the
OIB-type Payenia volcanism has occurred at times of
slab roll-back and slab detachment where the back-arc
mantle upwelling is expected to be strongest; this has
also been suggested for the Somuncura province further south (de Ignacio et al., 2001; Kay et al., 2007). A
strong upwelling could possibly lead to entrainment
and decompression melting of stored recycled material
in the form of pyroxenite, which is not mobilized under
normal conditions. This would be in line with the observation that back-arc volcanism far from the arc triggered by slab fragmentation or windows, or vigorous
return flow, is often found to be alkaline and OIB-like,
and not necessarily isotopically similar to the expected
composition of the sub-slab mantle [see discussion by
Faccenna et al. (2010)] as is also the case for the
Patagonian slab window basalts (e.g. Gorring et al.,
2003).
CONCLUSIONS
The Payenia olivines indicate that their host basalts
were formed by mixing between peridotite and pyroxenite melts and that pyroxenite melts dominate in
southern Payenia in contrast to the dominance of peridotite melts in the north. Melt contributions from a
pyroxenitic mantle source are indicated by higher Fe/
Mn, Zn/Fe, Zn, Co and Ga/Sc, and lower Co/Fe in the
olivines from high Nb/U samples. The mixing proportions apparently extend from 0 to 100% pyroxenite melt
and Fe/Mn is found to correlate with a wide number of
trace element concentrations and ratios, Pb isotope
ratios and fO2 estimates. The correlations suggest the
involvement of two mantle sources: (1) a pyroxenitic
source with EM-1 OIB-type trace element characteristics
and unradiogenic Pb, but fO2 similar to MORB-type
mantle; (2) a more oxidized peridotitic source with more
radiogenic Pb, which was metasomatized by subduction-zone fluids and melts. The pyroxenitic mantle
source component is interpreted to have been upwelling beneath southern Payenia along with unmetasomatized peridotite mantle owing to mantle disturbances
1514
Journal of Petrology, 2015, Vol. 56, No. 8
Fig. 13. Tectonic model for the late Pliocene–early Pleistocene period. The slab break-off southwards from 38 S and south generates an upwelling of mantle carrying pyroxenite components. The shallowly subducting slab beneath Payenia is gradually steepening from north to south, causing suction of the pyroxenite-bearing mantle from the south and east into the mantle wedge beneath
Payenia. The melts formed by decompression melting of the pyroxenite in some cases mix with melts of the metasomatized South
Atlantic mantle. CSVZ is the Central Southern Volcanic Zone arc.
caused by slab detachment just south of Payenia in the
Pliocene and steepening and roll-back in the subducting
plate beneath Payenia.
Crystallization temperature estimates for the southern Payenia magmas are primarily above 1200 C, indicating a mantle temperature of 1350 C at pressures of
28 GPa, showing that the magmas are asthenospheric
melts from a mantle with normal upper mantle temperature. The sample from Laguna Blanca south of
Payenia indicates a lower temperature of 1155 C, which
is, however, still within the range of MORB.
A positive correlation between Fe/Mn and wholerock FeOT* among the southern Payenia basalts and
the lack of correlation with whole-rock MnO* and CaO*
suggests that the olivine Fe/Mn and Ca/Fe ratios and
probably also Fo contents are controlled by the varying
Fe-enrichment of the melts. The high FeO, MgO and Ni
of the basalts relative to pure pyroxenite melts suggests
that the pyroxenite melts reacted with peridotite by partial reactive crystallization before extraction or that the
pyroxenite source or melts thereof were mixed with
peridotite. The pyroxenite source is indicated to have
been Fe-rich and CO2-poor.
ACKNOWLEDGEMENTS
We greatly appreciate the reviews from Andrey
Gurenko, Cin-Ty Lee and one anonymous reviewer.
Christina Bonanati is thanked for assistance and company during fieldwork. We are grateful to Mario Thöner,
Ulrike Westerströer and Silke Hauff for their analytical
assistance. Heidi Wehrmann, Guillaume Jacques and
Roman Golowin are thanked for their practical assistance and good discussions.
FUNDING
This work was supported by the Danish Council for
Independent Research, Natural Sciences (grant no.
0602-02528B to N.S.), the GEOMAR Helmholtz Center
for Ocean Sciences Kiel and the Russian Science
Foundation (grant no. 14-17-00582 to M.P.).
SUPPLEMENTARY DATA
Supplementary data for this paper are available at
Journal of Petrology online.
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