Streck et al. (2007) state that primitive andesite lavas (Mg# > 0.6; 53–63 wt% SiO2) from Mt. Shasta are not primary melts, but instead are mixtures of evolved, high-Sr/Y, low-Mg# dacite with primary, low-Sr/Y, high-Mg# basalt. They propose that this mixing forms primitive andesites worldwide, and they dismiss the idea that primitive andesite is an end member in arc magmatism.

Magma mixing is evident in primitive Aleutian andesites (e.g., Kay, 1978; Kelemen et al., 2003a; Yogodzinski and Kelemen, 1998) and xenoliths (e.g., Conrad et al., 1983; Yogodzinski and Kelemen, 2007). Streck et al. imply that our data support their hypotheses, but Aleutian mixing requires a primitive andesite end member with the highest Mg#, Cr, Ni, Th, Ba, Sr and light rare Earth elements (REE), and lowest Y and heavy REE. We show this with Sr/Y versus Mg# (Fig. 1A). High-Sr/Y lavas are mixtures of a low-Mg#, low-Sr/Y component and a high Sr/Y, high Mg# component. The high Sr/Y component is an andesite (Fig. 1B) with Mg# ~0.7, in Fe/Mg equilibrium with mantle olivine. There are no low Mg# lavas with higher Sr/Y, so the Streck et al. process did not form end-member Aleutian primitive andesites.

Figure 1

Compositions of lavas from the oceanic Aleutian arc (compiled by Kelemen et al. [2003a], plus lavas dredged on the 2005 Western Aleutian Volcano Expedition). Yb*9.6 used instead of Y for samples with Sr data but no Y. One sample with discrepant Y and Yb is omitted.

Figure 1

Compositions of lavas from the oceanic Aleutian arc (compiled by Kelemen et al. [2003a], plus lavas dredged on the 2005 Western Aleutian Volcano Expedition). Yb*9.6 used instead of Y for samples with Sr data but no Y. One sample with discrepant Y and Yb is omitted.

Mt. Shasta data (Fig. 2) are similar to that from the Aleutians, with a mixing trend extending toward a high-Sr/Y, high-Mg# andesite end member, nearly perpendicular to the mixing proposed by Streck et al. If their process formed any lavas at Mt. Shasta, they are rare. In any case, by analogy with the Aleutians, their dacite end member (Mg# 0.61) formed from a primitive andesite.

Figure 2

Compositions of lavas from Mt. Shasta (Grove et al., 2002,2005). Right panel, with expanded vertical axis, illustrates lava data and mixing trajectories from Streck et al. (2007). Along these mixing trajectories, symbols are as follows: Circle—“dacite;” square—“high alumina olivine tholeiite” (HAOT); diamond—“basaltic andesite” (BA); filled symbols—lava mixing alone; open symbols—lava mixing after harzburgite is added to mafic end member in proportions required by Streck et al.

Figure 2

Compositions of lavas from Mt. Shasta (Grove et al., 2002,2005). Right panel, with expanded vertical axis, illustrates lava data and mixing trajectories from Streck et al. (2007). Along these mixing trajectories, symbols are as follows: Circle—“dacite;” square—“high alumina olivine tholeiite” (HAOT); diamond—“basaltic andesite” (BA); filled symbols—lava mixing alone; open symbols—lava mixing after harzburgite is added to mafic end member in proportions required by Streck et al.

Streck et al. also misrepresent Pb isotope data for arc lavas. Citing our work (Kelemen et al., 2003b), Streck et al. (2007, p. 353) write, “elevated Pb isotopic signatures of [primitive andesites] worldwide could be interpreted in terms of a crustal contribution.” However, Kelemen et al. (2003b, our Figure 9; 2003a, our Figures 7–11) show that Aleutian primitive andesites have the most depleted, least “crustal” Sr, Nd, and Pb isotopes of any arc magmas worldwide.

REFERENCES CITED

Conrad
W.K.
Kay
S.M.
Kay
R.W.
1983
,
Magma mixing in the Aleutian arc: Evidence from cognate inclusions and composite xenoliths
:
Journal of Volcanology and Geothermal Research
 , v.
18
,
p
.
279
295
,
doi: 10.1016/0377-0273(83)90012-4
.
Grove
T.L.
Parman
S.W.
Bowring
S.A.
Price
R.C.
Baker
M.B.
2002
,
The role of H2O-rich fluids in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N. California
:
Contributions to Mineralogy and Petrology
 , v.
142
,
p
.
375
396
.
Grove
T.L.
Baker
M.B.
Price
R.C.
Parman
S.W.
Elkins-Tanton
L.T.
Chatterjee
N.
Müntener
O.
2005
,
Magnesian andesite and dacite lavas from Mt. Shasta, northern California: Products of fractional crystallization of H2O-rich mantle melts
:
Contributions to Mineralogy and Petrology
 , v.
148
,
p
.
542
565
,
doi: 10.1007/s00410-004-0619-6
.
Kay
R.W.
1978
,
Aleutian magnesian andesites: Melts from subducted Pacific ocean crust
:
Journal of Volcanology and Geothermal Research
 , v.
4
,
p
.
117
132
,
doi: 10.1016/0377-0273(78)90032-X
.
Kelemen
P.B.
Yogodzinski
G.M.
Scholl
D.W.
2003a
,
Along-strike variation in lavas of the Aleutian island arc: Implications for the genesis of high Mg# andesite and the continental crust, Chapter 11
,
in
Eiler
J.
ed
.,
Inside the Subduction Factory
 :
American Geophysical Union Monograph
138
,
p
.
293
311
.
Kelemen
P.B.
Hanghøj
K.
Greene
A.
2003b
,
One view of the geochemistry of subduction-related magmatic arcs, with an emphasis on primitive andesite and lower crust
,
in
Rudnick
R.L.
ed
.,
The Crust, Vol. 3, Treatise on Geochemistry
  (
Holland
H.D.
Turekian
K.K.
,
eds.)
:
Oxford, UK
,
Elsevier-Pergamon
,
p
.
593
659
.
Streck
M.J.
Leeman
W.P.
Chesley
J.
2007
,
High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive mantle melt
:
Geology
 , v.
35
,
p
.
351
354
,
doi: 10.1130/G23286A.1
.
Yogodzinski
G.M.
Kelemen
P.B.
1998
,
Slab melting in the Aleutians: Implications of an ion probe study of clinopyroxene in primitive adakite and basalt
:
Earth and Planetary Science Letters
 , v.
158
,
p
.
53
65
,
doi: 10.1016/S0012-821X(98)00041-7
.
Yogodzinski
G.M.
Kelemen
P.B.
2007
,
Trace elements in clinopyroxenes from Aleutian xenoliths: Implications for primitive subduction magmatism in an island arc
:
Earth and Planetary Science Letters
 , v.
256
,
p
.
617
632
,
doi: 10.1016/j.epsl.2007.02.015
.
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