Figure S.1. Plots of the contents of mobile elements [Na

(1)

0.0 0.5 1.0 1.5 1.0

30 50

Mg#

K2O (wt%) K2O (wt%)

0 1 2

10 30 50

Nb (ppm)

O'Higgins guyot Alpha seamount R. Crusoe Island A. Selkirk Island

U (ppm)

2.0 3.0 4.0 5.0

Mg#

Na2O (wt%)

Figure S.1. Plots of the contents of mobile elements [Na²O and K2O (wt%) and U (ppm)] vs.

an immobile element (Nb) for lavas from O’Higgins Guyot, Alpha Seamount, Robinson Crusoe Island, and Alejandro Selkirk Island. The correlations indicate that even though low- temperature alteration may have affected the mobile elements (Na, K, and U), they can still be used with caution for petrogenetic interpretations (e.g., TAS diagram).

(2)

5 8 11 14

40 50 60 70 80

Mg#

O'Higgins guyot Alpha seamount R. Crusoe Island A. Selkirk Island

Al

2

O

3

0.2 0.7 1.2 1.7

40 50 60 70 80

Mg#

K

2

O

0.0 0.4 0.8 1.0

40 50 60 70 80

Mg#

P

2

O

5 10

12 14

16

Fe

2

O

3T

1.0 2.0 3.0

4.0

TiO

2

2.0 3.0 4.0

Na

2

O

Figure S.2. Plots of Mg number (Mg#) vs. major elements (Fe²O3T, TiO2, Na2O, Al2O3, K2O,

1

and P2O5) for JFR shield-stage lavas.

2

(3)

1 10 100

Rb Ba Th U Nb Ta K La Ce Pr Sr Nd Sm Zr Hf Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

Sample / Primitive mantle

O'Higgins guyot Alpha seamount Robinson Crusoe Island Alejandro Selkirk Island

Figure S.3. Primitive mantle-normalized (PM from Sun and McDonough, 1989) patterns 1

showing the abundance of incompatible elements for samples from the O’Higgins (yellow), 2

Alpha (blue), Robinson Crusoe (red), and Alejandro Selkirk (green) volcanoes. The data 3

reveal the relative chemical enrichment of Robinson Crusoe/Alpha samples with respect to 4

Alejandro Selkirk/O’Higgins samples.

5

(4)

1 10 100

Th Nb Ta La Ce Pr Sr Nd Sm Zr Hf Eu Gd Tb Dy Y Ho Er Tm Yb Lu LL250711-7 (H2O in the source=0.03 wt%; n=41)

Tp: 1325 °C Pmt : 2.43 GPa Pxfr : 3.0 wt%

TotF_mt: 3.6 wt%

PerFmt : 2.5 wt%

PxFmt : 40.1 wt%

1 10

100 MF-6 (H20 in the source=0.03 wt%; n=172)

Tp: 1356 °C Pmt : 2.47 GPa Px_fr: 9.3 wt%

TotFmt : 8.8 wt%

PerFmt : 4.0 wt%

PxFmt : 50.9 wt%

Figure S.4. Primitive mantle-normalized (PM from Sun and McDonough, 1989) patterns showing the trace-element contents of primitive melts from the Robinson Crusoe and Alejandro Selkirk Islands (red and green, respectively). Samples LL250711-7 and MF-6 represent a few samples with relatively low ratios of La/Yb and were modeled in OBS1 to evaluate the proposed model. See Figure 9 in the main text for details.