Appendix A - Methods 
1. 40Ar/39Ar Methods
The 40Ar/39Ar ages presented here were determined in Rutgers’ Earth and Planetary Sciences Noble Gas Laboratory on a MAP-215-50 mass spectrometer. Using ion-counting collection technique as described by Turrin et al. (2010). Samples were petrographically evaluated, selecting samples that had no visible weathering and minimal interstitial glass. The samples were crushed, sieved to 300 μ-600 μ size range, washed in distilled water in an ultrasonic bath, and dried in an oven at ~ 80 °C. Mineral separates were made using a Frantz Isodynamic Separator. Samples were then hand picked under a binocular microscope to remove phenocrysts. Samples where it was not possible to completely remove the phenocrysts are designed as “whole rock”. The groundmass of those in which larger plagioclase and other phenocrysts were completely removed is designated as “matrix”. The samples were then loaded into Al irradiation disks along with multiple splits of monitor minerals and wrapped in Al foil, and neutron irradiated at 1000kW for 0.25 hour using Cd-foil shielding in the central thimble facility of the USGS TRIGA reactor in Denver (Irradiation 56, 58 and 60). 
The irradiated samples aliquots were loaded into six-mm diameter wells milled into a stainless steel disk (~60 mm diameter) and baked overnight at ~ 100 °C. A 40-watt CO2 laser was used as the thermal source for the incremental-heating experiments. The 6-mm square laser beam was jogged to make sure to evenly heat the round sample pit. Samples were incrementally heated, by stepped increases in laser wattage output until the samples were fused into glass balls and minimal 39ArK was released.
The irradiation parameter J was determined by multiple total-fusion analyses of two co-irradiated monitor mineral combined, Fish Canyon Sanidine (FC-2) and the Alder Creek Sanidine (AC-2). The primary monitor mineral used was FC-2 at an age of 28.201 ± 0.046 Ma; Kuiper et al. (2008). In addition, the AC-2 standard was used as an internal check.  The measured AC-2 values (Table A1) are between 1.188 ± 0.004 Ma and 1.211 ± 0.008 Ma and average 1.204 ±0.006 Ma with a standard error of 0.0007 (n=78). These values are in agreement with the recalculated AC-2 age relative to the FC-2 age of 28.201 of Kuiper et al. (2008).
The following interfering neutron reaction from Ca and K values were used: (36Ar/37Ar)Ca = 2.64 ± 0.02 x10-4; (39Ar/37Ar)Ca= 6.73 ± 0.04 x10-4; (38Ar/39Ar)K = 1.34 ± 0.02 x10-2 from Dalrymple et al. (1993; 1981); and (40Ar/39Ar)K = 2.85 ± 0.5 x10-4 correction determined from measurements of kalsilite glass. Age calculations were made using the currently accepted decay constants and isotopic abundances of Steiger & Jäger (1977): λε=5.81x10−11 yr−1, λβ=4.962x10−10 yr−1, 40K/Ktotal=1.167x10−4.
During the analysis of the samples and standards, background and mass fractionation were regularly monitored through measurement of blanks and air aliquots. Automated software written by A. Deino (MassSpec) was used to calculate regression curves fitting the variations of the system baselines, Ar isotope backgrounds and mass discrimination through time.
All ages measurements are 40Ar/39Ar incremental-heating experiments and include integrated (total-fusion) ages, plateau ages if determined, and inverse-isochron ages of all steps and/or of the plateaus steps only. The resultant ages are summarized in Table 1. Full release spectra details and analytical conditions are given in Appendix C. The incremental-heating plateau ages are defined by at least three contiguous increments representing at least 50% of the total 39ArK released from the sample (where 39ArK=39Ar produced from 39K), and with any two adjacent fractions that must be analytically indistinguishable at the 95% confidence level (Fleck et al. 1977). The mean square of weighted deviates (MSWD) values of Ar isotopic data of reliable ages should be close to the unity, nonetheless a “geological scatter” is present in near all experiments (McIntyre et al. 1966). Hence, we follow the Brooks et al. (1972) “rule of thumb” that rejects data arrays with MSWD > 2.5. See Turrin et al. (2007) for a discussion of using MSWD for an evaluation age data. The inverse isochron ages are deduced by the fit obtained from plotting the atmospheric related 36Ar/40Ar component against the radiogenic 39Ar/40Ar (McDougall & Harrison 1999). Reliable isochron ages should be analytically indistinguishable from the plateau age and intercepting at 40Ar/36Ar values indistinguishable at the 95% confidence level from the accepted atmospheric ratio of 298.61 ± 0.09 (value calculated as the average of published results from Lee et al. (2006) and Valkiers et al. (2010)). 
One of the difficulties of obtaining precise radiometric ages on young volcanic rocks by the 40Ar/39Ar method is the possible need to apply large corrections because of the subtraction of atmospheric Ar from the total Ar concentration measured in the sample. Therefore, to obtain accurate and precise 40Ar/39Ar ages on young volcanic rocks, an accurate “mass discrimination correction” must be applied to all of the measured isotopic ratios used to calculate the ages. This correction was applied to the NTD samples using the method described by Turrin et al. (2008) and Turrin et al. (2010). The mass discrimination was monitored by measurement of an air aliquot of approximately 1x10-13x (1 through 5) moles of 40Ar, delivered via an online automated air pipette system, after every seventh isotope measurement. The mass discrimination data, determined from the measured apparent 40Ar/36Ar ratio of the air aliquots, was then plotted through time, tracking any temporal drift in the mass discrimination and modeled with a best-fit regression through the time series data. The resultant curve was then applied to the standards and unknowns. Modeling the mass discrimination data throughout the run period significantly improves the accuracy of the analytical data for both the standard and unknowns.
A summary of the ages is given in Table 1, where the ages that are considered more reliable are reported in bold. As a general rule, we interpret as the best approximation of the age of a sample the plateau age whenever the total fusion age, plateau age and isochron age are consistent (at the a-95% confidence level), and the 40Ar/36Ar ratio intercept is atmospheric. In the instances in which the isochron results indicate that the “trapped” component is not of atmospheric origin, the best age estimate for the sample is considered to be the isochron age. In the case in which two splits of the same sample were run (e.g. matrix and plagioclase) and the plateau ages produced were not statistically indistinguishable (a-95% confidence level) because of the low statistical errors, we compiled the plateau steps of the two splits all together on an inverse isochron correlation diagram, obtaining what we consider the best estimate of the age of these particular samples.

Table A1. Co-irradiated Alder Creek values
Co-irradiated Alder Creek values. These values are in agreement with the recalculated AC-2 age (1.202 Ma) to the used Fish Canyon Sanidine value (FC-2 = 28.201 ± 0.046 Ma; Kuiper et al. 2008). All the AC-2 ages were calculated after fitting a plane through the Js calculated for the FC = 28.201 using all grains available for each layer altogether. The Js were calculated subdividing each pit and fitting a plane through it after calculating a weighted mean, J errors were calculated as SEM.

2. Geochemistry Methods
Alteration-free rock chips, selected using a stereoscopic microscope, were powdered with the employment of an alumina mill. About 5 grams of each unknown and USGS natural standards (W-2, BCR-2 and BHVO-2) were fused into homogeneous glass disks as described by Hannah et al. (2002) at Michigan State University by flux fusion together with lithium tetraborate (Li2B4O7). Glass disks were analyzed by X-ray fluorescence (XRF) in a Bruker S4 Pioneer for major elements. Trace elements were obtained on the same glass disks by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Samples from Ketumbeine and Gelai (Table 2) were run at Michigan State University by a Micromass Platform ICP-MS with a Cetac LSX 200+ Nd: YAG laser (266 nm) while samples from Burko, Monduli, Tarosero, and Meru were run at Virginia Tech employing an Agilent 7500ce ICP-MS coupled with a GeoLas Pro laser ablation system (ArF 193 nm Excimen Laser). 
Pb, Sr, and Nd isotope analyses were conducted at Rutgers University through sample digestion and element chromatography performed in a Class 1000 clean room, equipped with Class 100 laminar flow hoods with downdraft exhaust. About 500 mg of whole rock powder of each sample were weighted and dissolved in an open Teaflon-beaker at ~150 °C, by successive steps of dissolution and hydration acids digestion (HF, HNO3 concentrate, 7.3 N HCl and 0.5 N HBr). Sr and some Nd isotope ratios were determined using a GV Isoprobe-T multicollector thermal ionization mass spectrometer (TIMS) in dynamic multicollection mode (Table 4). The average 87Sr/86Sr ratio for the NIST standard SRM 987 was 0.710239 ± 5 (2σ, n=20, STDEV=13). Nd isotopes in a second set of samples were determined with a Thermo Scientific Neptune Plus high-performance multicollector ICP-MS. The samples were collected after ion-exchange chromatography, dry down and re-dissolution in 2% ultrapure (Fisher Optima Grade) Nitric acid. Two standards, one provided by Thermo (containing multiple elements) and ShinEtsu, were analyzed for comparison. The average 143Nd/144Nd ratio for the Thermo standard was 0.512398 ± 5 (2σ, n=5, STDEV=16), and for the average value for ShinEtsu was 0.512095 ± 4 (2σ, n=5, STDEV=15). The latter value is within error of the accepted value of Tanaka et al. (1996). 
For Pb, samples from Ketumbeine were measured using a FISONS PLASMA 54 multi-collector as described in Mana et al. (2012), while the other samples were measured with the Plus Neptune MC-ICP-MS. In summary, the samples were collected after an ion-exchange chromatography and were dried down and redissolved with 1 ml of 0.5 N HNO3. An aliquot of processed sample was diluted with 2% ultrapure (Fisher Optima Grade) Nitric acid and spiked with Tl standard. In the presence of Hg, measurement of 202Hg is used to correct for the contribution of 204Hg to the total 204 measured. Mass fractionation was determined by comparison of the 205/203 measured ratio of added Tl with the theoretical value of 2.3871.  An aliquot of NIST 981 was processed though the ion-exchange chromatography in order to confirm the absence of contamination and/or artifacts from sample dissolution and separation. The NIST981 measurements using both instruments were consistent with the values reported by Platzner et al. (2001) confirming the absence of cross contamination. Average obtained ratios are 206Pb/204Pb 16.9329, 207Pb/204Pb 15.4855, 208Pb/204Pb 36.6813 (n=13). Measurement precision (2σ) is 100 ppm or lower.

3. LA-ICP-MS at Virginia Tech
Trace element data were obtained at the Geosciences Department at Virginia Tech. The laboratory features an Agilent 7500ce ICP-MS coupled with a GeoLas Pro laser ablation system (ArF 193 nm Excimen Laser) connected to a beam homogenizing optics. The system offers the ability of measuring 40Ca with low detection limits thanks to an octopole reaction cell. Data were collected as following: ~60-second of background (only He flow, laser off) followed by ~60-second ion signal collection (laser on). Sample analyses were bracketed by analyses of USGS natural standards BCR-2g and BHVO-2g (Kelley et al. 2003) for external calibration and 40Ca ion signal intensities were employed as the internal standard to monitor instrumental drift. In the case of the samples from Tarosero volcano 49Ti ion signal intensities were used instead because of the really low concentrations of CaO, which were resulting in highly dispersed data. Standard reference material W-2 was run as unknown to monitor the accuracy of the data obtained (Table A2). Standards and unknown glasses were produced at Michigan State University by flux fusion of ~5 grams of sample powder plus lithium tetraborate (Li2B4O7) fused into homogeneous glass disks as described by Hannah et al. (2002). Samples were ablated using a 90 µm spot size. Instrument setting and analytical conditions are listed in Table A3. He was used as carrier gas in the ablation cell and mixed with Ar prior to entering the torch assembly. The average precision for three replicate analyses is <<1% RSD. Accuracy varies by element: 5% or better (Rb, Sr, Y, Zr, Nb, Ba, Ce, Pr, Nd, Sm, Ta, Pb, Th, U); 10% or better (Sc, V, Ni, Cr, Cu, Zn, Cs, La, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf).

Table A2. Virginia Tech LA-ICP-MS USGS standards results

Table A3. Virginia Tech LA-ICP-MS instrument and data acquisition parameters

LA-ICP-MS instrument and data acquisition parameters	
Excimer 193nm ArF laser GeoLasPro	
Output Energy	150 mJ	
Energy Density on sample	~7  10 J/cm2	
Repetition Rate	5 Hz	
Pit Size	90 m	
Ablation Cell Volume	~1.5 cm3	
Cell Gas Flow (He)	~1 L/min	
Agilent 7500ce Quadrupole ICP-MS	
Auxiliary gas flow	1.03 l/min Ar	
RF power	1500 V	
Detector Mode	Dual 8 orders of magnitude linear dynamic range	
Quadrupole Settling Time	2 ms	
Data acquisition parameters	
Sweeps per reading	1	
Reading per replicate	200  300	
Replicates	1	
Dwell time per isotope	10 ms	
Points per peak	1 per measurement	
External Standard	BCR-2g and BHVO-2g 	
Isotopes Analyzed	7Li, 11B, 23Na, 25Mg, 27Al, 28Si, 39K, 40Ca, 45Sc, 49Ti, 51V, 52Cr, 55Mn, 56Fe, 62Ni, 63Cu, 66Zn, 85Rb, 88Sr, 89Y, 90Zr, 93Nb, 133Cs, 138Ba, 139La, 140Ce, 141Pr, 143Nd, 147Sm, 153Eu, 157Gd, 159Tb, 163Dy, 165Ho, 166Er, 172Yb, 175Lu, 178Hf, 181Ta, 208Pb, 232Th, 238U	

References
Brooks, C., Hart, S., Wendt, I., 1972. Realistic Use of Two-Error Regression Treatments as Applied to Rubidium-Strontium Data. Reviews of Geophysics and Space Physics 10, 551–577.
Dalrymple, G.B., Alexander, E.C.J., Lannphere, M.A., Kraker, G.P., 1981. Irradiation of samples for 40Ar/39Ar dating using the Geological Survey TRIGA reactor. U.S. Geological Survey, Professional Paper 1176.
Dalrymple, G.B., Ryder, G., 1993. 40Ar/39Ar Age Spectra of Apollo 15 Impact Melt Rocks by Laser Heating and Their Bearing on the History of Lunar Basin Formation. Journal of Geophysical Research 98, 13085–13095.
Fleck, R.J., Sutter, J.F., Elliot, D.H., 1977. Interpretation of discordant 40Ar/39Ar age-spectra of mesozoic tholeiites from antarctica. Geochimica et Cosmochimica Acta 41, 15–32.
Hannah, R., Vogel, T., Patino, L., Alvarado, G., Pérez, W., Smith, D., 2002. Origin of silicic volcanic rocks in Central Costa Rica: a study of a chemically variable ash-flow sheet in the Tiribí Tuff. Bulletin of Volcanology 64, 117–133.
Kelley, K.A., Plank, T., Ludden, J., Staudigel, H., 2003. Composition of altered oceanic crust at ODP Sites 801 and 1149. Geochemistry, Geophysics, Geosystems 4, n/a–n/a.
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, 500–504. 
Lee, J.-Y., Marti, K., Severinghaus, J.P., Kawamura, K., Yoo, H.-S., Lee, J.B., Kim, J.S., 2006. A redetermination of the isotopic abundances of atmospheric Ar. Geochimica et Cosmochimica Acta 70, 4507–4512.
Mana, S., Furman, T., Carr, M.J., Mollel, G.F., Mortlock, R.A., Feigenson, M.D., Turrin, B.D., Swisher III, C.C., 2012. Geochronology and geochemistry of the Essimingor volcano: Melting of metasomatized lithospheric mantle beneath the North Tanzanian Divergence zone (East African Rift). Lithos 155, 310–325.
McDougall, I., Harrison, T.M., 1999. Geochronology and thermochronology by the 40Ar/39Ar method.
McIntyre, G.A., Brooks, C., Compston, W., Turek, A., 1966. The statistical assessment of Rb-Sr isochrons. Journal of Geophysical Research 71, 5459–5468.
Platzner, I., Ehrlich, S., Halicz, L., 2001. Isotope-ratio measurements of lead in NIST standard reference materials by multiple-collector inductively coupled plasma mass spectrometry. Fresenius’ journal of analytical chemistry 370, 624–8.
Tanaka, T., Tanimizu, M., Asahara, Y., Yonezawa, C., Togashi, S., Kamioka, H., 1996. A variety of 143Nd/144Nd ratios among six high purity Neodymium oxide reagents. Journal of Mass Spectrometry Society of Japan 44, 79–83.
Turrin, B.D., Muffler, L.J.P., Clynne, M.A., Champion, D.E., 2007. Robust 24±6 ka 40Ar/39Ar age of a low-potassium tholeiitic basalt in the Lassen region of NE California. Quaternary Research 68, 96–110.
Turrin, B.D., Gutmann, J.T., Swisher III, C.C., 2008. A 13 ± 3 ka age determination of a tholeiite , Pinacate volcanic field , Mexico , and improved methods for 40Ar/39Ar dating of young basaltic rocks. Journal of Volcanology and Geothermal Research 177, 848–856.
Turrin, B.D., Swisher, C.C., Deino, A.L., 2010. Mass discrimination monitoring and intercalibration of dual collectors in noble gas mass spectrometer systems. Geochemistry, Geophysics, Geosystems 11, n/a–n/a.
Valkiers, S., Vendelbo, D., Berglund, M., de Podesta, M., 2010. Preparation of argon Primary Measurement Standards for the calibration of ion current ratios measured in argon. International Journal of Mass Spectrometry 291, 41–47.