Supplementary Table 12. Results of laser fusion 40Ar/39Ar dating of anorthoclase crystals from two pumice clasts from the Burgi Tuff, Burgi Member, Koobi Fora Formation.  Area 207, in cliff on Il Ingumwai, photolocality 1729/091-090

Sample	36Ar/39Ar
x 10-4	37Ar/39Ar
x 10-3	40Ar/39Ar	K/Ca	40Ar*/39ArK
± c.v.	% 40Ar*	39Ar
10-15mol	Calculated age
Ma ± 1 s.d.	
									
83-286B anorthoclase; J = 4.144E-4 ± 0.35%; level 13, ANU69	
	  1	5.658 ±  6.16%	4.232 ± 23.70%	3.732 ± 0.17%	124	3.525 ± 0.34%	94.5	7.55	2.634 ± 0.013	
	  2	3.007 ± 13.92%	4.112 ± 17.99%	3.651 ± 0.15%	128	3.523 ± 0.38%	96.5	7.70	2.632 ± 0.014	
	  3	6.739 ±  1.54%	5.947 ±  3.62%	3.690 ± 0.21%	89	3.452 ± 0.24%	93.5	10.53	2.579 ± 0.011	
	  4	2.549 ±  8.56%	5.287 ±  3.50%	3.655 ± 0.21%	100	3.541 ± 0.28%	96.9	21.19	2.645 ± 0.012	
	  5	3.264 ±  4.03%	8.349 ±  4.34%	3.652 ± 0.12%	63	3.517 ± 0.17%	96.3	18.98	2.627 ± 0.010	
	  6	4.871 ±  2.82%	4.804 ±  1.66%	3.742 ± 0.15%	110	3.558 ± 0.20%	95.1	28.08	2.658 ± 0.011	
	  7	3.332 ±  8.96%	5.777 ±  3.53%	3.606 ± 0.33%	91	3.468 ± 0.42%	96.2	5.81	2.591 ± 0.014	
	 [8	4.041 ±  7.22%	4.818 ± 20.31%	3.851 ± 0.18%	109	3.692 ± 0.30%	95.9	5.38	2.758 ± 0.013]	
	  9	1.314 ± 15.21%	4.556 ±  9.75%	3.623 ± 0.16%	116	3.545 ± 0.23%	97.8	6.51	2.648 ± 0.011	
	10	0.120 ± 99.95%	3.273 ± 13.37%	3.569 ± 0.25%	161	3.526 ± 0.55%	98.8	5.35	2.634 ± 0.017	
	11	6.749 ±  8.13%	4.576 ± 10.22%	3.761 ± 0.18%	115	3.522 ± 0.50%	93.6	7.70	2.631 ± 0.016	
	12	2.960 ±  5.21%	6.077 ±  3.05%	3.652 ± 0.17%	87	3.525 ± 0.22%	96.5	6.73	2.634 ± 0.011	
	13	3.039 ± 20.06%	5.302 ±  5.74%	3.615 ± 0.23%	99	3.486 ± 0.57%	96.4	6.65	2.604 ± 0.017	
	14	8.513 ±  1.64%	6.622 ±  8.60%	3.758 ± 0.13%	79	3.467 ± 0.18%	92.3	5.62	2.590 ± 0.010	
	15	7.612 ±  7.11%	5.055 ±  8.76%	3.733 ± 0.18%	104	3.469 ± 0.50%	92.9	7.02	2.592 ± 0.016	
	Arithmetic mean K/Ca (n = 14)	105 ± 23	Arithmetic mean age (n = 14)
Weighted mean age (n = 14)	2.621 ± 0.025
2.623 ± 0.009	
									
83-287A anorthoclase; J = 4.156E-4 ± 0.35%; level 14, ANU69	
	  1	6.188 ±  3.57%	6.787 ± 14.05%	3.716 ± 0.17%	78	3.494 ± 0.26%	94.0	9.07	2.618 ± 0.011	
	  2	5.192 ±  3.42%	5.133 ±  2.68%	3.639 ± 0.15%	103	3.446 ± 0.22%	94.7	6.98	2.582 ± 0.011	
	  3	5.534 ±  5.55%	4.327 ±  8.22%	3.647 ± 0.15%	122	3.444 ± 0.31%	94.4	4.84	2.581 ± 0.012	
	  4	3.411 ± 11.53%	3.823 ±  6.05%	3.634 ± 0.12%	138	3.494 ± 0.35%	96.1	7.38	2.618 ± 0.013	
	  5	1.370 ± 45.78%	4.664 ± 11.83%	3.587 ± 0.17%	113	3.507 ± 0.56%	97.8	6.20	2.627 ± 0.017	
	  6	4.660 ± 12.94%	6.290 ±  7.20%	3.644 ± 0.22%	84	3.467 ± 0.56%	95.1	5.39	2.598 ± 0.017	
	  7	4.731 ±  5.67%	4.397 ±  7.55%	3.665 ± 0.18%	120	3.485 ± 0.29%	95.1	5.49	2.612 ± 0.012	
	  8	5.696 ±  9.06%	5.277 ± 13.62%	3.705 ± 0.17%	100	3.497 ± 0.47%	94.4	7.77	2.620 ± 0.015	
	  9	3.726 ± 20.42%	5.602 ±  5.53%	3.655 ± 0.12%	94	3.506 ± 0.65%	95.9	7.03	2.626 ± 0.019	
	10	3.642 ± 14.70%	4.395 ± 11.60%	3.642 ± 0.12%	120	3.495 ± 0.47%	96.0	5.75	2.618 ± 0.015	
	11	6.761 ±  2.46%	4.274 ±  3.83%	3.730 ± 0.21%	123	3.491 ± 0.26%	93.6	5.58	2.616 ± 0.011	
	12	7.224 ±  7.74%	1.499 ± 33.29%	3.686 ± 0.17%	351	3.432 ± 0.51%	93.1	4.84	2.572 ± 0.016	
	13	6.951 ±  2.72%	2.979 ±  9.21%	3.690 ± 0.21%	177	3.445 ± 0.28%	93.4	5.66	2.581 ± 0.012	
	14	3.982 ±  3.50%	4.677 ±  8.46%	3.630 ± 0.16%	113	3.473 ± 0.20%	95.7	7.55	2.602 ± 0.011	
	15	7.266 ±  2.70%	5.593 ±  8.62%	3.742 ± 0.15%	94	3.488 ± 0.23%	93.2	5.77	2.614 ± 0.011	
	Arithmetic mean K/Ca (n = 15)	128 ± 66	Arithmetic mean age (n = 15)
Weighted mean age (n = 15)	2.606 ± 0.018
2.603 ± 0.009	

l = 5.543 x 10-10 a-1   Sensitivity 2.6 (± ~15%) x 10-17 mol/mV        Bracketed results excluded from mean age calculation
(36Ar/37Ar)Ca = 3.50 x 10-4	(39Ar/37Ar)Ca = 7.86 x 10-4	(40Ar/39Ar)K = 0.040
40Ar* – radiogenic 40Ar	39ArK – K derived 39Ar	
Mass of alkali feldspar used in each analysis in range:  83-286B – 1.2 to 6.4 mg; 83-287A – 1.1 to 2.2 mg
Irradiated 24 hours in X33 or X34, HIFAR reactor
Fluence monitor: 92-176 sanidine from Fish Canyon Tuff; K-Ar reference age utilized: 28.1 Ma
For sample 83-286B analyses were done on single crystals except for analyses 11 to 15 for which 2 crystals were used
For sample 83-287A, analyses 1 to 6 were made on single crystals, analyses 7 to 12 on 2 crystals, and analyses 13 to 15 on 3 crystals
All errors are one standard deviation. Uncertainty on the 40Ar*/39ArK ratio is the analytical error. This error is combined quadratically with the estimated error of J to yield the quoted error for each age