Isotopes of osmium

Osmium (₇₆Os) has seven naturally occurring isotopes, five of which are stable: ¹⁸⁷Os, ¹⁸⁸Os, ¹⁸⁹Os, ¹⁹⁰Os, and (most abundant) ¹⁹²Os. The other natural isotopes, ¹⁸⁴Os, and ¹⁸⁶Os, have extremely long half-lives (1.12×10¹³ years and 2.0×10¹⁵ years, respectively) and for practical purposes can be considered to be stable as well. ¹⁸⁷Os is the daughter of ¹⁸⁷Re (half-life 4.12×10¹⁰ years) and is most often measured by the ¹⁸⁷Os/¹⁸⁸Os ratio. This ratio, as well as the ¹⁸⁷Re/¹⁸⁸Os ratio, have been used extensively in dating terrestrial as well as meteoric rocks. It has also been used to measure the intensity of continental weathering over geologic time and to fix minimum ages for stabilization of the mantle roots of continental cratons.

Isotopes of osmium (₇₆Os)

Main isotopes[1] Decay abun­dance half-life (t1/2) mode pro­duct 184Os 0.02% 1.12×1013 y α 180W 185Os synth 92.95 d ε 185Re 186Os 1.59% 2.0×1015 y α 182W 187Os 1.96% stable 188Os 13.2% stable 189Os 16.1% stable 190Os 26.3% stable 191Os synth 14.99 d β− 191Ir 192Os 40.8% stable 193Os synth 29.83 h β− 193Ir 194Os synth 6.0 y β− 194Ir
Standard atomic weight Ar°(Os)
	190.23±0.03[2] 190.23±0.03 (abridged)[3]
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There are also 31 artificial radioisotopes,^[4] the longest-lived of which are ¹⁹⁴Os with a half-life of 6.0 years, ¹⁸⁵Os with 92.95 days, and ¹⁹¹Os with 14.99 days; others are under 30 hours, with most under seven minutes. There are also 19 listed nuclear isomers, the longest-lived of which is ^191mOs with a half-life of 13.10 hours. All isotopes and nuclear isomers of osmium are either radioactive or observationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed.

Osmium isotopes in radiometric dating

The isotopic ratio of osmium-187 and osmium-188 (¹⁸⁷Os/¹⁸⁸Os) can be used as a window into geochemical changes throughout the ocean's history.^[5] The average marine ¹⁸⁷Os/¹⁸⁸Os ratio in oceans is 1.06.^[5] This value represents a balance of the continental riverine inputs of Os with a ¹⁸⁷Os/¹⁸⁸Os ratio of ~1.3, and the mantle/extraterrestrial inputs with a ¹⁸⁷Os/¹⁸⁸Os ratio of ~0.13.^[5] The lighter isotope, ¹⁸⁷Os, is produced by beta decay of ¹⁸⁷Re.^[6] This decay has actually increased the ¹⁸⁷Os/¹⁸⁸Os ratio of the bulk silicate earth (Earth less the core) by 33%.^[7] The difference between crust and mantle ratios is explained this way: crustal rocks have a much higher level of rhenium^[why?], which produces an excess of ¹⁸⁷Os.^[6] The combined input of the two sources to the marine environment results in the observed ratio in the oceans, and has fluctuated over the geologic history. These changes in the isotopic values of marine Os can be observed in the marine sediment that is deposited, and eventually lithified in that time period.^[8] This allows for researchers to estimate weathering fluxes, flood basalt volcanism, and impact events that may have caused some of our largest mass extinctions. The marine sediment Os isotope record has corroborated the K-T boundary impact, for example.^[9] The impact of this ~10 km asteroid massively altered the ¹⁸⁷Os/¹⁸⁸Os signature of marine sediments at that time - the average extraterrestrial ¹⁸⁷Os/¹⁸⁸Os of ~0.13 and the huge amount of Os this impact contributed (equivalent to 600,000 years of present-day riverine inputs) lowered the global marine ¹⁸⁷Os/¹⁸⁸Os value of ~0.45 to a minimum of ~0.2.^[5]

Os isotope ratios may also be used as a signal of anthropogenic impact.^[10] The same ¹⁸⁷Os/¹⁸⁸Os ratios that are common in geological settings may be used to gauge the addition of anthropogenic Os through things like catalytic converters.^[10] While catalytic converters have been shown to drastically reduce the emission of NO_x and CO, they are introducing platinum group elements (PGE) such as Os, to the environment.^[10] Other sources of anthropogenic Os include combustion of fossil fuels, smelting chromium ore, and smelting of some sulfide ores. In one study, the effect of automobile exhaust on the marine Os system was evaluated. Automobile exhaust ¹⁸⁷Os/¹⁸⁸Os has been recorded to be ~0.2 (similar to extraterrestrial and mantle derived inputs)^[10][5]. The effect of anthropogenic Os can be seen best by comparing aquatic Os ratios and local sediments or deeper waters. Surface waters thought to be affected have depleted values compared to deep ocean and sediments by a ratio larger than can be explained by cosmic inputs.^[10]

The alpha decay of ¹⁸⁴Os into ¹⁸⁰W (with a rate perhaps large enough for detection) has been proposed as a radiometric dating method for osmium-rich rocks or for differentiation of a planetary core.^[11][12][13]

List of isotopes

Nuclide [n 1]	Z	N	Isotopic mass (Da)[14] [n 2][n 3]	Half-life[1] [n 4]	Decay mode[1] [n 5]	Daughter isotope [n 6][n 7]	Spin and parity[1] [n 8][n 9]	Natural abundance (mole fraction)
	Excitation energy							Normal proportion[1]	Range of variation
160Os[15]	76	84		97+97 −32 μs	α	156W	0+
160mOs[15]	1844(18) keV			41+15 −9 μs	α	156W	8+
161Os	76	85	160.98905(43)#	0.64(6) ms	α	157W	(7/2–)
162Os	76	86	161.98443(32)#	2.1(1) ms	α	158W	0+
163Os	76	87	162.98246(32)#	5.7(5) ms	α	159W	7/2–
					β+ ?	163Re
164Os	76	88	163.97807(16)	21(1) ms	α (96%)	160W	0+
					β+ (4%)	164Re
165Os	76	89	164.97665(22)#	71(3) ms	α (90%)	161W	(7/2–)
					β+ (10%)	165Re
166Os	76	90	165.972698(19)	213(5) ms	α (83%)	162W	0+
					β+ (17%)	166Re
167Os	76	91	166.971552(87)	839(5) ms	α (51%)	163W	7/2–
					β+ (49%)	167Re
167mOs	434.3(11) keV			0.672(7) μs	IT	167Os	13/2+
168Os	76	92	167.967799(11)	2.1(1) s	β+ (57%)	168Re	0+
					α (43%)	164W
169Os	76	93	168.967018(28)	3.46(11) s	β+ (86.3%)	169Re	(5/2–)
					α (13.7%)	165W
170Os	76	94	169.963579(10)	7.37(18) s	β+ (90.5%)	170Re	0+
					α (9.5%)	166W
171Os	76	95	170.963180(20)	8.3(2) s	β+ (98.20%)	171Re	(5/2−)
					α (1.80%)	167W
172Os	76	96	171.960017(14)	19.2(9) s	β+ (98.81%)	172Re	0+
					α (1.19%)	168W
173Os	76	97	172.959808(16)	22.4(9) s	β+ (99.6%)	173Re	5/2–
					α (0.4%)	169W
174Os	76	98	173.957063(11)	44(4) s	β+ (99.98%)	174Re	0+
					α (.024%)	170W
175Os	76	99	174.956945(13)	1.4(1) min	β+	175Re	(5/2−)
176Os	76	100	175.954770(12)	3.6(5) min	β+	176Re	0+
177Os	76	101	176.954958(16)	3.0(2) min	β+	177Re	1/2−
178Os	76	102	177.953253(15)	5.0(4) min	β+	178Re	0+
179Os	76	103	178.953816(17)	6.5(3) min	β+	179Re	1/2–
179m1Os	145.41(12) keV			~500 ns	IT	179Os	(7/2)–
179m2Os	243.0(8) keV			783(14) ns	IT	179Os	(9/2)+
180Os	76	104	179.952382(17)	21.5(4) min	β+	180Re	0+
181Os	76	105	180.953247(27)	105(3) min	β+	181Re	1/2−
181m1Os	49.20(14) keV			2.7(1) min	β+	181Re	7/2−
181m2Os	156.91(15) keV			262(6) ns	IT	181Os	9/2+
182Os	76	106	181.952110(23)	21.84(20) h	EC	182Re	0+
182m1Os	1831.4(3) keV			780(70) μs	IT	182Os	8–
182m2Os	7049.5(4) keV			150(10) ns	IT	182Os	25+
183Os	76	107	182.953125(53)	13.0(5) h	β+	183Re	9/2+
183mOs	170.73(7) keV			9.9(3) h	β+ (85%)	183Re	1/2−
					IT (15%)	183Os
184Os[n 10]	76	108	183.95249292(89)	1.12(23)×1013 y	α[n 11]	180W	0+	2(2)×10−4
185Os	76	109	184.95404597(89)	92.95(9) d	EC	185Re	1/2−
185m1Os	102.37(11) keV			3.0(4) μs	IT	185Os	7/2−
185m2Os	275.53(12) keV			0.78(5) μs	IT	185Os	11/2+
186Os[n 10]	76	110	185.95383757(82)	2.0(11)×1015 y	α	182W	0+	0.0159(64)
187Os[n 12]	76	111	186.95574957(79)	Observationally Stable[n 13]			1/2−	0.0196(17)
187m1Os	100.45(4) keV			112(6) ns	IT	187Os	7/2−
187m2Os	257.10(7) keV			231(2) μs	IT	187Os	11/2+
188Os[n 12]	76	112	187.95583729(79)	Observationally Stable[n 14]			0+	0.1324(27)
189Os	76	113	188.95814595(72)	Observationally Stable[n 15]			3/2−	0.1615(23)
189mOs	30.82(2) keV			5.81(10) h	IT	189Os	9/2−
190Os	76	114	189.95844544(70)	Observationally Stable[n 16]			0+	0.2626(20)
190mOs	1705.7(1) keV			9.86(3) min	IT	190Os	10−
191Os	76	115	190.96092811(71)	14.99(2) d	β−	191Ir	9/2−
191mOs	74.382(3) keV			13.10(5) h	IT	191Os	3/2−
192Os	76	116	191.9614788(25)	Observationally Stable[n 17]			0+	0.4078(32)
192m1Os	2015.40(11) keV			5.94(9) s	IT	192Os	10−
					β−?	192Ir
192m2Os	4580.3(10) keV			205(7) ns	IT	192Os	(20+)
193Os	76	117	192.9641496(25)	29.830(18) h	β−	193Ir	3/2−
193mOs	315.6(3) keV			121(28) ns	IT	193Os	(9/2−)
194Os	76	118	193.9651794(26)	6.0(2) y	β−	194Ir	0+
195Os	76	119	194.968318(60)	6.5(11) min	β−	195Ir	(3/2−)
195mOs	427.8(3) keV			47(3) s	IT	195Os	(13/2+)
					β−?	195Ir
196Os	76	120	195.969643(43)	34.9(2) min	β−	196Ir	0+
197Os	76	121	196.97308(22)#	93(7) s	β−	197Ir	5/2−#
198Os	76	122	197.97466(22)#	125(28) s	β−	198Ir	0+
199Os	76	123	198.97824(22)#	6(3) s	β−	199Ir	5/2−#
200Os	76	124	199.98009(32)#	7(4) s	β−	200Ir	0+
201Os	76	125	200.98407(32)#	3# s [>300ns]	β−?	201Ir	1/2−#
202Os	76	126	201.98655(43)#	2# s [>300ns]	β−?	202Ir	0+
203Os	76	127	202.99220(43)#	300# ms [>300ns]	β−?	203Ir	9/2+#
					β− n?	202Ir
This table header & footer: view

1. ^mOs – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. Bold half-life – nearly stable, half-life longer than age of universe.
5. Modes of decay:

   EC:	Electron capture
   IT:	Isomeric transition
   p:	Proton emission

6. Bold italics symbol as daughter – Daughter product is nearly stable.
7. Bold symbol as daughter – Daughter product is stable.
8. ( ) spin value – Indicates spin with weak assignment arguments.
9. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
10. primordial radionuclide
11. Theorized to also undergo β⁺β⁺ decay to ¹⁸⁴W
12. Used in rhenium-osmium dating
13. Believed to undergo α decay to ¹⁸³W with a half-life over 3.2×10¹⁵ years
14. Believed to undergo α decay to ¹⁸⁴W with a half-life over 3.3×10¹⁸ years
15. Believed to undergo α decay to ¹⁸⁵W with a half-life over 3.3×10¹⁵ years
16. Believed to undergo α decay to ¹⁸⁶W with a half-life over 1.2×10¹⁹ years
17. Believed to undergo α decay to ¹⁸⁸W or β⁻β⁻ decay to ¹⁹²Pt with a half-life over 5.3×10¹⁹ years

See also

Daughter products other than osmium

• Isotopes of iridium
• Isotopes of rhenium
• Isotopes of tungsten