1. Sachiko Amari Washington University St. Louis MO, USA
Presolar diamond in meteorites: A few outstanding problems (and how we plan to investigate them)
Sachiko Amari
Washington University
St. Louis MO, USA

2. Presolar Grains Dust produced in the stellar outflow or stellar ejecta
Incorporated into meteorites 4.6 billion years ago (=older than the solar system)
STARDUST IN METEORITES
(not to be confused with the dust recovered from the STADUST mission)
Abundance: ≤ 0.1 weight percent of a bulk meteorite
Allende meteorite

3. Presolar diamond
First mineral type that was isolated from meteorites (1987)
One of the least understood presolar grain types
Abundance: ~1000 ppm in primitive meteorites
Small grain size: 3nm
Only bulk (=aggregate) analysis is possible
Contains an isotopically anomalous Xe called Xe-HL
Enriched in
L -- p-process only isotopes 124 and 126
H -- r-process only isotopes 134 and 136
Xe-L
Xe-H

4. Puzzle 1
The excess in the p-only isotopes (124 & 126) and that in the r-process only isotopes (134 & 136) are always correlated. (= Xe-H and Xe-L are ALWAYS correlated.) This correlation has been observed in all diamond separates from any type of primitive meteorites. Close association of the two different nucleosynthetic processes. Possible explanation Diamond grains are very small (average size: 3nm) and grains with Xe-L and those with Xe-H are homogeneously mixed.

5. Puzzle 2
r- and p-Process patterns inferred from Xe-HL are NOT what is expected from the Solar System abundance and models of stars.
Xe-HL = R-process in diamond + P-process in diamond + solar Xe
(Pure r- and p-process Xe does not contain 130)
R-process in diamond = measured Xe-H - solar Xe
P-process in diamond = measured Xe-L - solar Xe
(134/136)Diamond-R = 0.699, (124/126)Diamond-P = 2.205
R- and p-process Xe in the Solar System
(134/136)SS-R = 1.207*
* Solar Xe - (s-process Xe in SiC) (Ott, 1996)
(124/126)SS-P = 1.157
Theoretical models (Rayet et al., 1995)
(124/126)SN20Msun = 1.188

6. Effort to explain the r-process pattern in diamond
1. Rapid separation model (Ott, 1996)
Precursors of 134Xe and 136Xe (Te and I) have different half-lives.
136Te (T1/2 = 17.5 s)
136I (T1/2 = 1.39 m)
134Te (T1/2 = 42 m)
134I (T1/2 = 53 m)
Difficulties
Separation of Xe and the unstable precursors (Te and I) need to take place in a timescale of hours.
Needs a VERY fine tuning of the timing of the separation.
124Xe and 126Xe can be explained in the same manner with Ba Cs precursors (Ott, 1996).

7. 2. Mix with n-burst material
Effort to explain the r-process pattern in diamond (continued)
2. Mix with n-burst material
Neutron burst (Meyer et al., 2000)
Occurs in shocked He-rich matter
T=109K
~1017 neutrons/cm3
Mix of neutron burst and solar material
Solar = 0.79
N-burst = 0.21
Xe-L: left unexplained

8. Other elements in diamond
Isotopic analyses of heavy elements that were produced in the same processes as diamond may shed light on the origin of Xe-HL.
Isotopic anomalies in other elements are very small compared with those in Xe (Richter et al., 1998; Mass et al., 2001)
Tellurium [Te] (128 & 130 r-only, 124 s-only)
128Te/124Te & 130Te/124Te < times solar
Pd (104 s-only, 110 r-only)
110Pd/104Pd: 1.01 times solar
Possible explanation
Presence of contaminants (Ir-nuggets, SiC etc.)

9. Isotopic analysis of heavy elements in diamond
Low concentrations
Any contaminants compromise the analysis
Our Plans
First Step
Produce diamond separates of ultra-high purity.
Complete removal of SiC, Ir nuggets
Will be done at Washington University
Second Step
Perform isotopic analyses of heavy elements
SIMS -- high mass resolution power (M/M > 10,000 required)
RIMS (Resonant Ionization Mass Spectrometry)
(Lewis et al., 1991)

10. RIMS analysis at Argonne National Laboratory (Michael Pellin and Michael Savina and their coworkers)
Atoms are desorbed from the sample with a laser, resonantly ionized with several tuned lasers, and analyzed with a time-of-flight mass spectrometer.
Advantage of the instrument
An element of interest is selectively ionized and detected.
CHARISMA (The Chicago Argonne Resonance Ionization Spectrometer for Microbeam Analysis)
SARISA (Surface Analysis by Resonant Ionization of Sputtered Atoms)
Useful yields (ions detected/atoms removed)
CHARISMA - 1 to 2%
SARISA - up to 25%
(Nicolussi et al., 1998)

11. Summary Diamond One of the least understood presolar grains Puzzles
1) Excesses in the p-only isotopes (124 & 126) and in the r-process only isotopes are always correlated.
2) Their patterns are different from what is derived from the solar system abundance and from models.
Future plans
1) Produce ultra-high purity diamond
2) Analyze isotopic ratios of heavy elements using RIMS