mid-term exam

Mid-term answer key & score distribution

Answer Key:

Question Points Answer

1a 2 Sample 1 type = 6 (could be 5, except for coarseness of feldspar). Sample 2 type = 2 (could be 3, except for abundance of clay minerals).

1b 4 Sample 1 class = ordinary chondrite, based on O-isotope composition (which rules out enstatite chondrites which lie on TF line, and C-chondrites which lie below TF line), intermediate oxidation state (which reflects presence of iron partly in silicates, partly in metal; inconsistent with most highly oxidized carbonacous chondrites and the highly reduced enstatite chondrites); and presence and composition of olivine (which rules out enstatite chondrites). Sample 2 class = carbonaceous chondrite, based on the O-isotope composition and the elevated water and C contents.

2 6 IA and IIICD: uniform & low cooling rate suggest deep burial, possibly in a core. IIIAB: low but somewhat non-uniform cooling rate suggest deep burial, either in a large core, or in deep metal bodies coalescing into a core. IVA, IVB, IIC: variable cooling rates suggest origin in metallic bodies at different depths within parent bodies, possibly reflecting incomplete core formation.

3 5 Here I was looking for a plot with temperature on the y-axis, Ni on the x-axis, and three fields labelled kamacite (or alpha-iron), kamacite + taenite, and taenite (or gamma iron). For full credit the shape of the fields had to be correct. An annotated version of the subsolidus Fe-Ni phase diagram is reproduced here.

4 6 In all cases, at high temperatures the stable metal phase is taenite. This can convert either completely, partially, or not at all into kamacite, depending on the bulk Ni content. Hexahedites have low (<6 wt%) Ni, and they always cool into the kamacite field to form an iron meteorite consisting only of this mineral. Octahedrites have intermediate Ni contents and cool into the kamacite + taenite field to produce the Widmannstatten texture. Ataxites have the highest Ni contents (e.g., 15 wt%). As kamacite is stable at lower temperatures for higher Ni (see the phase diagram), kamacite either does not form before the closure temperature is reached, or just begins to form before the closure temperature is reached, resulting in an iron meteorite consisting mainly of taenite. (Cooling rate can be important, too, but in practice this only affects the width of kamacite lamellae that form in octahedrites and ataxites.)

5 6 A = FeNi metal (coarse). B = pyroxene (FeO-bearing). C = olivine (FeO-bearing).

6 4 A chondrule is a polymineralic, ferromagnesian object found in chondrites, typically < 1 mm in diameter, which has textures indicative of having cooled relatively rapidly from a molten state.

7a 4 A Type I chondrule is low in FeO and volatile elements; a Type II chondrule is high in FeO and volatile erlements ("solar" composition).

7b 3 The mian uncertainty is whether type I and II chondrules were derived from the same precursor involving open-system behavior (e.g., removal of volatiles from Type II to form Type I), or whether they were derived from different precursors.

8 6 Object A formed at high temperature, either as a condensate or a residue of vaporization. Object B must have formed by condensation at intermediate temperatures in one of two ways: either as a condensate that formed after the removal of the most refractory elements as in object A, or as a condensate from a gas produced by partial vaporization, missing the most refractory elements as contained in object A. (Object A is equivalent to a superefractory CAI; object B is equivalent to a Group II CAI.)

9 8 Here I was looking for a plot with temperature on the y-axis and time on the x-axis, showing the following features for chondrules and CAIs. Chondrules-- low pre-heating temperatures (e.g., < 650 K), as inferred from the presence of Na and S in chondrule precursors; rapid, short duration (e.g., minute) heating to ~1550-2100 K; followed by rapid cooling at ~10-5000 K/hr back to the ambient temperature. CAIs-- pre-heat temperatures uncertain, but maybe ~1350-1700 K, the predicted condensation temperatures for CAIs; probable reheating with relatively long durations to maximum temperature of ~1700 K (evidence of remelting, evidence of equilibrium); subsequent cooling at ~1-50 K/hour.

Total = 54

Distribution:

Number of students 1 1 1 1 1 1 1 2 1 1 1 1 1

Original score 17.75 19 22.25 22.5 23 31 34 35.25 37.5 37.75 40 41.4 44.5

Curved score 82 83 84.5 85 85.5 89 90.5 91 93 93 94 95 96.5

8 A, 6 B, 0 C-F

curved score mean = 89.5

Comment: This appears to have been a rather difficult test, so I applied a generous curve.

Question	Points	Answer
1a	2	Sample 1 type = 6 (could be 5, except for coarseness of feldspar). Sample 2 type = 2 (could be 3, except for abundance of clay minerals).
1b	4	Sample 1 class = ordinary chondrite, based on O-isotope composition (which rules out enstatite chondrites which lie on TF line, and C-chondrites which lie below TF line), intermediate oxidation state (which reflects presence of iron partly in silicates, partly in metal; inconsistent with most highly oxidized carbonacous chondrites and the highly reduced enstatite chondrites); and presence and composition of olivine (which rules out enstatite chondrites). Sample 2 class = carbonaceous chondrite, based on the O-isotope composition and the elevated water and C contents.
2	6	IA and IIICD: uniform & low cooling rate suggest deep burial, possibly in a core. IIIAB: low but somewhat non-uniform cooling rate suggest deep burial, either in a large core, or in deep metal bodies coalescing into a core. IVA, IVB, IIC: variable cooling rates suggest origin in metallic bodies at different depths within parent bodies, possibly reflecting incomplete core formation.
3	5	Here I was looking for a plot with temperature on the y-axis, Ni on the x-axis, and three fields labelled kamacite (or alpha-iron), kamacite + taenite, and taenite (or gamma iron). For full credit the shape of the fields had to be correct. An annotated version of the subsolidus Fe-Ni phase diagram is reproduced here.
4	6	In all cases, at high temperatures the stable metal phase is taenite. This can convert either completely, partially, or not at all into kamacite, depending on the bulk Ni content. Hexahedites have low (<6 wt%) Ni, and they always cool into the kamacite field to form an iron meteorite consisting only of this mineral. Octahedrites have intermediate Ni contents and cool into the kamacite + taenite field to produce the Widmannstatten texture. Ataxites have the highest Ni contents (e.g., 15 wt%). As kamacite is stable at lower temperatures for higher Ni (see the phase diagram), kamacite either does not form before the closure temperature is reached, or just begins to form before the closure temperature is reached, resulting in an iron meteorite consisting mainly of taenite. (Cooling rate can be important, too, but in practice this only affects the width of kamacite lamellae that form in octahedrites and ataxites.)
5	6	A = FeNi metal (coarse). B = pyroxene (FeO-bearing). C = olivine (FeO-bearing).
6	4	A chondrule is a polymineralic, ferromagnesian object found in chondrites, typically < 1 mm in diameter, which has textures indicative of having cooled relatively rapidly from a molten state.
7a	4	A Type I chondrule is low in FeO and volatile elements; a Type II chondrule is high in FeO and volatile erlements ("solar" composition).
7b	3	The mian uncertainty is whether type I and II chondrules were derived from the same precursor involving open-system behavior (e.g., removal of volatiles from Type II to form Type I), or whether they were derived from different precursors.
8	6	Object A formed at high temperature, either as a condensate or a residue of vaporization. Object B must have formed by condensation at intermediate temperatures in one of two ways: either as a condensate that formed after the removal of the most refractory elements as in object A, or as a condensate from a gas produced by partial vaporization, missing the most refractory elements as contained in object A. (Object A is equivalent to a superefractory CAI; object B is equivalent to a Group II CAI.)
9	8	Here I was looking for a plot with temperature on the y-axis and time on the x-axis, showing the following features for chondrules and CAIs. Chondrules-- low pre-heating temperatures (e.g., < 650 K), as inferred from the presence of Na and S in chondrule precursors; rapid, short duration (e.g., minute) heating to ~1550-2100 K; followed by rapid cooling at ~10-5000 K/hr back to the ambient temperature. CAIs-- pre-heat temperatures uncertain, but maybe ~1350-1700 K, the predicted condensation temperatures for CAIs; probable reheating with relatively long durations to maximum temperature of ~1700 K (evidence of remelting, evidence of equilibrium); subsequent cooling at ~1-50 K/hour.
	Total = 54

Number of students	1	1	1	1	1	1	1	2	1	1	1	1	1
Original score	17.75	19	22.25	22.5	23	31	34	35.25	37.5	37.75	40	41.4	44.5
Curved score	82	83	84.5	85	85.5	89	90.5	91	93	93	94	95	96.5