Sutter’s Mill        38°48’14"N, 120°54’29"W

California, United States

Fell: 22 Apr 2012

Classification: Carbonaceous chondrite (C)

History: (P. Jenniskens, SETI): A bright daytime east-to-west moving fireball was seen on April 22, 2012, from locations over California and Nevada between 7:51:10 and 7:51:30 am local daylight time (UT-7). The meteoroid fragmented towards the end of its trajectory. A loud sonic boom was heard in a wide region around Lake Tahoe. Wind gusts were felt and houses shook. At least a kiloton of kinetic energy was released, based on the infrasound signal detected at two stations. Eye witnesses in the townships of Coloma and Lotus, El Dorado County, reported hearing whistling sounds and some smelled a "welding" odor. U.S. National Climatic Data Center’s "NEXRAD" Doppler weather radar sweeps detected the falling meteorites. In data analyzed by Marc Fries, PSI, and Robert Matson, SAIC, the radar-defined strewn field is centered on the Sutter’s Mill historic site. On April 24, Robert Ward searched under the radar footprint and collected the first 5.5 g meteorite in Henningsen-Lotus Park. Later that day, Peter Jenniskens recovered a crushed 4 g fragment in the parking lot of that same park. A third find was made by Brien Cook, before heavy rain descended on the area in the following two days. After the rains more fragments were found including at the Sutter’s Mill site in the James W. Marshall Gold Discovery State Historic Park.

Physical characteristics: As of November 27th, 2012, 90 fragments have been recovered with a total mass of 992.5 g. A tally is maintained at the Sutter’s Mill Meteorite Consortium website: http://asima.seti.org/sm/.

Petrography: (M. Zolensky, JSC): A small piece was examined for petrography, taken from the stone found by Jenniskens on the parking lot surface, which had been crushed by an automobile tire and had some adhering terrestrial soil. The meteorite is unusually hard compared to CM2 chondrites, and microprobe totals for the matrix are also high for CM2, suggesting possibly incomplete aqueous alteration or, alternatively, mild thermal metamorphism.  Various coarse-grained components are embedded within an opaque fine-grained matrix including chondrules, fine-grained porous olivine aggregates, fine-grained porous low-Ca pyroxene aggregates, large isolated lithic and mineral fragments (both olivine, low-Ca pyroxene), abundant CAIs, and grains of pyrrhotite and pentlandite. The components are often rimmed by fine-grained dust mantles. Chondrule diameters are generally less than 0.4 mm with a few exceeding 1 mm. Some chondrule contents are partially altered to tochilinite and cronstedtite and/or serpentine. Fe-Ni metal occurs (as a minor constituent) in chondrules and aggregate olivine crystals. The porous aggregates are irregular in shape and measure up to a couple of hundred microns across. The CAI are typical layered CM variety, with spinel cores, diopside rims and cronstedtite or Fe-rich serpentine between. The matrix includes abundant tochilinite intergrown with serpentine with a layered structure.  Veins are present consisting of pyrrhotite, pentlandite, and Fe-Ni-Cr phosphides. One small, embayed grain of oldhamite was observed in matrix.  Several grains of a refractory carbon phase were observed in the matrix, measuring up to 10 µm. No carbonates were observed.

Geochemistry: Most olivines in the chondrules and coarse-grained components are forsterite (below Fa₃), but some Fe-rich olivine is present (Fa₂₃). Coarse-grained olivine grains in the matrix are similar, although the iron-rich ones are up to Fa_37. Olivine, from 22 analyses, averaged Fa₄, with a range Fa_1-29. Low-Ca pyroxene, from 19 analyses averaged Fs₄Wo₂, with a range Fs₂Wo_{1 -} Fs₇Wo_4.

Classification: Carbonaceous chondrite.

Specimens: Current institutional holdings include ASU - 24.9 g, FMNH - 10.1 g, UAz - 15.9, and UNM - 11.0 g. 3 g of the crushed 4 g specimen and a 17 g specimen are located at ARC.