Watson: A new link in the HE iron chain
Authors: Olsen Edward , Davis Andrew , Clarke Roy S. , Schultz Ludolf , Weber Hartwig W. , Clayton Robert , Mayeda Toshiko , Jarosewich Eugene , Sylvester Paul , Grossman Lawrence , Wang Ming‐Sheng , Lipschutz Michael E. , Steele Ian M. , Schwade James 
Published: June 2012

Abstract— Watson, which was found in 1972 in South Australia, contains the largest single silicate rock mass seen in any known iron meteorite. A comprehensive study has been completed on this unusual meteorite: petrography, metallography, analyses of the silicate inclusion (whole rock chemical analysis, INAA, RNAA, noble gases, and oxygen isotope analysis) and mineral compositions (by electron microprobe and ion microprobe). The whole rock has a composition of an H‐chondrite minus the normal H‐group metal and troilite content. The oxygen isotope composition is that of the silicates in the HE iron meteorites and lies along an oxygen isotope fractionation line with the H‐group chondrites. Trace elements in the metal confirm Watson is a new HE iron. Whole rock Watson silicate shows an enrichment in K and P (each ∼2X H‐chondrites). The silicate inclusion has a highly equilibrated igneous (peridotite‐like) texture with olivine largely poikilitic within low‐Ca pyroxene: olivine (Fa20), opx (Fs17Wo3), capx (Fs9Wo41) (with very fine exsolution lamellae), antiperthite feldspar (An1–Or5) with <1 μm exsolution lamellae (An1–3Or>40), shocked feldspar with altered stoichiometry, minor whitlockite (also a poorly characterized interstitial phosphate‐rich phase) and chromite, and only traces of metal and troilite. The individual silicate minerals have normal chondritic REE patterns, but whitlockite has a remarkable REE pattern. It is very enriched in light REE (La is 720X C1, and Lu is 90X C1, as opposed to usual chonditic values of ∼300X and 100–150X, respectively) with a negative Eu anomaly. The enrichment of whole rock K is expressed both in an unusually high mean modal Or content of the feldspar, Or13, and in the presence of antiperthite.

Whole rock trace element data for the silicate mass support the petrography. Watson silicate was an H‐chondrite engulfed by metal and melted at > 1550 °C. A flat refractory lithophile and flat REE pattern (at ∼1x average H‐chondrites) indicate that melting took place in a relatively closed system. Immiscible metal and sulfide were occluded into the surrounding metal host. Below 1100 °C, the average cooling rate is estimated to have been ∼1000 °C/Ma; Widmanstätten structure formed, any igneous zoning in the silicates was equilibrated, and feldspar and pyroxene exsolution took place. Cooling to below 300 °C was completed by 3.5 Ga B. P. At 8 Ma, a shock event took place causing some severe metal deformation and forming local melt pockets of schreibersite/metal. This event likely caused the release of Watson into interplanetary space. The time of this event, 8Ma, corresponds to the peak frequency of exposure ages of the H‐chondrites. This further confirms the link between HE irons and the H‐chondrites, a relationship already indicated by their common oxygen isotope source.

Watson metal structures are very similar to those in Kodaikanal. Watson, Kodaikanal and Netschaëvo form the young group of HE meteorites (ages 3.7 ± 0.2 Ga). They appear to represent steps in a chain of events that must have taken place repeatedly on the HE parent body or bodies from which they came: chondrite engulfed in metal (Netschaëvo); chondrite melted within metal (Watson); and finally melted silicate undergoing strong fractionation with the fractionated material emplaced as globules within metal (Kodaikanal). Watson fills an important gap in understanding the sequence of events that took place in the evolution of the IIE‐H parent body(ies). This association of H‐chondrite with HE metal suggests a surface, or near surface process‐a suggestion made by several other researchers.