Theoretically long gamma-ray bursts ( GRBs ) are expected to happen in low-metallicity environments , because in a single massive star scenario , low iron abundance prevents loss of angular momentum through stellar wind , resulting in ultra-relativistic jets and the burst .
In this sense , not just a simple metallicity measurement but also low iron abundance ( [ Fe/H ] \lesssim -1.0 ) is essentially important .

Observationally , however , oxygen abundance has been measured more often due to stronger emission .
In terms of oxygen abundance , some GRBs have been reported to be hosted by high-metallicity star-forming galaxies , in tension with theoretical predictions .

Here we compare iron and oxygen abundances for the first time for GRB host galaxies ( GRB 980425 and 080517 ) based on the emission-line diagnostics .
The estimated total iron abundances , including iron in both gas and dust , are well below the solar value .
The total iron abundances can be explained by the typical value of theoretical predictions ( [ Fe/H ] \lesssim -1.0 ) , despite high oxygen abundance in one of them .
According to our iron abundance measurements , the single massive star scenario still survives even if the oxygen abundance of the host is very high , such as the solar value .
Relying only on oxygen abundance could mislead us on the origin of the GRBs .

The measured oxygen-to-iron ratios , [ O/Fe ] , can be comparable to the highest values among the iron-measured galaxies in the Sloan Digital Sky Survey .
Possible theoretical explanations of such high [ O/Fe ] include the young age of the hosts , top-heavy initial mass function , and fallback mechanism of the iron element in supernova explosions .