We have studied 23 very metal-poor field turnoff stars , specifically chosen to enable a precise measurement of the dispersion in the lithium abundance of the Spite Li plateau .
We concentrated on stars having a narrow range of effective temperature and very low metallicities ( [ Fe/H ] ^ { < } _ { \sim } -2.5 ) to reduce the effects of systematic errors , and have made particular efforts to minimise random errors in equivalent width and effective temperature .
A typical formal error for our abundances is 0.033 dex ( 1 \sigma ) , which represents a factor of two improvement on most previous studies .

One of the 23 stars , G186-26 , was known already to be strongly Li depleted .
Of the remaining 22 objects , 21 ( i.e .
91 % of the original sample ) have abundances consistent with an observed spread about the Spite Li plateau of a mere 0.031 dex ( 1 \sigma ) .
As the formal errors are 0.033 dex , we conclude that the intrinsic spread \sigma _ { int } is effectively zero at the very metal-poor halo turnoff .
( Inclusion of the twenty-second star would inflate the observed spread to only 0.037 dex , leaving \sigma _ { int } < 0.02 . )
Furthermore , we have established this at a much higher precision than previous studies ( \sim 0.06–0.08 dex ) .

Our sample does not exhibit a trend with effective temperature , though the temperature range is limited .
However , for -3.6 ~ { } < [ Fe/H ] < ~ { } -2.3 we do recover a dependence on metallicity at d A ( Li ) /d [ Fe/H ] = 0.118 \pm 0.023 ( 1 \sigma ) dex per dex , almost the same level as discussed previously .
Earlier claims for a lack of dependence of A ( Li ) on abundance are shown to have arisen , in all likelihood , from the use of noisier estimates of effective temperatures and metallicities , which have erased the real trend .
The dependence is concordant with theoretical predictions of Galactic chemical evolution ( GCE ) of Li ( even in such metal-poor stars ) and with the published level of ^ { 6 } Li in two of the stars of our sample , which we use to infer the GCE ^ { 7 } Li contribution .
The essentially zero intrinsic spread ( \sigma _ { int } < 0.02 dex ) inferred for the sample leads to the conclusion that either these stars have all changed their surface Li abundances very uniformly , or else they exhibit close to the primordial abundance sought for its cosmological significance .
Although we can not rule out a uniform depletion mechanism , economy of hypothesis supports the latter interpretation .
The lack of spread in the A ( Li ) abundances limits permissible depletion by current rotationally-induced mixing models to < 0.1 dex .

Correcting for the GCE contribution to both ^ { 6 } Li and ^ { 7 } Li , we infer a primordial abundance A ( Li ) _ { p } \simeq 2.00 dex , with three systematic uncertainties of up to 0.1 dex each depending on uncertainties in the effective temperature scale , stellar atmosphere models , and correction for GCE .
( The effective-temperature zeropoint was set by Magain ’ s and Bell & Oke ’ s b - y calibrations of metal-poor stars , and the model atmospheres are Bell ’ s , without convective overshoot . )
We predict that observations of Li in extremely low-metallicity stars , having [ Fe/H ] < -3 , will yield smaller A ( Li ) values than the bulk of stars in this sample , consistent with a low primordial abundance .

The difference between our field star observations and the M92 data in the literature suggests that real field-to-cluster differences in Li evolution may have occurred .
This may indicate different angular momentum evolutionary histories , with interactions between protostellar disks in the dense globular cluster environments possibly being responsible .
Further study of Li in globular clusters and in very metal-poor field samples is required to clarify the situation .