** Mannatech MTEX.O surges ~24 pct to $19.70 as
nutritional supplements seller commences $16 mln buyback
** Stock biggest pct gainer across U.S. exchanges with
volumes well above average
** Starts modified Dutch auction to buy back stock at
between $18.50 and $21.00 each urn:newsml:reuters.com:*:nBwb9gBLwa
** Buyback value constitutes about a third of company's
market cap
** About 172k MTEX shares traded, ~a tenth of free float and
on course for busiest day since November 2014
** More than erases the ~16 pct loss in 7 sessions since
reporting Q1 results
(Reporting by Savio D'Souza)
((RM:savio.dsouza.thomsonreuters.com@reuters.net))