FORT WORTH — Chamber music fans have often pondered the origins of all the new professional string quartets, which suddenly appear as if from nowhere.This question was emphatically answered Wednesday evening as two newly formed string quartets of 20-something virtuosi took the stage at TCU’s PepsiCo Hall as part of the Mimir Chamber Music Festival.Both quartets are part of the revamped young artist program, which in the past was aimed at giving emerging solo players chamber music experience. This time it is designed for coaching two already formed quartets and giving them exposure. First up was the Vivant String Quartet, making its first appearance. The members are violinists Kevin Smith and Leah Greenfield, violist Robert Switala and cellist Elaine Whitmire. Three of these friends played together in the University of North Texas orchestra; the outstanding violist was discovered by the others last summer when they were all students at Mimir. They set a fearsome task for themselves by playing Prokofiev’s String Quartet in F Major, Op. 92, subtitled On Kabardinian Themes (because of the regional folk elements the composer so liberally scattered throughout the piece.) No worry. The four gave it a first-class performance. It was hard to believe it was their first performance and, the expert coaching from the Mimir artists notwithstanding, their natural synergy bodes well for the future. All of the composer’s rough-hewn percussiveness was there, yet never once was it overplayed. The more reflective passages were given room to sing, and the finale flashed with virtuosity. The Axiom Quartet took over for the second half of the program. This group is made up of alumni from the Shepherd School of Music at Houston’s Rice University. They are now affiliated with the much more remote Music Preparatory School at the University of St. Thomas. The members are violinists Juliette Javaheri and Dominika Dancewicz, violist Dawson White and cellist Hellen Weberpal. While they are not newly assembled, and although this group overflows with talent, they are still finding their “quartet” voice. The two violinists switched places for the second selection and that seemed to be a better arrangement, but balance was an issue in everything they played. The spirit of Prokofiev must have hung in the air, because the Axiom gave Antonín Dvorák’s more genteel “American” String Quartet in F Major, Op. 96, a more aggressive performance than necessary. Their style of playing was much better suited to the work that followed: Astor Piazzola’s Four for Tango for String Quartet. While this piece wasn’t nearly as bare-knuckled as Stephen Seleny’s program notes led us to believe, the Axiom gave it plenty of the sweaty, sultry and subversive nature that makes the tango so popular.